Severe speed "wobble"
cy...@freemail.co.za
developed a severe speed wobble or whip - it was very difficult to
keep the handlebars steady and bring the bike to a stop. My cycle
dealer has checked the bike out but can find nothing mechanically
wrong with it. The wheels are relatively new (DRC 32spoke rims,
Campy Daytona hubs) but are running true. I'm not sure what the fork
is made from, possibly cromoly (sp?). Frame is Columbus 7005
aluminium.
Anybody had a similar experience or have ideas on why this might
occur? Until I find an answer I'm not too keen on any high speed
descents!
Thanks
Doug Huffman
8h.5 Shimmy or Speed Wobble
--
The conspiracy of ignorance masquerades as common sense.
<cy...@freemail.co.za> wrote in message
news:3c612a50...@news.telkomsa.net...
Qui si parla Campagnolo
Campy Daytona hubs) but are running true. >>
Sometimes tough to diagnose. Might ensure that you have relativley balanced
wheels, that is not having a magnet at the same spot as the valve stem. Fishing
weights can be used to balance wheels.
Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl ST.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
Ken
http://groups.google.com/groups?hl=en&th=b9d5ed6fc4c91650&rnum=7
Ken
<cy...@freemail.co.za> wrote in message
news:3c612a50...@news.telkomsa.net...
A Muzi
http://draco.acs.uci.edu/rbfaq/
There's nothing "wrong" with your bike.
--
Andrew Muzi
http://www.yellowjersey.org
Open every day since 1 April 1971
<cy...@freemail.co.za> wrote in message
news:3c612a50...@news.telkomsa.net...
A Muzi
> Campy Daytona hubs) but are running true. >>
"Qui si parla Campagnolo" <vecc...@aol.com> wrote in message
news:20020206103609...@mb-fx.aol.com...
> Sometimes tough to diagnose. Might ensure that you have relativley
balanced
> wheels, that is not having a magnet at the same spot as the valve stem.
Fishing
> weights can be used to balance wheels.
>
> Peter Chisholm
Do you really think dynamic balance of a racing bicycle's wheels is an
issue? The old two-inch relectors were significant but I find it hard to
get concerned about a magnet. Try spinning a rear wheel in high gear in
midair (speeds at or above real cycling speeds). Does the wheel and/or bike
vibrate? Since this is a worst-case scenario (no damping by tires or
meat-engine) it's a good test. If you want an example of "siginificant",
attach an old 2" wheel reflector and try it again!
Denny Coyle
>
> cyclo-<< The wheels are relatively new (DRC 32spoke rims,
> Campy Daytona hubs) but are running true. >>
> Qui si parla Campagnolo wrote:
> Sometimes tough to diagnose. Might ensure that you have relativley balanced
> wheels, that is not having a magnet at the same spot as the valve stem. Fishing
> weights can be used to balance wheels.
Lead tape for weighting golf clubs works well. Stick on inside of rim
surface.
dc
Tom Kunich
getting a bit worried or did something scare you such as hitting a pothole?
Did you tense your arms up? If so there is a strong possibility that you
caused the speed wobble yourself. As it turns out a speed wobble -- the
shaking of the front end -- happens at about the same frequency as your
muscle response. So if your arms are tense and you hit something that
deflects the front wheel you jerk it straight again which causes and over
correction, which causes and over correction and so forth.
There are several ways to stop this sort of wobble but the best way is to
allow your arms to go completely slack while retaining a good hold on the
bars. This is a whole lot easier said that done. The idea is to be relaxed
on the bike no matter what it does. That helps you to retain control because
you don't over-react.
<cy...@freemail.co.za> wrote in message
news:3c612a50...@news.telkomsa.net...
Alex Rodriguez
tku...@earthlink.net says...
>
>There's something that you have to truthfully ask yourself first -- were you
>getting a bit worried or did something scare you such as hitting a pothole?
>Did you tense your arms up? If so there is a strong possibility that you
>caused the speed wobble yourself. As it turns out a speed wobble -- the
>shaking of the front end -- happens at about the same frequency as your
>muscle response. So if your arms are tense and you hit something that
>deflects the front wheel you jerk it straight again which causes and over
>correction, which causes and over correction and so forth.
>
>There are several ways to stop this sort of wobble but the best way is to
>allow your arms to go completely slack while retaining a good hold on the
>bars. This is a whole lot easier said that done. The idea is to be relaxed
>on the bike no matter what it does. That helps you to retain control because
>you don't over-react.
If it's cold out and you are shivering, it too can cause the same condition.
I've had this happen on the first lap of a race when I was cold. Subsequent
laps there was no shimmy down the same hill.
-----------------
Alex __O
_-\<,_
(_)/ (_)
C_ronin_V
common problem. I solved mine by changing the saddle. But there are dozens
of possible causes. Maybe experiment by trying different wheels, tires,
stem etc. The faq says something about the energy getting trapped in the
frame, that's what causes the wobble. But what traps the energy is the
question. Maybe someone with more knowledge of this can comment. But I
agree with you, it's a terrifying experience, and it is strange, how hard it
is to stop when that happens.
<cy...@freemail.co.za> wrote in message
news:3c612a50...@news.telkomsa.net...
Dave Balfour
helps.
Dave Balfour
Bushnell IL
Scott Hendricks
Ha. You're not "dampening" anything. The word is "damping".
>===== Original Message From "Dave Balfour" <dave...@hotmail.com> =====
Doug Goodwin
I had a wheel wobble at 42mph 2 weeks ago. Turns out my headset was a tad
loose. Not so much that I would have found it, but someone unfamiliar with
how my bike handles, spotted it within 5 minutes. A touch of torque, and I
was good to go!
--
Doug Goodwin
YMMV
"We have met the enemy, and they is us." - Pogo
__o
_`\<,_
(*)/ (*)
Mark Hickey
>Are you saying you wet yourself when you do this?
>
>Ha. You're not "dampening" anything. The word is "damping".
The other day when I was riding my "big" road bike (a Honda V65 Magna,
a scary-fast 1100cc cruiser), I took my hands off the bars to zip up
my jacket. Almost immediately I got a "near tank-slapper" going with
the bars (some VERY large deviations going on).
Wetting myself was NOT out of the question.... ;-)
Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame
Doug Milliken
On Fri, 8 Feb 2002, Mark Hickey wrote:
> The other day when I was riding my "big" road bike (a Honda V65 Magna,
> a scary-fast 1100cc cruiser), I took my hands off the bars to zip up
> my jacket. Almost immediately I got a "near tank-slapper" going with
> the bars (some VERY large deviations going on).
What road speed? About what frequency was the oscillation?
I did some instrumented wobble testing with a Honda 900 Custom, early 80's.
This was for a major tire company. The test was to roll off the throttle
at about 50 mph and then ride hands-off as the bike slowed down. When it
wobbled, the worst was when the tire once-around frequency matched the
first torsional resonance of the front fork -- about 35 mph and 7Hz. The
peak-to peak amplitude of the wobble could be 8 inches or more at the end
of the handlebar. Disconcerting!
But, in all the cases I rode (with ~20 different tire designs) the
motorcycle always went basically straight hands-off, even when the bars
were a blur and the front tire was chirping every half-cycle.
Different front tires made a big difference in the behavior, some were
terrible. Others were no problem, even if I tried to start an oscillation
by hitting the handlebar on one side as the bike slowed down.
It was an interesting job...
-- Doug Milliken
http://www.millikenresearch.com
Mark Hickey
>On Fri, 8 Feb 2002, Mark Hickey wrote:
>
>> The other day when I was riding my "big" road bike (a Honda V65 Magna,
>> a scary-fast 1100cc cruiser), I took my hands off the bars to zip up
>> my jacket. Almost immediately I got a "near tank-slapper" going with
>> the bars (some VERY large deviations going on).
>
>What road speed? About what frequency was the oscillation?
I was going about 35-40mph, and I'd guess the frequency of the
oscillations was about 5-7 cycles per second. The excursion at the
end of the bars (which are pretty long pull-back bars) was probably
around 10" (total, both ways). The bike DID go straight throughout
the episode though, and has never shown a hint of doing it when I'm
hanging on to the bars (a very, very good idea on that particular
bike).
Not like the Kawasaki LTD1000 I had many years ago. That pig would go
like blazes (4 into 1 Kerker header, carb and head work), but would
break into a strange oscillation at between 80 and 125mph (never knew
when it would hit, but if you stayed on the throttle, it would). It
made up for it by not handling or braking. Seriously dangerous bike.
>I did some instrumented wobble testing with a Honda 900 Custom, early 80's.
>This was for a major tire company. The test was to roll off the throttle
>at about 50 mph and then ride hands-off as the bike slowed down. When it
>wobbled, the worst was when the tire once-around frequency matched the
>first torsional resonance of the front fork -- about 35 mph and 7Hz. The
>peak-to peak amplitude of the wobble could be 8 inches or more at the end
>of the handlebar. Disconcerting!
Do some of the Harley folks put those leather streamers on the end of
the grips to damp those osciallations out? ;-)
>But, in all the cases I rode (with ~20 different tire designs) the
>motorcycle always went basically straight hands-off, even when the bars
>were a blur and the front tire was chirping every half-cycle.
>
>Different front tires made a big difference in the behavior, some were
>terrible. Others were no problem, even if I tried to start an oscillation
>by hitting the handlebar on one side as the bike slowed down.
>
>It was an interesting job...
I hope it paid well (and that they provided new leathers ever time you
wore through the old ones....).
Doug Milliken
Well, sounds like what your Magna does is the same kind of wobble that I
was working on. The 900 Custom had a long, stylishly-slender fork and
wide-heavy pull-back bars (think about rotational inertia at the top of the
fork assembly).
The other prominent mode in the 900 Custom was weave at 90-110 mph. This
happened hands-on (never tried to take my hands off at that speed!) and got
worse if I tried to correct for it, since the frequency was 2Hz, just
beyond typical human "frequency response". I think that the best response
was to keep my arms fairly "limp" and just slow down (or speed up if the
weave was not severe) to get out of the critical speed range. The weave
was a lot more disconcerting than the wobble, since the whole bike followed
a sine-wave shaped path with 1+ foot peak-peak amplitude on the road.
Are you using the original equipment tires or some other tire design?
We found a good reverse correlation with this 900 -- front tires that
minimized low-speed wobble gave the worst high-speed weave behavior.
Re leathers: Never went down during any of this testing. I think the key
was that we took our time and worked up to each new thing gradually, so
there were no big surprises.
I met another test rider at a different company (a former road racer, not
an engineer) who was in a hurry--he went from empty to full load in a set
of rear luggage boxes, all in one step...and was quite badly injured (but
recovered).
No idea about your Kawasaki, only rode one once.
On the Harley dresser we tested, the ancient Speed Grip tire (4 rib,
squared off tread, sort of looks like an original VW Beetle tire) worked
very well to minimize wobble...although I was the butt of a good practical
joke from the other rider on that project. He rode the Speed Grip first
and came back all shaking and muttering about that terrible "Death Grip"
tire...then it turned out to be the second best tire of the bunch. Lots of
laughs after that round!
-- Doug Milliken
www.millikenresearch.com
Tom H
> This is covered well in the FAQ:
> http://draco.acs.uci.edu/rbfaq/
> There's nothing "wrong" with your bike.
>
I can't agree with this. Bikes shouldn't wobble hands-on at moderate
speeds. I would say there is something fundamentally "wrong" with it!
Tom
Doug Huffman
I'll note the weasel words "moderate" and "shouldn't."
Whose "moderate speeds?"
Please produce a bicycle that can not wobble. The world will beat a path to
your door.
--
The conspiracy of ignorance masquerades as common sense.
"Tom H" <thomas....@bakeratlas.com> wrote in message
news:5c69a497.02021...@posting.google.com...
A Muzi
Some bikes under some riders shimmy at some times. Anything I can do
(alignment, bearing service, wheels round, fatseners tight) makes no
difference and then another rider on another copy of the same bike will not
experience shimmy. If something's "wrong", what is it?
David L. Johnson
> Please produce a bicycle that can not wobble. The world will beat a path to
> your door.
No, actually, they wouldn't. Such a bike would have to be so much more damped
than any bike sold today that it would feel dead.
--
David L. Johnson
__o | "It doesn't get any easier, you just go faster." --Greg LeMond
_`\(,_ |
(_)/ (_) |
Matthew Temple
Andrew,
I've wondered about this myself. A very tall friend of mine from
Belgium had quite a severe shimmy problem. No one was ever
able cto figure out what the problem was to my knowledge. What _could_
account for the different experience of these two riders?
Matt Temple
--
=============================================================
Matthew Temple Tel: 617/632-2597
Director, Research Computing Fax: 617/632-4012
Dana-Farber Cancer Institute m...@research.dfci.harvard.edu
44 Binney Street, JF 314 http://research.dfci.harvard.edu
Boston, MA 02115 Choice is the Choice!
Tom H
This is really what I meant. It's a fundamental problem with the bike.
The frame and forks are simply not up to the job. If you get shimmy
you have two choices: get a new bike and hope it works, or avoid the
terrain where you are likely to lose control.
Tom
Tom H
>
> I've wondered about this myself. A very tall friend of mine from
> Belgium had quite a severe shimmy problem. No one was ever
> able cto figure out what the problem was to my knowledge. What _could_
> account for the different experience of these two riders?
>
> Matt Temple
Speed-wobble happens almost invariably to tall riders (heavy helps
too) on straight descents. It can be initiated by side-winds or rough
road surfaces. Large bicycles are inherently much more flexible than
small ones. Large bicycles also typically have less trail, which I
think causes more steering torque for small forced deviations of the
front wheel. It's a problem which the bicycle industry quietly
ignores, and one which I'm sure injures people every year.
