Craig_Calfee
new member
Reged: 12/19/03
Posts: 11
|
|
Calfee Design has identified a cause of speed wobble (a.k.a. shimmy) and instability that can be prevented. Speed wobble is a dangerous condition that can cause the rider to lose control of the bicycle and crash. While loose headsets and out of true wheels and frames can contribute to speed wobble, we have found that fork asymmetry can also cause speed wobble.
Fork symmetry is defined as the symmetrical position of the fork dropouts in relation to the steering axis. Specifically, the equality of the distances from the dropout faces to the steering axis must be within a certain tolerance for the bike to ride in a stable and confident manner.
Traditionally, steel forks were cold set after welding or brazing to realign them after possible distortion caused by the heating and cooling of the metal. A diligent steel frame builder can align the fork blades to within a millimeter of symmetry.
Carbon fiber forks cannot be cold set. They must be molded straight to begin with. We have found that a small percentage of carbon forks by various companies were molded with asymmetrical fork blades. Some are off by a little over a millimeter and others are off by two or more. Forks that are off by over 1.8 millimeters in symmetry have a good possibility of being prone to speed wobble. A symptom of a fork that is off by 1.8 mm or more is a noticeable difficulty when riding no hands at a slow speed (less than 10 mph). One has to lean to the side slightly to keep going straight. A bike with asymmetrical forks seems to corner better in one direction but not so well in the other. At speeds of 30 mph or more, the bike can develop speed wobble.
If your bike has the above-mentioned symptoms, the fork should be measured for symmetry. This is difficult to measure without proper tools. Calfee Design measures all forks for symmetry and is equipped to measure any fork. If any Calfee customer wishes to have their fork checked, please send it to us with a letter requesting a fork inspection. Non-Calfee customers may send their forks for inspection for a nominal fee. Replacements may be available for asymmetrical forks, depending on the individual fork maker’s policy.
Craig Calfee
|
bfd
journeyman
Reged: 12/22/03
Posts: 77
|
|
Jobst Brandt will disagree:
none of the items listed cause shimmy. We have had discussions in this newsgroup at length and theories tested, none of which had any effect on shimmy.
All of the following is irrelevant as the claim of Rolf Dietrich that the wheels prevent shimmy. That claim thoroughly failed a reality test by independent testers.
I think you should read the FAQ before hypothesizing on causes of shimmy:
http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html (update no yet posted)
# Subject: 8h.5 Shimmy or Speed Wobble
# From: Jobst Brandt <jbrandt@hpl.hp.com>
# Date: Mon, 25 June 2004 14:13:14 PDT
Shimmy, a spontaneous steering oscillation of the front wheel, usually occurs at a predictable speed when riding no-hands. The likelihood of shimmy is greatest when the only rider-to-bicycle contact is at the saddle and pedals. This position gives the least damping by hands, arms, and legs. When shimmy occurs on descents, with hands on the bars, it is highly disconcerting because the most common rider response, of gripping the bars firmly, only increases
it.
