Help troubleshooting severe high speed wobble

[bluesea]

I'm 5'10" on a size 54, with 130 stem.

Have a new to me Legend ST with a (scary) high speed wobble. Actually its come on at 35-40 mph, and the one time coming to a stop, the wobble increased to practically jumping at around 7-10 mph. Knees on toptube didn't seem to help, but have recovered a couple times as speed fell to ∽25 mph.

[GScot]

Headset adjustment can play a part in wobble. I'm not fluent in recognizing whether you have threaded or threadless but either way they have to be right. I had a friend with Calfee that kept having issues until I replaced the expander in the steerer tube. Whatever was in there didn't quite hold tight enough to preload the bearings.

[palincss]

I'm 5'10" on a size 54, with 130 stem.

Have a new to me Legend ST with a (scary) high speed wobble. Actually its come on at 35-40 mph, and the one time coming to a stop, the wobble increased to practically jumping at around 7-10 mph. Knees on toptube didn't seem to help, but have recovered a couple times as speed fell to ∽25 mph.

Speed wobble at 7 mph while slowing down? That sure doesn't sound like any speed wobble I've ever experienced. Are you sure this bike doesn't have some kind of issue with loose bearings somewhere?

[oldpotatoe]

I'm 5'10" on a size 54, with 130 stem.

Have a new to me Legend ST with a (scary) high speed wobble. Actually its come on at 35-40 mph, and the one time coming to a stop, the wobble increased to practically jumping at around 7-10 mph. Knees on toptube didn't seem to help, but have recovered a couple times as speed fell to ∽25 mph.

What kind of front wheel? Fat, aluminum spoke type? If so, try a round spoke type. Also, HS, and front hub adjust. Fork and frame alignment.

[bluesea]

Speed wobble at 7 mph while slowing down? That sure doesn't sound like any speed wobble I've ever experienced. Are you sure this bike doesn't have some kind of issue with loose bearings somewhere?

That was slowing down from 40 mph, where the wobble turned into hopping just before coming to a stop.


I've checked Headset/ compression plug a couple times--will check again later today.

[eddief]

me thinks. loose spokes can cause mysterious bouncing around at both low and high speeds.

[Ken Robb]

In the late 1990s I had a Trek bike with aluminum frame that did this. Changing wheels moved the onset speed of the wobble up 2 MPH. I tired everything I could think of to no avail. It was terrifying on descents of 30+ MPH.

I put the bike up for sale and told the first guy to look at the bike about my wobbles and insisted that he take it for a ride before buying the bike. He took it down La Jolla Shores Drive at 40+ MPH with no problem, paid me and left with a big smile on his face.

I had a Litespeed Natchez that I could get to wobble but I could stop it with a knee on the top tube.

Neither of my Serottas (CSi and Legend ti) EVER wobbled. My Rivendells never wobbled and my Hampsten by MOOTS with YBB descends like it's on rails even on rippled pavement.

You may find something wrong mechanically or you may just not be a good match for this frame/fork. Is this the same size you usually ride with no problems? 5'10" on a 54cm frame sounds like a frame that might be too small. When you compensate by using a 130mm stem you may be putting too much weight on the front wheel.

[Bradford]

I have a Legend that developed a high speed wobble and it was the glued in headset cap. I replaced that with a compression plug and has been fine for the last 13 years. Check that and see if it is ok.

It was kind of a funny story how it all went down. It was during an open house ride at the Serotta factory when it happened coming down a wicked steep hill. I was telling someone about it, I think it was Bruce K, when Ron Kiefel overheard us, pulled out a hex wrench, politely grabbed my bike, and starting working on it. He had it diagnosed in about 30 seconds and it was an easy fix.

[fogrider]

first try different front wheel. also try balancing your wheels.

[572cv]

Is the fork original to the bike? (Not all replacement forks have the same rake)
Getting the correct rake fork totally corrected this issue on my Meivici, though your issue sounds more severe than what i was experiencing. Of course, maybe you just go faster!

[Cicli]

also try balancing your wheels.

This I have to see.

Oh, and check the headset.

[BdaGhisallo]

I have a Legend that developed a high speed wobble and it was the glued in headset cap. I replaced that with a compression plug and has been fine for the last 13 years. Check that and see if it is ok.

It was kind of a funny story how it all went down. It was during an open house ride at the Serotta factory when it happened coming down a wicked steep hill. I was telling someone about it, I think it was Bruce K, when Ron Kiefel overheard us, pulled out a hex wrench, politely grabbed my bike, and starting working on it. He had it diagnosed in about 30 seconds and it was an easy fix.

What was the fix?

[ultraman6970]

All my life never had (or ride) a bike that wobbles.

[Peter P.]

...using a 130mm stem you may be putting too much weight on the front wheel.

I was thinking the same thing but not from a too much weight over the front wheel perspective, but from WHERE the weight is fore/aft of front wheel the contact patch. Maybe we're saying the same thing in different terms.

Checking the usual stuff has already been mentioned and hopefully will be tried.

I'm a guy that looks at some of the less than obvious stuff. Changing the fork rake is one item. Trying a different front wheel is another.

Calfee is big on carbon fork alignment. Unfortunately, carbon forks can't be aligned, but that doesn't mean they can't be checked on a jig. I'd pay to have the frame and fork alignment checked.

Report back and tell us whether you found a solution.

[flydhest]

I had a custom for me CSi that had a wobble. Couldn't figure it out. Serotta eventually gave me an oversized Legend to make sure the tubes were oversized enough at 60x60 not to have a wobble. Still did. Turns out, the fork had a loose fork tip.

