why are rear brakes weaker?
Theodore Sternberg
get alot less stopping power from the rear brakes?
Ted Sternberg
"I have a masters, but it's not in science."
,__0
_-\_<,
___~*: (*)/'(*)
'/|~~\
Mr. S.J. Wooding
: How come, even with everything spanking clean and lubricated, you still
: get alot less stopping power from the rear brakes?
'Cos the cable run to the rear is longer by several times.
When you pull the brake lever to apply the brakes, the cables go tight and take
up any slack *before* the brake pads actually move. This slack may be caused by
stretch in the cables, cable outers compressing, or the cable itself tightening
against the cable outers around bends.
The longer the cable run, the more stretch in the cable, and generally the
more bends it has to go through in the case of the back brake. so the less
of the lever movement actually gets to apply the pads to the rims, hence the
loss in power.
This slack can be minimised by:
1. Making the run to the rear (or front) brake as direct as possible
(Basically a frame design point, and pretty easy to do.)
2. Keep the length of cable outer to a minimum. Again, starts as a frame design
point, but by sloppy building can be made completely ineffective.
3. Don't run the cable through any tight bends, or kink the outers. Cable runs
should be smooth and flowing to allow the cable itself to move freely.
4. Keep the cables well lubricated, and change them if they show signs of
rust or fraying - this produces weak spots where stresses will concentrate,
causing even more slack/stretch and eventually failure.
Anyone think of anything I've missed......
Steve JW (MadBritishMountainBikerAndMaterialsScientist)
!faster and faster until the thrill of speed overcomes the fear of death!
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Dept. of Materials Science and Engineering ____ ._.. /\,
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fax: (051) 794-4675 tel: (051) 794-5384 radd...@liv.ac.uk
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Jobst Brandt
> How come, even with everything spanking clean and lubricated, you still
> get alot less stopping power from the rear brakes?
With weight transfer caused by negative acceleration, there is less weight
on the rear wheel and it easily skids. Depending on you wheelbase and
height, your rear brake can have about 1/3 or less effect than the front
brake. However, this may not be your question at all, but rather that
below the skidding threshold the rear brake is less effective.
With identical caliper brakes, it is easier to make the comparison because
you can see how far down the brake pad adjustment slot the pads are
located. The rear wheel usually has the pads farther down the slot, making
the mechanical advantage poorer than the closer front brake. For
cantilever brakes and other less apparent layouts, you'll have to look
carefully to determine that front and rear are actually identical in geometry.
As was mentioned, friction plays a role but your brakes are in good condition
so that has little effect. I assume you are comparing the force required and
not the 'sponge' in the feel. Good cable should not make this substantially
different. In any case, manufacturers realize that you can't get as much
braking at the rear than the front and make no special effort to make the
rear identical to the front. Too bad, it would make things easier on the
user.
Joe Riel
: Theodore Sternberg (ster...@haas.berkeley.edu) wrote:
: : How come, even with everything spanking clean and lubricated, you still
: : get alot less stopping power from the rear brakes?
As pointed out by another poster the principle reason is the weight transfer
during braking; however, the rear brake is also usually less efficient at
generating braking forces because of increased cable friction losses.
The increased friction is caused primarily by the increased total curvature of
the rear brake cable, not by its greater length. Cable stretch and housing
compression should not affect the force transfer efficiency of the brakes but
will increase the travel required at the brake lever and thus increase the
energy (force * distance) needed to activate the brakes.
: 'Cos the cable run to the rear is longer by several times.
: When you pull the brake lever to apply the brakes, the cables go tight and take
: up any slack *before* the brake pads actually move. This slack may be caused by
: stretch in the cables, cable outers compressing, or the cable itself tightening
: against the cable outers around bends.
The cables don't "go tight" *before* the pads move. They tighten significantly
only after the pads contact the rims and begin to exert a "back force". The
amount of stretch is proportional to the tension in the cable (which
varies along the length of the cable due to friction loss).
Does anyone know the stiffness of brake cable and housing? How does it compare
to the other elements of the brake system (levers, calipers, pads)?
: The longer the cable run, the more stretch in the cable, and generally the
: more bends it has to go through in the case of the back brake. so the less
: of the lever movement actually gets to apply the pads to the rims, hence the
: loss in power.
:
: This slack can be minimised by:
:
: 1. Making the run to the rear (or front) brake as direct as possible
: (Basically a frame design point, and pretty easy to do.)
:
: 2. Keep the length of cable outer to a minimum. Again, starts as a frame design
: point, but by sloppy building can be made completely ineffective.
The length is not so important as the total curvature.
:
: 3. Don't run the cable through any tight bends, or kink the outers. Cable runs
: should be smooth and flowing to allow the cable itself to move freely.
So long as the cable is not kinked, the radius of the bend does not affect
friction loss within the cable. A 90 degree bend with a one inch radius will
have the same loss as a 90 degree bend with a three inch radius.