# Shimmy is not related to frame alignment or loose bearings, as
# is often claimed. Shimmy results from dynamics of front wheel
# rotation, mass of the handlebars, elasticity of the frame, and where
# the rider contacts the bicycle. Both perfectly aligned bicycles and
# ones with wheels out of plane to one another shimmy nearly equally
# well. It is as likely with properly adjusted bearings as loose
# ones. The idea that shimmy is related to bearing adjustment or
# alignment has been established by repetition.
# Bicycle shimmy is the lateral oscillation of the head tube about the
# road contact point of the front wheel and depends largely on frame
# geometry and the elasticity of the top and down tubes. It is driven
# by gyroscopic forces of the front wheel, making it largely speed
# dependent. It cannot be fixed by adjustments because it is inherent
# to the geometry and elasticity of the bicycle frame. The longer the
# frame and the higher the saddle, the greater the tendency to shimmy,
# other things being equal. Weight distribution also has no effect on
# shimmy although where that weight contacts the frame does. Bicycle
# shimmy is unchanged when riding no-hands, whether leaning forward or
# backward.
# Among parameters that supposedly cause shimmy, spoke pattern and
# balance had no effect. Tests with wheels balanced and purposely
# unbalanced and ones with paired spokes as well as low spoke count
# caused no change in shimmy. Filling the front tire with water,
# doubling its mass, had no effect other than to change its frequency
# of oscillation slightly.
# Shimmy requires a spring and a mass about which to oscillate and
# these are furnished by the frame and seated rider. Unloading the
# saddle (without standing up) will stop shimmy. Pedaling or rough
# road will also reduce the tendency to shimmy. In contrast, coasting
# no-hands downhill on a smooth road at more than 20mph with the
# cranks vertical seems to be the most shimmy prone condition.
# When coasting no-hands, laying one leg against the top tube is the
# most common way to inhibit shimmy and also one of the most common
# ways to coast no-hands. Compliant tread of knobby tires usually
# have sufficient squirming damping to suppress shimmy. Weight of the
# handlebar and its extension from of the steering axis also affects
# shimmy.
# Shimmy is caused by the gyroscopic force of the front wheel whose
# tilt is roughly at right angles to the steering axis, making the
# wheel steer to the left when it leans to the left. This steering
# action twists the toptube and downtube, storing energy that both
# limits travel and causes a return swing. Trail (caster) of the fork
# acts on the wheel to limit these excursions and return them toward
# center.
# The shimmy that concerns riders the most occurs with hands firmly on
# the bars. This is rider generated by muscular effect whose natural
# response is the same as the shimmy frequency, about that of Human
# shivering. Descending in cold weather can be difficult for this
# reason. The rider's "death grip" only enhances the incidence of
# shimmy. Loosely holding the bars between thumb and forefinger is a
# way of avoiding shimmy when cold.
|
Kahuna
Forum Admin
Reged: 12/11/03
Posts: 162
Loc: Maui, Hawaii
|
|
Quote:
bfd wrote:
Jobst Brandt will disagree:
[..]
I solved a very bad shimmy problem once by methodically changing tires and wheels, making one change at a time until the problem was isolated to a damaged headset. Replacing this part completely eliminated a shimmy that would start at 15mph and get progressively worse as speed inreased. After replacing the headset there was no more shimmy - at least none that I could detect up to about 45mph. I can't test for shimmy faster than that because I'm too afraid to take my hands off the handlebars.
As far as Mr. Brandt's FAQ goes, I read it, but came to the conclusion that sometimes there's a gap between theory and practice.
I'm inclined to believe Craig because he does his homework.
-K
|
 Lon
sage
Reged: 12/20/03
Posts: 595
Loc: Pittsburgh, PA
|
|
Well Gary you solved wobble with a headset change and I eliminated wobble with a wheel change. Between us we have two of the factors proven.
My wobble actually started at 45 mph and it could almost shake you right off the bike. It was scary. I eventually put on a different front wheel and I never had a wobble again.
I think bfd needs to read more carefully. Craig's point as I took it was that there were a multiplicity of reasons and this was one more to check.
Brandt is the fellow that wrote the wheel building book that is so well received. I googled him and he also tours in the Alps extensively. Although everyone I read respected his knowledge it also came up more than once that he is highly opinionated and does not like to be told he could possibly be wrong or not completely correct about an issue.
|
Schwingding
new member
Reged: 06/09/04
Posts: 22
|
|
You bet your butt he does. I will never read or listen to anything that guy says again. As a total newbie, long, long ago, he read me the riot act for improper usage of "coast" vs. "drift". Nasty man.
|
Lon
sage
Reged: 12/20/03
Posts: 595
Loc: Pittsburgh, PA
|
|
It is funny you would use the word nasty. I ran across several threads in just a quick search that used that term in reference to him. You evidently have plenty of company in being a "victim" of his temper.
One thread was a comparison of him to Sheldon Brown. Essentially it was how both knew a great deal about bicycles but had completely opposite personalities. I believe one person posting had met them both and had discussions. Sheldon was on the great guy end of things.
|
Schwingding
new member
Reged: 06/09/04
Posts: 22
|
|
Lon I couldn't agree more, having had interactions with both of them. IIRC, Sheldon emailed me a wonderfully detailed response to a problem I was having that no one else even bothered responding to on rec.bicycles.somethingorother. Could not have been nicer, in direct contrast to Brandt. Both of those interactions were long ago in a far away galaxy, but funny how pungent the one still smells. Time usually tempers my emotions, but not always.
Back to on topic, Dan Empfield has strong opinions on wheel wobble, he tends, if I remember right, to feel that it inherent characterstics in a frame build that cause it. I might refer him to this thread to get his take.