Fixed that. Great bike. It is for sale now ...

[cmg]

This I have to see.

Oh, and check the headset.

wheel balancing

https://www.youtube.com/watch?v=4vG7iKoXeD0 and https://www.youtube.com/watch?v=yXezppKLgaE and this https://www.youtube.com/watch?v=dcgiUcbS64c

maybe not the cause but it's an easy fix.

[bluesea]

Had an uncharacteristically busy today. Will try to address as many points as I can for now.

- Ran two sets of wheels with same results: a Shimano RS 10 I had from new with >2000mi--no past mishaps with those. A brand new set of Dura Ace C24 clinchers. Same results with both wheelsets.

- Have checked my compression plug several times, using carbon paste for the last few rides. I'll try a different one, and file the steerer tube top closer to flat at the same time. There's a fat millimeter of space showing around half its circumference.

- The 130 stem could be contributing a negative effect, although I've been using 130/140 stems for quite a while on a number of different bikes with no issues. All frames are different of course.

- I ride a one size smaller frame for my height, mainly due to a short inseam and long torso. Seatpost is a Fizik Cyrano R3 w/25mm setback. Saddle center is positioned aft of center of seatpost, although I ride further forward than most.

- Frame/fork geometry: Not 100% sure of either. It steers a bit different from what I'm used to--something like a Colnago, but not really. It rides like a high trail frontend.

Love the ride otherwise.

[Peter P.]

While you may think the stem length isn't the culprit, the stem's torsional stiffness could be.

An expensive experiment/possible solution is a stem with an oversized extension or a steel extension. Steel will be stiffer than aluminum for a given diameter hence the greater stiffness.

Actually, I just got a great idea for another test, and it's just a test.

If your stem's extension is hollow and you can access it from either the steerer or handlebar end, remove the stem and fill it with lead shot. Don't panic; I'm not suggesting it as a solution but to prove whether it's a resonance or vibration from the cockpit that's setting up the shimmy. You can buy the lead shot cheap at a fishing tackle shop or gun shop.

[Rusty Luggs]

..... and file the steerer tube top closer to flat at the same time. There's a fat millimeter of space showing around half its circumference.


????

millimeter of space between what and what?

[estilley]

A guy racing in the same crit I was last week had a wobble on lap 1. On lap 8 his spokes blew.

Luckily everyone was fine.


Sent from my iPhone using Tapatalk

[Fatty]

- Ran two sets of wheels with same results: a Shimano RS 10 I had from new with >2000mi--no past mishaps with those. A brand new set of Dura Ace C24 clinchers. Same results with both wheelsets.


You are trying different tires on the other wheelsets?

[Bradford]

What was the fix?

I replaced the glue-in plug with a compression plug, re-tightened the cap and stem, and have never had the wobble again.

[Ti Designs]

Speed wobble isn't black magic any you would be hard pressed to find any item on the bike that can generate that much lateral force, so changing your tires or tubes or shorts isn't going to work. It's some pretty basic physics. Any tube can be seen as a torsional spring. Take the tube by itself, hold it solidly on one side, attach a rod on the other end with a weight on the end of it, and you have a pendulum. The period (how long it takes for the weight to swing to the left, back to the right, and back to center) is determined by the length and stiffness of the tube, along with the weight and length of the rod. On the bike you have two tubes connecting the front to the rear, the top tube and the down tube. Take out either one and every frame has wobble, it's called simple harmonic motion. The question is how the harmonic of the top tube and the harmonic of the down tube add or cancel. Given that the tubes intersect at an angle, it's pretty rare to find a bike where they don't cancel at some point, the question is if it's in the normal parameter range. The parameters in question are the restoring constants applied at either end of the tube. Tires on the ground are the best example, which is why a loose wheel bearing can make a speed wobble seem worse. Rider's weight on the saddle and their hands on the bars are the other factors.

A few years back I came across a frame (which I had just ordered for a customer) with a serious speed wobble. Having access to both physics students, labs and machine shops, I came up with a way to control the torsion of a tube. We put stand-off cable guides on a tube, then we ran a boron cable around the tube at a 60 degree spiral in both directions. When the cable was loose the tube's characteristics were unchanged. Adding tension to the cable increased the torsional strength of the tube.

[bluesea]

Won't get a chance to get at bike for a day or two. Will report back.

[Pierre]

Had an uncharacteristically busy today. Will try to address as many points as I can for now.

- Ran two sets of wheels with same results: a Shimano RS 10 I had from new with >2000mi--no past mishaps with those. A brand new set of Dura Ace C24 clinchers. Same results with both wheelsets.

- Have checked my compression plug several times, using carbon paste for the last few rides. I'll try a different one, and file the steerer tube top closer to flat at the same time. There's a fat millimeter of space showing around half its circumference.

- The 130 stem could be contributing a negative effect, although I've been using 130/140 stems for quite a while on a number of different bikes with no issues. All frames are different of course.

- I ride a one size smaller frame for my height, mainly due to a short inseam and long torso. Seatpost is a Fizik Cyrano R3 w/25mm setback. Saddle center is positioned aft of center of seatpost, although I ride further forward than most.

- Frame/fork geometry: Not 100% sure of either. It steers a bit different from what I'm used to--something like a Colnago, but not really. It rides like a high trail frontend.

Love the ride otherwise.

Can you confirm that you are able to control the wobble? If so, keep testing...if not please be cautious!!!