: 4. Keep the cables well lubricated, and change them if they show signs of
: rust or fraying - this produces weak spots where stresses will concentrate,
: causing even more slack/stretch and eventually failure.
Should lined brake cables be lubricated?
Joe Riel
Ken Lee
>How come, even with everything spanking clean and lubricated, you still
>get alot less stopping power from the rear brakes?
Because your center of gravity shifts forward (i.e., to the front wheel)
when you're slowing. Hint: you can get more stopping power out of your
rear brake by pushing your butt to the back of the seat (or farther back).
---
Ken Lee, kl...@synoptics.com
Michael V. Scully
Todd Kelley
ster...@haas.berkeley.edu (Theodore Sternberg) writes:
>How come, even with everything spanking clean and lubricated, you still
>get alot less stopping power from the rear brakes?
I'll answer a different question, hoping that you'll consider it
equivalent.
That question is, ``Why can I bring a bike to a stop faster when
using only the front brake rather than when using only the rear brake,
all else being equal?"
Here's the sloppy answer:
Any braking force, whether from the front or rear wheel, tends
to lift the rear wheel off the road, and push the front wheel
into the road. Braking force is limited by how hard the wheel
being used for braking is pushed against the road.
Here's the careful answer:
We assume that the bike does not rotate about an axis parallel
to the wheel axles (i.e. the rider doesn't go over the bars).
The torque on the bike and rider about the center of mass is
therefore zero. Also assume the rider is in a normal position,
so the center of mass is somewhere in a vertical plane between
the wheel axles.
There are four forces acting on the bike and rider:
gravity (G),
the force pushing up on the rear wheel (F_rear),
the force pushing up on the front wheel (F_front),
the braking force pushing back on the wheel used for braking (F_braking).
During braking, F_braking results in a torque about the center of mass
pointing to the rider's left. Since the total torque is zero, and
G acts through the center of mass (no torque), F_rear and F_front
must result in a torque of the same magnitude, and pointing to the
right. The sum of F_rear and F_front is constant, and the center of
mass doesn't change, so the bigger F_braking is, the bigger F_front is.
(F_front can be no bigger than G, since F_rear can be no less than 0.
That means if F_braking gets really big, the torques don't cancel
anymore, and the rider goes over the bars.)
All this is true, regardless of which wheel is used for braking.
Now, the maximum value possible for F_braking is proportional
to F_Front if the front wheel is used, and it's proportional
to F_rear if the rear wheel is used (F_braking is a frictional force).
F_Front can be as big as the manitude of G, but F_rear is
always smaller than G. Therefore, F_braking can be bigger when
the front wheel is used for braking than when the rear wheel
is used.
Todd
--
Todd Kelley t...@cs.toronto.edu
Department of Computer Science
University of Toronto
Stephen Moss
>From: ster...@haas.berkeley.edu (Theodore Sternberg)
>Subject: why are rear brakes weaker?
>Date: 8 Dec 1993 05:37:04 GMT
>How come, even with everything spanking clean and lubricated, you still
>get alot less stopping power from the rear brakes?
>
>Ted Sternberg
>"I have a masters, but it's not in science."
> ,__0
In a single word... WEIGHT TRANSFER ... well 2.
When you are stopping most or all your weight is transferred to your front
wheel increasing the friction between the tire and the ground. The back is
also lightened reducing the friction.
Stephen
Oivind Toien
B.t.w. I think this only applies to maximum force, and not to the situation
where both wheels touch the ground and do not slide.
My question is: My rear brake pads tend to wear 2-3 times as fast at those at
the front. I have some difficulty beleaving that this only is due to more dirt
on the rear rim, or that I use the rear brakes more than the front. Most of
the wear occur during wet conditions. Any opinions? (A possible control
would be if someone are riding their bikes with clean rims all the time..).
--
Oivind Toien <oiv...@fagmed.uit.no>
Dept. of Arctic Biology, Institute of Medical Biology, University of Tromso
9037 Tromso, NORWAY Phone+47-776-45661 Fax+47-776-45770
Jobst Brandt
> My question is: My rear brake pads tend to wear 2-3 times as fast at
> those at the front. I have some difficulty beleaving that this only is
> due to more dirt on the rear rim, or that I use the rear brakes more
> than the front. Most of the wear occur during wet conditions. Any
> opinions? (A possible control would be if someone are riding their
> bikes with clean rims all the time..).
Assuming you are using the same pads front and rear, it seems reasonable
that your rear brake should wear faster because it is the one that usually
gets the most use in bad traction and bad traction with wet grit eats rims.
In dry conditions, rims hardly wear and in dry conditions you can break
hard with the front brake. This heavier use of the front brake in the dry
does not make up for the wear in a few gritty wet miles.
