TEENAGERS ON MOTOR
Dave Williams
-> brake, that I never used the back brake, I.E. not properly utilizing
-> both brakes.
Proper utilization of the back brake is to keep from rolling when
you're at a stop light. Otherwise, you should be able to stop hard
enough to lift the rear wheel clear of the ground, whence it doesn't
matter if you're using the rear brake or not. <grin>
Noemi Berry
> Proper utilization of the back brake is to keep from rolling when
>you're at a stop light. Otherwise, you should be able to stop hard
>enough to lift the rear wheel clear of the ground, whence it doesn't
>matter if you're using the rear brake or not. <grin>
i've heard this argument many times, and it always leaves me with
two questions. i have always wondered why people say they don't use
the rear. ok, so 75% + of your braking is in the front, and max
braking is when you've done a "stoppie". but until the time the
rear wheel leaves the ground, isn't that rear brake contributing to
your stopping? up to 25%? i guess i don't see any reason NOT to
use it, though many people say they don't.
also, i suppose MSF advocates using both for a Good Reason.
at the very least, i don't see any reason NOT to use both.
forgive me for dredging up another cyclical discussion (if this is).
but i've been reading r.m. less than a year and haven't seen this
cycle yet :-).
noemi
-------------------------------------------------------------------------------
no...@cs.ucla.edu KotSBL '82 kawasaki kz305csr dod #443
4580 miles to ride until NM (Next Motorcycle)
reduce traffic: increase your speed
-------------------------------------------------------------------------------
Jonathan E. Quist
>In "A Twist of the Wrist," Keith Code introduces the concept that each
>rider, at any given time, has $10 of concentration, to "spend" on
>whatever they want. Once the $10 is spent, there is no more (ie, human
>concentration is finite).
Unless you have a BMW, in which case you can spend as much as you want,
or a Harley, in which case you've already spent your $10.
--
Jonathan E. Quist INTERACTIVE Systems Corporation
j...@i88.isc.com Naperville, IL
'71 CL450-K4 "Gleep" - "Worth the Oppression"
DoD #094 "I'm not mad; just terribly, terribly hurt." HDM #007
Mark Joseph Andy
[noemi :-) asks about rear brake usage]
I ride on both the track and the street, and I find that I use
different braking techniques for both. On the street (on my Seca) I
tend to use both brakes. I'm almost never stopping with full power,
and use the rear brake to help out and fine tune the front brake. On
the track, I very rarely use the rear brake. The few times I have
have been when I thought that the front wouldn't slow me fast enough.
Also, on the track, usually engine braking is all the rear brake I
need. Also, I'm concentrating on being smooth, while slowing down
fast, and getting to a proper gear, as well as lines, position, etc.
I really don't have time for the rear brake. Now that I'm riding an
RZ on the track (no engine braking), I may modify this a bit, but at
present it seems that there are other areas where I have much more
room for improvement.
Mark
| Mark J. Andy WERA NV #813 AMA #700560 |
| ma...@cmu.edu KotR DoD #813 |
| Co-founder/Rider, Team RISC and Two Geeks Racing |
| '82 Seca 650 (street), '85 RZ350 (track), '78 KM100 (commute-in-snow) |
| I want RZ350 parts like steering damper, fork brace, SS lines, etc... |
Robert Fridman
> > Proper utilization of the back brake is to keep from rolling when
> >you're at a stop light. Otherwise, you should be able to stop hard
> >enough to lift the rear wheel clear of the ground, whence it doesn't
> >matter if you're using the rear brake or not. <grin>
>
> i've heard this argument many times, and it always leaves me with
> two questions. i have always wondered why people say they don't use
> the rear. ok, so 75% + of your braking is in the front, and max
> braking is when you've done a "stoppie". but until the time the
> rear wheel leaves the ground, isn't that rear brake contributing to
> your stopping? up to 25%? i guess i don't see any reason NOT to
> use it, though many people say they don't.
>
Since the rear wheel is offloaded, it looses traction (it only gets
about 25% of the available traction that is) which means it would take much
less force to lock up the rear wheel (about 25% of the force it would take
if rear wheel was not offloaded ;).
As it was mentioned before in this group, a locked up rear wheel will
tend to slide since the traction it experiences is equal no matter which way
it goes. Now, since a sliding rear wheel goes wherever it wants to or
wherever the road and the chassi push it, chances are that it will move
out from the line of motion since the road is not perfectly flat and the
chassi is compressing from the front breaks (ie: fishtail). So if by this
time your heart has not blocked your throat and started asfixiating you, you will
realize that the front wheel is pointing straight ahead (assuming you
countersteer correctly) and the rest of the bike is pointing to the side
while both of you are actually going forward with some speed. Don't forget
that you are on a motorcycle (and not a bicycle) which weighs about 500lbs + you.
Now, if you release the rear brake and the back wheel regains traction, it
will prefere (in fact insist) on traveling along the path of least
resistence, which in this case is to the side remember (since its already pointing
that way). But your front wheel is pointing forward! So as a compromise
between your front and rear tyres, your motorcycle will sort of flip over
and buck you off!
Voilla, you are experiencing a high-side!
The morale of this story is that the rear break does not offer signifficat
stopping power in a HARD stop from speeeeed. The chance of it getting locked
up is so high in a pannic stop that it is not worth risking it. Use the rear
at very low speeds only.
RF.
Martyn J. Wheeler
>Hmmm, $10 bills for concentration and ice cream cones for wheels. Am I
>Mr. Analogy or what? :^)
So how many ice cream cones do you get for $10, and how much of that
goes to the AMA Museum? :-)
--
Martyn
---------...@unx.sas.com----(Martyn J. Wheeler)----DoD #293------------
SAS Institute, Inc: (919) 677-8000 ext.7954 H: (919) 839-0092 (Raleigh, NC)
"If you spin, you deserve to die" -- Mike Hawthorn
"I love competition, but I hate conflict" -- Alain Prost
Kenneth J. Hendrickson
This is such a good idea--I wonder how long it will take before this is
standard on all bikes. I'm a conservative, and I think big government
is bad government, but this is one case where some stinking lawyer might
be able to do some good: pass a law mandating anti-skid brakes (at least
on the front) for all new motorcycles.
--
"Arguing about predestination is virtually irresistible." --RC Sproul
Ken Hendrickson N8DGN/6 k...@usc.edu ...!uunet!usc!pollux!kjh
Chris Malcolm
>>you should be able to stop hard
>>enough to lift the rear wheel clear of the ground, whence it doesn't
>>matter if you're using the rear brake or not. <grin>
>i have always wondered why people say they don't use
>the rear. ok, so 75% + of your braking is in the front, and max
>braking is when you've done a "stoppie". but until the time the
>rear wheel leaves the ground, isn't that rear brake contributing to
>your stopping? up to 25%?
>also, i suppose MSF advocates using both for a Good Reason.
>at the very least, i don't see any reason NOT to use both.
It is true that in good conditions a maximum stop needs only the front
brake, since the rear wheel is on the point of leaving the ground. Now
let us suppose that you are called upon to do a 1/2 maximum stop,
which we _assume_ happens to be a 1/2G, and that under this braking
effort it happens that 60% of the weight is on the front wheel, and
40% on the back. If you stop with only the front wheel you are
retarding one bike weight at 1/2 G with the wheel bearing 60% of its
weight, and consequently require a coefficient of friction of 50/60 or
.83 to stop without losing the front wheel. If you stop with (say)
equal braking on each wheel (this is not the optimum case), then you
will need 25/40 or .63 coeff of friction to stop the back wheel
skidding, and .42 to stop the front wheel from skidding.
So, now let us suppose two bikers braking at 1/2G side by side, one of
them using only the front wheel, and one using 50/50 (non-optimal)
front/rear braking. They are both stopping at the same rate. Then
their wheels encounter a little patch of slightly greasy road with a
coefficent of friction of .8 -- still quite a good value. The biker
using only his front brake goes into a front wheel skid and crashes.
The biker using both brakes still has a safety margin of (.8-.63)/.8 =
21%, and brakes quite safely to a stop.
Now let us suppose that the coefficient of friction actually drops to
.6. The stupid front-wheel-only biker still crashes of course. The
50/50 two-wheel braking biker goes into a rear-wheel skid. These are
easy to control if you are going in a straight line, and he stills
comes safely if excitingly to a stop.
And the third biker, on the Moto Guzzi with linked brakes, didn't even
skid, and didn't even have to think about balancing the braking :-)
In other words, when doing less than the maximum stop, using both
brakes, even distinctly non-optimally, gives you substantially more
safety margin against skidding.
And just recall where you are most likely to find patches of greasy
road? That's right, at halts, where cagers and Harleys :-) like to
leave their mark, not to mention the residue of the occasional
accident.
(And if using both brakes is beyond you, you'ld be a lot safer on a
Moto Guzzi.)
--
Chris Malcolm c...@uk.ac.ed.aifh +44 (0)31 650 3085
Department of Artificial Intelligence, Edinburgh University
5 Forrest Hill, Edinburgh, EH1 2QL, UK DoD #205
John Robert Nadzam
being a lowly undergrad I'll be taking off for the summer, somewhere in
the pennstate area in 2-3days. You can e-mail to me I just wont get it
for 3 months.
see ya in about 6,000 mi
NADZ
DoD #528
Thomas Price
>
> [stuff deleted about the dangers of using the back brake and locking
> it up when it's out of line with the front, and high-siding]
>
>The morale of this story is that the rear break does not offer signifficat
>stopping power in a HARD stop from speeeeed. The chance of it getting locked
>up is so high in a pannic stop that it is not worth risking it. Use the rear
>at very low speeds only.
Jerry, Mojo, anyone, I thought that the MSF answer to this was to
get into the habit of straightening the bike up and getting on both
brakes.
If someone would like to make a reasoned argument that doing so is
not the best thing to do with a modern sportbike (on the street)
I'd like to hear it.
(Additionally, if anyone would like to contradict the MSF flat out, that
might be interesting, although I doubt I'd agree. That is, of course,
presuming that I've correctly represented the MSF's position.)
Tom
PS I skidded my rear wheel in a turn once, but when I hooked up
again, it was very controllable -- probably because I run a Conti
Blitz on the back (ugh!) and it's never sticky enough to flip me :-)
******************************************************************************
tp...@cs.cmu.edu | Mr Ethical | "Tune in, turn on, drop out" - Timmy Leary
1981 KZ440LTD | DoD #518 | "First do no harm" -- Hippyocrates
******************************************************************************
Sean Casey
have? I wonder how small and lightweight it could be. Being able to
simply jam both brakes is an extremely attractive concept. I wonder if
it could be made small enough to fit most bikes?
Sean
--
|``Wind, waves, etc. are breakdowns in the face of the
Sean Casey | commitment to getting from here to there. But they are the
se...@s.ms.uky.edu | conditions for sailing -- not something to be gotten rid
U of KY, Lexington| of, but something to be danced with.''
John Robert Nadzam
my .02$
I dunno, I find my rear brake very useful, Yes I know my front brake
provides the majority of my braking power, but I often use it to help
bleed off speed in corners I've gone into too fast, to help me jockey my
suspension in corners, to sit at red lights and make my rear end bounce
up and down........well,
but yes I have taken the MSF and I do find it very helpful in stopping
quickly.I didn't notice how much I missed that 25% untill recently
(read:now) my rear brake is inoperative due to the fact I think I have
air in the lines somewhere.
NADZ
DoD #528
Dances With Bikers
>This makes me wonder. How much extra hardware does a bike with ABS
>have? I wonder how small and lightweight it could be. Being able to
>simply jam both brakes is an extremely attractive concept.
Attractive perhaps, but a good way to dump it, even with ABS. Because
all current ABS systems work on a lock-release-lock-release cycle, they
ONLY work when the bike is vertical. If you rely on ABS in a corner the
lock-release-lock-release cycle will work more like lock-slide-crash.
Whether the crash is a low side or a high side is left as an excersize
for the student with an ABS bike.
--
Blaine Gardner @ Evans & Sutherland 580 Arapeen Drive, SLC, Utah 84108
blga...@javelin.sim.es.com uunet!javelin.sim.es.com!blgardne BIX: blaine_g
DoD #46 My other motorcycle is a Quadracer. FJ1200
Dave Tharp CDS
>>you're at a stop light. Otherwise, you should be able to stop hard
>>enough to lift the rear wheel clear of the ground, whence it doesn't
>>matter if you're using the rear brake or not. <grin>
>
transfer of weight to the front wheel under racing conditions is close
to 100%. If you watch the GP riders on TV braking at the end of a
straight, light is often seen under the rear tire. Keith Code teaches
his students to ignore the rear brake entirely at the California
Superbike school. The restriction on this technique is that it must be
done with the bike bolt upright, and on smooth, dry pavement. Locking
the front wheel leads to immediate disaster, and in a corner, traction
is shared between cornering forces and braking forces. Therefore, in
racing, braking and gear selection must be done *before* entering a
corner because all traction is spent in cornering forces if you're doing
it right.
>i've heard this argument many times, and it always leaves me with
>two questions. i have always wondered why people say they don't use
>the rear. ok, so 75% + of your braking is in the front, and max
>braking is when you've done a "stoppie". but until the time the
>rear wheel leaves the ground, isn't that rear brake contributing to
>your stopping? up to 25%? i guess i don't see any reason NOT to
>use it, though many people say they don't.
>
On the street, the surface conditions seldom approach a good racetrack,
and your mission does not require maximum cornering speed. The rear
brake must be used in combination with the front brake, particulary on
loose surfaces. Learning to use both brakes to produce best braking
performance under varying road conditions is difficult and only comes
with lots of riding experience. For example, in deep gravel or on sandy
surfaces, the rear brake is much more effective because the front will
wash out immediately (unless, of course, you're on a steep downhill on
one of the above surfaces, in which case either brake with crash you).
Riders with off-road experience have an advantage over street-trained
riders, because they know how to deal with variable surfaces.
>also, i suppose MSF advocates using both for a Good Reason.
>at the very least, i don't see any reason NOT to use both.
>
The MSF folks have the right idea, but they cannot possibly teach all
the necessary techniques of braking in the brief time alloted to them. The
best that they can do is to try to give you some ideas that might keep
you alive during the time in which you are gaining experience.
>no...@cs.ucla.edu KotSBL '82 kawasaki kz305csr dod #443
Oh, you might add '55-'69 BMW 500 and 600cc twins to the SBL. They're
kickstart only, but are incredibly easy. And the seat height is
adjustable on the solo saddle down to about 27".
-----------------------------------------------------------------------------
| Dave Tharp | DoD #0751 | "You can't wear out |
| da...@loowit.WR.TEK.COM | MRA #151 | an Indian Scout, |
| '88 K75S '48 Indian Chief | AHRMA #751 | Or its brother the Chief.|
| '75 R90S(#151) '68 CB450(#751) | AMA #524737 | They're built like rocks |
| '65 R50/2/Velorex '57 NSU Max | | to take the knocks, |
| | (Compulsive | It's the Harleys that |
| My employer has no idea. | Joiner) | give you grief." |
-----------------------------------------------------------------------------
Jonathan E. Quist
>Is a Moto Guzzi the *only* motorcycle with anti-skid brakes?
>
>This is such a good idea--I wonder how long it will take before this is
>standard on all bikes. I'm a conservative, and I think big government
>is bad government, but this is one case where some stinking lawyer might
>be able to do some good: pass a law mandating anti-skid brakes (at least
>on the front) for all new motorcycles.
You never heard of a small manufacturer by the name of "BMW"?
And no, this would not be a good thing a law required anti-lock brakes
(there's no such thing as anti-skid, no matter what they tell you) on
only the front wheel. Might actually cause more accidents then it
would prevent. You need the ABS on the rear, for stability. If you
lose braking traction on the rear, but still have good braking traction
on the front, the rear end get's envious and decides it wants to be
in front instead. That's why there is a law requiring rear ABS on some
vehicles (like passenger vans, which have brakes designed to handle
max gross weight, but spend most of their time near empty weight, making
wheel lockup much more likely).
Technical issues aside, there are some of us who _like_ to be permitted
to control our machines; some who occaisionally want to lock a rear wheel
to break the rear end loose.... and some of us who are simply willing
to accept the responsibility of learning our limits and staying within them.
David Svoboda
|The biker
|using only his front brake goes into a front wheel skid and crashes.
Just a small point: a front wheel skid != a crash. I suggest that if you
find yourself terrified by skidding the front, take a ratbike out in the
rain and learn how to do it. Just because you lose stability for a
moment doesn't mean you can't regain it the next (let off the damn brake,
okay? :-), and a front skid doesn't cause a highside. It's not
comfortable, exactly, but you don't have to crash, either.