Edited by Schwingding (08/17/04 04:28 PM)
|
Bruce
contributor
Reged: 12/27/03
Posts: 133
Loc: North of NYC
|
|
Seems to me Craig's diagnosis of fork misalignment jibes with Brandt's observations. Notice Brandt does not postulate a theory, only observations. Therefore is he is not really taking a scientific approach to the problem. On the other hand, Craig reports a specific finding. We are not sure how he derived this conclusion, but he has data to back up his claim. His theory matches the observations of Brandt. So if Craig theorizes that fork misalignment causes speed wobble, then theory looks correct, and scientifically we would say frame misalignment would be at least of cause of speed wobble.
Score one for Craig, none for Brandt.
|
PsyDoc
friend
Reged: 01/14/04
Posts: 37
|
|
The only "scientific" statement one could correctly make is that the data currently provide support that fork misalignment contributes to speed wobbles. Next step would be to conduct several controlled experiments using the scientific method to determine the exact nature of the relationship between fork misalignmnet and speed wobbles.
|
Nev
captain
Reged: 05/03/04
Posts: 376
Loc: Never where I want to be
|
|
Quote:
bfd wrote:
Jobst Brandt will disagree:
# The shimmy that concerns riders the most occurs with hands firmly on
# the bars. This is rider generated by muscular effect whose natural
# response is the same as the shimmy frequency, about that of Human
# shivering. Descending in cold weather can be difficult for this
# reason. The rider's "death grip" only enhances the incidence of
# shimmy. Loosely holding the bars between thumb and forefinger is a
# way of avoiding shimmy when cold.
I'm a newbie to the road things so please excuse my greeness.
Where exactly does the rider factor come in?
When I first got my 02' Vortex in May (ouzo pro fork, ksyruim SLs) every now and then out of the blue I'd feel a strong and distinct "wobble" from my front wheel. I had a ticking headset and got that replaced. The ticking is now gone, but I've also noticed the front wheel is WAY more stable. I could sit up with no hands on the bars before, but now it tracks way better. Very stable bike. I've gone around a curve with no hands at about 28mph (slow, I know) with no problems totally steady ride.
However, every now and then I still do feel a shake in the front wheel. Although much less distinct with the new headset. My thinking is it's my bad technique. That I'm leaning forward too much, all my weight on the bars, almost resting my whole upper body on them. Probably a combination of too much of my weight forward on the bars, and probably uneven force pressing forward between left and right arms, left and right side of my body. If I'm sharp and quick enough to tune into this, I can make it go away by relaxing my upper body, grip and getting my weight a little further back. I think it happens when I notice myself "leaning" too much onto the bars.
So Nev's theory:
Maybe there's something to rider induced wobble. I'm not a pro so I'm light years away from being focused on my technique and body position throughout my entire ride. Maybe some riders get lazy (not an insult here, there's a better word somewhere) or simply every now and then not pay attention to their form and they just kinda find themselves holding themselves up on the bars with weight going to far foward and the shakes happen. I've noticed the same on my mountain bike during long rides or 24hr races, I kinda get lazy and realize I'm too far forward, like I'm pushing the front end into the ground, I'm gripping the bars too hard and it totally affects steering and the fork. I losen my grip and sit up a bit and the bike feels free again. No real wheel wobble issues with mtb bikes though.
Just wondering,
Nev
|
garygromet
friend
Reged: 01/28/04
Posts: 36
Loc: Miami, FL, USA
|
|
September 15, 2004
John Adams of Lake Oswego describes how Cycle Oregon rider Karen Holmes of West Linn was losing control before riding off the road. Holmes died in the crash Tuesday.
JACKSONVILLE — A West Linn woman died Tuesday when she lost control of her bicycle on a twisting mountain road near Williams, marring Cycle Oregon’s swing through Jackson and Josephine counties.
Karen Holmes, 60, was the first person to die from an accident in Cycle Oregon’s 17-year history.
Holmes was three or four miles into a long descent down Holcomb Peak, in Josephine County, on the way to Williams, where riders were scheduled to stop for lunch on their way to Jacksonville, their destination for Tuesday night.
"It happened real fast," said John Adams of Lake Oswego, who had stopped to check his brakes just before the accident occurred. He happened to look up and saw Holmes coming downhill fast.
"She was really movin’," Adams said, "and her front tire was wobbling like crazy. She was totally out of control and clearly panicked."
Adams said Holmes shouted something unintelligible that made him think she was in trouble as she hurtled toward a sharp curve in the road.