I've a small aside to demonstrate how differently large and small
bicycles behave. My girlfriend and I have matching his'n'hers bikes.
We were descending side-by-side (from Femes in Lanzarote if anyone
knows it) along a newly laid road which looked super-smooth. I told
her to look at my front wheel. It was bouncing up and down at high
frequency. There must have been some underlying irregularity in the
road. Her bike was unaffected.
Tom
Peter Cole
> too) on straight descents. It can be initiated by side-winds or rough
> road surfaces. Large bicycles are inherently much more flexible than
> small ones. Large bicycles also typically have less trail, which I
> think causes more steering torque for small forced deviations of the
> front wheel. It's a problem which the bicycle industry quietly
> ignores, and one which I'm sure injures people every year.
I'm 6'10" (210 cm), weigh 235 (105 kg), I have never experienced shimmy at all,
in several bike frames, made from steel & aluminum, at speeds up to exceeding 50
mph.
Mark Hickey
>Doug Huffman wrote:
>
>> Please produce a bicycle that can not wobble. The world will beat a path to
>> your door.
>
>No, actually, they wouldn't. Such a bike would have to be so much more damped
>than any bike sold today that it would feel dead.
Well, FWIW - I've never had a customer mention experiencing shimmy on
their Habanero. Since there's nothing really unusual about the
geometry of the Habanero road frame, I can only surmise that
large-diameter (stiff) titanium tubes must do a good job at doing
whatever it is that prevents shimmy and that it's mainly because of
the frame's lateral and torsional stiffness, rather than the material
itself.
David L. Johnson
> Well, FWIW - I've never had a customer mention experiencing shimmy on
> their Habanero. Since there's nothing really unusual about the
> geometry of the Habanero road frame, I can only surmise that
> large-diameter (stiff) titanium tubes must do a good job at doing
> whatever it is that prevents shimmy and that it's mainly because of
> the frame's lateral and torsional stiffness, rather than the material
> itself.
There are many variables that go into the resonance that is shimmy. Stiffness
is part of it, of course. But any reasonably responsive frame can't be free
of shimmy, it can, however, happen that shimmy would not occur except at
speeds over 70mph, which is just as good.
--
David L. Johnson
__o | When you are up to your ass in alligators, it's hard to remember
_`\(,_ | that your initial objective was to drain the swamp. -- LBJ
(_)/ (_) |
Wayne Morrison
An experience of mine tends to confirm what you have found. I am 6 feet 3
inches tall and well over 200lb in weight. One of my main riding road bikes (an
older Raleigh) handles well at all speeds. I put over 3000 Km. on this bike
last year, so obviously I know it well, and it works well for me.
On one longer trip (80 Km.) I decided to carry some supplies with me and set up
a container on my rear carrier (which has always been there). I put about 7
pounds of stuff in a box on this carrier, and this was enough to cause a VERY
mild repeatable shimmy at about 29 to 32 Km. per hour. After the run I serviced
the bike well, looking for things like loose head bearings etc. I found nothing
wrong, and once the extra weight was removed the shimmy dissappeared.
That happened during the summer of 2000, and this last summer (2001) I did a
longer trip again with the extra weight, and once more this occurred. Obviously
there is something in the balance or weight distribution of the bike that is
causing this.
If weight distribution IS a contributing factor, then obviously moving saddle
etc. forward or back will change what is happening.
Wayne
Mark Hickey
>Mark Hickey wrote:
>
>> Well, FWIW - I've never had a customer mention experiencing shimmy on
>> their Habanero. Since there's nothing really unusual about the
>> geometry of the Habanero road frame, I can only surmise that
>> large-diameter (stiff) titanium tubes must do a good job at doing
>> whatever it is that prevents shimmy and that it's mainly because of
>> the frame's lateral and torsional stiffness, rather than the material
>> itself.
>
>There are many variables that go into the resonance that is shimmy. Stiffness
>is part of it, of course. But any reasonably responsive frame can't be free
>of shimmy, it can, however, happen that shimmy would not occur except at
>speeds over 70mph, which is just as good.
I think that's probably the case here - that any bike probably does
have a "shimmy speed" but if that speed's faster than the bike will
ever go, it's a non-issue.
Jon Isaacs
>have a "shimmy speed" but if that speed's faster than the bike will
>ever go, it's a non-issue.
Probably this is indeed the case. It is likely Habanero's would suffer from
speed wobble problems that if Mark ever built one capable of being ridden over
20 mph. <g>
jon isaacs
David L. Johnson
> I put about 7
> pounds of stuff in a box on this carrier, and this was enough to cause a VERY
> mild repeatable shimmy at about 29 to 32 Km. per hour. After the run I serviced
> the bike well, looking for things like loose head bearings etc. I found nothing
> wrong, and once the extra weight was removed the shimmy dissappeared.
> That happened during the summer of 2000, and this last summer (2001) I did a
> longer trip again with the extra weight, and once more this occurred. Obviously
> there is something in the balance or weight distribution of the bike that is
> causing this.
If you think of the bike as a mechanical system (spring-damper sort of thing),
then simple differential equations will show you what is happening.
Side-to-side a bike is very lightly damped, and so prone to resonance, where
certain input frequencies get reinforced rather than damped out, building to
large amplitudes quite quickly. The input frequencies in this case come from
the steering corrections we make to keep upright, countered by the gyroscopic
effects of the wheel. At certain speeds, the input frequency is reinforced by
the natural frequency of the bike+rider, and shimmy is the result.
There are only two ways to change this, either change the damping of the
system (which would be a dead frame, and probably a challenge to build,
anyway) or change the natural frequency to something beyond what you encounter
riding. There are many ways to do this; certinly buying a new bike might do
it -- but then a new bike might produce, with you on board, more shimmy than
the old one. There are too many factors to control for. Another alternative,
and a lot cheaper, is to change the characteristics on the fly. Get off the
saddle. The whole system rotates around the most massive part, which is the
rider. Change the connection from the relatively rigid plant of seat on seat,
to the well-sprung legs and arms, and the shimmy will disappear. It worked
for me.
Since you don't want to do a lot of out-of saddle downhilling on a tour with a
loaded bike, certainly experiment with re-distributing the weight. There is
no way to tell what change will work -- but it is quite likely that almost any
change will have some effect.
--
David L. Johnson
__o | Deserves death! I daresay he does. Many that live deserve
_`\(,_ | death. And some that die deserve life. Can you give it to
(_)/ (_) | them? Then do not be too eager to deal out death in judgement.
-- J. R. R. Tolkein
Toby Hamilton
> Probably this is indeed the case. It is likely Habanero's would suffer
from
> speed wobble problems that if Mark ever built one capable of being ridden
over
> 20 mph. <g>
::Sigh:: Now you tell me? It's a good thing I only plan to ride mine in
km/h...
--
Toby Hamilton (th-...@rogers.com)
Mark Hickey
>Jon Isaacs wrote:
>
>> Probably this is indeed the case. It is likely Habanero's would
>> suffer from speed wobble problems that if Mark ever built one
>> capable of being ridden over 20 mph. <g>
>
>::Sigh:: Now you tell me? It's a good thing I only plan to ride mine in
>km/h...
Of course, the wobble is entirely different if you're going to be
riding in km/h instead of mph. But even though the wobble might start
at only 20kph, the wobbles themselves will be in millimeters instead
of inches, so you won't even notice 'em.
Denny Coyle
"David L. Johnson" wrote:
> If you think of the bike as a mechanical system (spring-damper sort of thing),
> then simple differential equations will show you what is happening.
> Side-to-side a bike is very lightly damped, and so prone to resonance, where
> certain input frequencies get reinforced rather than damped out, building to
> large amplitudes quite quickly. The input frequencies in this case come from
> the steering corrections we make to keep upright, countered by the gyroscopic
> effects of the wheel. At certain speeds, the input frequency is reinforced by
> the natural frequency of the bike+rider, and shimmy is the result.
I'm sure this has been dealt with ad nauseum in other threads but why
has wheel imbalance been placed in a secondary role to steering
corrections. I think that the primary force that instigates the
oscillations leading to this resonance is a dynamic imbalance of the
wheel-side/side with increasing rpms and as you say very lightly damped.
Try sticking weight on the side of the tire to bring it into a static
balance and the oscillations initiate with greater amplitudes and bring
on resonance. It turns out to be a trial and error procedure to bring
the wheel into a dynamic balance(I don't know of anyone with a wheel
adapter for a SnapOn balancer)-I use stick on lead tape that's available
at golf pro shops, use leather punches to cut various sizes and keep
them small to get them to the outside of the spoke holes,better to use
two/three small ones to one larger one. This has minimzed my problems
with shimmy.
The problem with steering corrections in my experience is that the whole
scenario plays out with you hands off the bars if downhill speed is
there. Induced oscillations at low speeds damp out rather rapidly.
dc
Bluto
> Some bikes under some riders shimmy at some times. Anything I can do
> (alignment, bearing service, wheels round, fatseners tight) makes no
> difference and then another rider on another copy of the same bike will not
> experience shimmy. If something's "wrong", what is it?
I have observed that my motorcycle will flap the handlebars around on
certain combinations of speed, engine load, grade, and camber, *when
the front tire is well worn*. The same conditions that cause a shimmy
when there's a comparatively flattened strip of tread will not do it
when the tires are fresh.
Likewise, some of the boys in the local chopper bicycle club built a
chopper that had two front wheels side-by-side about 5 inches apart.
Cruising downhill, this craft would shudder, flutter, and flail its
bars without changing overall direction. It was basically stepping
from the left front tire to the right front tire and vice versa at a
high frequency, exerting heavy feedback to the bars as it went.
My tentative conclusion is this: Just like a flat-treaded tire or a
paired front wheel, many tire/rim/pressure/load/head angle
combinations have an intrinsic self-servo flutter frequency, with a
smaller magnitude than the aforementioned examples. When this
frequency is harmonically coupled to the period of say, frame flex at
certain speed and load conditions, the small and easily damped
intrinsic flutter can be stoked up to frightening proportions.
If this is the case, then changing to a front tire with a
significantly different tread contour would either eliminate the
shimmy or dramatically shift the set of requisite conditions for the
shimmy to occur. In particular, a more peaked tread cross-section (a
la Panaracer Pasela or similar) would probably help. That type of
cross-section is the opposite of the flattened front treads that seem
to exaggerate shimmy issues in my observation.
Even experimenting with significantly different tire pressures could
pay dividends in this regard.
I made a mental note to do some experiments on shimmy with a
Schwinn-type "cheater slick" square-treaded tire, but I have had no
good opportunity to acquire one. Someone who owns a 20" bike fitted
with such a rear tire could do some testing just by swapping the tires
back to front and coasting downhill to try to initiate some shimmying.
Chalo Colina
David L. Johnson
> I'm sure this has been dealt with ad nauseum in other threads but why
> has wheel imbalance been placed in a secondary role to steering
> corrections.
First, because it is in the wrong direction. It is not an up-and-down motion
that is being translated to the frame. Bike frames are nearly completely
rigid in this plane, anyway. It's perpendicular to that direction, side to
side, where the shimmy comes from. A shimmy will indeed be side-to-side
motion, and needs input force in the same direction to start it.
Secondly, out-of-balance tires don't cause that much motion even in the
vertical direction. The compliance (spring) of the tire is not high enough,
it is too damped, and the force from the few grams of out-of balance that any
wheel is is not sufficient.
> I think that the primary force that instigates the
> oscillations leading to this resonance is a dynamic imbalance of the
> wheel-side/side
Side-to-side imbalance? Miniscule.
> with increasing rpms and as you say very lightly damped.
> Try sticking weight on the side of the tire to bring it into a static
> balance
How are you measuring side-to-side tire imbalance?
> The problem with steering corrections in my experience is that the whole
> scenario plays out with you hands off the bars if downhill speed is
> there. Induced oscillations at low speeds damp out rather rapidly.
What, you don't steer with your hands off the bar? Of course you do, or you'd
fall over.
--
David L. Johnson
__o | You will say Christ saith this and the apostles say this; but
_`\(,_ | what canst thou say? -- George Fox.
(_)/ (_) |
Tom H
>
>
> There are only two ways to change this, either change the damping of the
> system (which would be a dead frame, and probably a challenge to build,
> anyway) or change the natural frequency to something beyond what you encounter
> riding. There are many ways to do this; certinly buying a new bike might do
> it -- but then a new bike might produce, with you on board, more shimmy than
> the old one. There are too many factors to control for. Another alternative,
> and a lot cheaper, is to change the characteristics on the fly. Get off the
> saddle. The whole system rotates around the most massive part, which is the
> rider. Change the connection from the relatively rigid plant of seat on seat,
> to the well-sprung legs and arms, and the shimmy will disappear. It worked
> for me.
>
Are you saying that you had one of those violent speed-wobbles where
you can barely hang onto your handlebars and simply stood up off your
saddle and the shimmy stopped? I've had speed wobbles so violent that
I wasn't sure whether I was standing or not. I've also had speed
wobbles where I've stood up only to cause my rear wheel to skip from
side to side. I've never had a wobble that simply disappeared the
instant I stood up. The only thing that really works is to slow down.
> Since you don't want to do a lot of out-of saddle downhilling on a tour with a
> loaded bike, certainly experiment with re-distributing the weight. There is
> no way to tell what change will work -- but it is quite likely that almost any
> change will have some effect.
You don't need to be on a loaded tour to get speed-wobble. In my
experience, changing wheels/tyres (within reason) makes no practical
difference.
Tom
Tom H
> This has been beat to death here and in years past, hence the FAQ.
>
> I'll note the weasel words "moderate" and "shouldn't."
>
> Whose "moderate speeds?"
>
> Please produce a bicycle that can not wobble. The world will beat a path to
> your door.