I dropped my beloved Connie twice, emergency braking in the rain, before
I said "screw it", and practiced sliding with her. (Those double disks are
*powerful*.) I haven't made a braking mistake since. And, of course,
there are the slippy-slide sessions with the ratbike on an icy parking lot
in the middle of the winter. (Wear armor, and don't care about the bike. :-)
Don'cha just hate it when ya drop a new bike before ya make the first
loan payment? :-(
Dave Svoboda (ru...@gagme.chi.il.us) | An Angel on a Harley pulls
90 Concours 1000 (Mmmmmmmmmmmm) | across to greet a fellow
81 KZ550 Rat (The Little Engine That Could) | Rolling Stone.
AMA ...let me get it out..oh yes... 583905 | He says, "Where ya been,
DoD #0330 "LtF,FtL Geeks R Us" | where ya 'goin?" -- R Waters
Ed Green - Pixel Cruncher
> Therefore, in
>racing, braking and gear selection must be done *before* entering a
>corner because all traction is spent in cornering forces if you're doing
>it right.
To pick a nit, on the street, braking and gear selection should be done
before entering a corner, also.
>On the street, the surface conditions seldom approach a good racetrack,
>and your mission does not require maximum cornering speed.
OTOH, on the track, unexpected hazards never approach those on the
street. Since emergency straight-line braking is often necessary on
the street, seldom with any warning (unlike entering a turn on the
track), using the technique best suited to that, for all one's braking,
reenforces the technique, and the skill with which it can be applied.
IMHO, that technique is 100% concentration on the front, including
letting off when it starts to slide.
---
Ed Green, former Ninjaite |I was drinking last night with a biker,
Ed.G...@East.Sun.COM |and I showed him a picture of you. I said,
DoD #0111 (919)460-8302 |"Go on, get to know her, you'll like her!"
(The Grateful Dead) --> |It seemed like the least I could do...
Tim Keller
>will prefere (in fact insist) on traveling along the path of least
>resistence, which in this case is to the side remember (since its already pointing
>that way). But your front wheel is pointing forward! So as a compromise
>between your front and rear tyres, your motorcycle will sort of flip over
>and buck you off!
>
>Voilla, you are experiencing a high-side!
>
>The morale of this story is that the rear break does not offer signifficat
>stopping power in a HARD stop from speeeeed. The chance of it getting locked
>up is so high in a pannic stop that it is not worth risking it. Use the rear
>at very low speeds only.
>
> RF.
the front brake and the rear wheel is locked, you have three forces: the front
braking force, the rear tire sliding force, and your momentum acting through
the center of gravity of the bike (above the wheel axis). When the rear tire
slides to the side your cg shifts out of line with the front tire, and the
only thing preventing you from "passing" your front tire is the force exerted
by the sliding rear wheel. When you release the rear wheel, there is no longer
any force opposing this "passing" effect. Your cg is now to one side of the
bike and instead of your momentum compressing the front forks as it would if
your cg was in line with the front tire, it simply pivots the bike around the
steering column and the front axle. Thus the "bucking" effect, and a rather
unpleasant feeling of freedom from the bike.
I suppose you could argue that by releasing the front brake you might
be able to unlock the rear wheel, but something tells me you'll crash either
way. Maybe that's why the MSF instructors told us to keep that rear wheel
locked once you lock it.
Tim,
Charlie Preston
It sounds almost like the buses in this area. It especially effective for
warning pedistrians and drivers who have windows open.
David H. Wise
>>you're at a stop light. Otherwise, you should be able to stop hard
>>enough to lift the rear wheel clear of the ground, whence it doesn't
>>matter if you're using the rear brake or not. <grin>
>
>i've heard this argument many times, and it always leaves me with
>two questions. i have always wondered why people say they don't use
>the rear.
Me too. Even though I can get by using only the front (or only the rear!)
the majority of times I brake, I always use both to keep in practice. That
one time when maximum braking can save your life, you'd better do it right.
Both wheels at impending skid for the entire duration of the maneuver. You
just can't stop quicker than that.
> ok, so 75% + of your braking is in the front, and max
>braking is when you've done a "stoppie".
Unless your bike doesn't do stoppies. Lots of bikes lock the front wheel
first.
> but until the time the
>rear wheel leaves the ground, isn't that rear brake contributing to
>your stopping? up to 25%? i guess i don't see any reason NOT to
>use it, though many people say they don't.
>
>also, i suppose MSF advocates using both for a Good Reason.
>at the very least, i don't see any reason NOT to use both.
The front/rear ratio varies with time. When you start to brake,
the rear wheel still has a lot of weight on it. As the forks compress,
some of that weight shifts to the front, and you can increase front
braking force if you want, but you _must_ decrease rear braking force.
To get the most braking, this is a delicate balancing act and needs
mucho practice, and it's different for every bike.
>
>forgive me for dredging up another cyclical discussion (if this is).
>but i've been reading r.m. less than a year and haven't seen this
>cycle yet :-).
I use the metaphor of a blender with a small outlet at the bottom. Sure,
you keep throwing new things in the top, but most of the stuff inside goes
round and round quite a few times before it leaves.
>noemi
>-------------------------------------------------------------------------------
>no...@cs.ucla.edu KotSBL '82 kawasaki kz305csr dod #443
> 4580 miles to ride until NM (Next Motorcycle)
> reduce traffic: increase your speed
>-------------------------------------------------------------------------------
--
--
David Wise (DoD#427, dav...@orca.wv.tek.com, Packy -- 1980 CX500C)
Dave Tharp CDS
Yes, it's the only bike whose brakes are so bad that they cannot be
locked, and which has so little power , the rear tire cannot be spun.
>
>This is such a good idea--I wonder how long it will take before this is
>standard on all bikes. I'm a conservative, and I think big government
>is bad government, but this is one case where some stinking lawyer might
>be able to do some good: pass a law mandating anti-skid brakes (at least
>on the front) for all new motorcycles.
>
>--
least as much sense as this post.
Dave Tharp CDS
>This makes me wonder. How much extra hardware does a bike with ABS
>have? I wonder how small and lightweight it could be. Being able to
>simply jam both brakes is an extremely attractive concept. I wonder if
>it could be made small enough to fit most bikes?
>
battery. The big hydraulic actuators are the main problem. The pickups
consist of gear-like gismos on the wheels and what appears to be a
reluctance coil on the brake mounts. The computer box is under the
seat. The rear actuator is behind the right footpeg, and the front is
buried under the tank behind the radiator.
Martyn J. Wheeler
>Is a Moto Guzzi the *only* motorcycle with anti-skid brakes?
>
>This is such a good idea--I wonder how long it will take before this is
>standard on all bikes. I'm a conservative, and I think big government
>is bad government, but this is one case where some stinking lawyer might
>be able to do some good: pass a law mandating anti-skid brakes (at least
>on the front) for all new motorcycles.
A couple of points:
1) Moto Guzzis do not have "anti-skid" (by which I assume you mean
anti-lock) brakes, they have linked brakes. So do Goldwings and (I
think) a few others. What this does is to operate the rear brake disc
and one of the front discs with the foot pedal, and the remaining
front disc with the hand lever. You can still lock up the brakes if
you stab them.
2) Lawyers are the reason we *didn't* get anti-lock brakes in the US
until BMW, then Honda & Yamaha, were brave enough to say "stuff the
product liability suits." Putting safety improvements on some bikes
in your line and not others is tantamount to admitting you make some
unsafe bikes, at least in US law.
Anyway, (I suppose this is a third point?) you only *need* anti-lock
brakes if you are deficient in braking skills. What about the Honda
TCS -- should that be mandatory? I assume you ride an ST1100, because
it's the only safe bike? I suppose you would also support passing a
law installing governors on bikes so they can't exceed the national
speed limit? Brakes are still limited by the laws of physics, which
(simplified) say "you can't brake really hard while leaned over."
Law student, are you? :-)
S.Fielding-Isaacs [WWFO Eng Wordsmith]
>In article <1992May14....@cs.ucla.edu> no...@maui.cs.ucla.edu (Noemi Berry) writes:
>>
>>> Proper utilization of the back brake is to keep from rolling when
>>>you're at a stop light. Otherwise, you should be able to stop hard
>>>enough to lift the rear wheel clear of the ground, whence it doesn't
>>>matter if you're using the rear brake or not. <grin>
>>
>>i've heard this argument many times, and it always leaves me with
>>two questions. i have always wondered why people say they don't use
>>the rear.
>
>Me too. Even though I can get by using only the front (or only the rear!)
>the majority of times I brake, I always use both to keep in practice. That
>one time when maximum braking can save your life, you'd better do it right.
>Both wheels at impending skid for the entire duration of the maneuver. You
>just can't stop quicker than that.
Yet Another Cyclical Argument (YACA).
Until recently, I was of the "use only the front brake" school of thought.
I have years of experience riding modern sportbikes and racing. I firmly
believe that in a panic situation, using only the front brake is the
correct action. By definition, one only has split seconds to react
(because if one were prepared, one wouldn't be in a panic situation, would
one? :->). In this sort of situation, those actions which we have
practiced and mastered will go a long way to saving us. In this case,
the full-bore stop with the front brake.
I would venture to say that 95% or better of street riders don't know
the limits of their front brake. Consequently, they use it timidly or
not to its full capacity. The only way around this is to practice
high-speed and low-speed full-bore stops.
Even after you have mastered this skill, you still have to worry about
processing what is happening around you, choosing which one of your
escape routes will keep your ass intact, and making sure that no one
else is going to clean your clock while you execute your avoidance
maneuver.
Lots and lots to think about, while your adrenaline is kicking in,
and you are trying to hold that front brake at impending lockup and
keep your weight positioned so that you can move the bike rapidly.
You have NO time to waste. No concentration to waste on the rear brake.
Some argue (the MSF proponents out there) that practice makes a big
difference, that you get better braking from using both brakes.
I would argue in response that this is in a test-bed condition.
Real life really agrees so nicely with the test strip. It is near
impossible to simulate the rush of adrenalin (unless you want to
dig up that inhaler :->) that you get when your life is threatened.
I think it evident that the vast majority of people would have a
great deal of difficulty writing their name intelligibly after
such an incident. It is too much to ask of the average rider that
s/he feather the rear brake in proportion to the weight shift
(so as to prevent rear wheel lockup). It's just not going to happen.
Most riders will just jump on that rear brake and lock the rear,
unable to steer the bike with any confidence.
>> ok, so 75% + of your braking is in the front, and max
>>braking is when you've done a "stoppie".
>
>Unless your bike doesn't do stoppies. Lots of bikes lock the front wheel
>first.
>
>The front/rear ratio varies with time. When you start to brake,
>the rear wheel still has a lot of weight on it. As the forks compress,
>some of that weight shifts to the front, and you can increase front
>braking force if you want, but you _must_ decrease rear braking force.
Having said all I have said above, I rode my old roommates Beemer.
What a piece of junk (R75/6 I think). Front brake was a total joke.
Just had to use the rear brake a lot (while carrying a passenger).
Seemed like it didn't have a hope in hell of lifting the rear under
braking (and it was in good to excellent mechanical condition).
So I suppose that I must narrow the focus of my comments/statement
above. If your bike is anywhere near a modern sportbike I think
you should be using the front brake to the exclusion of the rear.
Point being that a well-used front brake will outperform a poorly
used front and rear brake combination.
However, in the event that you ride a bike with a less than optimal
weight distribution I can see that one would have to make the
best of a bad situation and use the rear brake. I think the
MSF comments are pertinent here. I think that they should consider
revising their curriculum in the light of advances in technology
and construction of bikes. Also keeping in mind that test-bed
conditions to not approximate the reactions of riders in real-life
crises.
--
Stefan Fielding-Isaacs me: (415) 599-4876 me at them: (415) 688-9400
dba Live-Wire Technologies (a consulting technical writing firm)
AMA/CCS #5, Regional Lightweight Supersport, Superbike, Grand Prix Champion
mail: race-r...@formula1.Corp.Sun.COM to join motorcycle roadracing list
Noemi Berry
>the rear wheel still has a lot of weight on it. As the forks compress,
>some of that weight shifts to the front, and you can increase front
>braking force if you want, but you _must_ decrease rear braking force.
but WHY?! (said the annoying five-year-old repeatedly)
not that i don't believe you, but i don't understand why holding that rear
brake will make any difference. provided you were to use the front
brake with maximum effectiveness (not always a given, i know), why
must you _decrease_ rear braking force? at the very worst, won't
it just become ineffective as the forks compress and weight is shifted
forward, or will it truly hamper the front braking?
a lot of answers to the rear-brake-usage question have gone by, including
$10 allocation of concentration (Code's method, a la Ed Green), "because
if you lock the rear and release it you high-side" (not exactly an
*answer*!), because MSF says so, because it's not useful on the racetrack
... the usual DoD consensus! :-)
i am inclined to agree with MSF about using both, but learned
a lot from going on a rear-brake "diet" for 2 weeks, that is,
only using the front. i learned that i was getting a LOT more
braking from the rear than i thought (in a normal street situation)
(hence my question about why people say they don't use it), and
that i was cheating with the rear to create the illusion of smooth
front braking, and that really, my "front braking technique" is
an oxymoron.
and so i find myself in MSF:RSS to get the very basics in (again),
more for braking technique than anything. just in time -- before
my experience level tips over into the "i know what i'm doing"
skepticism, and out of the beginner's "tell me more, fill me
with knowledge O Great Ones" open mind. :-)
>I use the metaphor of a blender with a small outlet at the bottom. Sure,
>you keep throwing new things in the top, but most of the stuff inside goes
>round and round quite a few times before it leaves.
heh heh.
noemi
-------------------------------------------------------------------------------
no...@cs.ucla.edu KotSBL '82 kawasaki kz305csr dod #443
4440 miles to ride until NM (Next Motorcycle)
Mark Slagle
[...a lot of hand-waving arguments for front brake only...]
> and construction of bikes. Also keeping in mind that test-bed
> conditions to not approximate the reactions of riders in real-life
> crises.
When I was up in BC a few years ago, riding at an extremely high
rate of speed between Prince George and Jasper (daylight til around
11pm, neat), a good sized elk dashed into the road in front of me
and grew roots when he saw me. I got on both of my brakes hard,
let up some on the back one, and had the front tire howling like
a banshee all the way down to about two or three miles an hour.
At this point I was about fifteen feet from the beast who, seeing
that I was just some speed-crazed hoodlum on a motorcycle, lost
interest and pranced off into the woods. Heaving a great sigh
of relief, and rushing like a freight train on adrenaline, I
forgot to put my feet down. The stories about adrenaline strength
must be true, because I eventually got a foot off the peg and
onto the pavement and caught the bike and hauled it back from
about thirty degrees lean without more than a grunt. Of course,
I then almost dropped it over the other side from the momentum
and vertigo, but soon was able to negotiate it over to the side
of the road and onto the side stand whereafter I hyperventilated
and thanked all the deities I could think of until I got back on
the mellow side of my self.
So what's the point of this ramble? Well, I was about fifteen
feet from becoming an antler ornament for an animal about as
big as a good sized, ten-year old Cadillac Coupe de Ville. It
is my contention that if I hadn't used both brakes, I would now
be road kill in the middle of nowhere Canada where the pavement
never wears out. If I hadn't taken the time and effort to practice
panic stops over the years, I would not have the great pleasure of
regaling you all with this tale (some would no doubt find that an
improvement, but I will stubbornly disagree). So don't give ME
that crap about real life not being unlike test conditions. Sure
it's different: In the test you don't die if you fuck it up.
That's the difference. But the brakes still work best when you use
both. You use only one if you want, but I want to stop before
planting my face in elk intestines.
--
----
Mark E. Slagle PO Box 61059
sla...@lmsc.lockheed.com Sunnyvale, CA 94088
408-756-0895 USA
Mark Slagle
> not that i don't believe you, but i don't understand why holding that rear
> brake will make any difference. provided you were to use the front
> brake with maximum effectiveness (not always a given, i know), why
> must you _decrease_ rear braking force? at the very worst, won't
> it just become ineffective as the forks compress and weight is shifted
> forward, or will it truly hamper the front braking?
As you apply the brakes the weight of the bike shifts to the
front wheel. The reduced weight on the back wheel causes it
to be more inclined to lock up and skid. When the back wheel
is sliding already it has the unpleasant tendency to slide
sideways as well as in the direction of travel. This is due
to the condition of being in a state of unstable equilibrium
with respect to the hinge at the front forks, sorta like trying
to hold a pendulum upside down. You can do it, but it's not
easy and requires constant corrections to keep it there. This
distracts from the task at hand, which is not hitting some
obstacle in your path.
Now, not only does having the rear wheel out to the side make
it a hell of a lot more difficult to stop (and it takes farther),
the rear wheel also has the disconcerting tendency to grab the
pavement abruptly if you let off the rear brake when it is out
there. This causes the unwary rider to do a Superman imitation
over the "high side". Speaking from experience, this is only
enjoyable for the first few moments. Upon performing an expert
three point landing on the pavement, the experience becomes
somewhat less delightful thereafter. You may confirm this
information for yourself if you wish, although I do recommend
against it.