I'm not convinced it's the stem and I'm concerned that it's something more nefarious (i.e. loose fork drop out or something). Having said this, switching out the stem is probably the easiest thing to do next I suppose. I'm assuming that you've checked the tension on each spoke and verified that hub/axle are tight and smooth, right?

[David Kirk]

The Ti man hit the nail on the head above - speed wobble is the frame winding up torsionally and then springing back and setting up a wobble.

Typically it starts when something small bumps the bike off line....a gust of wind, a bump in the road...etc. This causes the frame to twist ever so slightly and the frame becomes a spring that then returns to its unloaded position and of course overshoots it and goes past center. And here's the kicker - when it does return and the frame suddenly stops twisting one way and is heading back in the other direction the weight of the stem, bars, levers...etc (all far ahead of the center of rotation of the headset) will continue to swing just a bit....this in turn makes the bike steer slightly to the side....and then the frame whips back the other way and with it the bar/stem/levers and they then steer the other way. This is "speed wobble."

A few things about wobble -

- all bikes can speed wobble. Every single bike ever made can be made to do it. Zero exceptions. I'm sure there are a lot of folks reading this and thinking "BS - my bike won't"....but I'm here to tell you that I can get on it and make it wobble every time. Fly me to your house and I'll be happy to prove it :)

- big bikes are more likely to wobble. The longer main tubes will be softer in torsion than the same tubes in a shorter frame and therefore they are more prone to wobble.

- bikes with smaller main tubes are more likely to wobble because the tubes are not as stiff in torsion and it's easier for the cycle to set up.

- longer stems contribute to the likelihood of wobble. A long stem moves the weight of the bars and levers further away from the center of steering rotation and makes the bike more likely to steer to the side as the wobble cycles from one side to the other.

- heavy levers make a bike more likely to wobble. Back in the day, when STi was new we suddenly saw a rash of bikes of all brands wobble. It was the added weight of the levers so far from the center of rotation and make it more likely to steer.

- alignment is almost never the cause of wobble.....in fact the closer the alignment is to perfect the more likely it is to wobble. It sounds crazy until you think of it. If the bike is so out of line that it pulls hard to the left all the time it's very hard to get a cycle to set up where it will want to go right on its own. So rather than going right-left-right-left it just goes left. One can test this by purposely misaligning a bike by cocking the wheels in the frame or fork. Cock the wheels in there so it pulls hard to one side and it won't wobble. Not exactly a fix but certainly interesting.




So....if all bikes can wobble how can the rider prevent it from happening or deal with it on the very rare occasion when it does happen? The most surefire way to stop it NOW is to lift your weight out of the saddle. It can take some large balls to do this but it works every time. The reason is that the twisting of the main triangle needs something to twist against and the rider's weight in the saddle is just that. Unweight the saddle and it will stop in one cycle as there is nothing to work against.

The rider can also fight instinct and keep a loose hold on the bars. The stiffer the arms and the harder one holds on the more the cycle can work against the rider's body weight. A loose hold damps the cycling and tight one helps it. This is one of the reasons that riders see more wobble when it's really cold...their bodies are stiff in the cold and the bike more prone to wobble. I once read a thing online by a "top builder" that said bike are more prone to wobble in the cold not because the rider is stiff but because it changes the stiffness of the metal tubing. Uhmm....no.

Run lower front tire pressure. A lower pressure give s larger foot print and more twisting friction when steering and this can help damp the system.

I spent countless hours studying this issue during my Serotta days and it was my job to take bikes to the top of the hill near the factory and see if i could get them to wobble and figure out how to change the design so that they would't. it was during this time that I found I could make every bike wobble....especially if the bike was dead straight. It's interesting stuff and when one can set up a safe low speed wobble and just sit and observe it it becomes fairly obvious what is happening mechanically.

One last thing (finally I know)....you don't need to dig too far online to find people that will say that they had a bike wobble once and they made a change and it never did it again. This is a red herring at best. If the guy rode the bike for years and thousands of miles and it only wobbled the one time how does he know if he fixed it? He doesn't. He could have let it alone and it would have gone years before it did it again. It's just takes the right circumstances and every single bike ever made will wobble. So throwing salt over your shoulder of putting pins a doll won't fix it.

Back to sanding stainless for me.

dave

[eddief]

and based on it one would think your manhood is (or used to be) strongly intact :).

The Ti man hit the nail on the head above - speed wobble is the frame winding up torsionally and then springing back and setting up a wobble.

Typically it starts when something small bumps the bike off line....a gust of wind, a bump in the road...etc. This causes the frame to twist ever so slightly and the frame becomes a spring that then returns to its unloaded position and of course overshoots it and goes past center. And here's the kicker - when it does return and the frame suddenly stops twisting one way and is heading back in the other direction the weight of the stem, bars, levers...etc (all far ahead of the center of rotation of the headset) will continue to swing just a bit....this in turn makes the bike steer slightly to the side....and then the frame whips back the other way and with it the bar/stem/levers and they then steer the other way. This is "speed wobble."

A few things about wobble -

- all bikes can speed wobble. Every single bike ever made can be made to do it. Zero exceptions. I'm sure there are a lot of folks reading this and thinking "BS - my bike won't"....but I'm here to tell you that I can get on it and make it wobble every time. Fly me to your house and I'll be happy to prove it :)

- big bikes are more likely to wobble. The longer main tubes will be softer in torsion than the same tubes in a shorter frame and therefore they are more prone to wobble.

- bikes with smaller main tubes are more likely to wobble because the tubes are not as stiff in torsion and it's easier for the cycle to set up.

- longer stems contribute to the likelihood of wobble. A long stem moves the weight of the bars and levers further away from the center of steering rotation and makes the bike more likely to steer to the side as the wobble cycles from one side to the other.