"The road turned left," he said, "and she just went right over the side.
Adams said the road was surfaced with a mixture of rock chips and oil, a mixture known as "chip seal," that’s commonly used on rural roads.
"I’ve been on worse (roads)," he said. "It was not wet, not slippy, but very steep."
Nicholas announced Holmes’ death to riders who had gathered at the Britt Grounds for the nightly entertainment that is one of the features of the tour.
Nicholas said in an interview it was still too early to know why Holmes lost control of her bike. All cyclists had their brakes checked by mechanics at the top of the descent.
Adams, who saw the accident, said the descent was so steep that some riders’ tires blew out from heat generated by friction from their brakes. Hearing so many tires pop prompted him to stop and check his own brakes just before Holmes barreled past him out of control.
Adams said he initially thought Holmes had blown a tire, too, because her front wheel was wobbling badly, but he later saw her bike in camp at Jacksonville and the front tire was not flat.
For brakes to fail completely on a bike would be rare, said Brian Combs of Portland, who is following the tour selling equipment for Bike Gallery, a Portland-based bicycle store.
Combs did not ride the course, but said friends who did told him they were riding their brakes "80 to 90 percent of the time."
You can find this story online at:
http://www.mailtribune.com/archive/2004/0915/local/stories/01local.htm
|
Lon
sage
Reged: 12/20/03
Posts: 595
Loc: Pittsburgh, PA
|
|
Gary,
Do you know what fork it was? That is really a tragic story.
I had that happen at about 47 mph once. The rider behind me thought it would shake me right of of my bike. That was not tied to the fork however. It is scary though to have your bike shaking like that.
Take care.
|
JamesC
new member
Reged: 12/24/03
Posts: 19
|
|
Why this is such an important, much discussed issue:
A Cycle Oregon participant from West Linn died after losing control of her bicycle on a twisting mountain road in Southern Oregon.
“She was really moving,” Adams told The Mail Tribune in Medford, “and her front tire was wobbling like crazy. She was totally out of control and clearly panicked.”
http://news.statesmanjournal.com/article.cfm?i=86767
|
Wisdom
new member
Reged: 04/09/04
Posts: 3
|
|
Quote:
Nev wrote:
Where exactly does the rider factor come in?
True story: Neighbor borrows my old steel frame and finds that it wobbles severely at 40+ mph. I observe him almost going down as tank slappers begin on a steep downhill. Never having experienced this myself in a decade plus of riding that bike, I take the bike back and test ride. Approaching 50 mph, the bike is as stable as ever - no problems whatsoever.
Moral: The rider definitely influences wobble. Some bikes may have geometry problems that make wobble more likely but even the most stable bike can be ridden badly.
Postscript: My Calfee tracks perfectly at all speeds.
|
flythebike
captain
Reged: 08/26/04
Posts: 272
Loc: N. Virginia, USA
|
|
People used to say to grab the top tube with your knees to stabilize it.
|
Lon
sage
Reged: 12/20/03
Posts: 595
Loc: Pittsburgh, PA
|
|
I left my bike shop on Sunday and it is downhill. There are not any cross streets until the bottom and having recently ridden it I knew the road was fine. 
I got into a deep tuck and took off. When I hit the bottom at the light I looked at my max speed to see if I had hit 50 mph. I wasn't sure since there was not any obvious perception of speed. 
Top Speed...60.5 mph.  It is a new Fly with the Easton EC 90 fork. It is one awesome bike. 
First time I have gone over 60...58 and change was the best prior to this. 
|
SAVAGEP
new member
Reged: 11/13/04
Posts: 7
|
|
Interesting thoughts on this horrible topic.
My experience. On my Calfee D'Fly the only instance of shimmy was descending Whiteface Mtn after the hillclimb.
Cause - shivering. Adding clothing fixed it!
On my old Colnago Mexico - probably the most flexiblw steel frame ever - it developed the wobbles at about 30 mph - but only when I tried to use very light wheels. With the usual Campy Sigma (?) never a problem.
I've also heard of people having this problem gradually develop and then go away when changing to new tires.
Way back (40 yrs ago!) I worked on Airframe design - stability issues. I suspect that bike shimmy is about as complex and multi sourced.
|
cgarch
new member
Reged: 03/25/05
Posts: 1
Loc: sonoma county, ca
|
|
Interesting discussion. I'd like your take on the video that can be found at this link: http://www.bikeroute.com/Recumbents/News/Archives/000066.html where you can clearly see a great amount of wobble or oscillation in the crash of this high-wheeler. I think that one has to wonder if the wobble induced the tire off the rim.