Michial Gunter
>The front/rear ratio varies with time. When you start to brake,
>the rear wheel still has a lot of weight on it. As the forks compress,
>some of that weight shifts to the front, and you can increase front
>braking force if you want, but you _must_ decrease rear braking force.
The main reason the force at the front tire increases (and the force
on the rear decreases) is that the bike is attempting to rotate about
its center of mass:
\ _
\ \
---------------- ---> | (Rotation about the center of mass.)
| | /
| X | <--
xxx ^ xxx
x x | x x
x x | x x
x x | x x
<---xxx | <---xxx
|
|
Center of Mass
Try braking hard on a bicycle. No compression of suspension (unless your very lucky)
but the amout of force on the tires shiftes. In fact a bicycle is a
good place to practice maximum braking (and the only one I have at the moment:-(().
not that i don't believe you, but i don't understand why holding that rear
brake will make any difference. provided you were to use the front
brake with maximum effectiveness (not always a given, i know), why
must you _decrease_ rear braking force? at the very worst, won't
it just become ineffective as the forks compress and weight is shifted
forward, or will it truly hamper the front braking?
As the amount of down-force at the rear tire decrease, you better
decrease the force you expect it to exert to slow the bike down,
or it will do it for you (by sliding.)
LF @ C * DF
C = Coefficient of friction
(fixed for a given tire and surface --- that is, not fixed enough)
LF = Lateral force
DF = Downward force
Friction comes in two flavors: static and sliding. Static is
higher is all cases and is much higher for a tire.
You don't slide as long as @ is <=. When that is not the case the
back tire is going to slide causing a drastic decreases in the amount
of available stopping power there as well as the more troublesome
problems of the rear sliding out of line with the front.
mike
Mike Coleman
[about practicing front wheel skids]
I was intrigued by this, so I did a little playing with it in the parking lot
a few minutes ago. In runs starting at about 15-20mph, I applied either the
front or the rear brake and was able to lay long skids without any feeling of
side-to-side instability. Front skids were more unnerving because the front
end would start "chattering" (vibrating heavily) when the wheel hit a
less-slick spot. I should add that the concrete in the parking lot was very
smooth, making skidding easy.
Mike
--
--Mike Coleman (col...@cs.ucla.edu), Ringmaster, Boelter Hall Roach Circus--
It could be worse--you could be bicycling in Florida.
Dances With Bikers
>>Is a Moto Guzzi the *only* motorcycle with anti-skid brakes?
> ^^^^^^^^^^^^^^^^
>Yes, it's the only bike whose brakes are so bad that they cannot be
>locked, and which has so little power , the rear tire cannot be spun.
For heaven's sake, don't let Mike Tiberio know about this, he's been
doing pretty well racing one of the beasts. :-)
Al Bowers
>>The front/rear ratio varies with time. When you start to brake,
>>the rear wheel still has a lot of weight on it. As the forks compress,
>>some of that weight shifts to the front, and you can increase front
>>braking force if you want, but you _must_ decrease rear braking force.
Actually, it varies as a function of wheel loading and braking force.
time is only because there is a time constant in the raction to the
forces applied.
>but WHY?! (said the annoying five-year-old repeatedly)
A thought experiment. If you *SLAM* on the front brake, the wheel
locks and you go down. Now, if you apply progressivly more and more
front brake pressure, the forks compress, the front wheel loads up and
you can achieve higher front brake lever force without locking the
front wheel (with control) than if you simply jerked on the lever and
locked the wheel. The reason is that with the additional load of the
weight shift, the wheel could continue to turn the brake with that
amount of braking. Without the weight shift, there wasn't enough
available traction to keep the wheel turning.
>not that i don't believe you, but i don't understand why holding that rear
>brake will make any difference. provided you were to use the front
>brake with maximum effectiveness (not always a given, i know), why
>must you _decrease_ rear braking force? at the very worst, won't
>it just become ineffective as the forks compress and weight is shifted
>forward, or will it truly hamper the front braking?
Now as the weight shifts forward, you have less available traction at
the back. So ease up on the rear.
>a lot of answers to the rear-brake-usage question have gone by, including
>$10 allocation of concentration (Code's method, a la Ed Green), "because
>if you lock the rear and release it you high-side" (not exactly an
>*answer*!), because MSF says so, because it's not useful on the racetrack
>... the usual DoD consensus! :-)
The most important thing that the MSF emphasizes is that using both
brakes is best. The reason they do this is the result of the data
from the Hurt report. Now, here is where the divergence of opinion
occurs. Both the MSF and Code/CLASS/whoever-racer are right. The
reason has to do with the equipment being used. No matter how hard
you try, you will find it impossible to do a stoppie on a Goldwing
Aspencade. In this case, there is still a significant portion of the
weight being carried by the rear wheel. In this case it is a very
good idea to use the rear brake, balanced with the front (the front
will still carry more braking). Most modern sportbikes carry so much
weight bias and have such exceptional brakes that _lifting_ the rear
wheel off the ground in a all-out-front brake only stop that use of
the rear brake is _entirely_ superfluous. Cruisers likewaise have
their own requirements.
Now here is where it gets sticky. You never know when you will be
confronted by a brain-dead cager attempting to make you and their door
handle occupy the same point in the time-space continum. You might be
solo on your GSXRZFJFZRVFR whatever. And in this case, if that is the
only way you ride, you'll possibly do the correct thing (provided you
_know_ what the right thing is to start with). If you are out on your
first ever ride on a XLH after learning to ride on a G00F2, you'll
probably have a problem cause you didn't use enough rear brake. On
the otherhand, if you've just gotten off of a KZ1300, and are getting
familiar with a ZX-6, you have the converse problem.
>i am inclined to agree with MSF about using both, but learned
>a lot from going on a rear-brake "diet" for 2 weeks, that is,
>only using the front. i learned that i was getting a LOT more
>braking from the rear than i thought (in a normal street situation)
>(hence my question about why people say they don't use it), and
>that i was cheating with the rear to create the illusion of smooth
>front braking, and that really, my "front braking technique" is
>an oxymoron.
In this case, you have your particular machine well dialed in and
understand that a dual brake usage is suited to it. This is *A GOOD
THING*. If you tried this same experiment on a sportbike, you'll find
that there isn't too much difference. Or, conversely, if you try it
on a cruiser you'll find that a lot of rear brake is required to
optimaize your stops.
>and so i find myself in MSF:RSS to get the very basics in (again),
>more for braking technique than anything. just in time -- before
>my experience level tips over into the "i know what i'm doing"
>skepticism, and out of the beginner's "tell me more, fill me
>with knowledge O Great Ones" open mind. :-)
:-) indeed. But always being open to ideas and discovering what works
best is a good attitude to have. Especially, if you are doing
something unfamiliar. And that can be something as simple as new
tires on your old bike, or a different bike, even of the same model.
Who knows, the brake pads might be different and react different
compared to what you are used to.
So not only does the style of what you ride affect the way you
approach problems, but the differences in equipment also should affect
you approach. You must adapt to your surroundings. And you should
try not to adapt to the brain-dead cager's door handle. IMHO.
Safe riding.
--
"Hokey religions and ancient weapons are no match for a good blaster
at your side, kid." - Han Solo, Star Wars
Al Bowers bow...@rigel.dfrf.nasa.gov
Iaido Takamine Hobie 16 Rossignol Alfa GTV6 Ducati MHR DOD #900
Tim Keller
>
>The main reason the force at the front tire increases (and the force
>on the rear decreases) is that the bike is attempting to rotate about
>its center of mass:
>
> \ _
> \ \
> ---------------- ---> | (Rotation about the center of mass.)
> | | /
> | X | <--
> xxx ^ xxx
> x x | x x
> x x | x x
> x x | x x
> <---xxx | <---xxx
> |
> |
> Center of Mass
>
There are three forces: forward momentum acting through the cg, front braking,
and rear braking. As long as the front tire is rotating relative to the frame
of the bike it is imposible for the bike to rotate about any other points
except the front and rear axles.
If you sum moments about the front axle, you will find that the
three forces all contribute to a moment about that axle. Once those forces
are great enought to overcome the opposing moment of the bike weight acing
through the cg the rear tire will come off of the groung.
>
>You don't slide as long as @ is <=. When that is not the case the
>back tire is going to slide causing a drastic decreases in the amount
>of available stopping power there as well as the more troublesome
>problems of the rear sliding out of line with the front.
>
One other point, the reason you do not want to lock any of the wheels is this:
when the wheels are spinning they posess kinetic energy, when trying to stop
the bike you have two forms of energy dissipation- the heat from friction in
the brakes and the kinetic energy of the wheel rotation (and of course, the
friction of the tires). When the tire stops rotating, you lose this additional
energy dissipation and it takes longer to stop. This is the primary reason why
cars with anti-lock brakes stop better than those without.
Tim.
S.Fielding-Isaacs [WWFO Eng Wordsmith]
>In article <GUNTER.92M...@covet.Berkeley.EDU> gun...@covet.Berkeley.EDU (Michial Gunter) writes:
>>
>>The main reason the force at the front tire increases (and the force
>>on the rear decreases) is that the bike is attempting to rotate about
>>its center of mass:
>>
>Nice try, but the bike will rotate about the front axle.
>
Hmmm.. your explanation seems incorrect as well. Here you assume that
the frame of the bike (including forks) is rigid. Undoubtedly to make
the calculation of forces simpler.
Would it not be the case that the bike tends to rotation around the
center of mass until the forks are completely compressed (ie. become
rigidly coupled) and then the bike will begin to rotate around the
front axle?
It would seem (only intuitively however) that you are both to some
degree correct.
NB: It is possible to calculate the ideal center of mass (CM) for
a bike and optimize a bike for performance under braking. The relationship
between the CM and the front axle is a critical measure in this
calculation.
Waddya think?
Stef
Sean Casey
:I was intrigued by this, so I did a little playing with it in the parking lot
:a few minutes ago. In runs starting at about 15-20mph, I applied either the
Every once in a few years, I practice skids in my car in an icy
parking lot. It's helped immensely. I know the limits of the thing. I
know what to expect when it happens. I know what to do to recover. I
know when to speed-pray :).
Sean Casey
:energy dissipation and it takes longer to stop. This is the primary reason why
:cars with anti-lock brakes stop better than those without.
:Tim.
While it is true that the brakes dissipate the kinetic energy of the
bike as long as the wheel rotates, the real issue is in the friction
of the tires with the pavement.
The reason a locked tire is less effective is that the coefficient of
sliding friction is always less than that of static friction,
sometimes substantially so. That's Physics 101. In laymans terms, it
means that it's harder to start something sliding (more friction),
than it is to keep something sliding (less friction).
When the tire is not sliding, the tire rubber is not moving relative
to the surface of the road, and thus is demonstrating static friction.
This probably explains why a locked back tire, when released, suddenly
"grabs" the road, often resulting in a high-side crash.
When ABS brakes for cars first came out, one of Car and Driver's test
drivers was astounded to find it decreased braking distance about 20%
for a straight stop on dry pavement.
Most ABS systems for cars work by detecting when one of the wheels
moves at a different rate than the other wheels. From what people have
said on this group, it wouldn't work for motorcycle front wheels,
since even the briefest front wheel lockup results in a crash. I
wonder if that's actually been tested? A system could pulse thousands
of times per second. I'd like to hear if someone knows.
Mark Joseph Andy
Perhaps I can answer some of your questions, as a person that uses the
rear on the street but not on the track.
no...@hana.cs.ucla.edu (Noemi Berry) writes:
> >some of that weight shifts to the front, and you can increase front
> >braking force if you want, but you _must_ decrease rear braking force.
>
> but WHY?! (said the annoying five-year-old repeatedly)
The reason is that as weight transfers to the front, there is less
traction available for the rear wheel (since weight over the contact
patch is an important component of traction). Consequently, if you
are at maximum rear braking at time zero (when brakes are first
applied), as weight transfers forward, you must ease the rear brake
off, so that you don't exceed the available traction (which is
becoming less as the weight transfers forward). None of this stuff is
steady state...
> a lot of answers to the rear-brake-usage question have gone by, including
> $10 allocation of concentration (Code's method, a la Ed Green), "because
> if you lock the rear and release it you high-side" (not exactly an
> *answer*!), because MSF says so, because it's not useful on the racetrack
> ... the usual DoD consensus! :-)
This is just sorta true. Someone pointed out that the rear brake will
help quite a bit on "non-race-type" bikes (ie, standards/crusiers).
You might take what Ed says in regard to the track with a grain of
salt, as well, as he is not a racer. For instance, his recent
assertion that all braking/ gear changing is done straight up is
absolutely wrong...
> i am inclined to agree with MSF about using both, but learned
> a lot from going on a rear-brake "diet" for 2 weeks, that is,
> only using the front. i learned that i was getting a LOT more
> braking from the rear than i thought (in a normal street situation)
> (hence my question about why people say they don't use it), and
> that i was cheating with the rear to create the illusion of smooth
> front braking, and that really, my "front braking technique" is
> an oxymoron.
Your bike, which I believe is a standard (?) will almost definately
benifit from the help that a rear brake can give it. However, even if
your bike has front drums (?), realize that the argument that some
people give about "the concentration required for using the rear
properly isn't worth the return" may still be true. Also realize that
different people are capable of different things...
Using the rear in normal braking situations certainly makes things
more smooth for me. I usually use the rear on my Seca on the street
when I'm coming to a stop, but for fine tuning speeds entering
corners, or when I'm on the track (when max braking is nearly always
happening), I usually only use the front.
> and so i find myself in MSF:RSS to get the very basics in (again),
> more for braking technique than anything. just in time -- before
> my experience level tips over into the "i know what i'm doing"
> skepticism, and out of the beginner's "tell me more, fill me
> with knowledge O Great Ones" open mind. :-)
Hell, it's easy for us racer's to avoid the "large-head-syndrome",
especially when people like Kurt Hall and Team Suzuki are around...
:-)
Take anything you get off the net (including this) with some
scepticism. YOU are the rider and the person thats riding *your*
bike. There may be things special about you or your bike or your
environment that require something other than what "net wisdowm"
provides (even assuming that net wisdom is related to wisdom :-)
So tell me about your bike sometime, eh? I don't think I've ever seen
one...
Mark
| Mark J. Andy WERA NV #813 AMA #700560 |
| ma...@cmu.edu KotR DoD #813 |
| Co-founder/Rider, Team RISC and Two Geeks Racing |
| '82 Seca 650 (street), '85 RZ350 (track), '78 KM100 (commute-in-snow) |
| I want RZ350 parts like steering damper, fork brace, SS lines, etc... |
Mark Joseph Andy
sla...@lmsc.lockheed.com (Mark Slagle) writes:
> >In article <l18jra...@jethro.Corp.Sun.COM>, s...@formula1.Corp.Sun.COM (S.F\
> ielding-Isaacs [WWFO Eng Wordsmith]) writes:
>
> [...a lot of hand-waving arguments for front brake only...]
> So don't give ME
> that crap about real life not being unlike test conditions. Sure
> it's different: In the test you don't die if you fuck it up.
> That's the difference. But the brakes still work best when you use
> both. You use only one if you want, but I want to stop before
> planting my face in elk intestines.
I usually try and keep an open mind about how someone else chooses to
ride. Perhaps you could try the same. Its a FACT that most modern
sport bikes (with brakes in good repair) will loft the rear wheel
under maximum braking. Why the hell shouldn't riders of sport bikes
(which Stephan is) tell other riders of sport bikes to concentrate on
the front only? Just because you had one case where the rear brake
helped? Please.
Tim Keller
>In article <1992May18....@news.Hawaii.Edu> kel...@hubble.ifa.hawaii.edu (Tim Keller) writes:
>>In article <GUNTER.92M...@covet.Berkeley.EDU> gun...@covet.Berkeley.EDU (Michial Gunter) writes:
>>>
>>>The main reason the force at the front tire increases (and the force
>>>on the rear decreases) is that the bike is attempting to rotate about
>>>its center of mass:
>>>
>>> [nice picture deleted]
>>>
>>Nice try, but the bike will rotate about the front axle.
>>
>> [explanation of why not deleted]
>
> Hmmm.. your explanation seems incorrect as well. Here you assume that
> the frame of the bike (including forks) is rigid. Undoubtedly to make
> the calculation of forces simpler.
>
> Would it not be the case that the bike tends to rotation around the
> center of mass until the forks are completely compressed (ie. become
> rigidly coupled) and then the bike will begin to rotate around the
> front axle?
that the center of mass is stationary and the rest of the bike rotates around
it. When you apply the brakes the forward momentum is being arrested through
the front forks (because that is the only thing connecting the front wheel
to the rest of the bike). Because the front forks are angled towards the
rear of the bike a component of the forward momentum will act as a force
along the axis of the front forks, the other component (parallel to the
ground) will act to rotate the bike (create a moment) about the front axle.
When the forks are completely compressed, the bike will raise the rear tire
with less force (i.e. it is now completely rigid).