- heavy levers make a bike more likely to wobble. Back in the day, when STi was new we suddenly saw a rash of bikes of all brands wobble. It was the added weight of the levers so far from the center of rotation and make it more likely to steer.

- alignment is almost never the cause of wobble.....in fact the closer the alignment is to perfect the more likely it is to wobble. It sounds crazy until you think of it. If the bike is so out of line that it pulls hard to the left all the time it's very hard to get a cycle to set up where it will want to go right on its own. So rather than going right-left-right-left it just goes left. One can test this by purposely misaligning a bike by cocking the wheels in the frame or fork. Cock the wheels in there so it pulls hard to one side and it won't wobble. Not exactly a fix but certainly interesting.




So....if all bikes can wobble how can the rider prevent it from happening or deal with it on the very rare occasion when it does happen? The most surefire way to stop it NOW is to lift your weight out of the saddle. It can take some large balls to do this but it works every time. The reason is that the twisting of the main triangle needs something to twist against and the rider's weight in the saddle is just that. Unweight the saddle and it will stop in one cycle as there is nothing to work against.

The rider can also fight instinct and keep a loose hold on the bars. The stiffer the arms and the harder one holds on the more the cycle can work against the rider's body weight. A loose hold damps the cycling and tight one helps it. This is one of the reasons that riders see more wobble when it's really cold...their bodies are stiff in the cold and the bike more prone to wobble. I once read a thing online by a "top builder" that said bike are more prone to wobble in the cold not because the rider is stiff but because it changes the stiffness of the metal tubing. Uhmm....no.

Run lower front tire pressure. A lower pressure give s larger foot print and more twisting friction when steering and this can help damp the system.

I spent countless hours studying this issue during my Serotta days and it was my job to take bikes to the top of the hill near the factory and see if i could get them to wobble and figure out how to change the design so that they would't. it was during this time that I found I could make every bike wobble....especially if the bike was dead straight. It's interesting stuff and when one can set up a safe low speed wobble and just sit and observe it it becomes fairly obvious what is happening mechanically.

One last thing (finally I know)....you don't need to dig too far online to find people that will say that they had a bike wobble once and they made a change and it never did it again. This is a red herring at best. If the guy rode the bike for years and thousands of miles and it only wobbled the one time how does he know if he fixed it? He doesn't. He could have let it alone and it would have gone years before it did it again. It's just takes the right circumstances and every single bike ever made will wobble. So throwing salt over your shoulder of putting pins a doll won't fix it.

Back to sanding stainless for me.

dave

[Ti Designs]

I spent countless hours studying this issue during my Serotta days and it was my job to take bikes to the top of the hill near the factory and see if i could get them to wobble and figure out how to change the design so that they would't. it was during this time that I found I could make every bike wobble....especially if the bike was dead straight. It's interesting stuff and when one can set up a safe low speed wobble and just sit and observe it it becomes fairly obvious what is happening mechanically.

To think I wasted time modeling a bike frame on a computer when I could have been out riding...

Yes, you can cause any frame to wobble, because tubes have torsion. The problem frames are the ones where the two tubes set up waves that are around the same frequency, so the amplitude (how far the top of the head tube swings) is the sum of the two waves. The best case is where the down tube's resonance is a multiple of the top tube's, so they will always get to a point where they cancel.

[MikeD]

David Kirk, can you comment on the influence of fork trail on shimmy?

[chrisroph]

I bought a used legend once. The guy who sold it to me (second owner, affiliated with a serotta shop) in the spirit of full disclosure told me the guy who originally bought it said it wobbled horribly. He did some sort of return. The guy I bought it from rode it for a while and it never wobbled. I thought it was as solid as a rock, beautifully built and perfectly aligned. I did let it go at a time where I had too many bikes but it never wobbled.

[redir]

All my bikes high speed wobble except for my Moots and I always chalked that up to it being a compact frame. My bikes are big frames and I think big frame bikes have a tendency to wobble. Yours is not big so I don't know but I'm also surprised that a knee to the TT doesn't stop it, that leads me to believe it's something mechanical and not inherent to the frame.

[David Kirk]

David Kirk, can you comment on the influence of fork trail on shimmy?

Back in the Serotta days we'd have a person get a bike and they would complain that it wobbled so we would have them send the entire assembled bike back to us to test ride just as he owner rode it. It was extremely rare for the bike that wobbled under the customer would so so under me unless I provoked it. It was during this time that I learned I could make any bike wobble if I wanted using a rather foolish method.

We really didn't fully understand the issue at the time and we tried all kind of things to try to fix the problem. One of the obvious ones was to change the fork rake to give more or less trail to see how that influenced things. This was back when all the forks were steel so I would re-rake the fork and then retest. Simple stuff.

This was a very long time ago and the details are fuzzy but I don't recall ever "fixing" a bike by changing the rake/trail. It would wobble just the same (either on it's own or with provocation) but .... it would be more prone to wobbling at either higher or lower speeds. More rake (less trail) meant it would be more prone to acting up at lower speeds and visa versa. So the take-away was that the trail wasn't the cause or the fix but it did influence the speed at which it was most prone to wobble.




One last thing that might be hard to explain but here goes......if I ride a bike and induce wobble one of two things will happen -

- the wobble will set up and last for a short period of time before it damps itself out and reverts to being stable. This is a good thing.

- the wobble will set up and gradually build in amplitude becoming progressively more violent with each cycle until I stop it or it throws me to the pavement. This is not good.