BTW, my Calfee Tetra-Tetra purrs at 50+, even in cross-winds descending highway 1 at Scott Creek, north of Santa Cruz.
|
Riccardo
new member
Reged: 12/17/05
Posts: 14
|
|
Hi,
This is an interesting thread even though its a bit old. The age doesn't affect much, though, because the fundementals of bike dynamics still aren't well known.
If you doubt the latter, assemble 3 or more frame-builders to discuss handling and frame/fork design. They'll disagree. The best seem to recognize that more than one combination of design factors can work well and that humans adapt easily to their bikes. So, what's this got to with speed wobbles?
Inputs of some cause cause the wobble, but what are they? The problem with tests to date is that in their attempts to isolate a variable, they limit other variables that may have synergistic consequences. The weight of ones wheels may be a causal factor for speed wobble, but only with certain configurations of bike and rider. The same can be true of the headsets condition/adjustment/fit, or frame flexibility, or alignment of the fork. Proving that one, or a limited set of instances of a mis-aligned fork doesn't prove that a mis-aligned fork isn't a causal factor in speed wobble.
The experienced mechanic or builder will attack speed wobble by looking first for more common and easier to solve solutions and then work towards more difficult and less likely solutions. Speed wobble hasn't a single cause nor a single solution.
If anyone doubts the impact of alignment, think of the last time you pushed a grocery cart with a bent front wheel. It oscillates. Nuff said on that.
In any case, grossly misaligned carbon forks are a crime. For their price, high precision should be expected.
As to technique, someone with more experience and better honed bike handling skills will probably have a better chance of recovering from a wobble. However, no one should become too proud of their skills. That you've never succumbed to a wobble only means you haven't yet met a wobble you couldn't handle. Sort of like someone from New Orleans sayinhg a year ago that a hurrican couldn't wipe out the town. Same goes for your bike, regardless of who built it.
|
Insightdriver
captain
Reged: 03/07/04
Posts: 472
|
|
Hello Riccardo!
I like what you said. And I do agree that a carbon fork, to be out of alignment means that the molds that were used to manufacture it were defective. Any manufacturer knows the accuracy of their molds because they have inspectors to check for that before they are used to produce something that would be scrap if the mold was wrong. There are those manufacturers, though, that might de-mold a product before it was properly cured, then putting it back into an autoclave or oven without being in a mold to fully cure it. That would indicate very poor quality assurance procedures. I do not think it would be fair, though, to label it criminal. More acurate in my opinion that it would show gross incompetence.
|
Krash
new member
Reged: 09/04/05
Posts: 17
|
|
Quote:
Riccardo wrote:
As to technique, someone with more experience and better honed bike handling skills will probably have a better chance of recovering from a wobble. However, no one should become too proud of their skills. That you've never succumbed to a wobble only means you haven't yet met a wobble you couldn't handle.
Are you saying that we'll all wobble eventually?
Does pride goeth before the fall?
Am I just proud enough or too proud?
Should I attend a Carmichel clinic before riding again?
This is scary stuff. I'm selling my bikes today. Check Ebay now.
|
Allan
journeyman
Reged: 05/04/04
Posts: 198
Loc: Bds,W I
|
|
Having re-read the postings here its easy to form a lot of opinions on what causes the dreaded shimmy situation, and while it does happen to some bikes it can usually be traced down to the area by changing out suspect parts.
I'm not going to debate it other than to say ive never had it happen on any bikes ive built up, and using frames with headtube angles from 73degrees to 75.6degrees there were no problems i encountered.
Granted i build and true my own wheels so the tension i use is something i know, but i couldn't speak for the bikes that had shimmy and what the tension on their wheels were, something to also consider.
Also most front forks are not quite accurate and i've had to file quite a few dropouts to centre a perfectly symetrical front wheel.
As for speed, where i live there is a hill that decends close to 900 feet of drop in less than two miles. The maximum speed i've done on it without assisted pedalling is 72 MPH, and yup the slight bends it has really come up fast at those speeds.
--------------------
Its time to ride.
|
Dave_Thompson
prophet
Reged: 12/19/03
Posts: 720
Loc: Spokane, Washington
|
|
See Jobst Brandt's article regarding speed wobble: http://www.sheldonbrown.com/brandt/shimmy.html
--------------------
Steel lover, but then I like Ti with carbon too.