If the forks were straight up and down, they would not be compressed by braking
at all.
> It would seem (only intuitively however) that you are both to some
> degree correct.
>
> NB: It is possible to calculate the ideal center of mass (CM) for
> a bike and optimize a bike for performance under braking. The relationship
> between the CM and the front axle is a critical measure in this
> calculation.
>
The optimum center of mass for a bike would be below the wheel axles. Then the
forward momentum would rotate the bike in the opposite direction, i.e. it
would oppose the reaction torques of the front and rear brakes, and front forks
would not compress, and the rear tire would not rise.
An interesting feature of such a bike is that it would be impossible
to do a wheelie. It would instead shift the weight to the front tire, not quite
what you want when trying to accelerate.
Tim.
Tim Keller
will tend to rise, and the weight will shift to the rear (just about the
opposite of the way things behave now).
Tim.
Chris Pirih
conditions. Even on sport bikes, stoppies are not automatic.
Most of the sport bikes I've owned (and that's quite a few)
would slide the front wheel before lifting the back wheel on
all but the most ideal surfaces. I was out doing braking
practice on my FZR 400 in a parking lot, and skidded the
front tire quite a few times (up to 10' or so!) trying to
get the back wheel in the air. (Gotta replace those stock
Bridgerocks with something sticky...)
---
chris
Dances With Bikers
>Most ABS systems for cars work by detecting when one of the wheels
>moves at a different rate than the other wheels. From what people have
>said on this group, it wouldn't work for motorcycle front wheels,
>since even the briefest front wheel lockup results in a crash.
Absolutely not true, personal experience shows that front wheel slides
can be recovered. Not that it's a preferred mode of operation, but it is
not an instant death sentence.
>I wonder if that's actually been tested? A system could pulse thousands
>of times per second. I'd like to hear if someone knows.
Would you settle for a handfull of cycles per second? I don't have the
numbers handy, but both BMW & Yamaha ABS systems have a very slow cycle,
at least for those of us used to thinking of electrical systems instead
of hydraulic systems. :-)
Chris Malcolm
>Since emergency straight-line braking is often necessary on
>the street, seldom with any warning (unlike entering a turn on the
>track), using the technique best suited to that ...
>IMHO, that technique is 100% concentration on the front brake.
If you brake only with the front on a wet road, if you don't crash, you
will need a lot more stopping distance than someone using both brakes.
And what about braking in turns? The nasty hazards of street riding often
require one to brake while cornering, and maximum braking while cornering
requires the use of both brakes.
If you train yourself to hit the front hard in emergencies, you will come
unstuck on wet roads and in cornering braking.
--
Chris Malcolm c...@uk.ac.ed.aifh +44 (0)31 650 3085
Department of Artificial Intelligence, Edinburgh University
5 Forrest Hill, Edinburgh, EH1 2QL, UK DoD #205
Chris Malcolm
> I firmly
> believe that in a panic situation, using only the front brake is the
> correct action.
> In this sort of situation, those actions which we have
> practiced and mastered will go a long way to saving us. In this case,
> the full-bore stop with the front brake.
I'm beginning to suspect that these US front-brake-only enthusiasts must
stay indoors or drive cars when the roads are wet, and go round corners
so infrequently that they are never called upon to brake when cornering.
The ONLY time a full-bore front brake stop is safe in on a dry road when
travelling in a straight line!
Well, I suppose such a riding style would explain the popularity of HDs!
:-)
Seth Zirin
> Absolutely not true, personal experience shows that front wheel slides
> can be recovered. Not that it's a preferred mode of operation, but it is
> not an instant death sentence.
I agree here. This past weekend the leader of our parade decided he didn't
want to miss a left turn to a side road so he slammed on his brakes. About half
of the cycles locked up one or both tires and the guy behind me screeched past me
(over the yellow line!) with both wheels locked. Nobody went down and we
all ragged at the jackass that was leading. The leader was none other than
the club safety director and the local MSF instructor...
- Roid
From the jumbled mind of Steve Andersen
>One other point, the reason you do not want to lock any of the wheels is this:
>when the wheels are spinning they posess kinetic energy, when trying to stop
>the bike you have two forms of energy dissipation- the heat from friction in
>the brakes and the kinetic energy of the wheel rotation (and of course, the
>friction of the tires). When the tire stops rotating, you lose this additional
>energy dissipation and it takes longer to stop. This is the primary reason why
>cars with anti-lock brakes stop better than those without.
>Tim.
I wasn't going to touch this one, but I must...
Without even getting involved with your kinetic energy argument,
I wish to look at the statement about "this is why cars with
anti-lock brakes stop better"
Cars with ABS do not "stop better" than those without.
ABS simply serves to keep the driver's natural reaction of
jamming the brake pedal to the floor from killing them by
locking the brakes, increasing stopping distance, and losing
directional control. If a tire is sliding it can't steer...
A car without ABS, with a skilled driver under "ideal" conditions
can outbrake ABS by braking with the wheels at iompending skid.
Maximum braking occurs at some percent slip (say 10% for sake
of argument). A skilled driver can attain and maintain this
level, getting maximum braking "all the time". However
ABS systems pulse the brakes causing braking to change as a
function of time. This does not allow that maximum slip
to be achieved for long. For this reason in IDEAL conditions
ABS is not superior. It is only from the standpoint that the
wheels can't be locked and therefore braking can be maintained
along with directional control that abs can be considered
superior. I have nothing against ABS, I feel it is in general
a "good thing (tm)" particularly in foul road
conditions where some abs systems can control each wheel
individually (which a non abs driver cannot do).
However remember that abs cannot provide traction that
does not exist, and it is not the end all- do all- save
all of the automobile and the braindead driver...
Steve
______________________________________________________________________________
Steve Andersen ande...@oscar.ccm.udel.edu
(302) 324-0888 ande...@zr1.ccm.udel.edu
"I own two bikes, brand unimportant" -- Jeff Kaskey
DoD #0239
Jim Brewer
>The rear brake can be useful on any bike under most street
>conditions.
> I was out doing braking
>practice on my FZR 400 in a parking lot, and skidded the
>front tire quite a few times (up to 10' or so!) trying to
>get the back wheel in the air.
So were you also using the rear brake while doing these front
tire slides? I'll bet you weren't.
The major point of the front brake only argument with respect
to sport bikes is it takes away attention. Go try it.
On sport bikes, the rear brake is useful when traction doesn't
allow for full front brake usage. The decision as to when to use it
is a matter of experience.
>(Gotta replace those stock Bridgerocks with something sticky...)
Well I should hope so. Try doing front wheel slides with an ME1F.
Jonathan E. Quist
>>One other point, the reason you do not want to lock any of the wheels is this:
>>when the wheels are spinning they posess kinetic energy, when trying to stop
>>the bike you have two forms of energy dissipation- the heat from friction in
>>the brakes and the kinetic energy of the wheel rotation (and of course, the
>>friction of the tires). When the tire stops rotating, you lose this additional
>>energy dissipation and it takes longer to stop.
No, no, no, 1000x no.
There is no energy dissipated in rotating the wheels, except for the negligible
amount of energy lost in the wheel bearings, and small aerodynamic losses
which vary with wheel construction. Think "conservation of
momentum". Once the wheel is spinning, it takes no energy to keep it
spinning, except for the aforementioned losses. Running on the road,
you also have frictional losses in the tire, due to the deformation of
the tire casing. (Demonstration: After a long, fast ride, feel your
tires. They will be warm.) Both are small relative to energy dissipation
in the braking system.
--
Jonathan E. Quist INTERACTIVE Systems Corporation
j...@i88.isc.com Naperville, IL
'71 CL450-K4 "Gleep" - "Worth the Oppression"
DoD #094 "I'm not mad; just terribly, terribly hurt." HDM #007
Tim Keller
>In article <1992May18....@news.Hawaii.Edu>, kel...@hubble.ifa.hawaii.edu (Tim Keller) writes...
>
>
>ABS simply serves to keep the driver's natural reaction of
>jamming the brake pedal to the floor from killing them by
>locking the brakes, increasing stopping distance, and losing
>directional control. If a tire is sliding it can't steer...
>
>A car without ABS, with a skilled driver under "ideal" conditions
>can outbrake ABS by braking with the wheels at iompending skid.
Considering that in a panic stop when the abs will be employed that
most drivers react by stomping on the brake, abs equiped cars will
stop better. From a pure physics standpoint you are correct, it does
not matter HOW the impending skid is maintained.
I suppose I should have said that is stops better given that
99.99% of the drivers out there lack sufficient skill to administer the
braking control equal or better than the abs.
>Maximum braking occurs at some percent slip (say 10% for sake
How do you come about this? Once the slip starts you are no longer dissipating
energy with the brakes, only with the tires. Ideally the tires would never
stop rotating. If you are refering to what is actually physically possible
given the braking systems and reaction times of the skilled driver, then I'll
accept that.
Tim.
Ron Miller
in front of motorcycle" in about 7 years (at the front gate here at
H-P), I can offer some adrenaline-seasoned commentary.
>Stefan:
> practiced and mastered will go a long way to saving us. In this case,
> the full-bore stop with the front brake.
Yup. Do this often, do it for fun, do it to save your life.
>
> Even after you have mastered this skill, you still have to worry about
> processing what is happening around you, choosing which one of your
> escape routes will keep your ass intact, and making sure that no one
> else is going to clean your clock while you execute your avoidance
> maneuver.
Yea, also remember that braking should be done while vertical. In my case,
while looking at the side of the obstacle's door and evaluating where
the butthead will be going next. Once speed is reduced, steering can resume.
(Thanks, ERC! God I love the light steering of a d-p bike :-)
In this case, I steered *behind* the butthead since she isn't likely to
go *that* direction in time to get me. (They can GO, they can STOP, but
they have a hard time reversing quickly enough to get you. This was also
a basic of maneuvering ships at sea. Aim for them, because if they are
moving -check for bow wave- they won't be there when you arrive.)
> I think it evident that the vast majority of people would have a
> great deal of difficulty writing their name intelligibly after
> such an incident. It is too much to ask of the average rider that
> s/he feather the rear brake in proportion to the weight shift
> (so as to prevent rear wheel lockup). It's just not going to happen.
I have not been able to master modulating the rear brake. Therefore I have
set the pedal travel to require lots of pedal movement to begin to engage the
brake. This is one way to make the machine compensate for the rider.
It also activates the light switch before braking occurs.
>
> >
> >Unless your bike doesn't do stoppies. Lots of bikes lock the front wheel
> >first.
Some won't do either on dry pavement. (XT350, R100GS. Yes, I've tried.)
> >
> >The front/rear ratio varies with time. When you start to brake,
> >the rear wheel still has a lot of weight on it. As the forks compress,
> >some of that weight shifts to the front, and you can increase front
> >braking force if you want, but you _must_ decrease rear braking force.
Doesn't the RSS teach to "lead " with the rear brake to get the transfer
going? Note that front-end dive can be a useful rider cue as to the
status of weight transfer. A dirt suspension can be dramatic!
>
> However, in the event that you ride a bike with a less than optimal
> weight distribution I can see that one would have to make the
> best of a bad situation and use the rear brake. I think the
> MSF comments are pertinent here. I think that they should consider
> revising their curriculum in the light of advances in technology
> and construction of bikes. Also keeping in mind that test-bed
> conditions to not approximate the reactions of riders in real-life
> crises.
Or maybe MSF could get more specific as to bike types? (Lest we do it
ourselves......)
Ron Miller
S.Fielding-Isaacs [WWFO Eng Wordsmith]
>In article <v18ig...@sixgun.East.Sun.COM> egr...@East.Sun.COM writes:
>
>>Since emergency straight-line braking is often necessary on
>>the street, seldom with any warning (unlike entering a turn on the
>>track), using the technique best suited to that ...
>>IMHO, that technique is 100% concentration on the front brake.
>
>If you brake only with the front on a wet road, if you don't crash, you
>will need a lot more stopping distance than someone using both brakes.
Not necessarily true. If it takes 50ft to brake in the dry, it can
take as little as 60ft to brake in the wet (given the right tire and
brake pads).
>And what about braking in turns? The nasty hazards of street riding often
>require one to brake while cornering, and maximum braking while cornering
>requires the use of both brakes.
I never, ever, ever, brake at speed in turns. The only time this
behavior is conceivable to me is if I was say doing 20mph in a 45mph
turn (ie. upright). To do otherwise is insane. Of course, I'm
talking about braking in a sweeper, not while turning a corner from
one road to another at a rational pace.
>If you train yourself to hit the front hard in emergencies, you will come
>unstuck on wet roads and in cornering braking.
I brake very hard in the rain (enough to lift the rear). It's really
very little different until you initiate a turn.
Jeffrey Picciotto
>> skidded the front tire quite a few times (up to 10' or so!) trying to
>> get the back wheel in the air.
Rate of brake application can affect the results, too. Slamming on
the front brakes is more likely to result in a skid than rapidly squeezing
them. (The latter allows for weight transfer.)
But does that mean my bullet goes slower if I squeeze the trigger? Hmmm.
--jeff picciotto
j...@mitre.org
Paul Thompson
of it seemingly contradictive. Ed Green says in a panic stop you very
likely don't have enough attention to spend on trying to keep both tires
at the limit of adhesion, so don't use the rear brake. Chris Malcolm
shows that when traction is less than optimal, using the front brake
only results in significantly longer stopping distances, and also carries
the danger of lockup, which presents a high risk of crashing - thus both
brakes should be used.
Who's right? They both are! Over the years I've gone the full range -
from mostly rear, to front only, and finally to "it depends". I
believe to be a truly proficient braker, you need to always be aware of
what you're riding on. On very slippery surfaces, you should emphasize
the rear brake, since with so little traction available there will be
little weight transfer, and thus the contribution to braking force by
each wheel will be about the same. Using the rear somewhat MORE than
the front will ensure that it will be the first one to start sliding,
which is important because it is so much easier to control a rear wheel
slide.
Now let's consider the other extreme - a nice clean pavement. Since
much traction is now available, there will be lots of weight transfer,
and the front brake will do the majority of the work. How much? It
depends quite a bit, depending on what kind of bike you have and just
how much traction is available. In general, long low bikes, like
cruisers, have less weight transfer, and short high bikes, like sport
bikes, have more. Again at the extreme, a modern sportbike can have
enough weight transfer so there is little to no weight on the rear and
thus little to be gained from using the rear brake. Indeed, with so
little weight on it, it can be impossible to tell if it is actually
sliding. On my Hurricane, locking up the rear when braking hard with
the front causes no noise, no sliding sensation, and leaves no skid
mark. I don't even know it is locked until the rear of the bike starts
to come around. Thus I don't use the rear brake on a sportbike when
traction is good. Cruisers are quite a different story, however.
Because of the low center of gravity, there is always going to be a
fair amount of weight on the rear, and you should always use the rear
brake.
There's no substitute for practicing braking to see what works for you
on your bike. Go to a parking lot with a friend, some chalk, and a
tape measure, and try the front brake only versus both, and in various
ratios. I've done this and found it extremely instructive. I was able
to start from what I thought was a good technique and improve on it
substantially. I'll dig up the article I wrote last time I did this
and repost it.
And finally, when you're on the road, be aware of the road conditions
so you have an intuitive feel of the traction available. It affects
not only your ability to stop, but turn as well. I can appreciate the
MSF preaching to use both brakes always, since a new rider would likely
be overwhelmed trying to think of all this, but once a rider is more
experienced I believe it's time to really understand what holds your
bike to the road.
--
Paul Thompson Apple Computer DoD #47 Would you remember a 1-line .sig?
Paul Thompson
their butts and on their bikes to find out what the real best way for
them to brake is. This weekend elef and I did just that.
We started by finding a deserted parking lot and putting chalk marks
each foot. Then one of us would make several runs, while the other
watched for locked wheels and recorded distances. My bike is a 600
Hurricane with Comp-K Metzlers, and hers is a 250 Rebel with stock
tires. The lot was old asphalt, with a touch of loose grit thrown in
for good measure. We calibrated our speedometers, and found them
both to read 33 at an actual 30 MPH.
I went first, starting from 30 MPH and using the front brake only.
After several non-ideal runs resulting in the front sliding or the rear
in the air, I honed in on a better technique, resulting in quicker
stopping without the excitement. My best stop was 25 feet, with the
front just starting to slide, while most of my runs were 27-28 feet.
My main revelation was to get on the brakes harder and quicker - this
stopped me about 7 feet sooner! The common wisdom is to s-q-u-e-e-z-e
on the brakes, but this is relative. On my bike I got best results by
being full on the brakes in less than a second. I also found that
enormous amounts of braking can be applied at first without sliding the
front tire - my best stops required me to actually let up slightly as
my speed decreased, to keep from locking the front. I also found that
front wheel slides are easier to handle at higher speeds. A slide near
the end of my stop (~10 MPH) would often wrench the bike violently,
while at 25 it kept going straight. The difference in traction also
seemed my more pronounced at slow speeds - too much to be explained
merely by the difference between static and sliding coefficients of
friction. I'll postulate that at slow speeds, bits of sand build up
under the sliding tire and keep it from making a good contact with the
road (there were deposits of sand on the lot, much like you often find
on older, seldom traveled roads.)