>
> --
> The conspiracy of ignorance masquerades as common sense.
>
>
The shimmy in question occurred at 35mph. That is a totally reasonable
speed! It is also perfectly reasonable to assume that any frame sold
for the purposes of racing should not shimmy at racing speeds. Many
(in fact most) bikes do not shimmy under reasonable conditions - ie
within their design parameters. Some bikes do shimmy, and the ones
that do, and the conditions under which they shimmy have a lot in
common.
Some people love banging on about the faq. I guess it's far easier
than thinking for themselves. Trouble is, the faq really concerns
itself with the phenomenon of hands-off shimmy. This is a subtley
different beast to hands-on speed-wobble. There are several aspects of
speed-wobble that are different from shimmy, at least they don't tie
in with the analysis in the faq. I hope noone is so brain dead as to
take these comments as an attack on the faq's author. I know better
than that! I'm in total agreement with the faq as far as it goes.
Jobst is careful to give zero advice on what changes to make to reduce
the chances of speed-wobble.
There may not be such a thing as a bike that "can not" wobble, but
there may be such a thing as a bike that will not wobble under
reasonable conditions. There certainly should be!
Tom
--
Ignorant faq-quoting masquerades as knowledge
Tom H
I'm willing to volunteer to definitevely shimmy-test your bikes - all
you have to do is lend me one ;-).
Seriously though, I know a descent which guarantees a shimmy. I've
ridden it several times (~8) on three different bikes ( Cannondale
2.8i, CAAD3, Concorde steel) and got shimmy every time. It's a really
depressing as it's a beautiful decent that begs high speed. The
experience basically puts me in a really bad mood for the rest of the
day, especially as I watch others (it's a bit of a cycling mecca) tear
down the hill at speeds of 50mph+. It's got the three essential shimmy
ingredients: Straight line (about 8k dead-straight), wind, rough
road-surface. It's the descent down Fire Mountain on Lanzarote. Next
time I go there, I'm considering taking an MTB. I've never shimmied an
MTB!
Tom
Denny Coyle
Denny Coyle wrote:
> > I'm sure this has been dealt with ad nauseum in other threads but why
> > has wheel imbalance been placed in a secondary role to steering
> > corrections.
"David L. Johnson" wrote:
> First, because it is in the wrong direction. It is not an up-and-down motion
> that is being translated to the frame.
Nobody said anything about an up/down motion. That's just my point.
You're looking at this balance as a static balance, I would guess you're
thinking in terms of nulling out the imbalance with the wheel in a
truing stand and countering the wheel rotation by adding weights to get
the wheel to remain motionless in any set position. This type of wheel
balancing has long ago been abandoned, I'm sure your well aware of this.
What you want to do is counter the oscillatory force that is causing the
wheel to deflect in a direction normal to wheel direction by adding
weight equal but one eighty and on the opposite side.
> Bike frames are nearly completely
> rigid in this plane, anyway. It's perpendicular to that direction, side to
> side, where the shimmy comes from. A shimmy will indeed be side-to-side
> motion, and needs input force in the same direction to start it.
That's what I'm saying.
> Secondly, out-of-balance tires don't cause that much motion even in the
> vertical direction. The compliance (spring) of the tire is not high enough,
> it is too damped, and the force from the few grams of out-of balance that any
> wheel is is not sufficient.
Not my experince with high dollar car wheels(spendy alloys with
outrageous tires to match-not mine). I have wheels here with less than a
quarter ounce of weight to bring them in dynamic balance.
> > I think that the primary force that instigates the
> > oscillations leading to this resonance is a dynamic imbalance of the
> > wheel-side/side
> Side-to-side imbalance? Miniscule.
I suppose the imbalance in car wheels comes from steering wheel input as
well. There's a certain wheel speed at which point a dynamically
imbalanced wheel shimmy translates to resonance. Getting beyound that
speed can sometimes alleviate the resonance and it's smooth traveling,
it has nothing to do with rockin&rollin in the cabin.
> > with increasing rpms and as you say very lightly damped.
> > Try sticking weight on the side of the tire to bring it into a static
> > balance
>
> How are you measuring side-to-side tire imbalance?
Well to start you need to do the static balance, then you use conical
blocks with the wheel axel vertical and rim/tire blocked. Remove side
blocks,note dip and counterbalance. Then it's trial/error at the local
hill.
> > The problem with steering corrections in my experience is that the whole
> > scenario plays out with you hands off the bars if downhill speed is
> > there. Induced oscillations at low speeds damp out rather rapidly.
>
> What, you don't steer with your hands off the bar? Of course you do, or you'd
> fall over.
Well we can play semantics all day long. Of course you're maintaining
the bike upright by using body motion, but you lead one to believe
that's it's hand twitching or some such motion which sets this
oscillatory motion in action.While this might be true in some instances,
I believe there are other cases where this is not the force causing the
imbalance and the subsequent resonance.
I recall reading-it was one of Zinn's tech reports- during last year's
TF, that one of the wheel manufacturers was going to be incorprating a
sliding weight system into one of rims to better achieve this balance.
Maybe someone could dig this up, can't seem to find this.
dc
Peter Cole
> than thinking for themselves. Trouble is, the faq really concerns
> itself with the phenomenon of hands-off shimmy. This is a subtley
> different beast to hands-on speed-wobble. There are several aspects of
> speed-wobble that are different from shimmy, at least they don't tie
> in with the analysis in the faq. I hope noone is so brain dead as to
> take these comments as an attack on the faq's author. I know better
> than that! I'm in total agreement with the faq as far as it goes.
> Jobst is careful to give zero advice on what changes to make to reduce
> the chances of speed-wobble.
I didn't remember the FAQ saying all that, so I re-read it & it doesn't say
that!
David L. Johnson
> > Side-to-side imbalance? Miniscule.
>
(My comment)
> I suppose the imbalance in car wheels comes from steering wheel input as
> well.
What? No. But the vast majority of car wheel balance problems come from
uneven weight distribution radially -- translating into up-and-down motion.
It may be possible to balance in the direction of the axle, but I've never
seen the car wheel that has been balanced that way, and never had one that
needed it. And car wheels are a lot heavier, and move faster, than bike
wheels. The amount in the direction of the axle that a bike wheel could be
out-of balance is totally insignificant.
> There's a certain wheel speed at which point a dynamically
> imbalanced wheel shimmy translates to resonance.
And this resonance is in the direction that the car is capable of responding,
vertical.
> it has nothing to do with rockin&rollin in the cabin.
What?
> Well to start you need to do the static balance,
Your use of "static" and "dynamic" balance suggests that they are measuring
imbalance in different directions. Static simply means that the wheel is not
rotating, dynamic means that it is (which allows greater precision).
> then you use conical
> blocks with the wheel axel vertical and rim/tire blocked. Remove side
> blocks,note dip and counterbalance.
This is still measuring imbalance in the same sense.
The imbalance you are talking about will still measure only whether one side
of the wheel is heavier than the other. The terminology is imprecise, so let
me try again. All you are doing is being more careful about eliminating the
tendency of the wheel, when rotating freely, to end up with the valve at the
bottom when it stops rotating. Spin such a wheel fast, and any imbalance
would result in reciprocating forces up and down (OK, forward and back as
well) since the wheel rotates around the center of mass, not the axle.
> > > The problem with steering corrections in my experience is that the whole
> > > scenario plays out with you hands off the bars if downhill speed is
> > > there. Induced oscillations at low speeds damp out rather rapidly.
> >
> > What, you don't steer with your hands off the bar? Of course you do, or you'd
> > fall over.
>
> Well we can play semantics all day long. Of course you're maintaining
> the bike upright by using body motion, but you lead one to believe
> that's it's hand twitching or some such motion which sets this
> oscillatory motion in action.
The body motion turns the wheel. That is how you turn a bike, hands-on or
off. Your point (> > > above) was supposedly that a no-hands shimmy implied
that this explanation was incorrect, but steering corrections are the same no
hands, which was my point, although you considered it "semantics".
> I recall reading-it was one of Zinn's tech reports- during last year's
> TF, that one of the wheel manufacturers was going to be incorprating a
> sliding weight system into one of rims to better achieve this balance.
Don't confuse engineering with marketing.
--
David L. Johnson
__o | "A foolish consistency is the hobgoblin of little minds, adored
_`\(,_ | by little statesmen and philosophers and divines." --Ralph Waldo
(_)/ (_) | Emerson
David L. Johnson
> Are you saying that you had one of those violent speed-wobbles where
> you can barely hang onto your handlebars and simply stood up off your
> saddle and the shimmy stopped?
Yep.
> I've had speed wobbles so violent that
> I wasn't sure whether I was standing or not. I've also had speed
> wobbles where I've stood up only to cause my rear wheel to skip from
> side to side.
? Well, I certainly have never seen that. I'm not suggesting it's
impossible, but what could be causing the bike -- the whole bike -- to jump
side to side? Any shimmy I've experienced, or have heard about, comes from
the front part of the bike shaking from side to side, actually rotating back
and forth with an axis about at the seat tube -- since it is rotating around
the rider's body. That's why unloading the saddle works, it removes the
pivot.
> You don't need to be on a loaded tour to get speed-wobble.
Never said you did.
--
David L. Johnson
__o | "What am I on? I'm on my bike, six hours a day, busting my ass.
_`\(,_ | What are you on?" --Lance Armstrong
(_)/ (_) |
Tom H
The only point I have made is that hands-on speed-wobble has some
characteristics that seem to be at odds with the analysis of hands-off
shimmy presented in the faq. As I am in total agreement with the faq
on shimmy, I am forming the opinion that the two phenomena are not
quite the same. I could of course be wrong. Either way, I will remain
of the opinion that bikes should be designed not to speed-wobble under
reasonable conditions.
Tom
Jon Isaacs
>Not my experince with high dollar car wheels(spendy alloys with
>outrageous tires to match-not mine). I have wheels here with less than a
>quarter ounce of weight to bring them in dynamic balance.
Car wheels are quite a different situation than bicycle wheels. Are car wheel,
especially those expensive trendy ones that fracture when they hit a curb, are
wide in relation to their diameter. This means that they can be out of balance
in both the radial direction which produces a vertical motion and across the
width which can produce a motion about the steering axis.
Bicycle wheels are very narrow in relation to their width and thus the moments
produced by being out of balance across their width is tiny.
Personally I have never experienced a wobble on either a motorcycle or a
bicycle though I have owned and ridden ones famous for their wobbles at speeds
that were know to provoke such things.
jon isaacs
Denny Coyle
> Denny Coyle wrote:
> > I suppose the imbalance in car wheels comes from steering wheel input as
> > well.
> "David L. Johnson" wrote:
> What? No. But the vast majority of car wheel balance problems come from
> uneven weight distribution radially -- translating into up-and-down motion.
> It may be possible to balance in the direction of the axle, but I've never
> seen the car wheel that has been balanced that way, and never had one that
> needed it. And car wheels are a lot heavier, and move faster, than bike
> wheels. The amount in the direction of the axle that a bike wheel could be
> out-of balance is totally insignificant.
>
> > There's a certain wheel speed at which point a dynamically
> > imbalanced wheel shimmy translates to resonance.
>
> And this resonance is in the direction that the car is capable of responding,
> vertical.
> > it has nothing to do with rockin&rollin in the cabin.
> What?
Just making reference to your top down solution of this
problem-hands,nohands,body- as opposed to what I believe is a bottoms up
problem-imbalances...
>
> > Well to start you need to do the static balance,
>
> Your use of "static" and "dynamic" balance suggests that they are measuring
> imbalance in different directions. Static simply means that the wheel is not
> rotating, dynamic means that it is (which allows greater precision).
> The imbalance you are talking about will still measure only whether one side
> of the wheel is heavier than the other. The terminology is imprecise, so let
> me try again. All you are doing is being more careful about eliminating the
> tendency of the wheel, when rotating freely, to end up with the valve at the
> bottom when it stops rotating. Spin such a wheel fast, and any imbalance
> would result in reciprocating forces up and down (OK, forward and back as
> well) since the wheel rotates around the center of mass, not the axle.
Dynamic is balancing around the longitudinal plane of the wheel. This
imbalance creates a side/side motion of the wheel. Whether there is
sufficient width of the bicycle wheel to render this imbalance
significant remains open to question. Some believe that this should play
out only with the wider/lower profile tires but here again most
motorcycle wheels-granted wider than bicycle wheels- are balanced using
dynamic machines, some operating in a super fine mode of one gram.
> > > Side-to-side imbalance? Miniscule.
> >
> (My comment)
Miniscule relative to what? Up/down? It doesn't necessarily mean
insignificant. Whose to say there's not a cumulative effect here which
might add up to the force needed to put this system into resonance. Add
all the imbalances, add the force variance from the wheel runout (maybe
miniscule maybe not depending on the wheel builder and quality of tire),
could there be a moment created by the air force on the sides of the
wheel? insignificant? maybe. Operate these forces (and probably others)
on the gyro of the wheel and I don't see why there shouldn't be a
resonance eventually set up in the blades of the forks and transmitted
via headtube throughout the bike.
>
> > > > The problem with steering corrections in my experience is that the whole
> > > > scenario plays out with you hands off the bars if downhill speed is
> > > > there. Induced oscillations at low speeds damp out rather rapidly.
> > >
> > > What, you don't steer with your hands off the bar? Of course you do, or you'd
> > > fall over.
> >
> > Well we can play semantics all day long. Of course you're maintaining
> > the bike upright by using body motion, but you lead one to believe
> > that's it's hand twitching or some such motion which sets this
> > oscillatory motion in action.
>
> The body motion turns the wheel. That is how you turn a bike, hands-on or
> off. Your point (> > > above) was supposedly that a no-hands shimmy implied
> that this explanation was incorrect, but steering corrections are the same no
> hands, which was my point, although you considered it "semantics".