G. E. Madine, AT&T-BL/AL, cbnewsm.
>
> Assuming you are using the same pads front and rear, it seems reasonable
> that your rear brake should wear faster because it is the one that usually
> gets the most use in bad traction and bad traction with wet grit eats rims.
> In dry conditions, rims hardly wear and in dry conditions you can break
> hard with the front brake. This heavier use of the front brake in the dry
> does not make up for the wear in a few gritty wet miles.
>
I agree on points 1.5 (wet + grit wears brakes (and rims)),
and 2 (almost no wear when dry), and its corollary, point 3 (hard
use of front brakes in dry conditions for a much larger proportion of
riding time does not make up for the extra wear on the rear brakes
in wet conditions).
But I use my front brakes just as hard in wet conditions. At least I
think I do, but may find out otherwise if objective measurements could be made.
The front brake handle sometimes gets an extra hard sqeeze when the
rims are wet (as all ocassional rain cyclists know). True, in a turn
in the wet I am a bit shy about using the front brakes. But
I ease up on the rear too. I absolutely don't want to skid the front
wheel. But I don't want to skid the rear while in a turn either.
Wet roads are not necessarilly bad traction roads when bicycling
on a straight line. My commute pattern includes many straight through
situations where I have to use the brakes. Come to think of it,
in low traction conditions, I consciously slow down while still
going straight before beginning any turn.
So I'm still clueless. Tonight, I'll have an opportunity to observe :-).
Gary E. Madine, Lehigh Valley Bicycle Riders Alliance
g...@aloft.att.com
Dave Hayes
E. Madine, AT&T-BL/AL, cbnewsm.) wrote:
> jbr...@hpl.hp.com (Jobst Brandt) writes:
> >
> > Assuming you are using the same pads front and rear, it seems reasonable
> > that your rear brake should wear faster because it is the one that usually
> > gets the most use in bad traction and bad traction with wet grit eats rims.
> > In dry conditions, rims hardly wear and in dry conditions you can break
> > hard with the front brake...
>
> I agree on points 1.5 (wet + grit wears brakes (and rims)),
> and 2 (almost no wear when dry), and its corollary, point 3 (hard
> use of front brakes in dry conditions for a much larger proportion of
> riding time does not make up for the extra wear on the rear brakes
> in wet conditions).
>
> But I use my front brakes just as hard in wet conditions. At least I
> think I do...
I think that the rear pads may wear more quickly _because_ the rear brakes
are less effective, and one must apply them for a longer time to achieve
adequate deceleration. And, because the deceleration can be so slight in
wet conditions, you tend to pull harder and Harder on the lever in an
effort to slow down. You learn from experience that the rear wheel is
unlikely to lock and skid, and that a short skid of the rear wheel is not
near as dangerous/unnerving as one in the front. I know that I have no
qualms about grabbing a LOT of rear brake in the wet but, consciously or
un, I tend to be conservative with the front. Another factor may be that,
in wet conditions, many riders tend to "ride" the rear brakes on downhills
to keep the rims/pads reasonably dry and to maintain a lower speed and a
greater sense of security.
Dave Hayes
G. E. Madine, AT&T-BL/AL, cbnewsm.
> > Hint: you can get more stopping power out of your
> >rear brake by pushing your butt to the back of the seat (or farther back).
> B.t.w. I think this only applies to maximum force, and not to the situation
> where both wheels touch the ground and do not slide.
>
If the weight shifts forward, then some weight shifts off the rear wheel.
Remember, even if the rear wheel is still rolling, the force of friction
parallel to the road surface is proportional to the normal force. The
normal force is the weight remaining on the wheel. But that has been reduced.
>My question is: My rear brake pads tend to wear 2-3 times as fast at those at
>the front. I have some difficulty beleaving that this only is due to more dirt
> on the rear rim, or that I use the rear brakes more than the front. Most of
> the wear occur during wet conditions. Any opinions?
>
I notice dramatically greater wear on the rear brake shoes (and greater
amounts of crud left on the rim) from cycling in wet weather. I am not
a cyclist who is shy about using the front brakes either. So I am left
with no explanation.
Gary E. Madine, Lehigh Valley Bicycle Riders Alliance.
g...@aloft.att.com
Callum F Wilson
jbr...@hpl.hp.com (Jobst Brandt) writes:
>
> Assuming you are using the same pads front and rear, it seems reasonable
> that your rear brake should wear faster because it is the one that usually
> gets the most use in bad traction and bad traction with wet grit eats rims.
> In dry conditions, rims hardly wear and in dry conditions you can break
> hard with the front brake. This heavier use of the front brake in the dry
> does not make up for the wear in a few gritty wet miles.
>
surely because most of the riders weight is at the rear, so the rider is
likely to pull the rear brake harder before the wheel starts to skid???
The problem is compounded if the rim is dirty - the rear rim gets more mud
than the front.
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