Next I tried using both brakes. I found that the rear slides very
easily, due to there being almost no weight on it. In fact, it made no
noise, left no skid mark, and I couldn't tell it was locked until the
rear of the bike started to come around, usually at the very end of the
stop. It was distracting to try to apply the rear the correct amount
while keeping track of the front brake too. This probably explains why
I was not able to duplicate my prior 25 foot record - 28 feet was the
best I could muster, with results varying all the way up to 35 feet. I
decided that for maximum braking on my bike under good traction
conditions, I should not use the rear brake, as it is distracting and
doesn't improve my braking distance. Some day if I get very brave I
may repeat this test stopping on sand - I would expect to find that the
rear brake would help me to stop quicker in this case.
Next I climbed on "gogo", elef's 250 Rebel. I quickly found that,
unlike my bike, using the rear brake stopped the bike significantly
sooner. I averaged 34 feet using both brakes, while it took 42 feet to
stop with the front only. I also noted that, while at speeds up to
about 20 MPH I could lock the front tire, the front brake just wasn't
up to the task at faster speeds, limiting the weight transfer and the
stopping distance. As an example, braking from only 25 MPH the
distances were 21 feet (front) versus 16 feet (both). The percentage
difference is greater because the weight transfer is more (because I
didn't run out of brakes at this speed) I also noted that to stop the
bike from 30 MPH required significant force on the front brake and
extremely little on the back - before this experiment I'm sure I was
inadvertently using the rear brake for the majority of my stopping
simply because it had so much of a mechanical advantage. Finally, I
noted that sliding the front tire was relatively uneventful. I'm not
sure whether this is due to differences in chassis or tires or what.
Elef also practiced her braking technique, at a
slightly-slower-known-only-to-her speed, as getting the 250 up to 30
MPH and down again in the confines of the parking lot was a bit dicey.
I'll estimate she was somewhere between 20 and 25 MPH. Like myself,
she found that it was quite easy to lock the rear brake, and being a
relatively new rider, found it quite distracting when the rear
squealed. This usually caused her to instinctively let up on both
brakes, resulting in rather long distances. With a little practice she
was stopping in 17 feet, and managed to uncork an especially nice 14
footer. This compares to 21 and 26 feet distances from reacting to a
locked rear.
So there you have it! We both learned quite a bit, and you can too.
Let's hear from some more riders on their experiences...
Mark Slagle
>>The optimum center of mass for a bike would be below the wheel
>>axles. Then the forward momentum would rotate the bike in the
>>opposite direction, i.e. it would oppose the reaction torques of the
>>front and rear brakes, and front forks would not compress, and the
>>rear tire would not rise. An interesting feature of such a bike
>>is that it would be impossible to do a wheelie. It would instead
>>shift the weight to the front tire, not quite what you want when
>>trying to accelerate.
> I'm sorry, I should have said the fronk forks will compress, and the
> front tire will tend to rise, and the weight will shift to the rear
> (just about the opposite of the way things behave now). Tim.
Well, you've had your two chances and you still don't have it
right. I'm sorry, you've been disqualified to speak on matters
of physics and mechanics for a period of time necessary to
complete a course in elementary free body diagrams (the best
things in life are, you know).
As long as the CG of the machine is above the road level, where
the forward force is from the road resistance is applied to the
rear wheel, there will be an inertial moment tending to rotate
the front wheel off the pavement and into squid heaven. The
effect is the same as on my own beloved motor, just somewhat
less. Now quit arguing and twist the grip.
--
----
Mark E. Slagle PO Box 61059
sla...@lmsc.lockheed.com Sunnyvale, CA 94088
408-756-0895 USA
From the jumbled mind of Steve Andersen
> I suppose I should have said that is stops better given that
>99.99% of the drivers out there lack sufficient skill to administer the
>braking control equal or better than the abs.
potential in a physical sense, not braking control as administered
by a driver or computer...
>>Maximum braking occurs at some percent slip (say 10% for sake
>
>How do you come about this? Once the slip starts you are no longer dissipating
>energy with the brakes, only with the tires. Ideally the tires would never
>stop rotating. If you are refering to what is actually physically possible
>given the braking systems and reaction times of the skilled driver, then I'll
>accept that.
>Tim.
Okay this is where it gets deep so for those of you not interested in
a long explanation of how pneumatic tires provide tractive (or
braking) effort and how that relates to tire slip, hit 'n' now...
Taken without permission from "Theory of Ground Vehicles" J. Y. Wong
Wiley, 1978 pp13-15
Note that any references to traction or driving torques also
hold for braking...
When a driving torque is applied to a pneumatic tire, a tractive
force is developed at the tire-ground contact patch. At the same time,
the tire tread in front of and within the contact patch is subjected to
compression. A corresponding shear deformation of the side wall of the
tire is also developed.
As tread elements are compressed before entering the contact
region, the distance that the tire travels when subject to a driving
torque will be less than that in free rolling. This phenomenon is
usually referred to as deformation slip. The slip of the vehicle
running gear, when a driving torque is applied is defined by:
i = (1- (V/rw)) * 100% = (1-(re/r)) * 100%
where V is the translatory speed of the tire center, w is angular
speed of the tire, r is the rolling radius of the free-rolling tire,
and re is the effective rolling radius of the tire, which is the ratio
of the translatory speed of the tire center to the angular speed of
the tire. When a driving torque is applied, the tire rotates without
the equivalent translatory progression; therefore, rw > V.
As the tractive force developed by a tire is proportional to
the applied wheel torque under steady state conditions, slip is a function
of tractive effort. Generally speaking, at first the wheel torque
and tractive force increase linearly with slip, because initially
slip is mainly due to elastic deformation of the tire tread. This
corresponds to section OA of the cheesey ascii curve below. Further
increase of wheel torque and tractive force results in part of the tire
tread sliding on the ground. Under these circumstances, the relationship
between the tractive force and slip is nonlinear. This corresponds
to section AB of the curve. Based on experimental data, the maximum
available tractive force of a pneumatic tire on hard surfaces is usually
reached somewhere between 15 and 20% of slip. Any further increase of
slip beyond that results in an unstable condition, with the tractive
effort coefficient, which is the ratio of the tractive effort to the
vertical load of a tire, falling rapidly from the peak value 'up'
to the purely sliding value 'us' as shown in the figure.
Steve's note: this last statement is explaining what occurs
as the unstable condition mentioned manifests
itself as "locked wheels". Also this alludes
to an aforementioned relationship similar to
that of simple static and sliding coefficients
of friction. Unfortunately pneumatic tires
can not simply be looked at in this way due to
the fact that they are deforming bodies (ie
not rigid, as normal "fizzicks" friction
assumes...)
| B
| ----
| - | -------
| - | -------------
| - | ------------- ---
| A- | |
| / | |
TEC | / | |
| / |up |
| / | us|
| / | |
| / | |
| / | |
| / | |
|/ | |
|_______________________________________________________ ---
O 15-20 50 100%
SLIP
TEC= Tractive Effort Coefficient
Variation of tractive effort coef. with longitudinal slip of tire.
Any questions????
Mark Slagle
> Cars with ABS do not "stop better" than those without.
> ABS simply serves to keep the driver's natural reaction of
> jamming the brake pedal to the floor from killing them by
> locking the brakes, increasing stopping distance, and losing
> directional control. If a tire is sliding it can't steer...
> A car without ABS, with a skilled driver under "ideal" conditions
> can outbrake ABS by braking with the wheels at iompending skid.
> Maximum braking occurs at some percent slip (say 10% for sake
> of argument). A skilled driver can attain and maintain this
> level, getting maximum braking "all the time". However
I am not even passingly familiar with the setup for ABS, but
it would seem that this is not necessarily so. Given that
automobiles do not give the driver independent control of the
brakes in the front and rear (not to mention each wheel, each
of which may experience different road surface conditions),
it may not be possible for even the most skilled driver to
maintain the front brakes at the optimum setting without
locking the rear ones and losing directional control. I
realize that a good design will have the brake system
allocate the force in a way that is close to optimal, but it
seems at least possible in principle for an ABS to keep all
four wheels at peak brake force, in contrast to a manual
system. Is this so? Does this not argue against the
braking superiority of the "skilled driver"?
Of course, this point is moot in the case of motorcycles, but
what's the deal with automotive ABS? Do they work each wheel
independently?
Mark Slagle
> Your bike, which I believe is a standard (?) will almost definately
> benifit from the help that a rear brake can give it. However, even if
> your bike has front drums (?), realize that the argument that some
> people give about "the concentration required for using the rear
> properly isn't worth the return" may still be true. Also realize that
> different people are capable of different things...
While everyone has different aptitudes, if a person cannot learn
to use motorcycle brakes effectively, without "concentration",
then that person would be well advised to avoid riding
motorcycles (or driving anything faster than a lawn mower in my
vicinity). This whole notion of concentration is a red herring.
It simply takes too much concentration to use your brakes, even
improperly, in the midst of a panic stop if you don't know what
you're doing. You will die before you get it right. The thing
to do is to find a quiet stretch of road or parking lot and
(practice,)^n where n is large. The only way to learn how to use
your brakes well and how your machine behaves under heavy braking
is to do it a lot. Better you should do it when your ass is not
on the line, then you'll do the right thing, without needing
excessive "concentration", when it counts.
Now, everybody go out there this weekend and practice going fast
and braking hard. See how much easier it's getting already?
Doesn't that seem natural now?
Mark Slagle
> Every once in a few years, I practice skids in my car in an icy
> parking lot. It's helped immensely. I know the limits of the thing. I
> know what to expect when it happens. I know what to do to recover. I
> know when to speed-pray :).
I recall one winter a few years back when I hit an icy patch during a
blizzard on the the NY State Thomas E. Dewey Thruway. Going backwards
at sixty five is a thrill you don't forget easily. Fortunately, I was
able to keep it on the road. All those years of doing doughnuts in the
A&P parking lot finally paid off big. The guy following me was pretty
amazed, too. I like to think he was rooting for me, though.
Maybe we should meet after the bike braking exercises and practice
doing our four-wheel drift moves in the cages?
Mark Slagle
> sla...@lmsc.lockheed.com (Mark Slagle) writes:
>> So don't give ME
>> that crap about real life not being unlike test conditions. Sure
>> it's different: In the test you don't die if you fuck it up.
>> That's the difference. But the brakes still work best when you use
>> both. You use only one if you want, but I want to stop before
>> planting my face in elk intestines.
> I usually try and keep an open mind about how someone else chooses to
> ride. Perhaps you could try the same. Its a FACT that most modern
> sport bikes (with brakes in good repair) will loft the rear wheel
> under maximum braking. Why the hell shouldn't riders of sport bikes
> (which Stephan is) tell other riders of sport bikes to concentrate on
> the front only? Just because you had one case where the rear brake
> helped? Please.
Perhaps you were insufficiently entranced by my deathless prose to
actually read to the end. Perhaps then you missed the last sentence
that you quoted. Perhaps if I were a nasty guy I would make some
uncalled for remarks about reading comprehension. But what, pray,
does the phrase "You use only one if you want..." mean to you if not
you may do as you please without my interference. How much more of an
"open mind", short of allowing a stiff breeze to traverse from point
to point between my ears, did you have in mind? I was just telling my
story. If you've got a different one then you go ahead and tell
yours. Seems like you've got a nasty streak of your own buried under
all that "howdy" and "please", else why get on my case? Why not just
argue the merits of your own case instead? Anyway, some people (whose
taste we will not get into) like my stories (which people are hereby
encouraged to help minister to my sorely bruised ego at this point).
I'm sorry that you didn't like it.
Now, as for the lifting of the rear wheel stuff. I hear this all
the time lately, so maybe it's true. Personally, I've never lifted
a rear wheel on anything but a bicycle. But if these bikes you buy
can lift one, then they're a pretty sorry design. Explain to me,
if you would be so kind, how does a person manage to maintain
directional control of a machine that's howling and jerking down
the road on the front wheel with the major part of its inertia
flailing about on a bearing hinge? If your back wheel is off the
ground for more than a brief instant, then you're in a pile of
deep shit, I'd think. What keeps it from reaching around and
biting you on the ass?
In any case, the audience here is a diverse one, with a lot of
different kinds of hardware. I think telling people (other than
racers, who have their own unique brand of craziness to contend with)
to use only their front brake is irresponsible. The ONLY valid advice
for all riders is that which I and others have repeatedly offered: Get
out on some deserted pavement and find out what your own machine does
best and then practice it until you get it right. Do it until it
comes, naturally (I'll let you literary types figure out if a comma is
really wise here or whether it should only be used if it's on the
front).
Happy riding. I hope you all stop in time, with or without your
back-end grabbers.
Jody Levine
(Jerry Lotto) writes:
>
>WAIT! You've established that premature release of the rear brake
>while the wheel is locked can result in a highside. So, (on good
>pavement) don't release it if it locks up! A locked rear wheel
>provides no directional control by itself, but it is attached to a
>motorcycle traveling straight ahead. As long as you look well ahead
>and keep the motorcycle vertical (front tire pointed straight), the
>rear wheel will follow.
In my bike course (which included the MSF videos) we were taught as follows
Do use the rear brake.
If the front locks up, release the brake. I once locked up the front on a
sandy road, in a turn, released the brake, the bike grabbed the road and
continued on its merry way. So it works.
If the bike is dead vertical, you're on pavement and the rear locks up,
leave it locked. You'll continue to track straight and you don't have to
lose stopping time to fiddling with the rear wheel. We had to try this, and
it worked.
If the bike is dead vertical, you're on gravel and the rear locks up, release
the brake until the wheel rolls again. Gravel will throw a locked wheel around
too much and may dump you. We didn't try this, there was no gravel in our
parking lot.
If you're leaned and the rear locks, you have a problem. They never told
us what to do here, they only told us to avoid it. Part of the government's
licensing required 'braking in a curve', which involves stopping in a preset
distance given your speed as told by the radar gun, and the point where the
light came on to tell you to stop. When we practised this, a few instructors
stood around the curve to grab the bike of anyone who unintentionally
attempted the rear wheel lock exercise. For some reason the Honda Rebel was
particularly bad for this, but the instructors managed to keep everyone's
shiny side up. The bottom line was caution on the brakes, and standing
the bike up as soon as possible.
I've bike like | Jody Levine DoD #275
got a you can if you -PF | ohrd!jle...@cs.toronto.edu
ride it | Toronto, Ontario, Canada
Jody Levine
>>The front/rear ratio varies with time. When you start to brake,
>>the rear wheel still has a lot of weight on it. As the forks compress,
>>some of that weight shifts to the front, and you can increase front
>>braking force if you want, but you _must_ decrease rear braking force.
>
Because a real wheel with less weight on it will lock up easier. This
is an oversimplification for tires/pavement, but think of the first-year
physics thing where friction (both dynamic and static, but static in the case
of a rolling object) is proportinal to the force pushing the two surfaces
together. If you continue to mash the brake pedal as the bike's weight
transfers to the front, the rear wheel will lock (too little road glue to
keep it rolling). Again, this is no problem if you're going straight, but if
you're leaning...
As a note, although I talked about the MSF stuff in the last posting,
I forget about the rear during full panic stops. I use the rear brake in normal
conditions, because I like using all of the fun features my bike came with.
Jonathan E. Quist
>
>> I firmly
>> believe that in a panic situation, using only the front brake is the
>> correct action.
>
>> In this sort of situation, those actions which we have
>> practiced and mastered will go a long way to saving us. In this case,
>> the full-bore stop with the front brake.
>
>I'm beginning to suspect that these US front-brake-only enthusiasts must
>stay indoors or drive cars when the roads are wet, and go round corners
>so infrequently that they are never called upon to brake when cornering.
Well, the actions which _this_ US biker practices is a full-bore stop
with balanced front/rear braking. So in a panic situation, not
all of us will grab the front only. Of course, part of my philosopy is
to never allow myself to get into panic situations...
>The ONLY time a full-bore front brake stop is safe in on a dry road when
>travelling in a straight line!
>
>Well, I suppose such a riding style would explain the popularity of HDs!
>:-)
Well, my next door neighbor doesn't ride his 49 Harley this way. Of course,
it's at least in part because he has no front brake.... (something about
the original cable not reaching the length of the chopper fork...)
Jody Levine
>
>since even the briefest front wheel lockup results in a crash. I
Bull. In the advanced courses around here they put a piece of wet wood on
the ground and tell you to lock your front wheel on it so that you realize
that it's not so scary and that it is recoverable. OK, it is always scary,
but I've locked my front wheel in the real world twice (once on a gravel
road, once in a sandy curve) and I have yet to crash. (At least never as a
result of a front wheel lock. And you wouldn't call what I did a crash,
anyway.)