Wouldn't it seem reasonable that faster speeds would necessitate less
steering corrections? Why it's harder to track stand than just ride.
> > I recall reading-it was one of Zinn's tech reports- during last year's
> > TF, that one of the wheel manufacturers was going to be incorprating a
> > sliding weight system into one of rims to better achieve this balance.
> Don't confuse engineering with marketing.
Engineering is growth! from supposition to application!
Just trying to keep an open mind here.
dc
jobst....@stanfordalumni.org
>> I suppose the imbalance in car wheels comes from steering wheel
>> input as well.
> What? No. But the vast majority of car wheel balance problems come
> from uneven weight distribution radially -- translating into
> up-and-down motion. It may be possible to balance in the direction
> of the axle, but I've never seen the car wheel that has been
> balanced that way, and never had one that needed it. And car wheels
> are a lot heavier, and move faster, than bike wheels. The amount in
> the direction of the axle that a bike wheel could be out-of balance
> is totally insignificant.
I guess it went by too long ago but we had this wheel balancing idea
discussed here at length.
http://yarchive.net/bike/wheel_balancing.html
Subsequently I put a significant wrap of 1/8" thick solder wire around
one spoke at the rim and coasted down a hill no-hands, observing the
incidence and intensity of shimmy. There was no difference between
balanced and imbalanced wheel. Rather than hypothesize about it, a
test of theory does a lot to put it into perspective.
Also interesting it that the line on the pavement barely shows any
deviation from a straight line, the top of the handlebar stem making
th largest excursions. I find putting weight on the handlebar more
enlightening. This affects both initiating speed and amplitude of
shimmy.
Jobst Brandt <jobst....@stanfordalumni.org> Palo Alto CA
Jon Isaacs
>insignificant. Whose to say there's not a cumulative effect here which
>might add up to the force needed to put this system into resonance.
Try this experiment.
Tape a steel bar weighing about 200 grams to front wheel against the spokes on
one side.
I contend this represents a much larger out of balance condition than is
possible with a real wheel.
Now spin the front wheel holding the bike by the top tube thus letting the
front end be free to move.
Spin it to 12 mph or so. You will find there is a significant vertical
component to the motion, no side to side component.
While the speeds are low, the 200 gram mass is so far beyond any possible out
of balance mass that I myself and pretty sure that "dynamic" wheel balance is
not an issue with bicycle wheels.
jon isaacs
jobst....@stanfordalumni.org
> I've a small aside to demonstrate how differently large and small
> bicycles behave. My girlfriend and I have matching his'n'hers bikes.
> We were descending side-by-side (from Femes in Lanzarote if anyone
> knows it) along a newly laid road which looked super-smooth. I told
> her to look at my front wheel. It was bouncing up and down at high
> frequency. There must have been some underlying irregularity in the
> road. Her bike was unaffected.
What sort of road bike was this and how hard were the tires inflated?
Vertical hop of a wheel on pavement needs a huge imbalance to occur.
jobst....@stanfordalumni.org
> Are you saying that you had one of those violent speed-wobbles where
> you can barely hang onto your handlebars and simply stood up off your
> saddle and the shimmy stopped?
Don't make it sound so dramatic. All you need to do is unload the
saddle. That's far from standing up. Without the rider mass on the
bicycle saddle, shimmy cannot resonate.
> I've had speed wobbles so violent that I wasn't sure whether I was
> standing or not. I've also had speed wobbles where I've stood up
> only to cause my rear wheel to skip from side to side. I've never
> had a wobble that simply disappeared the instant I stood up. The
> only thing that really works is to slow down.
You'll have to try it. Your tales of wheels skipping from side to
side make your accounts of shimmy to sound like imagined events.
There are many riders who have experienced shimmy and it isn't the way
you tell it.
jobst....@stanfordalumni.org
>> Please produce a bicycle that can not wobble. The world will beat
>> a path to your door.
>> The conspiracy of ignorance masquerades as common sense.
> The shimmy in question occurred at 35mph. That is a totally
> reasonable speed! It is also perfectly reasonable to assume that any
> frame sold for the purposes of racing should not shimmy at racing
> speeds. Many (in fact most) bikes do not shimmy under reasonable
> conditions - ie within their design parameters. Some bikes do
> shimmy, and the ones that do, and the conditions under which they
> shimmy have a lot in common.
I have not found a bicycle that doesn't shimmy if I coast no-hands
down a hill with increasing speed. At some point, less than 40mph, I
have experienced shimmy. I have not tried this on frames smaller than
23". I also believe that removing the mass of stem and handlebar,
shimmy would be rare because the only mass to induce steering would be
too small to cause noticeable excursions.
> Some people love banging on about the FAQ. I guess it's far easier
> than thinking for themselves. Trouble is, the FAQ really concerns
> itself with the phenomenon of hands-off shimmy.
Maybe you don't know what FAQ means. If it is FAQ then a summary
response should be written for that file for people to read. Besides,
when writing for the FAQ (frequently asked questions), a bit more
editing and choice of words than replies here on the NG can be used.
The FAQ item does not exclude hands-on shimmy, it even mentions when
shimmy is unavoidable at any speed with hands on, even on a bicycle
that has no propensity to shimmy.
> This is a subtley different beast to hands-on speed-wobble. There
> are several aspects of speed-wobble that are different from shimmy,
> at least they don't tie in with the analysis in the FAQ.
Oh no! Now it's semantics. Shimmy is well known in the auto,
motorcycle and bicycle business. It is defined at:
http://www.m-w.com/cgi-bin/dictionary
---------------------------------------------------------------------
Main Entry:
Pronunciation: 'shi-mE
Function: noun
Inflected Form(s): plural shimmies
Date: 1837
1 [by alteration] : CHEMISE
2 [short for shimmy-shake] : a jazz dance characterized by a shaking
of the body from the shoulders down
3 an abnormal vibration especially in the front wheels of a motor
vehicle
---------------------------------------------------------------------
> I hope noone is so brain dead as to take these comments as an attack
> on the FAQ's author.
Noon is lunchtime but who is noone? Calling readers "brain dead" and
your distaste for the FAQ doesn't strengthen you credibility.
> I know better than that! I'm in total agreement with the FAQ as far
> as it goes. Jobst is careful to give zero advice on what changes to
> make to reduce the chances of speed-wobble.
There you go again, so to speak, mixing shimmy with speed-wobble.
They are the same and the tank slapper people mean the same thing,
although with more drama. I think I mentioned some countermeasures a
rider can take. The resonance is self energizing and requires a mass
at the saddle and handlebars on an extended stem. Tire pressure
affects it slightly as does the weight of the tire. I think a fat
tubed Cannondale could isolate the elasticity because it has none to
speak of other than in the forks.
> There may not be such a thing as a bike that "can not" wobble, but
> there may be such a thing as a bike that will not wobble under
> reasonable conditions. There certainly should be!
I think that is where most sports and racing bicycles are. My bicycle
has served me well for many years and it will shimmy coasting no-hands
and when I'm cold to the extent of shivering.
jobst....@stanfordalumni.org
>> I suppose the imbalance in car wheels comes from steering wheel
>> input as well.
> What? No. But the vast majority of car wheel balance problems come
> from uneven weight distribution radially -- translating into
> up-and-down motion. It may be possible to balance in the direction
> of the axle, but I've never seen the car wheel that has been
> balanced that way, and never had one that needed it. And car wheels
> are a lot heavier, and move faster, than bike wheels. The amount in
> the direction of the axle that a bike wheel could be out-of balance
> is totally insignificant.
I guess it went by too long ago but we had this wheel balancing idea
discussed here at length.
http://yarchive.net/bike/wheel_balancing.html
Subsequently I put a significant wrap of 1/8" thick solder wire around
one spoke at the rim and coasted down a hill no-hands, observing the
incidence and intensity of shimmy. There was no difference between
balanced and imbalanced wheel. Rather than hypothesize about it, a
test of theory does a lot to put it into perspective.
Also interesting is that the line on the pavement (of a wet tire)
barely shows any deviation from a straight line, the top of the
handlebar stem making the largest excursions. I find adding weight to
the handlebar more enlightening. This affects both initiating speed
and amplitude of shimmy.
Jobst Brandt <jobst....@stanfordalumni.org> Palo Alto CA
Benjamin Weiner
> Miniscule relative to what? Up/down? It doesn't necessarily mean
> insignificant. Whose to say there's not a cumulative effect here which
> might add up to the force needed to put this system into resonance. Add
> all the imbalances, add the force variance from the wheel runout (maybe
> miniscule maybe not depending on the wheel builder and quality of tire),
> could there be a moment created by the air force on the sides of the
> wheel? insignificant? maybe. Operate these forces (and probably others)
> on the gyro of the wheel and I don't see why there shouldn't be a
> resonance eventually set up in the blades of the forks and transmitted
> via headtube throughout the bike.
If these wheel balancing concerns were important, everyone who has
ever had a spoke reflector on their front wheel and ridden down hill
would be dead from shimmy-induced crashes. They're not, and it
isn't important.
Try spinning a wheel with spoke reflector while holding
the bike up. It'll bob like crazy. But when you're
riding the bike, you can't feel any effect.
Denny Coyle
jobst....@stanfordalumni.org wrote:
> I guess it went by too long ago but we had this wheel balancing idea
> discussed here at length.
>
> http://yarchive.net/bike/wheel_balancing.html
Thanks, I'll not beat the dead horse.
dc
jobst....@stanfordalumni.org
>>> Well to start you need to do the static balance,
>> Your use of "static" and "dynamic" balance suggests that they are
>> measuring imbalance in different directions. Static simply means
>> that the wheel is not rotating, dynamic means that it is (which
>> allows greater precision).
>> The imbalance you are talking about will still measure only whether
>> one side of the wheel is heavier than the other. The terminology
>> is imprecise, so let me try again. All you are doing is being more
>> careful about eliminating the tendency of the wheel, when rotating
>> freely, to end up with the valve at the bottom when it stops
>> rotating. Spin such a wheel fast, and any imbalance would result
>> in reciprocating forces up and down (OK, forward and back as well)
>> since the wheel rotates around the center of mass, not the axle.
> Dynamic is balancing around the longitudinal plane of the
> wheel. This imbalance creates a side/side motion of the
> wheel. Whether there is sufficient width of the bicycle wheel to
> render this imbalance significant remains open to question. Some
> believe that this should play out only with the wider/lower profile
> tires but here again most motorcycle wheels-granted wider than
> bicycle wheels- are balanced using dynamic machines, some operating
> in a super fine mode of one gram.
For balance applications, the bicycle wheel is two dimensional, being
more than 25 times larger in diameter than its width. The center of
mass at any cross section of the circumference can be 1mm at most. I
think you are projecting from wide tires on cars which doesn't apply
here.
> Miniscule relative to what? Up/down? It doesn't necessarily mean
> insignificant. Whose to say there's not a cumulative effect here
> which might add up to the force needed to put this system into
> resonance. Add all the imbalances, add the force variance from the
> wheel runout (maybe miniscule maybe not depending on the wheel
> builder and quality of tire), could there be a moment created by the
> air force on the sides of the wheel? insignificant? maybe. Operate
> these forces (and probably others) on the gyro of the wheel and I
> don't see why there shouldn't be a resonance eventually set up in
> the blades of the forks and transmitted via headtube throughout the
> bike.
Minuscule to the balance of the wheel and in particular, shimmy.
Shimmy is enhanced by the smoothness of the road. On a bumpy road
with lots of bounce, I cant get my bicycle to shimmy. The forcing
function is trail and the gyroscopic forces of the front wheel.
David L. Johnson
> Just making reference to your top down solution of this
> problem-hands,nohands,body- as opposed to what I believe is a bottoms up
> problem-imbalances...
Lots -- damn near every -- cyclist has wheels that are out of balance. Pick
up any bike, and the front wheel will rotate until (usually) the valve is
down. If it doesn't, likely the bearings are gummed up. But not every bike
has shimmy problems. Most don't. Taller bikes tend to have more problems,
yet they have the same wheels. Where is a reason to believe that imbalance
causes shimmy?
>
> >
> > > Well to start you need to do the static balance,
> >
> > Your use of "static" and "dynamic" balance suggests that they are measuring
> > imbalance in different directions. Static simply means that the wheel is not
> > rotating, dynamic means that it is (which allows greater precision).
> Dynamic is balancing around the longitudinal plane of the wheel.
The word "dynamic" just means that the measurements are done with the wheel
spinning.
Maybe some auto racing wheels are balanced with respect to the direction of
the axle (longitude?), but passenger car wheels are not. And yet, you can
indeed get dynamic balance. I recall one shop tried to dynamically balance
the wheels in my car. For the rear wheels, they just jacked up the car and
put it in gear (they marked the edge to find the heavy spot). They were a bit
surprized, though, since my car then was front-wheel drive, and this was well
before they were common.
> Miniscule relative to what? Up/down? It doesn't necessarily mean
> insignificant.
It does in this case.
Whose to say there's not a cumulative effect here which
> might add up to the force needed to put this system into resonance.
Who is to say? Think about this. The amount of force would depend upon two
things, the extra mass, and the moment. But the moment depends on the
distance from the centerline, which on a bike wheel, near the rim where all
the mass is, is maybe 15mm. OTOH, in the directions away from the axle, where
what we have called "up-and-down" imbalance occurs, you have 350mm to work
with. So a gram or two, or even 3 or 4, with a distance of 15mm is trivial
when compared to a distance of 350mm in the other axis.
Of course you could conduct an experiment. In fact, I have been doing this
for 2 years now, ever since I got a computer with a magnet dangling on a
spoke. There is your side-to-side imbalance, placed there on purpose. No
shimmy. Amazing.