Licensed bike geek.
|
Craig_Calfee
new member
Reged: 12/19/03
Posts: 11
|
|
Update as of 5/21/2006
Since posting this on the forum and our website, we have had a chance to help ten people with "uncurable" speed wobble. All had forks that were out of symmetry by 2mm or more. We (through the various fork vendors) replaced their forks with forks that were less than 1 mm out. They now enjoy going downhill at speeds over 35 mph without fear of getting the wobbles. They also notice greater ease in going no hands.
Interestingly, through crash inspections and repairs, we found a few people with forks that were pretty bad yet didn't report having trouble with stability. These were typically very skilled riders.
So it appears that speed wobble can be prevented by having straight forks or with rider technique.
--------------------
Craig Calfee
|
jamo
new member
Reged: 04/27/07
Posts: 1
|
|
I'm glad to see this thread here as a Calfee owner. I experienced speed wobble on my Luna for the first time Monday. As I was descending a very steep hill, my bike began oscillating more and more until it was whipping like a noodle. Really, I had plenty of time to feel the bike flex underneath me as visions of myself as road pizza danced through my head. Although my speed was only 40mph, there was no taking my hands off the bars nor putting my knees against the top tube. I felt lucky to bring the bike back under control and not crash in front of the car behind me.
Now I'd like to figure out how to prevent this from occuring again. Calfee aligned my fork before shipping me the frame and fork, so I'd like to rule out fork and frame alignment. From the thread postings, that leaves wheels, headset, and most likely me as the remaining culprits. The hill is pretty steep with a nice hairpin at the bottom, so I'm pretty gripped (pun intended)as I descend. My tense grip on the handlebars and bakes, and rigid body position most likely brought on this scare. Therefore, I can work on rider technique to reduce and prevent reoccurence, but what is the best way to confirm that wheels nor headset are contributing to the problem?
Thanks for your help,
James
|
Lon
sage
Reged: 12/20/03
Posts: 595
Loc: Pittsburgh, PA
|
|
I had a similar wobble on my Kestrel a long time ago. It "hit" at 44 mph. I know your feeling as it was like riding a bronco. It had never happened prior to that instance.
I thought it could have been me or a fluke and I went back the next week. That wasn't the best idea in the world as I got to relive that experience.
I then changed the wheelset. With the wobble I had a Spinergy Rev X on the front and a conventional wheel on the back. I went back with another wheelset and the problem never came back.
I'd get the headset checked by a top mechanic as well. While you are there I'd tell him/her what happened and have the bike checked over.
Good luck. There was a heated discussion on this topic on the Serotta forum a few weeks ago. Some folks felt it was vitually always rider error. My experience spoke otherwise. Plus I live in Western PA so you have fast descents on every ride.
All I know is I hope I never experience one again!
|
mintyness
new member
Reged: 07/05/07
Posts: 1
|
|
A colleague of mine mentioned that on the last AIDS LifeCycle Ride (SF -> LA), there were more accidents than in years past -- one especially nasty one that was apparently the result of speed wobbles, or wheel shimmy. I have never read anything online about this, so I was surprised to find out that there were so many competing theories when I got back to the office and looked.
Before searching online, however, I simply said to her, "It's all physics. Remember that movie of the bridge up in Washington that we all saw in elementary school? Like that bridge, you plus your bike have a very deterministic and discreet set of resonant frequencies, depending on how you are seated, if you are pedaling, hands on or off, etc, etc. The rotation of the wheels (probably only the front really), in combination with your exact frame geometry and handlebar length and weight give rise to the minute oscillations that resonate with those discreet frequencies I just mentioned. If you get the wobbles, loosen up your grip (or put hands back on the bars, if they're off), clamp the top tube in between your legs (thighs, calves, whatever), and either slow down or speed up." [Yes, I actually talk like that. And I sound like even *more* of a pompous ass than you'd guess from that .-]
This is all based on my own experiences on 4 different road bikes and 3 different mountain bikes over the past 30 years of riding (Yes, I'm an old fart now), and on my Master's education in Physics.