Charlie Preston
>Doesn't the RSS teach to "lead " with the rear brake to get the transfer
>going? Note that front-end dive can be a useful rider cue as to the
>status of weight transfer. A dirt suspension can be dramatic!
>
Chas
Tim Keller
rear axle, not the contact point with the rear tire and the ground.
The force you mention is being applied buy the rear wheel torque, an equal and
opposite reaction torque is also being applied at the center of rotaion (i.e.
the rear axle). This reaction torque is being opposed by the weight of the bike
acting through the cg (equal to the weight x distance from cg to rear axle in
the horizontal direction) plus the inertial force = mass of bike x acceleration
x distance from cg to rear axle in the vertical direction.
Now, if the cg is lower than the rear axle the weight and inertial
forces add, if the cg is higher then the reaction torque and the inertial forces
add. The later case is what we experience every day, if you accelerated quick
enought both the inertial torque and the reaction torque add to overcome the
weight of the bike. If the cg was lower than the rear axle then the reaction
torque and the inertial torque would both increase proportionally and the
difference between these two would be the "net" torque available to overcome
the weight of the bike and lift the front tire.
There would come a point where the net torque would be great enough to
raise the front tire, however, you would most likely not have enough traction
to apply that force. It all depends how close the cg is to the ground, if the
cg is in line with the rear axle, then there is no inertial torque and only
the bike weight needs to be overcome. If the cg is halfway between the rear
axle and the ground, then the reaction torque will be twice the inertial torque,
and eventually, the difference will be greater than the weight torque and the
front tire will rise. But IMHO there would never be enough traction (standard
tires, not huge drag bike tires) traction to accomplish this and the tire would
break free before the wheelie.
Now I'm ready to go twist the throttle!
Tim,
.
Mark Joseph Andy
ohrd!jle...@cs.toronto.edu (Jody Levine) writes:
> In my bike course (which included the MSF videos) we were taught as follows
>
> Do use the rear brake.
>
> leave it locked. You'll continue to track straight and you don't have to
> lose stopping time to fiddling with the rear wheel. We had to try this, and
> it worked.
This is one of the reasons why I'm leary of the MSF. This is utter
garbage. If you lock the rear while the bike is dead vertical, just
release the damn brake and get some directional control back (you
can't swerve with the rear locked). Even if you don't want to swerve,
you could then try and bring the rear to impending skid and maximum
braking. I understand that the MSF is teaching beginners and all, but
I wish there was some way to impress upon people that the MSF isn't god.
> If the bike is dead vertical, you're on gravel and the rear locks up, release
> the brake until the wheel rolls again. Gravel will throw a locked wheel around
> too much and may dump you. We didn't try this, there was no gravel in our
> parking lot.
More horseshit. Go find a gravel/dirt road and try it. The rear
doesn't kick around any more than it does on asphalt. Of course, the
same reasoning regarding directional control and braking applies.
> If you're leaned and the rear locks, you have a problem. They never told
> us what to do here, they only told us to avoid it.
Just as some quick advice, if you can release the rear *quickly* (ie,
before the rear gets too much out of line) then do it. The bike will
buck some (trying to high side you), but if you react quickly enough,
the rear won't have stepped out enough for you to get thrown. If the
rear is coming around, however, you're probably best to just leave it
locked and try and steer into the slide. If you release the brake,
you're probably gonna highside. If you don't, you're probably gonna
lowside. As was recently mentioned, take the lowside :-)
I don't want to start a major flame war or anything, but please
realize that the MSF is a *starting* point, and that intelligence,
experience, and knowledge can take you beyond the MSF (at least the
intro course).
Mark
| Mark J. Andy WERA NV #813 AMA #700560 |
| ma...@cmu.edu KotR DoD #813 |
| Co-founder/Rider, Team RISC and Two Geeks Racing |
| '82 Seca 650 (street), '85 RZ350 (track), '78 KM100 (commute-in-snow) |
| I want RZ350 parts like steering damper, fork brace, SS lines, etc... |
ra...@ninja.megatek.uucp
|Perhaps I can answer some of your questions, as a person that uses the
|rear on the street but not on the track.
Same here - I use both brakes in traffic and in any straight line riding,
but take my foot completely off the rear brake lever when riding aggressively
in the mountains (I've really scared myself a couple of times when getting
the rear tire loose due to using the rear brake in corners - hence the complete
foot shift OFF the lever). You can (depending on bike?) brake quite deeply
into a corner as long as you don't TOUCH that rear brake. For straight line
stopping, the rear brake is invaluable.
|Using the rear in normal braking situations certainly makes things
|more smooth for me. I usually use the rear on my Seca on the street
|when I'm coming to a stop, but for fine tuning speeds entering
|corners, or when I'm on the track (when max braking is nearly always
|happening), I usually only use the front.
Exactly.
I've done enough normal street miles to already waste three sets of rear
pads in 15,000 miles. However, despite some very aggressive canyon riding
where I use the front brakes *only*, I'm only on my second set of front pads
(well, third set, but the second set was replaced for shitty performance
rather than because they wore out - replaced the EBC Golds with the much better
Ferodos).
Randy Davis UUCP: megatek!ra...@uunet.uu.net
DoD #00013 ucsd!megatek!randy
Tim Keller
>In article <1992May19.1...@news.Hawaii.Edu>, kel...@hubble.ifa.hawaii.edu (Tim Keller) writes...
>> I suppose I should have said that is stops better given that
>>99.99% of the drivers out there lack sufficient skill to administer the
>>braking control equal or better than the abs.
> This is true, but your original argument was exlaining braking
>potential in a physical sense, not braking control as administered
>by a driver or computer...
>
said...oh well, you are right in that I should have justified it in the physical
sense when in reality the abs suffers from the same limitations as conventional
braking systems (traction). I made the switch without even realizing it, so
much for short term memory.
Thanks for the info, this is the first time I've seen an actual analysis of
what is really happening at the tire/road interface (never had the time nor
energy to look it up myself)
>
>Any questions????
So according the the analysis the sequence of events is:Wheel stops rotating,
elastic deformation of the tire allows movement of tire relative to wheel with
no slip between tire and pavement, partial slip of tire and more relative
motion of wheel to tire, full slip of tire - both wheel and tire moving as a
single unit again. So by this you can argue that since the wheel has stopped
rotating BEFORE the tire slips, the abs will react to get the wheel rotating
again despite the fact that all of the traction potential has not be used up.
This then shows that abs will not be able to stop the vehicle in the absolute
shortest distance that is physically possible.
Is that summary correct???
If so, couldn't the abs systems be calibrated to at least take
advantage of the linear portion of the slip/torque relationship?
Tim.
Mark Slagle
> Better draw your free body diagram again...the bike is rotating
> about the rear axle, not the contact point with the rear tire and
> the ground. The force you mention is being applied buy the rear
> wheel torque, an equal and opposite reaction torque is also being
> applied at the center of rotaion (i.e. the rear axle). This
[...and so on it goes...]
I could draw my free body diagram again, but I like it where it
is. The beauty of free body diagrams is that you can draw them
wherever you like that is convenient. It sometimes involves a bit
of art to keep them simple, but in this case it is not that great
a challenge.
I had hoped that it would not come to this, but you have forced me
to speak physics. My choice of free body diagram, which I commend
to you and the rec.moto world, is that drawn about the boundary of
the motorcycle. The significant external forces acting on this
system are few, as there are only two contact points with the
outside world. For our purposes we may safely ignore such trivial
contributions as the balmy breeze blowing past our legally
mandated helmet and the minor horizontal force owing to the
rolling resistance of the front wheel. This leaves, unless I am
neglecting something which you will be alert enough to spot and
kind enough to point out, only three remaining forces with which
to contend, namely the vertical reactions at the front and rear
wheels and the horizontal force at the rear wheel in ultimate
reaction to the application of the twisted grip.
If we sum our horizontal forces, in timeless engineering fashion,
we quickly discover that the traction force at the rear wheel
is equal in magnitude and opposite in direction to the inertial
"force" acting through the center of mass of the system. This
combination is known as a couple in engineeringese, not to be
confused with the two riders of the motorcycle who seem to be
joined at the hips.
Now you can argue about the location of the center of rotation
until the cows come home if it pleases you and you can find
anybody to listen, but it is irrelevant to this discussion. A
couple is of a class of entities known as free vectors. They can
be applied wherever you think they are most aesthetically
satisfying. If you apply one at the point of contact of the rear
wheel with the road, or at the rear axle, or at the CG, or at the
conjunction of the aforementioned hips, whatever suits you, it
changes nothing. The simple fact remains that so long as the CG
is at a higher vertical position than the road surface, where the
tire meets road, the inertia of the machine will give rise to a
moment tending to raise the front end relative to the back end
(I mean the motorcycle's, but either of the buttock pairs
cushioning the hips will serve as well).
It is true, as you point out that the effect is reduced when the
CG is below the axle. But this is true because it is lower than
otherwise. It does not cause the effect to change sign or
direction until it passes beneath the pavement, a practical
restriction we must all learn to live with regardless of our
politics or level of education. By your own lengthy analysis, the
inertial "torque" is always less than the applied torque unless
the moment arm is greater than that of the tire. This can only
happen when the CG is farther below the axle than the tire contact
patch, i.e. below the surface, as I have claimed (ignoring the
case where the CG is above the top of the tire, which is of no
interest unless you enjoy having road rash on your butt).
Also, you needn't confuse the issue with matters of tire traction.
Whether or not the tire has enough traction to actually get the
front wheel off the ground is not the issue. The question at hand
is whether the effect of inertia tends to raise or lower the front
end (no discussion of bra straps will be entertained at this time).
As I have claimed, it tends to unweight the front wheel regardless
of the position of the CG relative to the axle. That's why everybody
except the squid contingent likes them as low as they can get them
without having to grind the alternator cover off in corners.
Now you have to repeat the course instead of riding. After you
have completed the course successfully we can then, time
permitting, take up the discussion of this other curious claim
of yours:
> If the forks were straight up and down, they would not be
> compressed by braking at all.
Bye for now. Gotta work or ride or something.
From the jumbled mind of Steve Andersen
>So according the the analysis the sequence of events is:Wheel stops rotating,
>elastic deformation of the tire allows movement of tire relative to wheel with
>no slip between tire and pavement, partial slip of tire and more relative
>motion of wheel to tire, full slip of tire - both wheel and tire moving as a
>single unit again. So by this you can argue that since the wheel has stopped
>rotating BEFORE the tire slips, the abs will react to get the wheel rotating
>again despite the fact that all of the traction potential has not be used up.
>This then shows that abs will not be able to stop the vehicle in the absolute
>shortest distance that is physically possible.
> Is that summary correct???
Hmmmmmm, I suppose that your summary is at least possible,
dunno about correct. I have no idea what the magnitude of the
tire deformations are, or the time delay between the wheel
stopping and the tire reaching full slip. I think the main
reason that ABS can be "bettered" is that the human feedback
control system can maintain that 15-20% slip at an almost
constant level for the duration of the stop, whereas ABS
will be oscillating about that maximum tractive effort
point on the curve, hence causing (marginally) increased
stopping distance.
A few have mentioned the fact that ABS does have the
ability to control each wheel independently, thereby allowing
near maximum braking effort to be reached at each wheel.
Obviously this could easily swing the argument in favor of
ABS aided stopping distances, even without foul conditions.
Although it doesn't matter much, I will still stand by my
statement that will a properly front/rear balanced braking
system, a skilled driver, and road conditions that are
equivalent at all four corners of the car (ie same traction
available), that a non-ABS car could be stopped in a slightly
shorter distance.
This argument has now become irrelevant however, because
the bottom line is that if I could have ABS or not on a car
I was buying, i would buy the car with ABS due to it's ability
to compensate for "non-ideal" conditions, which occur
about 99.99% of the time on the road...
"It's not exactly a perfect world, is it?" -- Batman
> If so, couldn't the abs systems be calibrated to at least take
>advantage of the linear portion of the slip/torque relationship?
>Tim.
If the braking was maintained within the linear portion of the
slip/torque curve, the ABS would never be activated because the
wheels would no be "slipping" at different rates (ABS senses
differences in rotational speeds between wheels). Some
race drivers have shown that the 15-20% slip on dry pavement
can be achieved on the racetrack without even activating the
ABS (and thereby increasing stopping distance). I would supply
a reference for this if its availability came quickly to mind...
Paul Thompson
>Just a small point: a front wheel skid != a crash. I suggest that if you
>find yourself terrified by skidding the front, take a ratbike out in the
>rain and learn how to do it.
My experience has been that it's easier to maintain control of a motorcycle
on good pavement rather than wet, when I'm sliding the front wheel. I've
slid the front in many different situations, and rain is the one situation
that seemed the riskiest.
>Don'cha just hate it when ya drop a new bike before ya make the first
>loan payment? :-(
I managed to avoid this problem - I paid cash!
Paul Thompson
>I think telling people (other than
>racers, who have their own unique brand of craziness to contend with)
>to use only their front brake is irresponsible. The ONLY valid advice
>for all riders is that which I and others have repeatedly offered: Get
>out on some deserted pavement and find out what your own machine does
>best and then practice it until you get it right.
Yes, by all means go out and try it and see what works for you! I did
just this, and on my sportbike I found I could stop QUICKER using only
the front brake. Now I know I should be able to stop a little quicker
using both, but I tried and tried, and couldn't duplicate my best stop
with the front only. So there's no doubt in my mind what the best
thing for me on that bike is. And lest you think I'm just a lousy
braker, see how long it takes you to stop from an ACTUAL 30 MPH. Let
me know if you beat 25 feet. (Ooh, a challenge, maybe some readers
will actually try this practice thing now!)
From the jumbled mind of Steve Andersen
>I am not even passingly familiar with the setup for ABS, but
>it would seem that this is not necessarily so. Given that
>automobiles do not give the driver independent control of the
>brakes in the front and rear (not to mention each wheel, each
>of which may experience different road surface conditions),
>it may not be possible for even the most skilled driver to
>maintain the front brakes at the optimum setting without
>locking the rear ones and losing directional control. I
>realize that a good design will have the brake system
>allocate the force in a way that is close to optimal, but it
>seems at least possible in principle for an ABS to keep all
>four wheels at peak brake force, in contrast to a manual
>system. Is this so? Does this not argue against the
>braking superiority of the "skilled driver"?
This is correct. If the automobile does not have a
near perfect balance between front and rear then
the independent control of ABS would defeat the
potential "advantage" of the non-ABS car to the
skilled driver.
>Of course, this point is moot in the case of motorcycles, but
>what's the deal with automotive ABS? Do they work each wheel
>independently?
While I don't think that all automotive ABS systems have
independent wheel control, most of the newer ones
do. This of course negates the potential stopping distance
reduction of a non-ABS in anything less than a near-perfectly
uniform ideal surface. As mentioned before IMHO ABS is
a "good thing" for both cars and bikes. It will not save you
all the time, but the few times where it can give you the
advantage are definitely worth it...
Jerry Lotto
Paul> braker, see how long it takes you to stop from an ACTUAL 30 MPH. Let
Paul> me know if you beat 25 feet. (Ooh, a challenge, maybe some readers
Paul> will actually try this practice thing now!)
Anything that gets people practicing maximum braking is a Good Idea
(tm). We should be able to put together some figures, but the
motorcycle, surface condition and grade, riders weight, tire condition
and pressure, etc., etc. will have some effect :-) Between the MRC and
ERC "Stopping in the Shortest Distance" exercises, there are three
maximum standards which most motorcycles and riders should be able to
make:
15 mph 13 ft
20 mph 23 ft
25 mph 34 ft
Obviously, these numbers can be improved upon! If you practice this
on your own, the MOST IMPORTANT THING is to improve your own results
over time.
Paul brings up a good point - actual speed is not the same as
indicated speed, especially this low on the speedo range. One way to
get actual speeds is have someone time you. In the MRC:RSS, we use a
44 foot timing zone -
10 20 30 40
timing zone """"""""""""""
Start : 100' min :------44'------: -> ~80' runoff
<--establish speed, 2nd gear and hold-->^
Braking starts here
Time front tire in to front tire out of timing zone, and include
simple reaction time in the distance (braking onset at or past end of
44' timing zone). Divide 30 by your time (in seconds) to get actual
speed (mph). The timer should stand off to the side in the center of
the timing zone. Any MRC:RSS range should already have this painted.
--
Jerry Lotto <lo...@lhasa.harvard.edu> "Have you hugged the curves today?"
Chemistry Dept., Harvard Univ. AMA #520019 DoD #018 HOG #0323880
Alex Matthews
experimental corroboration.