I suspect that the results of the modeling that Ed talked about above is behind this. If the resonant frequency is just right between the top and down tubes it will progress and try to throw you off....if not it will damp itself out and end the cycling.

dave

[Brian Smith]

(message snipped)
Back in the Serotta days we'd have a person get a bike and they would complain that it wobbled so we would have them send the entire assembled bike back to us to test ride just as he owner rode it. It was extremely rare for the bike that wobbled under the customer would so so under me unless I provoked it. It was during this time that I learned I could make any bike wobble if I wanted using a rather foolish method.

We really didn't fully understand the issue at the time and we tried all kind of things to try to fix the problem. One of the obvious ones was to change the fork rake to give more or less trail to see how that influenced things. This was back when all the forks were steel so I would re-rake the fork and then retest. Simple stuff.


One last thing that might be hard to explain but here goes......if I ride a bike and induce wobble one of two things will happen -

- the wobble will set up and last for a short period of time before it damps itself out and reverts to being stable. This is a good thing.

- the wobble will set up and gradually build in amplitude becoming progressively more violent with each cycle until I stop it or it throws me to the pavement. This is not good.


dave

This type of diagnostic activity continued well after Dave's tenure at Serotta, and more information was gathered, but with essentially similar results.

I can add a modicum of information to Dave's fine summary, in that even in the case of underdamped oscillation, the building amplitude does largely reach a maximal level, at which point the grotesque behavior ceases to become "more violent," although it is not a steady state that most riders would deem to be an acceptable system tuning outcome. The spring(s) in the system do have a positive rate, and the system reaches a point where the forces producing the displacement grow less rapidly than the effective spring rate resisting them. If the rider can tolerate the system acting this way, unfavorable though it may be, they can at least be advised that the unfavorable condition won't continue worsening until they are thrown to the ground. In practical terms, amplitudes at this point can easily exceed 3" of displacement at the top of the stem/steerer, whereas even only 1" is enough to shock most riders not anticipating the shimmy behavior.

[Bwana]

Back in the Serotta days we'd have a person get a bike and they would complain that it wobbled so we would have them send the entire assembled bike back to us to test ride just as he owner rode it. It was extremely rare for the bike that wobbled under the customer would so so under me unless I provoked it. It was during this time that I learned I could make any bike wobble if I wanted using a rather foolish method.

We really didn't fully understand the issue at the time and we tried all kind of things to try to fix the problem. One of the obvious ones was to change the fork rake to give more or less trail to see how that influenced things. This was back when all the forks were steel so I would re-rake the fork and then retest. Simple stuff.

This was a very long time ago and the details are fuzzy but I don't recall ever "fixing" a bike by changing the rake/trail. It would wobble just the same (either on it's own or with provocation) but .... it would be more prone to wobbling at either higher or lower speeds. More rake (less trail) meant it would be more prone to acting up at lower speeds and visa versa. So the take-away was that the trail wasn't the cause or the fix but it did influence the speed at which it was most prone to wobble.




One last thing that might be hard to explain but here goes......if I ride a bike and induce wobble one of two things will happen -

- the wobble will set up and last for a short period of time before it damps itself out and reverts to being stable. This is a good thing.

- the wobble will set up and gradually build in amplitude becoming progressively more violent with each cycle until I stop it or it throws me to the pavement. This is not good.

I suspect that the results of the modeling that Ed talked about above is behind this. If the resonant frequency is just right between the top and down tubes it will progress and try to throw you off....if not it will damp itself out and end the cycling.

dave


So how exactly do you MAKE them wobble?

Maybe explaining the process will show bluesea (and others that have wobble problems) that they might be unintentionally contributing/initiating the wobble in the first place.

[David Kirk]

So how exactly do you MAKE them wobble?

Maybe explaining the process will show bluesea (and others that have wobble problems) that they might be unintentionally contributing/initiating the wobble in the first place.


No one would do this unintentionally thereby setting up a wobble that much is for sure.

A wobble is the bike twisting one way and steering the other so to get that to happen I ride no-handed at about 20 mph down a gentle grade and I take my fist and I give the rearmost part of the stem that attaches to the steerer a firm back to the side. This twists the head tube relative to the seat tube and at the same time makes the bike steer to the side because the weight of the bar/levers in front of the steerer have no urge to follow the head tube. DO NOT hit the stem forward of the steerer. This is bad.

So....the bike twists one way and steers the other. After you whack it the frame will rebound in the other direction and when it gets to the end of that rebound the stem will swing in the other direction......and now we have wobble or shimmy.

Caveat - you need to hit it pretty hard for anything to happen. A light tap won't result in much. Secondly.....if you try this and end up on the floor it's not my fault and I'll tell your lawyer to pound sand. Be careful. It can result in your regretting ever doing it.

But with all that out of the way I've never ridden a bike that won't wobble under these conditions. Some will cycle half heartedly 4-5 times and damp right out some will progressively build to the point where if you don't take corrective action (grab the bars my friend) you will certainly crash.

I hope this is if nothing else informative.

dave

[572cv]

.......
We really didn't fully understand the issue at the time and we tried all kind of things to try to fix the problem. One of the obvious ones was to change the fork rake to give more or less trail to see how that influenced things. ........

dave

Following on this, it would seem to be consistent that if a fork one was using had say, a rake of 43, and the frame was designed for a fork with a rake of 50, that this would lead to some level of instability, yes?

[David Kirk]

Following on this, it would seem to be consistent that if a fork one was using had say, a rake of 43, and the frame was designed for a fork with a rake of 50, that this would lead to some level of instability, yes?