> Wouldn't it seem reasonable that faster speeds would necessitate less
> steering corrections? Why it's harder to track stand than just ride.
Why is it harder to track stand than to ride at 1 mph? Why is it just as easy
to balance at 2mph as at 10? It's harder to track stand because you can't
correct for a fall to one side just by turning the wheel, you have to push the
bike to compensate. Riding is more natural. But it is not reasonable to
imagine that faster speed lessens the need to balance, except for the slight
advantage of the angular momentum of the wheels.
> > > sliding weight system into one of rims to better achieve this balance.
>
> > Don't confuse engineering with marketing.
>
> Engineering is growth! from supposition to application!
> Just trying to keep an open mind here.
But you'll notice that they put the balance weight at the stem. Unless they
really loaded up the mass on the seam on the other side of the rim, the stem
side would be the heavier, anyway, so a balance weight would be better placed
opposite the stem. However, it would look cooler at the stem, so there it
went. A $10 solution to a problem no one has. That is not engineering.
--
David L. Johnson
__o | Do not worry about your difficulties in mathematics, I can asure
_`\(,_ | you that mine are all greater. -- A. Einstein
(_)/ (_) |
Jon Isaacs
>the axle (longitude?), but passenger car wheels are not.
Actually passenger car wheels are balanced inside and outside. Those machines
tell the operator where to put the weights, and that includes both where around
the rim and whether it goes on the inside or the outside of the wheel.
jon isaacs
Tom H
As I do all the maintenance and pre-ride checks, I know exactly what
pressure the tyres were. They were both pumped to 100psi, hers were
19mm Hutchinson (Profile-U I think they're called) and mine were
Schwalbe Blizzard in 23mm. The bikes were a ~50cm steel Concorde
(Gavina) and a 63cm steel Concorde (Colombo).
Tom
Jon Isaacs
>pressure the tyres were. They were both pumped to 100psi, hers were
>19mm Hutchinson (Profile-U I think they're called) and mine were
>Schwalbe Blizzard in 23mm. The bikes were a ~50cm steel Concorde
>(Gavina) and a 63cm steel Concorde (Colombo).
Did you happen to swap wheels to see if it was a problem with the wheel?
jon isaacs
Tom H
>
> I have not found a bicycle that doesn't shimmy if I coast no-hands
> down a hill with increasing speed. At some point, less than 40mph, I
> have experienced shimmy. I have not tried this on frames smaller than
> 23". I also believe that removing the mass of stem and handlebar,
> shimmy would be rare because the only mass to induce steering would be
> too small to cause noticeable excursions.
>
Rare? What do you mean by rare? Am I to assume that the mass of the
handlebars and stem is an order of magnitude more important than the
mass of the wheel and forks?
>
> The FAQ item does not exclude hands-on shimmy, it even mentions when
> shimmy is unavoidable at any speed with hands on, even on a bicycle
> that has no propensity to shimmy.
>
I've read the faq a few times. The emphasis has always seemed to me to
be towards hands-off shimmy. There are also one or two observations
with regard to hands-on speed-wobble that I and others have
experienced which don't totally concur with the faq. I put this down
to the faq concentrating on the hands-off phenomenon.
> > This is a subtley different beast to hands-on speed-wobble. There
> > are several aspects of speed-wobble that are different from shimmy,
> > at least they don't tie in with the analysis in the FAQ.
>
> Oh no! Now it's semantics. Shimmy is well known in the auto,
> motorcycle and bicycle business. It is defined at:
>
> http://www.m-w.com/cgi-bin/dictionary
Thanks for the reference, but I'm sure it is clear to you that I am
questioning whether hands-off and hands-on shimmys differ in more than
degree. I am not trying to play with semantics, but to draw a
distinction. I am fully aware this distinction may be imaginary, that
is what I am trying to discover.
>
> There you go again, so to speak, mixing shimmy with speed-wobble.
> They are the same and the tank slapper people mean the same thing,
> although with more drama. I think I mentioned some countermeasures a
> rider can take. The resonance is self energizing and requires a mass
> at the saddle and handlebars on an extended stem. Tire pressure
> affects it slightly as does the weight of the tire. I think a fat
> tubed Cannondale could isolate the elasticity because it has none to
> speak of other than in the forks.
>
If they are one and the same thing, then:
1) Why does hands-on speed-wobble invariably happen on descents? This
has nothing to do with speed as I and many others have ridden at
similar speeds on the flat with no problems on the same bike.
2) Why is hands-on speed-wobble so sensitive to weight distribution?
Hands on the drops tends to make matters worse.
3) Why does descending on rough roads make hands-on speed-wobble more
likely? My personal experience on this is that rough roads can cause
speed-wobble.
Funny you should mention Cannondales. I had my worst ever speed-wobble
on a 2.8i multisport frame. I changed the entire front-end, shortened
the stem, removed the STIs in favour of down-tube levers, and it would
still try to throw me off when the conditions were right. I have an
observation to make with regard to frame stiffness - when a stiffer
frame goes, the wobble is much more violent, and the onset much
quicker.
> > There may not be such a thing as a bike that "can not" wobble, but
> > there may be such a thing as a bike that will not wobble under
> > reasonable conditions. There certainly should be!
>
> I think that is where most sports and racing bicycles are.
If that were true, then why do we continue to get periodical postings
on this matter?
> My bicycle
> has served me well for many years and it will shimmy coasting no-hands
> and when I'm cold to the extent of shivering.
>
My experience of hands-on speed wobble and the experience of others is
that it is a much more violent phenomenon than this.
Tom
Tom H
> "Tom H" <thomas....@bakeratlas.com> wrote in message
> news:5c69a497.02021...@posting.google.com...
> >
> > too) on straight descents. It can be initiated by side-winds or rough
> > road surfaces. Large bicycles are inherently much more flexible than
> > small ones. Large bicycles also typically have less trail, which I
> > think causes more steering torque for small forced deviations of the
> > front wheel. It's a problem which the bicycle industry quietly
> > ignores, and one which I'm sure injures people every year.
>
> I'm 6'10" (210 cm), weigh 235 (105 kg), I have never experienced shimmy at all,
> in several bike frames, made from steel & aluminum, at speeds up to exceeding 50
> mph.
I've had speed-wobble several times, but mostly under rather
particular conditions. I also know several people who have suffered
speed-wobble on a variety of different types of frame, including one
carbon. If I recall correctly, you're the guy who rides a cannondale
touring frame. Well, I managed to track one down. I've recently
purchased a CAAD2 63cm frame and forks which had been languishing in a
bike-shop for a few years. I'm looking forward to building it up and
trying it out. The frame seems so stiff, and the forks so over-built
that if it does wobble, I'll be simultaneously depressed and amazed!
Do you find your bike easy to ride no-hands? I'm not sure if this is
relevant to the issue of speed-wobble, but I'm conducting a straw pole
to see if there is any correlation.
Tom
Tom H
> Thomas Harrigan writes:
>
> > Are you saying that you had one of those violent speed-wobbles where
> > you can barely hang onto your handlebars and simply stood up off your
> > saddle and the shimmy stopped?
>
> Don't make it sound so dramatic. All you need to do is unload the
> saddle. That's far from standing up. Without the rider mass on the
> bicycle saddle, shimmy cannot resonate.
>
My guess is that you have neither experienced nor witnessed a violent
speed-wobble. It is very difficult to raise yourself out of the saddle
when it happens. As I mentioned in another post, most of my wobbles
have occurred when cycling in Lanzarote. Many riders experience
problems there, because the conditions are ripe for a truly dramatic
ride. There are wind, rough roads and straight descents. Should you
end up in one of the lava fields that make up much of the island, then
you're going to be a real mess.
> > I've had speed wobbles so violent that I wasn't sure whether I was
> > standing or not. I've also had speed wobbles where I've stood up
> > only to cause my rear wheel to skip from side to side. I've never
> > had a wobble that simply disappeared the instant I stood up. The
> > only thing that really works is to slow down.
>
> You'll have to try it. Your tales of wheels skipping from side to
> side make your accounts of shimmy to sound like imagined events.
> There are many riders who have experienced shimmy and it isn't the way
> you tell it.
>
I see, we're all making this up now. Honestly Jobst, if you're going
to be insulting I'm sure you can do better than this. On one occasion,
as I was endeavoring to take my weight off the saddle (I had been
descending knee-on-top-tube when the shimmy began) it definately felt
and sounded like my rear tyre began to skip from side to side. I
cannot say for sure as I could not look, but it occurred just as I
took my weight off the saddle, but while it was still trapped between
my legs. The experience was akin to a node becomming an antinode. It
didn't last long. I have never ridden that particular descent without
getting some shimmy.
Tom
jobst....@stanfordalumni.org
>> I have not found a bicycle that doesn't shimmy if I coast no-hands
>> down a hill with increasing speed. At some point, less than 40mph,
>> I have experienced shimmy. I have not tried this on frames smaller
>> than 23". I also believe that removing the mass of stem and
>> handlebar, shimmy would be rare because the only mass to induce
>> steering would be too small to cause noticeable excursions.
> Rare? What do you mean by rare? Am I to assume that the mass of
> the handlebars and stem is an order of magnitude more important than
> the mass of the wheel and forks?
Rare only because I don't believe it cannot occur. Trail and fork
flexibility could be such that in spite of no mass at the handlebar,
it could shimmy. However, for the average road bicycle, I think it
would not occur.
My bicycle probably shimmies at a lower frequency and speed than most,
because it is tall. I use a 120mm stem and old steel Cinelli bars.
The speed is between 23 and 28 mph and I never gave it a thought, all
my previous bicycles having had the characteristic.
>> The FAQ item does not exclude hands-on shimmy, it even mentions
>> when shimmy is unavoidable at any speed with hands on, even on a
>> bicycle that has no propensity to shimmy.
> I've read the FAQ a few times. The emphasis has always seemed to me
> to be towards hands-off shimmy. There are also one or two
> observations with regard to hands-on speed-wobble that I and others
> have experienced which don't totally concur with the FAQ. I put
> this down to the FAQ concentrating on the hands-off phenomenon.
The term shimmy describes both phenomena. There is no difference
between it and speed wobble, semantically or otherwise. Hands off is
mainly a diagnostic of whether the bicycle has the tendency without
rider input. I think any bicycle can be made to shimmy with shivering
like input. This has always been a problem when descending in cold
weather. That is where I experienced the induced hands-on shimmy.
When this occurs, I realize it's me not the bicycle and I make sure to
flex my muscles to avoid oscillation while keeping my leg against the
top tube.
> If they are one and the same thing, then:
> 1) Why does hands-on speed-wobble invariably happen on descents?
> This has nothing to do with speed as I and many others have ridden
> at similar speeds on the flat with no problems on the same bike.
That is where the bicycle rolls undisturbed by pedaling side forces
and no forceful input from the hands. The hands add mass to the
handlebars and this increases the shimmy moment. Once it starts, it
picks up sympathetic input from the riders arms, this coincidentally
being a natural frequency for involuntary contractions. I also think
it is less common in warm or hot weather but that is only a guess
based on my inverse experience.
> 2) Why is hands-on speed-wobble so sensitive to weight distribution?
> Hands on the drops tends to make matters worse.
That seems logical because the mass of the hands is farther forward,
enhancing the mass of the bars while grasping them more tightly.
> 3) Why does descending on rough roads make hands-on speed-wobble
> more likely? My personal experience on this is that rough roads can
> cause speed-wobble.
My experience is that no-hands it is less with road roughness just as
it is with no-hands forceful pedaling. That a bumpy road makes it
occur with hands on goes back to the previous explanation in that it
excites natural involuntary muscular response.
> Funny you should mention Cannondales. I had my worst ever speed-wobble
> on a 2.8i multisport frame. I changed the entire front-end, shortened
> the stem, removed the STIs in favour of down-tube levers, and it would
> still try to throw me off when the conditions were right. I have an
> observation to make with regard to frame stiffness - when a stiffer
> frame goes, the wobble is much more violent, and the onset much
> quicker.
This is interesting because it narrows the culprit to the forks and
wheel. I haven't had that opportunity and that's why I proposed it.
In any case, there is a spring mass system at work here together with
gyroscopic forces. Old cars did it and I think their spring was
mainly the tires. Cars have vertical steering axes and almost no
trail.
>>> There may not be such a thing as a bike that "can not" wobble, but
>>> there may be such a thing as a bike that will not wobble under
>>> reasonable conditions. There certainly should be!
>> I think that is where most sports and racing bicycles are.
> If that were true, then why do we continue to get periodical
> postings on this matter?
I think that is primarily because some riders are less attuned to the
remedies and are caught of guard when it occurs. As I said, I have
always had a shimmying bicycle and it has never bothered me because I
know how to stop it and if I can't it's probably because I am cold.
>> My bicycle has served me well for many years and it will shimmy
>> coasting no-hands and when I'm cold to the extent of shivering.
> My experience of hands-on speed wobble and the experience of others is
> that it is a much more violent phenomenon than this.
It all depends on how much you let it get out of control. I am
certain that it would go away if you unloaded the saddle or laid you
leg against the top tube hear the head tube.
Peter Cole
news:5c69a497.02021...@posting.google.com...
> "Peter Cole" <pete...@nospamatallmediaone.net> wrote in message
news:<WUT98.2594$AY.10...@typhoon.ne.ipsvc.net>...
> >
all,
> > in several bike frames, made from steel & aluminum, at speeds up to
exceeding 50
> > mph.