To all you who have been neglecting your own riding skills, who have
been taking the easy way out by believing what someone else tells
you to do, I offer this challenge: TEST YOUR OWN SKILLS ON ANY
BIKE YOU CAN GET, UNDER CONTROLLED CONDITIONS, AND REPORT YOUR
FINDINGS ON THE NET. By controlled conditions I mean set things
up so the results are repeatable -- keep as many variables fixed
as possible, like road surface, temperature, wind direction, etc.
Be observant and note down things even if you're not sure they
affected your results, like how much fuel is in the tank, how
rested and alert you are, etc. In order to have complete and
meaningful results you should try as many as possible of the various
techniques discussed on the net, not just your personal favorite. You
will be allowed to draw conclusions about your technique on a particular
bike only if you have convinced us with numbers that you have
experimented.
Paul and Elaine's braking test is an excellent example for you to
follow, but feel free to improve on it (they did a good job of
improving on my original test, posted last year. I would repost
my article, but that would be cheating; hence I will redo the
braking test within the next week or so). Protective gear
is a must, even for slow-speed stuff.
See you in the parking lot,
--
-Alex Matthews (matt...@ajsh.colorado.edu)
DoD #0010
"Every particle continues in its state of rest or uniform motion in a
straight line except insofar as it doesn't." -- Sir Arthur Eddington
Tim Keller
>
>> Better draw your free body diagram again...the bike is rotating
>> about the rear axle, not the contact point with the rear tire and
>> the ground. The force you mention is being applied buy the rear
>> wheel torque, an equal and opposite reaction torque is also being
>> applied at the center of rotaion (i.e. the rear axle). This
> [...and so on it goes...]
>
>Now you can argue about the location of the center of rotation
>until the cows come home if it pleases you and you can find
>anybody to listen, but it is irrelevant to this discussion. A
>couple is of a class of entities known as free vectors. They can
>be applied wherever you think they are most aesthetically
>satisfying. If you apply one at the point of contact of the rear
>wheel with the road, or at the rear axle, or at the CG, or at the
>conjunction of the aforementioned hips, whatever suits you, it
>changes nothing. The simple fact remains that so long as the CG
>is at a higher vertical position than the road surface, where the
>tire meets road, the inertia of the machine will give rise to a
>moment tending to raise the front end relative to the back end
>(I mean the motorcycle's, but either of the buttock pairs
>cushioning the hips will serve as well).
My whole point was that *you* or anybody else would not be able to do a
wheelie on such a bike. Maybe I should have clarified that the weight
transfer to the rear wheel would be less, but not completely eliminated
(its hard to explain a concept when the boss is comming).
>
>It is true, as you point out that the effect is reduced when the
>CG is below the axle. But this is true because it is lower than
>otherwise. It does not cause the effect to change sign or
>direction until it passes beneath the pavement, a practical
If the front tire is going to come off the groung where is thre rest
of the bike going to rotate about???
The torque will manifest itself about the rear axle, and thus if the
cg is lower than the rear axle the inertia will have an opposing effect.
>
>Also, you needn't confuse the issue with matters of tire traction.
>Whether or not the tire has enough traction to actually get the
>front wheel off the ground is not the issue. The question at hand
But it is the issue because I am talking about being able to pull
the front wheel off of the ground under normal circumstances, with
all the limitations of traction included in the discussion.
>is whether the effect of inertia tends to raise or lower the front
>end (no discussion of bra straps will be entertained at this time).
>As I have claimed, it tends to unweight the front wheel regardless
>of the position of the CG relative to the axle.
Yes you are absolutely correct.
>
>Now you have to repeat the course instead of riding. After you
>have completed the course successfully we can then, time
>permitting, take up the discussion of this other curious claim
>of yours:
>
>> If the forks were straight up and down, they would not be
>> compressed by braking at all.
>
By this I meant that the inertial force acting throught the cg (I suppose
that the cg would have to be below the top of the fork) would not have
a component down the fork, that's all... so sue me :-)
Tim.
Tim Keller
> This argument has now become irrelevant however, because
>the bottom line is that if I could have ABS or not on a car
>I was buying, i would buy the car with ABS due to it's ability
>to compensate for "non-ideal" conditions, which occur
>about 99.99% of the time on the road...
>
>"It's not exactly a perfect world, is it?" -- Batman
>
>
>> If so, couldn't the abs systems be calibrated to at least take
>>advantage of the linear portion of the slip/torque relationship?
>>Tim.
>
>If the braking was maintained within the linear portion of the
>slip/torque curve, the ABS would never be activated because the
>wheels would no be "slipping" at different rates (ABS senses
>differences in rotational speeds between wheels). Some
>race drivers have shown that the 15-20% slip on dry pavement
>can be achieved on the racetrack without even activating the
>ABS (and thereby increasing stopping distance). I would supply
>a reference for this if its availability came quickly to mind...
>
>Steve
>
I beleive that abs on a motorcycle would be more effective than that on a car
because part of the slip is caused by deformation in the sidewall of the tire
according to the reference you provided. Now, as I am sure you are aware,
motorcycle tires have re-enforced side walls for cornering and are much stiffer
than car tires, thereby (I assume) limiting the deformation of the side wall
and at least part of the slip. It would then follow (If I am indeed close to
correct) that the slip rate may well be below 15% or so for car tires, as a
result, the abs would be able to utilize more of the traction potential (i.e.
less percentage of slip before wheel lock). Know of anyone doing a study on
traction for motocycle tires near impending skid??
Tim.
From the jumbled mind of Steve Andersen
>One other item under food for thought:
>I beleive that abs on a motorcycle would be more effective than that on a car
>because part of the slip is caused by deformation in the sidewall of the tire
>according to the reference you provided. Now, as I am sure you are aware,
>motorcycle tires have re-enforced side walls for cornering and are much stiffer
>than car tires, thereby (I assume) limiting the deformation of the side wall
>and at least part of the slip. It would then follow (If I am indeed close to
>correct) that the slip rate may well be below 15% or so for car tires, as a
>result, the abs would be able to utilize more of the traction potential (i.e.
>less percentage of slip before wheel lock). Know of anyone doing a study on
>traction for motocycle tires near impending skid??
>Tim.
I think that your assumption of less sidewall deformation in a
motorcycle tire (compared to a car) is probably correct. This
is possibly due to construction of the tire ,and also due to
the reduced per tire loading compared to a car. I would assume that
this would reduce the diffierence in stopping potential between
a non-ABS and an ABS bike. However this would also have an effect
of making the tractive effort curve more "peaky" (high and narrow)
and make it more difficult to control the ABS to keep it in the
correct part of the curve. Whether or not these things amount
to anything is solely a matter of speculation.
Unfortunately I doubt whether there are any formal studies
going on related to this (or anything else motorcycle related
except perhaps politicians inflating mc health care cost statistics.)
I think that the BMW's ABS is switchable (ie you can turn it
off)? If so perhaps one of them could volunteer to do the
parking lot braking test with and without to get some comparisons.
Unfortunately the validity of the data would be suspect due to
the variability in the rider's braking ability, and the added weight
of the ABS even when not in use, etc....
Jonathan E. Quist
>The torque will manifest itself about the rear axle, and thus if the
>cg is lower than the rear axle the inertia will have an opposing effect.
Why? If you have a two near-identical bikes, with the CM the same horizontal
distance from the rear axle, and one has the CM at the level of the
rear axle, and the other has the CM above the rear axle, guess which
one has a longer moment arm measured from the rear axle?
The one time I accidentally popped the Gleep's front wheel off the
ground, he didn't rotate about his rear axle. He rotated about
a point somewhere between the rear axle and the swingarm pivot bolt.
>>> If the forks were straight up and down, they would not be
>>> compressed by braking at all.
>>
>By this I meant that the inertial force acting throught the cg (I suppose
>that the cg would have to be below the top of the fork) would not have
>a component down the fork, that's all... so sue me :-)
>Tim.
No, the CM would have to be at ground level, since the opposing force
is there as well. Otherwise, you've got a moment (or two) tending to
rotate the front down, and the rear up.
Oh, yeah, CM == center of mass.
Ron Miller
I played a little on the local MSF range while it was unguarded. :-)
Bike: R100GS (full of gas)
Tires: Metzler Saharas (half of useful tread remaining)
Speed error: unknown
Wx: 75 F
Surface: asphalt with rubber stripes from MSF trainer bikes
Supervision: None. (would be helpful for critique)
Best 2 brake stop from 30 mph (indicated) was 18'. Rear was locked for
at least part of the stop. 20' was more typical.
The single front-only try yielded about 22'.
Never did go so far as to lock the front, even momentarily.
Any break in concentration at all added a minimum of 4' to a stop plus
tended to create more chaos by changing the front end compression.
Further experimentation is indicated for exploring the chaotic region
where braking is adjusted with the ensuing front end extension/recompression.
Perhaps a brake 'n swerve 'n brake exercise? (But I think I already passed
that one on Monday in saving my butt at the front gate to work.)
Ron
Alex Matthews
[Ron Miller describes his impromptu braking technique test.]
Aaaaaaaaaaand they're off! Ron wins some as yet undetermined prize
for leading off the 1992 Rec.motorcycles Official Unofficial Braking
Challenge. Who's next?
(Ohhh, lets not see the same hands every time. Someone who hasn't
done it before?)
Ben Sloss
>Most ABS systems for cars work by detecting when one of the wheels
>moves at a different rate than the other wheels. From what people have
>said on this group, it wouldn't work for motorcycle front wheels,
>since even the briefest front wheel lockup results in a crash. I
>of times per second. I'd like to hear if someone knows.
If people have said that any front-wheel lockup will cause the bike to crash,
then people are wrong. There is nothing inherent about locking the front
wheel which will make the bike crash. The problem is that after the front
wheel is locked, it usually begins to turn to one side or the other, causing
the bike to crash when the front brake is released. If you don't release the
brake, the bike slowly (1-2 secs) falls over, because it doesn't have the
gyro effects of the front wheel to keep it upright. Since neither the front
wheel's turning nor the bike's leaning occur instantly, an ABS system and/or
an alert rider can correct the problem by immediately reducing front braking
to allow the front wheel to begin turning again. For a good demonstration,
watch a demo BWM bike at a bike show where they do a wet-braking ABS stop.
You can see the front wheel repeatedly lock and release as the bike slows.
Another thing about ABS - on a consistent traction surface, a skilled driver
can stop faster than the ABS system. Maximum braking is developed when
the tire is turning at ~85% of road speed: impending lockup. ABS systems
currently go from nearly locked to completely unlocked and back again,
leading to somewhat longer braking distances. See Motorcyclist, June 1992.
>
>Sean
--
Ben Sloss b...@osc.com (415) 329-7500x172
Toys: '93 CBR 900RR, '83 Nighthawk 650, '91 XR250L, '87 Mustang 5.0
Quote: "It's all fun and games until someone gets paid."
Ed Hackett
> I think that the BMW's ABS is switchable (ie you can turn it
>off)? If so perhaps one of them could volunteer to do the
>parking lot braking test with and without to get some comparisons.
>Unfortunately the validity of the data would be suspect due to
>the variability in the rider's braking ability, and the added weight
>of the ABS even when not in use, etc....
OK, here are may experiences with the BMW ABS system, but first a
few notes on how it works.
The system is not switchable (unless you pull the fuse). The switch
is for the ABS failure light. It switches it from blinking to a
somewhat less annoying steady light.
The system reads the rotation of both wheels, and under braking
modulates the brakes to try to keep both wheels turning at the same
speed. It is possible to loose the ABS function by doing wheelies
or burn-outs, which causes the wheel rotation to be out of sync.
The light will tell you if you no longer have the ABS function.
You have to trun off the key, or in an extreem case, take it to a
dealer to have the computer cleared.
It is possible to beat the braking distance of the ABS without
having it off. If you don't lock the wheels the ABS doesn't do
anything. As long as you can maintain braking right to the point
of lock, the bike behaves as if the ABS wasn't there. If you do go
ahead and lock the brakes, you can feel them release slightly as
they pulse on and off. This results in a stopping distance of a
few feet longer than if the brakes had not locked. (This distance
is much shorter than if they were locked and sliding, even if you
did manage to keep control).
I have never taken measurements, but the shortest stopping
distances that I have observed with an ABS bike are as follows:
(From most to least effective).
1. Both front and rear near lock.
2. Front near lock, rear in ABS.
3. Both in ABS.
4. Front only near lock.
5. Front only in ABS.
6. Rear only near lock.
7. Rear only in ABS.
The rear only in ABS is short of useless. The system is keeping
the rear from locking while matching rotational speed of the front
wheel. This is one case where the ABS increases the stopping
distance greatly. I think dragging your feet would be better. :-)
I think that one reason ABS can not be tuned to the ultimate stopping
distance at X slip, is that this is based on clean, dry pavement.
Tossing in oil, water, antifreeze, sand, snow, and ice mucks up the
equation. The ABS probably has to engage sooner than what would
give the shortest absolute distances to ensure that it worked
properly under less than ideal traction (where you _really_ need it
to be effective).
Ed Hackett KotLS The Desert Research Institute
e...@wheeler.wrc.unr.edu Reno, Nevada (702) 673-7380
DoD #200 WMTC BMWRA DIOC I'm not really a chemist, I'm just one of
DUCATI 900SS BMW K100RS them motorsicle sonsabitches. __=o&o>__
Tim Keller
>In article <1992May21.1...@news.Hawaii.Edu> kel...@hubble.ifa.hawaii.edu (Tim Keller) writes:
>>The torque will manifest itself about the rear axle, and thus if the
>>cg is lower than the rear axle the inertia will have an opposing effect.
>
>Why? If you have a two near-identical bikes, with the CM the same horizontal
>distance from the rear axle, and one has the CM at the level of the
>rear axle, and the other has the CM above the rear axle, guess which
>one has a longer moment arm measured from the rear axle?
the one in line with the axis will have no moment arm, and thus will not
contribute to raise the front tire off of the ground. The second case will
act to contribute to the raising of the front wheel. Resolve the force acting
throught the cm into tangent and radial components: if the cm is in line with
the rear axle then no tangent component and no moment, if it is above, there is
wheel. If it is below, then there is a moment in the opposite direction from
the previous case.
>The one time I accidentally popped the Gleep's front wheel off the
fo>ground, he didn't rotate about his rear axle. He rotated about
>a point somewhere between the rear axle and the swingarm pivot bolt.
>
Hummm, I have a hard time visualizing this...mainly because the rear axle is
forced stay the same vertical distance from the ground by the wheel (neglecting
the small deformations of the tire). Thus to rotate about any other point the
rear axle would have to move in the vertical direction, but it is restrained
from doing so by the ground.
What I think happens is this: once the front wheel comes off the
ground, the cm of the bike shifts towards the rear, and the moment of this
force (which opposes the torque reaction lifting the front) becomes less and
less because the moment arm is getting shorter and shorter. This accelerates
the lifting of the front wheel, and makes it feel like it is rotating about
a point other than the rear axle. Maybe there is enough travel in the rear
suspension to allow it to rotate about a point other than the rear axle, and
that would explain your experience.
Tim.
*******Before I was uncertain, now I'm not so sure*******
Bob Pasker
> You can see the front wheel repeatedly lock and release as the bike slows.
doesnt this affect handling, re: chaning the compression in the front forks?
--
-- bob pasker
-- b...@halfdome.sf.ca.us
--
Jim Brooking
Hey, I haven't seen anybody noticing this: Why are DoDers talking
about using brakes, anyway? Can't you get kicked out for using
them? Don't you have to remove them? The added weight 'n stuff...
8^) 8^)
--
ji...@access.digex.com | (Cage) 1991 323se ZCP-710 | Might replace GSXR
Merry Land (MD) U.S.A. | (Bike) 1986 GSXR750 (sold) | with Katana 1100
-------------------------------------------------------------------------------
Daddy grips the wheel & stares alone into the distance... (Police)
Paul Thompson
>Part of the government's
>licensing required 'braking in a curve'
>When we practised this, a few instructors
>stood around the curve to grab the bike of anyone who unintentionally
>attempted the rear wheel lock exercise. For some reason the Honda Rebel was
>particularly bad for this, but the instructors managed to keep everyone's
>shiny side up.
Hmm, "bike spotters". I'm not sure I'd like that job.
Perhaps the reason the Rebel's seemed more prone to this is because, being
a cruiser style, there is quite a mechanical advantage to the rear brake and
when your weight shifts forward it can tend to make you push the lever
harder.
Ed Green - Pixel Cruncher
>
>While everyone has different aptitudes, if a person cannot learn
>to use motorcycle brakes effectively, without "concentration",
>then that person would be well advised to avoid riding
>motorcycles (or driving anything faster than a lawn mower in my
>vicinity).
Equistrian excrement.
>This whole notion of concentration is a red herring.
No, it isn't. I don't care how perfect you think you are, or how much
you practice, there is a finite amount of information a human brain can
process at any given moment.
>It simply takes too much concentration to use your brakes, even
>improperly, in the midst of a panic stop if you don't know what
>you're doing.