Hmmmm?

Trail is the number that we are looking for here and the amount of trail will largely contribute to the way the bike steers and its stability.

If the frame was designed for a 50 mm fork one would assume they did a good job (not a safe assumption btw) of combining this rake with the head angle of the frame to give a trail number of say about 55 - 60 mm for a road bike. If you then put a 43 mm fork in it that will increase the trail substantially. How far? it depends on the head angle. But if you had about 57 mm of trail with the 50 mm fork you'd have something like 68 (rough guess) and this would be too stable for most road bikes.

Too stable? How can that be? Well it becomes so hard and unintuitive to steer that the bike will only want to go straight and that is it's own type of instability.

-----------------

Your question and my response have little to nothing to do with the topic at hand - wobble. It's a separate deal.

dave

[benb]

If the same bike has a problem/no problem with two different riders then the fit must impact things.

One of my current bikes will be more wobble prone with a longer stem. Not really an issue for me as the longer stem didn't fit well but it sure was interesting, particularly with the "longer stems are always more stable" stuff that goes around. It's rock solid with stems that are unfashionably short.

Pretty great info in this thread. Seeing what Dave is doing for testing makes a ton of sense. Most of the times I've experienced temporary wobbles involve hitting something in the road just right in a way that must induce a similar force through the steering axis. I've seen it on probably every bike I've ever owned, including motorcycles. None of them have ever had a "resonant" wobble that would get worse, but I've also always known about relaxing and/or unweighting the saddle.

For my road bikes it seems like they've gotten more resistant/stable as time has gone on. I think the most wobble prone bike I had was my first one, an aluminum Trek I got in 2000.

[572cv]

Hmmmm?

Trail is the number that we are looking for here and the amount of trail will largely contribute to the way the bike steers and its stability.

If the frame was designed for a 50 mm fork one would assume they did a good job (not a safe assumption btw) of combining this rake with the head angle of the frame to give a trail number of say about 55 - 60 mm for a road bike. If you then put a 43 mm fork in it that will increase the trail substantially. How far? it depends on the head angle. But if you had about 57 mm of trail with the 50 mm fork you'd have something like 68 (rough guess) and this would be too stable for most road bikes.

Too stable? How can that be? Well it becomes so hard and unintuitive to steer that the bike will only want to go straight and that is it's own type of instability.

-----------------

Your question and my response have little to nothing to do with the topic at hand - wobble. It's a separate deal.

dave

My question betrays a certain conflation of terminology, sorry for the confusion. Thank you for the response, which, unlike the question, is clear!

So that makes sense, and was consistent with the behavior with the old fork. I have a lot to learn/internalize on this stuff.

[oliver]

You can find out more from here once the embargo is lifted: http://repository.tudelft.nl/islandora/object/uuid:a98d51c1-7754-4c29-b883-f130ba05136b?collection=education

[oliver]

Also Cane Creek came out with an anti-wobble headset somewhat recently that e-bike manufacturers requested. However, I haven't heard anything about how well it actually works.

https://www.canecreek.com/products/headsets/viscoset

[Rusty Luggs]

Back in the Serotta days we'd have a person get a bike and they would complain that it wobbled so we would have them send the entire assembled bike back to us to test ride just as he owner rode it. It was extremely rare for the bike that wobbled under the customer would so so under me unless I provoked it. It was during this time that I learned I could make any bike wobble if I wanted using a rather foolish method.

We really didn't fully understand the issue at the time and we tried all kind of things to try to fix the problem. One of the obvious ones was to change the fork rake to give more or less trail to see how that influenced things. This was back when all the forks were steel so I would re-rake the fork and then retest. Simple stuff.

This was a very long time ago and the details are fuzzy but I don't recall ever "fixing" a bike by changing the rake/trail. It would wobble just the same (either on it's own or with provocation) but .... it would be more prone to wobbling at either higher or lower speeds. More rake (less trail) meant it would be more prone to acting up at lower speeds and visa versa. So the take-away was that the trail wasn't the cause or the fix but it did influence the speed at which it was most prone to wobble.




One last thing that might be hard to explain but here goes......if I ride a bike and induce wobble one of two things will happen -

- the wobble will set up and last for a short period of time before it damps itself out and reverts to being stable. This is a good thing.

- the wobble will set up and gradually build in amplitude becoming progressively more violent with each cycle until I stop it or it throws me to the pavement. This is not good.

I suspect that the results of the modeling that Ed talked about above is behind this. If the resonant frequency is just right between the top and down tubes it will progress and try to throw you off....if not it will damp itself out and end the cycling.

dave

I'm not so sure that total stiffness / relative stiffness top/down tubes really encompasses why a given bike will wobble for one rider and not for another. (Wobble in the sense of increasing magnitude oscillation, it is pretty much a given that it will oscillate"at all" if deflected since there is a moment of inertia around steering axis, centering torque provided by trail, and little damping, hence "any bike will wobble")

I suspect it may include weight distribution/front wheel load, and/or, the rider's impact on damping. If you watch motorcycle road racing, you will often see wobble during acceleration off a corner when the front end is getting light (even having steering dampers).

Damping-wise, the mechanical parts of the bike that contribute are friction of headset, and the damping effects of the tire at road surface (friction/hysteresis). I think the lowering the tire pressure suggestion is a good one that is often overlooked. Impacts both damping and pneumatic trail effect of contact patch.

Also, some years ago I did see a video demonstration of motorcycle wobble being halted by the rider getting out of the saddle.

[David Kirk]

If the same bike has a problem/no problem with two different riders then the fit must impact things.