>
> I've had speed-wobble several times, but mostly under rather
> particular conditions. I also know several people who have suffered
> speed-wobble on a variety of different types of frame, including one
> carbon. If I recall correctly, you're the guy who rides a cannondale
> touring frame. Well, I managed to track one down. I've recently
> purchased a CAAD2 63cm frame and forks which had been languishing in a
> bike-shop for a few years. I'm looking forward to building it up and
> trying it out. The frame seems so stiff, and the forks so over-built
> that if it does wobble, I'll be simultaneously depressed and amazed!
That's the frame I have (in a 68 cm). It has never given me the slightest wobble
or shudder. I think it's a terrific frame. The first thing I noticed on my first
ride was how easy it was to climb no-hands, I think that's due to the long
chainstays. I do everything with it except race crits, & I might try that this
year.
> Do you find your bike easy to ride no-hands? I'm not sure if this is
> relevant to the issue of speed-wobble, but I'm conducting a straw pole
> to see if there is any correlation.
Yes, I ride no-hands a lot. Last summer we were doing a training century by
repeating the same 33 mile loop, out of boredom, I did one repeat no-hands. I
ride my other bikes no-hands also, but they are "sport tourers", I've never
owned a flat out "racing" frame, and I've never experienced shimmy or wobble,
no-hands or otherwise, perhaps that's not a coincidence. FWIW, some of my wheels
are typical: Ultegra, Open Pro, etc., and I've used typical 23 mm Continentals,
etc., although now I mostly use slightly fatter tires, like Avocet 28 mm.
Theodore Heise
jobst....@stanfordalumni.org <jobst....@stanfordalumni.org> wrote:
> Thomas Harrigan writes:
>
> >> My bicycle has served me well for many years and it will shimmy
> >> coasting no-hands and when I'm cold to the extent of shivering.
>
> > My experience of hands-on speed wobble and the experience of others is
> > that it is a much more violent phenomenon than this.
>
> It all depends on how much you let it get out of control. I am
> certain that it would go away if you unloaded the saddle or laid you
> leg against the top tube hear the head tube.
I had an extremely unsettling experience a few years ago. My
bicycle is a 60 cm Waterford 2200 with a 120 cm stem, Cinelli
bars, 32 spoke Mavic Open Pro wheels, and Conti 23 mm tires.
I'm 6'1" and weighed about 190 lbs at the time.
I was descending a pass in the Rockies and at just over 40 mph
the shaking started. Thomas's description of the bike trying
to "throw him off" was apt. A rider following behind told
me he was surprised I kept the bike upright. The onset was
very sudden, I don't recall any warning signs. It was most
definitely violent--my thought was I had blown out the tire.
I don't recall it being cold that day, but it was high enough
elevation that it probably was.
The thing I find curious is that I have descended with that
bike many times at speeds well over 50 mph with no problems
whatsoever. Many of those times it was cold, too. It was
a bit gusty though, on the day in question.
--
Ted Heise <the...@netins.net> West Lafayette, IN, USA
jobst....@stanfordalumni.org
>> It all depends on how much you let it get out of control. I am
>> certain that it would go away if you unloaded the saddle or laid
>> you leg against the top tube hear the head tube.
> I had an extremely unsettling experience a few years ago. My
> bicycle is a 60 cm Waterford 2200 with a 120 cm stem, Cinelli bars,
> 32 spoke Mavic Open Pro wheels, and Conti 23 mm tires. I'm 6'1" and
> weighed about 190 lbs at the time.
> I was descending a pass in the Rockies and at just over 40 mph the
> shaking started. Thomas's description of the bike trying to "throw
> him off" was apt. A rider following behind told me he was surprised
> I kept the bike upright. The onset was very sudden, I don't recall
> any warning signs. It was most definitely violent--my thought was I
> had blown out the tire. I don't recall it being cold that day, but
> it was high enough elevation that it probably was.
Although my bicycle will always shimmy when coasting on a smooth road
in the critical speed (no-hands), I achieve my maximum speeds,
essentially no-hands in a crouch with my hands on the stem. This is
the most stable condition I can imagine. I suspect that if I were to
ride next to someone experiencing shimmy, I believe I would see a
different scenario than what the rider gives after the experience.
> The thing I find curious is that I have descended with that bike
> many times at speeds well over 50 mph with no problems whatsoever.
> Many of those times it was cold, too. It was a bit gusty though, on
> the day in question.
I don't have the terrain to reach 50mph without a good tailwind, one
that is not also a cross wind due to turbulence. Most steep roads I
encounter are too crooked, and the straight ones aren't steep enough.
However, your bicycle makes it spooky, not knowing when it might
occur. I think getting to the bottom of the phenomenon and getting it
under control is important. As has been brought out in this thread,
most riders seem not to have the problem.
jobst....@stanfordalumni.org
>>> Are you saying that you had one of those violent speed-wobbles
>>> where you can barely hang onto your handlebars and simply stood up
>>> off your saddle and the shimmy stopped?
>> Don't make it sound so dramatic. All you need to do is unload the
>> saddle. That's far from standing up. Without the rider mass on
>> the bicycle saddle, shimmy cannot resonate.
> My guess is that you have neither experienced nor witnessed a
> violent speed-wobble. It is very difficult to raise yourself out of
> the saddle when it happens. As I mentioned in another post, most of
> my wobbles have occurred when cycling in Lanzarote. Many riders
> experience problems there, because the conditions are ripe for a
> truly dramatic ride. There are wind, rough roads and straight
> descents. Should you end up in one of the lava fields that make up
> much of the island, then you're going to be a real mess.
Lanzarote is a safe qualification, because I can't drop in and try
that road myself. I don't believe any particular road has a greater
tendency to shimmy other than that it is downhill and possibly in cold
weather. I have seen shimmies on flat ground but the rider was cold.
>>> I've had speed wobbles so violent that I wasn't sure whether I was
>>> standing or not. I've also had speed wobbles where I've stood up
>>> only to cause my rear wheel to skip from side to side. I've never
>>> had a wobble that simply disappeared the instant I stood up. The
>>> only thing that really works is to slow down.
>> You'll have to try it. Your tales of wheels skipping from side to
>> side make your accounts of shimmy to sound like imagined events.
>> There are many riders who have experienced shimmy and it isn't the
>> way you tell it.
> I see, we're all making this up now. Honestly Jobst, if you're
> going to be insulting I'm sure you can do better than this. On one
> occasion, as I was endeavoring to take my weight off the saddle (I
> had been descending knee-on-top-tube when the shimmy began) it
> definitely felt and sounded like my rear tyre began to skip from
> side to side. I cannot say for sure as I could not look, but it
> occurred just as I took my weight off the saddle, but while it was
> still trapped between my legs. The experience was akin to a node
> becomming an antinode. It didn't last long. I have never ridden
> that particular descent without getting some shimmy.
The reason I said that is that unloading the saddle does not unload the
rear wheel. Only the point of contact between rider and bicycle
changed. The load, with the same CG, is borne by the feet and hands
instead of the saddle, the saddle that is the anchor point using rider
as the mass about which the oscillation occurs. The axis for the
shimmy is a straight line between front tire contact and saddle. Once
that anchor is removed, the oscillation will die within one period.
Tim McNamara
considered normal. The worst of the shimmiers (?) was my Viscount
Aerospace Pro, which had a light straight-gauge frame and rather
flexible cast aluminum forks. My Schwinn Continental seemed to shimmy
rarely. My Rivendell rarely shimmies but it's smaller than my other
bikes (59.5 cm C-to-T) and has 26" wheels with 26 x 1" slicks. My
Ritchey road bike shimmies about 23-25 mph hands-off and I don't
recall it shimmying hands-on, but then, I probably don't notice a
small shimmy since it's "normal" IME.
IME the lighter the bike the more likely shimmy is to occur. Onset is
generally predictable, occurring at a lower speed when riding
hands-off. Now, I ride 60-63 cm frames generally speaking, and I
wonder if the tallness of the frame makes it more likely to shimmy.
Shimmy seems less severe with fatter tires and more likely with high
pressure skinny tires (23s at 120 psi). Has anyone else noticed this?
David L. Johnson
> 1) Why does hands-on speed-wobble invariably happen on descents? This
> has nothing to do with speed as I and many others have ridden at
> similar speeds on the flat with no problems on the same bike.
Since shimmy usually happens at 40+mph on descents, kindly explain how you
maintain that speed for long enough on the flat to notice.
>
> 2) Why is hands-on speed-wobble so sensitive to weight distribution?
> Hands on the drops tends to make matters worse.
You tend to grab the bars tighter when on the drops, improving the connection
between rider and bike. Nothing makes a shimmy worse than a "death grip" on
the bars.
>
> 3) Why does descending on rough roads make hands-on speed-wobble more
> likely? My personal experience on this is that rough roads can cause
> speed-wobble.
Disagrees with my experience.
Theodore Heise
jobst....@stanfordalumni.org <jobst....@stanfordalumni.org> wrote:
> Although my bicycle will always shimmy when coasting on a smooth road
> in the critical speed (no-hands), I achieve my maximum speeds,
> essentially no-hands in a crouch with my hands on the stem. This is
> the most stable condition I can imagine. I suspect that if I were to
> ride next to someone experiencing shimmy, I believe I would see a
> different scenario than what the rider gives after the experience.
It may have been less dangerous than it felt, but it surely felt like
I was going down.
> I don't have the terrain to reach 50mph without a good tailwind, one
> that is not also a cross wind due to turbulence. Most steep roads I
> encounter are too crooked, and the straight ones aren't steep enough.
> However, your bicycle makes it spooky, not knowing when it might
> occur. I think getting to the bottom of the phenomenon and getting it
> under control is important. As has been brought out in this thread,
> most riders seem not to have the problem.
There are several passes in Colorado that have fairly steep and very
straight runs. Southeast from Cucharas is one where I've seen 55 mph
on my Avocet before I hit the brakes. Since the serious shimmy incident
I've been afraid to go let it get much over 40. Guess I'm getting a
little cautious as I age. <grin>
Tom H
> .. snip
Thanks for the clear explanations!
>
> > Funny you should mention Cannondales. I had my worst ever speed-wobble
> > on a 2.8i multisport frame. I changed the entire front-end, shortened
> > the stem, removed the STIs in favour of down-tube levers, and it would
> > still try to throw me off when the conditions were right. I have an
> > observation to make with regard to frame stiffness - when a stiffer
> > frame goes, the wobble is much more violent, and the onset much
> > quicker.
>
> This is interesting because it narrows the culprit to the forks and
> wheel. I haven't had that opportunity and that's why I proposed it.
> In any case, there is a spring mass system at work here together with
> gyroscopic forces. Old cars did it and I think their spring was
> mainly the tires. Cars have vertical steering axes and almost no
> trail.
>
I'm glad you are of the opinion that the "problem" might be narrowed
down to the forks and wheel. I'd like to take this idea one step
further. I am of the opinion that in some hands-on wobbles (notably
the more violent experiences) the spring of the steering (resonating
at frquencies the rider cannot damp) somehow couples to the spring in
the frame. This is why I was trying to draw a distinction between
hands-off and hands-on. The torsional rigidity of the steering system
did not seem to be an issue in the hands-off case, whereas the
coupling of the steering resonance with the frame resonance seemed to
be precisely the issue with hands-on.
Tom
Tom H
>
> I had an extremely unsettling experience a few years ago. My
> bicycle is a 60 cm Waterford 2200 with a 120 cm stem, Cinelli
> bars, 32 spoke Mavic Open Pro wheels, and Conti 23 mm tires.
> I'm 6'1" and weighed about 190 lbs at the time.
>
> I was descending a pass in the Rockies and at just over 40 mph
> the shaking started. Thomas's description of the bike trying
> to "throw him off" was apt. A rider following behind told
> me he was surprised I kept the bike upright. The onset was
> very sudden, I don't recall any warning signs. It was most
> definitely violent--my thought was I had blown out the tire.
> I don't recall it being cold that day, but it was high enough
> elevation that it probably was.
>
> The thing I find curious is that I have descended with that
> bike many times at speeds well over 50 mph with no problems
> whatsoever. Many of those times it was cold, too. It was
> a bit gusty though, on the day in question.
I was beginning to feel very alone here. I hope that when I say I'm
glad you had that experience, you'll take it the right way.
Tom
Tom H
> Tom H wrote:
>
> > 1) Why does hands-on speed-wobble invariably happen on descents? This
> > has nothing to do with speed as I and many others have ridden at
> > similar speeds on the flat with no problems on the same bike.
>
> Since shimmy usually happens at 40+mph on descents, kindly explain how you
> maintain that speed for long enough on the flat to notice.
I work im kilometers. I have had violent shimmy at 50kph, which I have
been known to sustain on the flat(ish) for several minutes.
Tom
> >
> > 2) Why is hands-on speed-wobble so sensitive to weight distribution?
> > Hands on the drops tends to make matters worse.
>
> You tend to grab the bars tighter when on the drops, improving the connection
> between rider and bike. Nothing makes a shimmy worse than a "death grip" on
> the bars.
I've experimented with hand positions, and have found that while I
could ride on the hoods, on the drops it was almost impossible. I
attribute this to more weight on the bars, and a less torsionally
rigid steering.
> >
> > 3) Why does descending on rough roads make hands-on speed-wobble more
> > likely? My personal experience on this is that rough roads can cause
> > speed-wobble.
>
> Disagrees with my experience.
What can I say?
Tom
Tom H
>
> Lanzarote is a safe qualification, because I can't drop in and try
> that road myself. I don't believe any particular road has a greater
> tendency to shimmy other than that it is downhill and possibly in cold
> weather. I have seen shimmies on flat ground but the rider was cold.
>
I'm going to have to disagree with you here and leave it at that. I
have found certain _conditions_ make a shimmy far more likely and
that's all I can offer on this point. These conditions are straight
descents, rough roads, and wind. Lanzarote has these in abundance.