NEWS FLASH: !(Slagle's way) != (don't know what you're doing)
>The thing
>to do is to find a quiet stretch of road or parking lot and
>(practice,)^n where n is large. The only way to learn how to use
>your brakes well and how your machine behaves under heavy braking
>is to do it a lot.
True. That practice may very well teach you that you can stop faster,
safer, and with significantly more mental bandwidth available for the
other information processing tasks which tend to accompany panic
stopping in the real world, by using only the front brake. Then again,
it may not. People and motorcycles are different.
Preaching that if everyone doesn't do things your way, they should go
ride a lawn mower is presumptuous, at best.
---
Ed Green, former Ninjaite |I was drinking last night with a biker,
Ed.G...@East.Sun.COM |and I showed him a picture of you. I said,
DoD #0111 (919)460-8302 |"Go on, get to know her, you'll like her!"
(The Grateful Dead) --> |It seemed like the least I could do...
Jonathan E. Quist
>In article <1992May22.1...@i88.isc.com> j...@i88.isc.com (Jonathan E. Quist) writes:
>>In article <1992May21.1...@news.Hawaii.Edu> kel...@hubble.ifa.hawaii.edu (Tim Keller) writes:
>>The one time I accidentally popped the Gleep's front wheel off the
>>ground, he didn't rotate about his rear axle. He rotated about
>>a point somewhere between the rear axle and the swingarm pivot bolt.
>>
>Hummm, I have a hard time visualizing this...mainly because the rear axle is
>forced stay the same vertical distance from the ground by the wheel (neglecting
>the small deformations of the tire). Thus to rotate about any other point the
>rear axle would have to move in the vertical direction, but it is restrained
>from doing so by the ground.
No, no, no.... you missed my point. I do not ride a hardtail Harley.
The only parts of the bike constrained to rotate about the rear axle
are the rear wheel and the swingarm. The rest of the bike compromises,
depending upon conditions. If you sit on any bike with a non-rigid
rear suspension, and bounce up and down, then the frame of the bike is
either a) translating vertically, with no rotation, or b) translating
vertically and simultaneously rotating about a point which probably doesn't
coincide with any physical bearing on the bike. If you don't believe me,
make a paper cutout model, pin the joints together, and trace the motions
with a pencil.
Mark Slagle
> It's about a 60 pound penalty on BMW K-bikes, counting the larger
> battery. The big hydraulic actuators are the main problem. The pickups
> consist of gear-like gismos on the wheels and what appears to be a
> reluctance coil on the brake mounts.
That must be the gizmo that makes buyers reluctant to pay for it?
Mark Slagle
> In article 92May1...@sgi417.msd.lmsc.lockheed.com, sla...@lmsc.lockheed.com (Mark Slagle) writes:
>>While everyone has different aptitudes, if a person cannot learn
>>to use motorcycle brakes effectively, without "concentration",
>>then that person would be well advised to avoid riding
>>motorcycles (or driving anything faster than a lawn mower in my
>>vicinity).
> Equistrian excrement.
Cute. But I don't ride horses. Check your local dictionary
for details.
>>This whole notion of concentration is a red herring.
> No, it isn't. I don't care how perfect you think you are, or how much
> you practice, there is a finite amount of information a human brain can
> process at any given moment.
That is why practice is essential. Knee jerk reactions are processed
in the spinal column. The brain takes too much time. Knee jerk
responses to common sense arguments are a dime a dozen.
>>It simply takes too much concentration to use your brakes, even
>>improperly, in the midst of a panic stop if you don't know what
>>you're doing.
> NEWS FLASH: !(Slagle's way) != (don't know what you're doing)
I got a flash for you. Nowhere in the material by me quoted in
your article, or in any other posting by me, have I written
anything that a rational being would take to be a prescription
for "Slagle's way" for others to use their brakes. What does
that say about you? I have told people only to experiment and
practice to determine and perfect the best braking response that
they can get out of their particular brand of machine (see
below). As I have said before "You do what you want." But if
you try to get yourself killed, I'll appreciate it if you do it
somewhere where I'm not.
>>The thing
>>to do is to find a quiet stretch of road or parking lot and
>>(practice,)^n where n is large. The only way to learn how to use
>>your brakes well and how your machine behaves under heavy braking
>>is to do it a lot.
> True. That practice may very well teach you that you can stop faster,
> safer, and with significantly more mental bandwidth available for the
> other information processing tasks which tend to accompany panic
> stopping in the real world, by using only the front brake. Then again,
> it may not. People and motorcycles are different.
If you need to think about it in a panic situation, then you will
die. You make the call.
> Preaching that if everyone doesn't do things your way, they should go
> ride a lawn mower is presumptuous, at best.
I clearly wrote the words use the brakes "effectively", not "my
way", not the MSF way, just effectively and without the need for
concentration. An imminent collision presents quite enough to
engage the rider's concentration to the limit. The words are
right above these, at the top of the article, so it shouldn't be
too difficult for you to find them in case you'd like to try
actually reading them instead of making them up. Putting words
into my mouth is presumptuous. An inability to read and
comprehend simple prose is a disability and an educational
failure. Engaging in arguments ad hominem is a reprehensible
rhetorical device. Given the evidence you present, I think I'd
give you a wide berth even if you were only on a lawn mower.
Unlike panic braking, avoiding accidents requires thinking and
observing. You need more practice.
Eric Rescorla
>
>> In article 92May1...@sgi417.msd.lmsc.lockheed.com, sla...@lmsc.lockheed.com (Mark Slagle) writes:
>> No, it isn't. I don't care how perfect you think you are, or how much
>> you practice, there is a finite amount of information a human brain can
>> process at any given moment.
>
>That is why practice is essential. Knee jerk reactions are processed
>in the spinal column. The brain takes too much time. Knee jerk
>responses to common sense arguments are a dime a dozen.
Are you suggesting here that "conditioned reflexes" to use the
old Pavlovian term are processed in the spinal column? I don't think
that this is true.
-Ekr
--
-------------------------------------------------------------------------------
Eric Rescorla, DoD#431, Honda CM400 rider resc...@rtnmr.chem.yale.edu
Yale University Department of Chemistry We hack anything.
Golden Anniversary: It's bad beer, but it's beer.
Steve Garnier
>In article <SLAGLE.92M...@sgi417.msd.lmsc.lockheed.com>
>sla...@lmsc.lockheed.com writes:
>>In article <vu45b...@sixgun.East.Sun.COM>,
>>egr...@East.Sun.COM (Ed Green - Pixel Cruncher) writes:
>>>In article 92May1...@sgi417.msd.lmsc.lockheed.com,
>>>sla...@lmsc.lockheed.com (Mark Slagle) writes:
>>> No, it isn't. I don't care how perfect you think you are, or how much
>>> you practice, there is a finite amount of information a human brain can
>>> process at any given moment.
>>That is why practice is essential. Knee jerk reactions are processed
>>in the spinal column. The brain takes too much time. Knee jerk
>>responses to common sense arguments are a dime a dozen.
>Hold on a second, Slagle.
>Are you suggesting here that "conditioned reflexes" to use the
>old Pavlovian term are processed in the spinal column? I don't think
>that this is true.
This old dog can tell you with 1000% certainty that my excessive saliva
production has absolutely nothing to do with my spinal column. :-)
Bark quietly, and carry an extremely large milk-bone. ;-)
Steve Garnier - DoD #00000005 - '79 XS1100SF w/ 156K+ miles and *climbing*
KotF (Flame), KotAM (Anecdotal Motorcycle), The Daemon's Byte
st...@bucolix.ece.ncsu.edu "Ineluctably Prolix" - John Norman Sims, Jr.
Seth Zirin
> > Equistrian excrement.
>
> Cute. But I don't ride horses. Check your local dictionary
> for details.
You don't have to ride horses to enjoy Equestrian excrement! I know a guy that
came around a sharp turn and rode right through a large glop of the stuff. He
wasn't nearly as upset as the guy riding immediately behind him...
- Roid
Tim Keller
>
>>>The one time I accidentally popped the Gleep's front wheel off the
>>>ground, he didn't rotate about his rear axle. He rotated about
>>>a point somewhere between the rear axle and the swingarm pivot bolt.
>>>
>>Hummm, I have a hard time visualizing this...mainly because the rear axle is
>>forced stay the same vertical distance from the ground by the wheel (neglecting
>>the small deformations of the tire). Thus to rotate about any other point the
>>rear axle would have to move in the vertical direction, but it is restrained
>>from doing so by the ground.
>
>No, no, no.... you missed my point. I do not ride a hardtail Harley.
>The only parts of the bike constrained to rotate about the rear axle
>are the rear wheel and the swingarm. The rest of the bike compromises,
>depending upon conditions. If you sit on any bike with a non-rigid
>rear suspension, and bounce up and down, then the frame of the bike is
>either a) translating vertically, with no rotation, or b) translating
>vertically and simultaneously rotating about a point which probably doesn't
>coincide with any physical bearing on the bike. If you don't believe me,
>make a paper cutout model, pin the joints together, and trace the motions
>with a pencil.
>--
rotation for the whole system which can be anywhere and does not have to
be physically attached to the linkages.
I suppose it all depends upon how much the rear suspension is compressed
before the front tire is raised. If it is sufficiently compressed, then for
all practical purposes the bike behaves as a rigid body (assuming of course that
the shift in weight doesn't compress it further). Now what was it we were
talking about??
Mark Slagle
> In article <SLAGLE.92M...@sgi417.msd.lmsc.lockheed.com> sla...@lmsc.lockheed.com writes:
>>That is why practice is essential. Knee jerk reactions are processed
>>in the spinal column. The brain takes too much time. Knee jerk
>>responses to common sense arguments are a dime a dozen.
> Hold on a second, Slagle.
Holding. You'll tell me when to stop, won't you?
Just ring a bell or something. Or better still,
make some sort of bizarre claim that is contrary
to known physical laws. That will be sure to get
me going.
> Are you suggesting here that "conditioned reflexes" to use the
> old Pavlovian term are processed in the spinal column? I don't think
> that this is true.
Some of us are accustomed to maintaining a distinction between
the senses of the word reflex as used in the contexts of operant
conditioning and physiological conditioning. But lest I be
accused of being closed-minded, or of promoting "Slagle's way" as
the only, the one, true way, I hasten to add that you need not do
so. You must be prepared for the near certainty of the consequence
that you will be unintelligible, however.
But, "you do what you want."(tm)
Eric Rescorla
>In article <1992May27.1...@cs.yale.edu>, resc...@rtnmr.chem.yale.edu (Eric Rescorla) writes:
>
>> In article <SLAGLE.92M...@sgi417.msd.lmsc.lockheed.com> sla...@lmsc.lockheed.com writes:
>
>>>That is why practice is essential. Knee jerk reactions are processed
>>>in the spinal column. The brain takes too much time. Knee jerk
>>>responses to common sense arguments are a dime a dozen.
>
>> Hold on a second, Slagle.
>
>Holding. You'll tell me when to stop, won't you?
>Just ring a bell or something. Or better still,
>make some sort of bizarre claim that is contrary
>to known physical laws. That will be sure to get
>me going.
I wanted to make sure that you weren't. Perhaps you're confusing
me with someone else.
>> Are you suggesting here that "conditioned reflexes" to use the
>> old Pavlovian term are processed in the spinal column? I don't think
>> that this is true.
>
>Some of us are accustomed to maintaining a distinction between
>the senses of the word reflex as used in the contexts of operant
>conditioning and physiological conditioning. But lest I be
>accused of being closed-minded, or of promoting "Slagle's way" as
>the only, the one, true way, I hasten to add that you need not do
>so. You must be prepared for the near certainty of the consequence
>that you will be unintelligible, however.
I accused you of no such thing. Let us be clear here:
Are you maintaining that the reflexes that you pick up in order
to ride a bike are carried out in the spinal column in the same
way that knee jerk responses are? If so, I'd like to see a source.
If not, I have no problem with you. It strikes me that you are
the one who is being unintelligible, not me, Mr. Slagle.
Mark Slagle
Now let me get this straight. Some guy riding a horse
mooned the world and dropped a load on the pavement for
your acquaintance to distribute with his motorcycle tire?
Maybe we should chip in for a box of NEW IMPROVED adult
size Pampers.
Oh wait, I get it, you forgot to check your dictionary, didn't
you? I'm just gonna stop writing this stuff if all you people
are just gonna ignore the content and complain about my attitude
anyway. So there.
Jonathan E. Quist
I wrote:
>>The only parts of the bike constrained to rotate about the rear axle
>>are the rear wheel and the swingarm. The rest of the bike compromises,
>>depending upon conditions. If you sit on any bike with a non-rigid
>>rear suspension, and bounce up and down, then the frame of the bike is
>>either a) translating vertically, with no rotation, or b) translating
>>vertically and simultaneously rotating about a point which probably doesn't
>>coincide with any physical bearing on the bike. If you don't believe me,
>>make a paper cutout model, pin the joints together, and trace the motions
>>with a pencil.
>>--
>It is true, that for any linkage system you can find an instant center of
>rotation for the whole system which can be anywhere and does not have to
>be physically attached to the linkages.
> I suppose it all depends upon how much the rear suspension is compressed
>before the front tire is raised. If it is sufficiently compressed, then for
>all practical purposes the bike behaves as a rigid body (assuming of course that
>the shift in weight doesn't compress it further). Now what was it we were
>talking about??
We were talking about your insistence that a bike must rotate about
its rear axle. Why are you assuming that the rear suspension will be
fully compressed at this point, though? You're applying enough torque
at the rear wheel that the rear wheel that the front end is coming up.
Which is another way of saying there's enough upward force at the
swingarm pivot that it's lifting up. At which time, the center of mass
of the bike is in front of the swingarm pivot, so it's providing a down
force, which means there's a torque being applied to attempt to _unload_
the rear suspension, as inobvious as it sounds. Whether the
net torque in the system is positive or negative will depend upon
the exact geometry of the bike is question, and possibly the
initial conditions of the system as well. This is not a statics problem.
Mark Slagle
> In article <SLAGLE.92M...@sgi417.msd.lmsc.lockheed.com> sla...@lmsc.lockheed.com writes:
>>In article <1992May27.1...@cs.yale.edu>, resc...@rtnmr.chem.yale.edu (Eric Rescorla) writes:
>>> Hold on a second, Slagle.
>>
>>Holding. You'll tell me when to stop, won't you?
>>Just ring a bell or something. Or better still,
>>make some sort of bizarre claim that is contrary
>>to known physical laws. That will be sure to get
>>me going.
> I didn't do any such thing, Slagle.
> I wanted to make sure that you weren't. Perhaps you're confusing
> me with someone else.
Calm down, please. I didn't accuse you of anything yet, as can
be easily seen by slowly reading the statement you quote. I am
often confused, but not about that. I was only making a small,
perhaps imperceptible, joke to the effect that if a person wanted
to "get me going", i.e. light my fuse or cause me to babble
incessantly, that all that would be required would be to make
some small mistake in a interpretation of a known physical
principle. Get it?
> Are you maintaining that the reflexes that you pick up in order
> to ride a bike are carried out in the spinal column in the same
> way that knee jerk responses are? If so, I'd like to see a source.
> If not, I have no problem with you. It strikes me that you are
> the one who is being unintelligible, not me, Mr. Slagle.
I have suggested that there is some similarity, yes. I am not a
physiologist, so I don't have any good references, but I can
easily provide a plausibility argument. Is the phrase "muscle
memory" familiar to you? The phrase does not really mean that
muscles have memory, of course, but it does mean that you can
train your body to do things on reaction that short circuit the
more extensive brain processing for less tasks. In short, you
can make use of physiological reflex mechanisms by appropriate
training and visualization. That this is true is clearly
evidenced by such common, but truly amazing, feats as infield
dives and grabs of hot grounders or line drives. If your
favorite spectator sport is not baseball, then make up your own
examples.
Furthermore, anyone who has experienced a panic stop on a
motorcycle should be intimately familiar with the experience of
finding his hand squeezing the brake lever before even realizing
that he has done so. There is just no time for higher brain
functions to be employed. When you swat a mosquito on your neck,
are you really thinking and concentrating on what you are doing?
Mammalian survival depends on the ability to do a lot of things
that are processed in the spinal column or lower brain stem. I
am saying that panic reactions, to anything, are in that
category. I am further saying that, with practice, we can
improve our performance on these low-level reactions. Can this
really be in any doubt?
So yes, although I am not claiming that these reactions are as
truly fundamental as a knee-jerk response to stimulus (that was
a secondary insinuation used, due to its nifty parallelism, in
the process of flaming the pyromaniac that was deliberately
misrepresenting my previous statements), I am claiming that much
of the processing for panic stop braking is handled in the brain
stem, rather than requiring more extensive mental processing.
And to the extent that a rider has practiced and perfected his
ability to use the brakes, he can save vital time in the
performance of the task by employing lower level physiological
reaction mechanisms. A small fraction of a second is quite
enough to make the difference between life and death, so I
think it's worth it.