One of my current bikes will be more wobble prone with a longer stem. Not really an issue for me as the longer stem didn't fit well but it sure was interesting, particularly with the "longer stems are always more stable" stuff that goes around. It's rock solid with stems that are unfashionably short.

Pretty great info in this thread. Seeing what Dave is doing for testing makes a ton of sense. Most of the times I've experienced temporary wobbles involve hitting something in the road just right in a way that must induce a similar force through the steering axis. I've seen it on probably every bike I've ever owned, including motorcycles. None of them have ever had a "resonant" wobble that would get worse, but I've also always known about relaxing and/or unweighting the saddle.

For my road bikes it seems like they've gotten more resistant/stable as time has gone on. I think the most wobble prone bike I had was my first one, an aluminum Trek I got in 2000.


Yes fit has an effect as do other things.

If you place the rider in a very upright position is moves weight off the front wheel and the trail has less weight pushing down on it making the bike more prone to wobble. This is one of the reasons why wobble so often hits when riding no handed or sitting very upright.

The other thing that has a huge contributing factor is what the rider does. Does the rider sit upright with an arch in their back with locked elbows and a white knuckle grip? If so the chance of wobble goes way up. Are they relaxed and soft on the bike? Then chances of trouble are pretty low.

Typically people say a given bike has a problem and in some cases that is the deal....in many cases the bike is merely prone to wobble and the one rider will have real issues and another will find it perfect.

dave

[David Kirk]

I'm not so sure that total stiffness / relative stiffness top/down tubes really encompasses why a given bike will wobble for one rider and not for another. (Wobble in the sense of increasing magnitude oscillation, it is pretty much a given that it will oscillate"at all" if deflected since there is a moment of inertia around steering axis, centering torque provided by trail, and little damping, hence "any bike will wobble")

I suspect it may include weight distribution/front wheel load, and/or, the rider's impact on damping. If you watch motorcycle road racing, you will often see wobble during acceleration off a corner when the front end is getting light (even having steering dampers).

Damping-wise, the mechanical parts of the bike that contribute are friction of headset, and the damping effects of the tire at road surface (friction/hysteresis). I think the lowering the tire pressure suggestion is a good one that is often overlooked. Impacts both damping and pneumatic trail effect of contact patch.

Also, some years ago I did see a video demonstration of motorcycle wobble being halted by the rider getting out of the saddle.

I agree - the rider fit and actions are one of the biggest deals. If he shifts his weight up and back (upslope stem, lots of spacers...etc) and rides with a death grip they are taking weight off the front wheel and not helping damp the system.

dave

[benb]

Isn't the death grip + locked out arms with lots of weight on them going to be even worse? (Within reason)

It seems like either case can be an issue.. with motos it's definitely the weight coming off the wheel as the bike tries to lift the wheel.. you get the shake when the front wheel gets light at the same time it hit something a little off center. You can kind of feel that on "comfort bikes" that have the bars jacked up to the sky and no weight on the front wheel too.

But flip that around and put too much weight on the front wheel your arms are going to tense up and you're not going to be relaxed enough to be any kind of damper.

I can really feel the negative effect on handling if I'm off balance to the front and using my arms to hold myself up. And little changes seem to have a big effect.

[David Kirk]

Isn't the death grip + locked out arms with lots of weight on them going to be even worse? (Within reason)

It seems like either case can be an issue.. with motos it's definitely the weight coming off the wheel as the bike tries to lift the wheel.. you get the shake when the front wheel gets light at the same time it hit something a little off center. You can kind of feel that on "comfort bikes" that have the bars jacked up to the sky and no weight on the front wheel too.

But flip that around and put too much weight on the front wheel your arms are going to tense up and you're not going to be relaxed enough to be any kind of damper.

I can really feel the negative effect on handling if I'm off balance to the front and using my arms to hold myself up. And little changes seem to have a big effect.

Yes - that is what I tried to, and intended to, say.

dave

[Peter P.]

Here's on example from youtube. Search "bicycle shimmy wobble" for more.

www.youtube.com/embed/xODNzyUbIHo

P.S. What's the trick to embed youtube videos in posts?

[Ti Designs]

No one would do this unintentionally thereby setting up a wobble that much is for sure.

A wobble is the bike twisting one way and steering the other so to get that to happen I ride no-handed at about 20 mph down a gentle grade and I take my fist and I give the rearmost part of the stem that attaches to the steerer a firm back to the side. This twists the head tube relative to the seat tube and at the same time makes the bike steer to the side because the weight of the bar/levers in front of the steerer have no urge to follow the head tube. DO NOT hit the stem forward of the steerer. This is bad.

So....the bike twists one way and steers the other. After you whack it the frame will rebound in the other direction and when it gets to the end of that rebound the stem will swing in the other direction......and now we have wobble or shimmy.

Caveat - you need to hit it pretty hard for anything to happen. A light tap won't result in much. Secondly.....if you try this and end up on the floor it's not my fault and I'll tell your lawyer to pound sand. Be careful. It can result in your regretting ever doing it.



I love this! We have lots of people spending time on trainers over the winter, learning how to produce power by pulling on parts of the bike in directions and ways that would cause road rash if done on the road, and we have Dave spending time on the road trying to simulate learned bad habits...

I'm not saying that speed wobble is caused by the rider, it's still the sum of the harmonics between the top tube and down tube. As Dave found, wobble can be induced, either his way or something as simple as hitting a small rock which moves the front wheel over a fraction of an inch. Some riders have all kinds of problems with speed wobble, others on the same bike don't.