>
> > I see, we're all making this up now. Honestly Jobst, if you're
> > going to be insulting I'm sure you can do better than this. On one
> > occasion, as I was endeavoring to take my weight off the saddle (I
> > had been descending knee-on-top-tube when the shimmy began) it
> > definitely felt and sounded like my rear tyre began to skip from
> > side to side. I cannot say for sure as I could not look, but it
> > occurred just as I took my weight off the saddle, but while it was
> > still trapped between my legs. The experience was akin to a node
> > becomming an antinode. It didn't last long. I have never ridden
> > that particular descent without getting some shimmy.
>
> The reason I said that is that unloading the saddle does not unload the
> rear wheel. Only the point of contact between rider and bicycle
> changed. The load, with the same CG, is borne by the feet and hands
> instead of the saddle, the saddle that is the anchor point using rider
> as the mass about which the oscillation occurs. The axis for the
> shimmy is a straight line between front tire contact and saddle. Once
> that anchor is removed, the oscillation will die within one period.
>
Surely you are familiar with a well known phenomenon of winter
ascents, that the road is so steep that you need to stand up, but on
standing you lose traction and need to sit down? Clearly the CG
changes.
Tom
Tom H
>
> That's the frame I have (in a 68 cm). It has never given me the slightest wobble
> or shudder. I think it's a terrific frame. The first thing I noticed on my first
> ride was how easy it was to climb no-hands, I think that's due to the long
> chainstays. I do everything with it except race crits, & I might try that this
> year.
>
Do you have the original forks fitted? I'm slightly tempted to fit
lighter ones as there are some very sturdy ali and carbon cyclocross
forks available.
>
> > Do you find your bike easy to ride no-hands? I'm not sure if this is
> > relevant to the issue of speed-wobble, but I'm conducting a straw pole
> > to see if there is any correlation.
>
> Yes, I ride no-hands a lot. Last summer we were doing a training century by
> repeating the same 33 mile loop, out of boredom, I did one repeat no-hands. I
> ride my other bikes no-hands also, but they are "sport tourers", I've never
> owned a flat out "racing" frame, and I've never experienced shimmy or wobble,
> no-hands or otherwise, perhaps that's not a coincidence. FWIW, some of my wheels
> are typical: Ultegra, Open Pro, etc., and I've used typical 23 mm Continentals,
> etc., although now I mostly use slightly fatter tires, like Avocet 28 mm.
I'm quite excited now. I may have found a bike to do the Lanzarote
Ironman on at last. All I have to do now is get over my ruptured disk
and start training!
Tom
Peter Cole
news:<OKdc8.19587$jj4.9...@typhoon.ne.ipsvc.net>...
> >
> > That's the frame I have (in a 68 cm). It has never given me the slightest
wobble
> > or shudder. I think it's a terrific frame. The first thing I noticed on my
first
> > ride was how easy it was to climb no-hands, I think that's due to the long
> > chainstays. I do everything with it except race crits, & I might try that
this
> > year.
> >
>
> Do you have the original forks fitted? I'm slightly tempted to fit
> lighter ones as there are some very sturdy ali and carbon cyclocross
> forks available.
Yes, because of my size & weight, I felt it was better to stay with a slightly
overbuilt touring fork, fork failure is probably the very last thing I ever want
to worry about. I do a lot of long distance riding with this bike (doubles+) and
I don't find the fork too heavy or stiff, but then I'm not really racing.
terry morse
> > The reason I said that is that unloading the saddle does not unload the
> > rear wheel. Only the point of contact between rider and bicycle
> > changed. The load, with the same CG, is borne by the feet and hands
> > instead of the saddle, the saddle that is the anchor point using rider
> > as the mass about which the oscillation occurs. The axis for the
> > shimmy is a straight line between front tire contact and saddle. Once
> > that anchor is removed, the oscillation will die within one period.
> >
>
> Surely you are familiar with a well known phenomenon of winter
> ascents, that the road is so steep that you need to stand up, but on
> standing you lose traction and need to sit down? Clearly the CG
> changes.
Not so clearly does the CG change. Another reason why you lose traction
when standing is because you have a higher peak force on your pedal.
Sitting down lets you produce a more even pedal force. This is more of
an issue in mountain biking, where the two choices for improving
traction are sitting and spinning or standing in a bigger gear.
--
terry morse
Palo Alto, CA
http://www.terrymorse.com/bike/
Pete Harris
around 35 MPH on five separate occasions: time trial and four
different descents. Each time, hitting a bump seemed to set off the
wobble. Loosening the grip only made things worse. Standing on the
pedals was no help either. Once the wobble began it did not go away
until I slowed to under about 20 MPH.
Cause:
Insufficiently tight quick release! Hitting a bump obliquely at speed
would break loose the frictional bond between the QR nut and fork
dropout, allowing for slight movement. The design of the QR, which has
an extreme over-center action, plus relatively smooth mating surfaces,
contributed.
I am convinced this was the cause, since there has been no tendancy to
wobble since the QR situtation was rectified over one hundred descents
ago.
Equipment:
Trek OCLV with Klein fork, Ksyrium with Mavic QR.
Moral: Lateral play in the front wheel can be the seed for a wobble
which involves the whole bicycle.
Dave Blake
> I'm glad you are of the opinion that the "problem" might be
> narrowed down to the forks and wheel....
[snip]
Look, shimmy is a classic oscillator.
You basically have forces from fork trail that tend to straighten
the front wheel, and precessional forces that work 90 degrees out
of phase with the fork trail forces. And damping. And it all
couples to the weight loaded on the bicycles seat to make a
complex second order system.
To get rid of it, some people move their weight from the saddle
to the pedals and handlebars. This rearranges the mass in the
system, which should shift the shimmying frequency much higher.
Some people try different tires, and potentially change damping.
When I have problems, I place my sternum over my stem, and that
seems to do it. MTBs almost never shimmy, I think because the
tires scrub too much to allow such fast motion (damping). But a
large bike frame with narrow slicks and a good headset will
almost always shimmy before 40 MPH or so.
But I think you should think real hard about whether this problem
is solvable, and what the cost to the rider would be. Nothing is
inherently broken, and dampers will have negative effects at all
other speeds.
--
Dave Blake
dbl...@phy.ucsf.edu
Tom Paterson
>Lateral play in the front wheel can be the seed for a wobble
>which involves the whole bicycle.
On my new touring bike with older panniers, I had a wobble coming and going.
Inspection showed that the elastic cords had given way (looked good until
ridden a few hours). Tying the bags on tight with spare bungee cords stopped
the wobble.
Also had severe wobble after running over thumbtack on long fast descent, front
tire. (Psst psst psst, "oh s___!) Putting front brake on made the bar snap back
and forth. Letting off brake let me straighten out bars and then reapply
brakes, letting off as oscillations increased toward painful get-off. Got
slowed down to hop-off speed just about the time the tire went completely flat.
Rode same bike/wheel/tire down that descent several times before/after that, no
wobble.
I don't discount reports of "violent" or "strong" wobble, as I couldn't catch
the timing of the swings or muscle the bars straight until I let off the brake.
FWIW
--Tom Paterson
Dave Blake
> 1) Why does hands-on speed-wobble invariably happen on
> descents? This has nothing to do with speed as I and many
> others have ridden at similar speeds on the flat with no
> problems on the same bike.
Presumably you have different speeds and different weight
distributions. Shimmy is an oscillation - it can be
described by a second order differential equation. The
distribution of mass will play a large part in both the
appropriate speeds for shimmy to occur and its magnitude.
Most people place their weight more over the rear wheel while
descending, and more centered while pedalling.
> 2) Why is hands-on speed-wobble so sensitive to weight
> distribution? Hands on the drops tends to make matters worse.
Try putting your sternum on the stem :) This shifts the
weight as far forward as possible. It is easier if you start
a descent this way and notice your speed, and abort if
you start to shimmy (I've never shimmied in this position).
The most common tendencies when shimmy starts is holding on
tighter and shifting the weight further back, which does little
to stop it. Raising your butt off the seat usually slows
it down noticeably.
> 3) Why does descending on rough roads make hands-on
> speed-wobble more likely? My personal experience on this is
> that rough roads can cause speed-wobble.
It doesn't happen on truly rough roads (for example, speed shimmy
is unheard of in mountain biking).
Bumps or shivering can be enough to get the motion started. Once
started, the second order dynamics take effect. Believe it or
not, some bike:rider combinations just shimmy violently on
absolutely smooth roads.
> Funny you should mention Cannondales. I had my worst ever
> speed-wobble on a 2.8i multisport frame. I changed the entire
> front-end, shortened the stem, removed the STIs in favour of
> down-tube levers, and it would still try to throw me off when
> the conditions were right. I have an observation to make with
> regard to frame stiffness - when a stiffer frame goes, the
> wobble is much more violent, and the onset much quicker.
I think this is a load of bull. Frame geometry will play a
heavy role, but not stiffness. I smell red herring.
> My experience of hands-on speed wobble and the experience of
> others is that it is a much more violent phenomenon than this.
Can be. Different tires often help. Different forks help
sometimes, sometimes they make it worse.
--
Dave Blake
dbl...@phy.ucsf.edu
Theodore Heise
Tom H <thomas....@bakeratlas.com> wrote:
> the...@netins.net (Theodore Heise) wrote in message news:<slrna72t3e...@worf.netins.net>...
> >
> > to "throw him off" was apt. A rider following behind told
> > me he was surprised I kept the bike upright. The onset was
> > very sudden, I don't recall any warning signs. It was most
> > definitely violent--my thought was I had blown out the tire.
> I was beginning to feel very alone here. I hope that when I say I'm
> glad you had that experience, you'll take it the right way.
Oh definitely. <grin>
jobst....@stanfordalumni.org
>> Lanzarote is a safe qualification, because I can't drop in and try
>> that road myself. I don't believe any particular road has a
>> greater tendency to shimmy other than that it is downhill and
>> possibly in cold weather. I have seen shimmies on flat ground but
>> the rider was cold.
> I'm going to have to disagree with you here and leave it at that. I
> have found certain _conditions_ make a shimmy far more likely and
> that's all I can offer on this point. These conditions are straight
> descents, rough roads, and wind. Lanzarote has these in abundance.
From testing this problem often, I find that pedaling and road
roughness diminish the propensity to shimmy. The hands-on shimmy is
harder to describe because it usually involves involuntary muscular
interaction by the rider.
>> The reason I said that is that unloading the saddle does not unload
>> the rear wheel. Only the point of contact between rider and
>> bicycle changed. The load, with the same CG, is borne by the feet
>> and hands instead of the saddle, the saddle that is the anchor
>> point using rider as the mass about which the oscillation occurs.
>> The axis for the shimmy is a straight line between front tire
>> contact and saddle. Once that anchor is removed, the oscillation
>> will die within one period.
> Surely you are familiar with a well known phenomenon of winter
> ascents, that the road is so steep that you need to stand up, but on
> standing you lose traction and need to sit down? Clearly the CG
> changes.
I see a misinterpretation of "unloading the saddle" versus "standing
up". As I have said often in this thread, you needn't stand up, only
get the weight off the saddle. This does not change rider CG with
respect to the bicycle. Standing and climbing, changes the CG
significantly.
I think this subject needs to be approached pragmatically. If this
were a major problem, then bicycle design would be changed. That some
riders are plagued by it, indicates to me that some if not all of the
problem lies with the rider. I've ridden with many people over many
miles over the years and none of them has had a problem with shimmy
although their bicycles could be made to do so.
jobst....@stanfordalumni.org
> I'm glad you are of the opinion that the "problem" might be narrowed
> down to the forks and wheel. I'd like to take this idea one step
> further. I am of the opinion that in some hands-on wobbles (notably
> the more violent experiences) the spring of the steering (resonating
> at frquencies the rider cannot damp) somehow couples to the spring
> in the frame. This is why I was trying to draw a distinction between
> hands-off and hands-on. The torsional rigidity of the steering
> system did not seem to be an issue in the hands-off case, whereas
> the coupling of the steering resonance with the frame resonance
> seemed to be precisely the issue with hands-on.
Torsional elasticity of steer tub and stem is enormous compared to
other elasticities in this system. Just imagine how much energy can
be stored in that "spring". This is in contrast to wheel and tire
lateral elasticity and frame torsion. I think this looks for the
culprit in the wrong places.
jobst....@stanfordalumni.org
> My experience with an uncontrollable high speed wobble: Happened at
> around 35 MPH on five separate occasions: time trial and four
> different descents. Each time, hitting a bump seemed to set off the
> wobble. Loosening the grip only made things worse. Standing on the
> pedals was no help either. Once the wobble began it did not go away
> until I slowed to under about 20 MPH.
> Cause:
> Insufficiently tight quick release! Hitting a bump obliquely at
> speed would break loose the frictional bond between the QR nut and
> fork dropout, allowing for slight movement. The design of the QR,
> which has an extreme over-center action, plus relatively smooth
> mating surfaces, contributed.
How do you explain that this caused the wheel to shimmy, especially
since this is a friction connection that when loose and wrenched side
to side will disengage the wheel. I suppose if you have "lawyer lips"
on the forks and the wheel is rattlingly loose, it could make the
shimmy audible. In any case, the discussion is about bicycles in
proper running order.
> I am convinced this was the cause, since there has been no tendancy
> to wobble since the QR situtation was rectified over one hundred
> descents ago.
> Equipment:
> Trek OCLV with Klein fork, Ksyrium with Mavic QR.
> Moral: Lateral play in the front wheel can be the seed for a wobble
> which involves the whole bicycle.
I don't believe it.
David L. Johnson
> Can be. Different tires often help. Different forks help
> sometimes, sometimes they make it worse.