If anyone is interested, here's what I experienced:
1.) 1981 Bob Jackson (19" steel frame/fork, Ambrosio tubulars, 32 h): perhaps the finest hand-made bike I've ever ridden. I had speed wobbles between 45-49 mph, but only at apparently random times. Changes to the bike (new tires, lowering/raising the seat, etc) did nothing to affect the range in which wobbles occurred. The headset is a 1982 Campy Super Record that has been checked by many mechanics. It's definitely not that part. On all bikes that I have had shimmies, I have found that when I keep my elbows locked, the wobbles are MUCH worse; keeping them loose significantly reduced the effect.
2.) 1983 Peugeot (steel, Mavic clinchers, 32 h): pretty much the same here, except the wobbles occurred at a bit lower speeds (40-44 mph).
3.) 2002 Raleigh Team (aluminum frame, carbon forks and seatstays (both Columbus Carve), Campy Proton clicher wheelset): this bike is a bit taller than my others (it's a 59cm), and the wobbles I have experienced are much more pronounced on this bike than any of my others. Also, I can predict each and every time it will occur (well, almost every time). It always happens when I hit 43-47 mph (depending on the road condition: curving, straight, bumps, smooth, etc). It's gotten so that I don't even freak out at all anymore... I simply clamp the top tube between my thighs and the wobble's frequency and amplitude are diminished pretty much to zero right away. I have also found that simply cranking up to bit higher speed does the same. Like a hockey puck traveling thru rough ice speeds up when it returns to smooth ice, the shimmies just disappeared when I got back above 49 mph or so.
[Note: I don't ride this bike anymore .-]
4.) 2006 Ridley Damocles (carbon, carbon, carbon ;-), Fulcrum Racing 1 clincher wheelset): yowza! this bike is insane! I have practically tried to induce wobbles on this thing and I can't get it to budge an inch. It descends like an arrow fired straight down from a building rooftop. Anyway, I've yet to experience any shimmies on this bad boy. (Of course, I've yet to break 60 mph... .-)
5.) Mountain bikes -- 1998 Giant MCM 980 (carbon front, aluminum rear tri,d/s), 1999 KHS (steel, hardtail), 1991 Cadex 3 (carbon w/ aluminum lugs, hardtail): never got any wobbles on any of these steeds. Lucky me!
So, what does all that mean? Basically, to me it means that wheel wobbles, or shimmies, are basically the result of hitting the resonant frequency of the system, in this case the rider + the bike. This doesn't mean that uneven (non-symmetrical) forks do not contribute to it in some way, or that rider position and action (ie, locked elbows and knees) are the sole cause -- it's all these things and more.
From the physical point of view Mr Brandt is most accurate in describing the action: "Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left." In this case, replace "shimmy" with "torque", and you're basically back to Physics 101. Very straightforward stuff. You spin the wheel, it wants to pull left. Lean left, and it wants to pull even more.
But the gyroscopic motion of the front wheel is only one of the variables involved: the height of the bike, the rigidity of the wheels (their tendency to resist deflection), the elasticity of the frame material(s), the center of gravity of the system, the list goes on and on (and on...). Just as the small oscillations of the wind across the bridge in WA caused massive resonant swings in the structure, tiny oscillations in the front wheel become vastly magnified when they resonate with the rest of the system.
To truly model the system accurately would be a nice doctoral thesis, I think. No kidding: there have been several doctoral theses on the physics of skipping stones across water, which, it turns out, is far more complicated that one would imagine just watching those stones fly across the pond. I can imagine that this would be of an equal or greater degree of complexity.
In the final analysis, what makes me convinced that the effect is mostly a resonant frequency issue is that when I get out of the "resonant band", either by slowing down (carefully) or speeding up (much more fun, when possible), they dissipate. One thing I NEVER do is maintain the same speed and attempt to "control" or "muscle" the wobbles away. Don't fight physics, use it .-)
Of course, after all that, you can just look at any of the many motorcycle crash videos on YouTube: it's pretty clear (to me at least), that asymmetrical forks ain't what's doing it. Or, to really feel it, jump on a skateboard and hit the nearest hill with your knees locked. When you begin to wobble, relax your knees: look ma! no wobble. (By relaxing your knees, you effectively change the resonant frequency of the system; at least until you speed up enough to hit the new, higher frequency about 100 yds and 15 mph+ later .-)
But, then, these are my own experiences and conclusions; YMMV.
Enjoy!
|
Previous
Thread Index
Next
Threaded
Instability and Speed Wobble
Edit
Reply
Quote
Re: Instability and Speed Wobble
Re: Instability and Speed Wobble
Solid As A Rock
Re: Instability and Speed Wobble