The problem with this topic, and the reason there are so many who are
convinced of some preposterous causes, is that changing just about anything
can help. The last time this started, someone claimed in all seriousness that
changing the stem cured the shimmy. I don't deny that, but I would not then
say that the stem was the _cause_ of it.
The cause is the resonate frequency of the system, which is the rider and the
bike together. The outside forces that set shimmy in motion are not clearly
visible, but reault from trail, and steering corrections we all make to stay
upright, which tends to cause an oscillation at high speeds, just what is
needed to produce resonance.
Anyway, if the system is changed in _any_ way, it can effect the resonant
frequencies. Since the oscillation is about the pivot, which is the rider,
the most dramatic change is to get the butt off the seat. That is a lot
cheaper than replacing perfectly good wheels.
Tim McNamara
> Pete Harris writes:
<snip>
> > Moral: Lateral play in the front wheel can be the seed for a wobble
> > which involves the whole bicycle.
>
> I don't believe it.
I guess this is what you'd call a "seed wobble?"
Jon Isaacs
>changing the stem cured the shimmy. I don't deny that, but I would not then
>say that the stem was the _cause_ of it.
>
I had a serious shimmy on my track bike that I cured by changing to a longer
stem. Of course that shimmy had nothing to do with the resonance of the
bicycle at speed because it was easily repeatable on mag trainer.
jon isaacs
David L. Johnson
If that is the case, that it had nothing to do with the resonance of the
bicycle at speed, then what could the stem have to do with it? On a trainer,
the stem is irrelevant, isn't it?
You had the same shimmy on a trainer as on the road? On a trainer, the
dynamics would be very much different, with the rear axle serving as a pivot
(since it is anchored). Pedalling forces can create the necessary driving
force, but I would imagine it to be a much different character.
--
David L. Johnson
__o | And what if you track down these men and kill them, what if you
_`\(,_ | killed all of us? From every corner of Europe, hundreds,
(_)/ (_) | thousands would rise up to take our places. Even Nazis can't
kill that fast. -- Paul Henreid (Casablanca).
Matthew Temple
thomas....@bakeratlas.com wrote on 2/19/02 7:07 AM:
Are all/most of these problems with shimmy constrained to bikes over some
particular size? (My bikes are small -- 48 - 51 cm and I've never gone
faster than about 45, which was fast enough for me. But on a wide variety
of bikes, I've _never_ experienced this wobble or even a hint of it.
Brief polling of short friends suggest that size of bike, height of rider
have a bearing on this. but why?
Matt Temple
David L. Johnson
> Are all/most of these problems with shimmy constrained to bikes over some
> particular size? (My bikes are small -- 48 - 51 cm and I've never gone
> faster than about 45, which was fast enough for me. But on a wide variety
> of bikes, I've _never_ experienced this wobble or even a hint of it.
> Brief polling of short friends suggest that size of bike, height of rider
> have a bearing on this. but why?
A larger frame would have more length of tubing to serve as a lever, and more
motion of the bars and headtube for a given angle of bend of the lugs (or
other joints). Larger frames simply have more room to wiggle. There may be
something about the steering geometry that encourages shimmy in large frames
as well. Small frames tend to have shallow head tube angles to get the front
wheel out of the way, but I'm not clear how that would affect the potential
for shimmy.
But your observation agrees with mine; this happens more often to people with
larger frames. Jobst is a case in point; his frame is quite big, and he
clearly has experienced shimmy. Mine is smaller, but bigger than yours (55
c-c). My current bike doesn't seem to be able to shimmy; it's aluminum, which
seems also less prone than steel or ti. My old, steel, bike would shimmy
pretty predicably on a good, fast, straight downhill. But I learned to cope
with it, as mentioned earlier, by unloading the saddle.
--
David L. Johnson
__o | Become MicroSoft-free forever. Ask me how.
_`\(,_ |
(_)/ (_) |
jobst....@stanfordalumni.org
> Are all/most of these problems with shimmy constrained to bikes over
> some particular size? (My bikes are small -- 48 - 51 cm and I've
> never gone faster than about 45, which was fast enough for me. But
> on a wide variety of bikes, I've _never_ experienced this wobble or
> even a hint of it. Brief polling of short friends suggest that size
> of bike, height of rider have a bearing on this. But why?
Since primarily the bars and stem oscillate, and these have a larger
swing the larger the bicycle, it seems reasonable that shimmy is less
prevalent on smaller bicycles.
Jon Isaacs
>bicycle at speed, then what could the stem have to do with it? On a trainer,
>the stem is irrelevant, isn't it?
>
Well, for some strange reason, or maybe not so strange reason, the body/bicycle
system would achieve reasonance at about 34 mph with the gearing I was using.
Setting the bike up on the trainer, getting into position and when I got to
speed the bars would start shaking, ie I would shake the bars as I pedalled.
A longer stem stopped this. I repeated it several times, changing the stems
back and forth because it seemed so odd but in fact that was the case.
>You had the same shimmy on a trainer as on the road? On a trainer, the
>dynamics would be very much different, with the rear axle serving as a pivot
>(since it is anchored). Pedalling forces can create the necessary driving
>force, but I would imagine it to be a much different character.
The issue here was the interaction of my body with the bicycle as I pedalled.
Realize that the bike is a small part of this equation, certainly each of my
legs weighs significantly more that the entire bike. So when at a cadence of
135 rpm or so there is a lot of potential for an imbalance. Just like a twin
cylinder engine with a 180 degree crank, there is a second order imbalance
which is a rocking couple.
jon isaacs
Pete Harris
> > Insufficiently tight quick release! Hitting a bump obliquely at
> > speed would break loose the frictional bond between the QR nut and
> > fork dropout, allowing for slight movement. The design of the QR,
> > which has an extreme over-center action, plus relatively smooth
> > mating surfaces, contributed.
>
> How do you explain that this caused the wheel to shimmy, especially
> since this is a friction connection that when loose and wrenched side
> to side will disengage the wheel. I suppose if you have "lawyer lips"
> on the forks and the wheel is rattlingly loose, it could make the
> shimmy audible. In any case, the discussion is about bicycles in
> proper running order.
Jobst, I'm pretty sure that only one side of the wheel was breaking
loose. I stumbled on the cause by kicking the wheel sharply at the
bottom to see how laterally stiff it was. When the QR wasn't tight
enough, I could break it loose to the extent that there was about 3/8
inch lateral play at the top and botom of the wheel as the axle moved
up and down just a tiny bit in the left dropout. It would stay loose
like this until the QR was reset.
As for the subject of this thread, the original poster was discouraged
about a speed wobble that no one could fix. I for one was living in
mortal fear until I found my problem.
> > Moral: Lateral play in the front wheel can be the seed for a wobble
> > which involves the whole bicycle.
>
> I don't believe it.
Try going really fast with a loose QR and you might become a believer.
- Pete
Tom H
> Torsional elasticity of steer tub and stem is enormous compared to
> other elasticities in this system. Just imagine how much energy can
> be stored in that "spring". This is in contrast to wheel and tire
> lateral elasticity and frame torsion. I think this looks for the
> culprit in the wrong places.
>
> Jobst Brandt <jobst....@stanfordalumni.org> Palo Alto CA
Try this test: Facing your bike, stand over your front wheel clasping
it firmly between your feet. Grab your handlebars and shake them for
all you're worth. If you put enough effort in, you'll get a rough idea
of the frequencies and forces involved in a vhos
(violent-hands-on-shimmy), especially if you shake your bike to the
point your grip gives out.
Another test: Coasting very slowly, shake your steering as hard as you
dare. There are two extremes of what tends to happen. Small bikes
ridden by small, light women, behave completely rigidly and just weave
a little. Tall bikes ridden by large heavy men on the other hand just
sqirm (even resonate) beneath the rider and go in a dead straight
line.
Whatever the culprit is, I'm still of the opinion that the the case of
vhos involves a tuning of the resonances of the steeing and the frame,
when both are effectively constrained by the riders weight (and
perhaps involuntary reflexes). The forces large, and as we know the
steering torque is effectively arbitrary, being dependant on the
angular velocity.
A couple of club-mates of mine ( engineering lecturers ) have built a
frame deflection test-jig at the Open University here in the UK. They
are failing to sell their services to frame manufacturers and are
always after frames to test. I'll see if they can rig up something for
me to test the torsional stiffness of a fork/stem and bar combination
and compare it with a wheel attached. I hope this whole fiasco isn't a
result of my failure to utilise 36 spokes ;-)
Tom H
>
> Presumably you have different speeds and different weight
> distributions. Shimmy is an oscillation - it can be
> described by a second order differential equation. The
> distribution of mass will play a large part in both the
> appropriate speeds for shimmy to occur and its magnitude.
>
I wouldn't be too sure about the order of the equation, perhaps you'd
like to write it down with a list of the relevant assumptions. I'd be
particularly interested in a second order equation, particularly as
we're dealing with gyroscopic forces and precession. We can leave
coupling this to the frame till later if you like.
> Most people place their weight more over the rear wheel while
> descending, and more centered while pedalling.
>
This is ineresting. The bicycle must be the only object in the
universe that when you tip it, it transfers its CoG the other way.
> > 2) Why is hands-on speed-wobble so sensitive to weight
> > distribution? Hands on the drops tends to make matters worse.
>
> Try putting your sternum on the stem :) This shifts the
> weight as far forward as possible. It is easier if you start
> a descent this way and notice your speed, and abort if
> you start to shimmy (I've never shimmied in this position).
>
Doesn't Pantani go one step further and put one arm behind his back? I
think you are probably right about this, but it will take this old guy
whose descending nerve shot a while to work up to it!
> > Funny you should mention Cannondales. I had my worst ever
> > speed-wobble on a 2.8i multisport frame. I changed the entire
> > front-end, shortened the stem, removed the STIs in favour of
> > down-tube levers, and it would still try to throw me off when
> > the conditions were right. I have an observation to make with
> > regard to frame stiffness - when a stiffer frame goes, the
> > wobble is much more violent, and the onset much quicker.
>
> I think this is a load of bull. Frame geometry will play a
> heavy role, but not stiffness. I smell red herring.
>
Feel free to argue this one with Jobst. Then again when you come up
with a second order equation describing the resonant frequencies of
the frame that does not involve its elasticity you'll have the proof.
Given how straightforward that is, it'll probably be quicker than
convincing him any other way.
>
> Can be. Different tires often help. Different forks help
> sometimes, sometimes they make it worse.
Again we agree, but the question is: In what way different???????????
Tom
Tom H
>
> I think this subject needs to be approached pragmatically. If this
> were a major problem, then bicycle design would be changed. That some
> riders are plagued by it, indicates to me that some if not all of the
> problem lies with the rider. I've ridden with many people over many
> miles over the years and none of them has had a problem with shimmy
> although their bicycles could be made to do so.
>
It's obviously not a major problem for cycling as a whole, and
doubtless only a tiny fraction of cyclists ever experience it. In
defence at the allegation of ineptitude, I used to ride with one
semi-pro rider who was plagued with it, and know several guys who have
experienced it who have been riding "all their lives". This thread was
started by someones real experiences, and in a month or so there will
be another.
Tom
Tom H
>
> A larger frame would have more length of tubing to serve as a lever, and more
> motion of the bars and headtube for a given angle of bend of the lugs (or
> other joints). Larger frames simply have more room to wiggle. There may be
> something about the steering geometry that encourages shimmy in large frames
> as well. Small frames tend to have shallow head tube angles to get the front
> wheel out of the way, but I'm not clear how that would affect the potential
> for shimmy.
>
The question of trail is one I've asked here before. I too think it
makes a difference, but noone seems to know for sure. Most bikes my
size tend to come with 74ish degree head angles and consequently less
trail. I also think that small bikes could be made to behave like big
ones if they supported enough weight. This might be several times the
weight of a typical rider.
> But your observation agrees with mine; this happens more often to people with
> larger frames. Jobst is a case in point; his frame is quite big, and he
> clearly has experienced shimmy. Mine is smaller, but bigger than yours (55
> c-c). My current bike doesn't seem to be able to shimmy; it's aluminum, which
> seems also less prone than steel or ti. My old, steel, bike would shimmy
> pretty predicably on a good, fast, straight downhill. But I learned to cope
> with it, as mentioned earlier, by unloading the saddle.
Jobst says he has never experienced nor seen the type of violent
hands-on-out-of-control-speed-wobble that I and several others report.
Almost to a man, everyone I have personally spoken to who has
experienced this is 6'4"+, rides 60+(C-C)cm frames, and of a heavier
build than your classic cyclist. My first experience occurred when I
was about 250lbs at 70kph.
Tom
Mark Hickey
If it were the mass times lever arm that were causing (or, allowing)
the shimmy, wouldn't there be an order of magnitude of difference
between a bike with regular brake levers and Ergo/STI?
And I have to wonder if there's any difference between the bikes set
up with four cables "damping" the movement of the bars and those with
only two (downtube shifters) or one or none (track bike).
Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame
Henrik Münster
wrote:
> On one longer trip (80 Km.) I decided to carry some supplies with me and set up
> a container on my rear carrier (which has always been there). I put about 7
> pounds of stuff in a box on this carrier, and this was enough to cause a VERY
> mild repeatable shimmy at about 29 to 32 Km. per hour.
I think that's pretty normal. When I put a child's seat on the rear of
my everyday-bike, I get a little shimmy when riding no-hands at medium
speed (15-20 km/h estimated). It's never there without the seat. With my
two year old son in the seat, the shimmy disappears again, and I can
ride no-hands without trouble. It's clearly a matter of balance.
I have also had a little high-speed shimmy on my road bike, once
I put a 1 cm shorter stem on it. Again, there's nothing "wrong" with the
bike. Just matter of coincidences resulting in a balance problem, which
causes shimmy.
--
Med venlig hilsen
Henrik Münster
Esbjerg