My solution is rollers - learn how to ride them. If you can ride rollers as relaxed as you ride down the road, and you take up maybe 3" of the roller (not wobbling around from side to side), and your bike has a speed wobble, blame the bike. If not, you have two possible causes, and one of them is you.

[jamesau]

Knees on toptube didn't seem to help...

I sometimes press one knee quite firmly against the top tube while descending a non-smooth road; do this before the onset of wobble. You might also experiment with moving the position of the knee further forward or back along the top tube (to whatever extent is possible/reasonably comfortable). A light but firm grip on bars and relaxed shoulders also seems to help. The idea is to dampen out those frequencies which might otherwise make the bike 'ring'.

This is great discussion.

My one memorable episode of high speed sustained (harmonic) wobble was aboard my 59cm Litespeed. It was on a straight and fast descent during the time of year when the walnut trees were shedding; my front wheel made a glancing impact on a walnut and then all mayhem ensued. Scared the crap out of me :eek:!

[Rusty Luggs]

I love this! We have lots of people spending time on trainers over the winter, learning how to produce power by pulling on parts of the bike in directions and ways that would cause road rash if done on the road, and we have Dave spending time on the road trying to simulate learned bad habits...

I'm not saying that speed wobble is caused by the rider, it's still the sum of the harmonics between the top tube and down tube. As Dave found, wobble can be induced, either his way or something as simple as hitting a small rock which moves the front wheel over a fraction of an inch. Some riders have all kinds of problems with speed wobble, others on the same bike don't.

My solution is rollers - learn how to ride them. If you can ride rollers as relaxed as you ride down the road, and you take up maybe 3" of the roller (not wobbling around from side to side), and your bike has a speed wobble, blame the bike. If not, you have two possible causes, and one of them is you.

IMO a rider's ability to ride a straight line has nothing whatsoever to do with the wobble phenomenon being discussed in this thread.

Most often occurs coasting downhill in a straight line with no rider steering input at all (no sloppy pedaling forces or yanking on bars), first deflecting force can be a tiny bump in the road or a little gust of wind. Dave Kirk's method of invoking wobble is essentially like hitting a bump in the road. And remember, he is doing it without his hands even being on the bars. Not emulating some bad riding habit.

Also, I don't agree that frame flexure harmonics are the only potential source of force that causes self amplification. Could also be caused by something as subtle as aerodynamic forces on the wheel changing as angle of wheel changes through the motion, causing a torque.

[William]

So....if all bikes can wobble how can the rider prevent it from happening or deal with it on the very rare occasion when it does happen? The most surefire way to stop it NOW is to lift your weight out of the saddle. It can take some large balls to do this but it works every time. The reason is that the twisting of the main triangle needs something to twist against and the rider's weight in the saddle is just that. Unweight the saddle and it will stop in one cycle as there is nothing to work against.

The rider can also fight instinct and keep a loose hold on the bars. The stiffer the arms and the harder one holds on the more the cycle can work against the rider's body weight. A loose hold damps the cycling and tight one helps it. This is one of the reasons that riders see more wobble when it's really cold...their bodies are stiff in the cold and the bike more prone to wobble. I once read a thing online by a "top builder" that said bike are more prone to wobble in the cold not because the rider is stiff but because it changes the stiffness of the metal tubing. Uhmm....no.

Run lower front tire pressure. A lower pressure give s larger foot print and more twisting friction when steering and this can help damp the system.

I spent countless hours studying this issue during my Serotta days and it was my job to take bikes to the top of the hill near the factory and see if i could get them to wobble and figure out how to change the design so that they would't. it was during this time that I found I could make every bike wobble....especially if the bike was dead straight. It's interesting stuff and when one can set up a safe low speed wobble and just sit and observe it it becomes fairly obvious what is happening mechanically.

One last thing (finally I know)....you don't need to dig too far online to find people that will say that they had a bike wobble once and they made a change and it never did it again. This is a red herring at best. If the guy rode the bike for years and thousands of miles and it only wobbled the one time how does he know if he fixed it? He doesn't. He could have let it alone and it would have gone years before it did it again. It's just takes the right circumstances and every single bike ever made will wobble. So throwing salt over your shoulder of putting pins a doll won't fix it.

Back to sanding stainless for me.

dave

Good explanation Dave.
This topic came up a couple of years ago and this is pretty much what I said but maybe explained slightly differently. Put your pedals at 12/6 and weight the bottom pedal, this unweights the saddle and sets your COG lower and also centers your weight more on the bike. You can even put a little pressure against to TT by shifting your weight. I ride big frames and I love to descend at mach 3. Early on I had a frame that would wobble until I started doing this and focusing on staying loose, I've never had an issue since on any bike that I ride.






William

[stien]

Also when it's cold you might be shivering to keep warm. I've felt those high frequency waves shake the whole bike, handling, everything. Not awesome going down hill and it happens mostly in the wet, even worse!

[NHAero]

William, not invalidating your experience at all, but I think that whether your body weight is supported primarily at the pedals or on the saddle isn't affecting the COG of the rider/bike system. After reading Dave's explanation, it seems to me that the effect that lifting your butt off the saddle has is to change the mass and solidity the torsional spring of the frame is acting upon, which stops the harmonics quickly.

Good explanation Dave.
This topic came up a couple of years ago and this is pretty much what I said but maybe explained slightly differently. Put your pedals at 12/6 and weight the bottom pedal, this unweights the saddle and sets your COG lower and also centers your weight more on the bike. You can even put a little pressure against to TT by shifting your weight. I ride big frames and I love to descend at mach 3. Early on I had a frame that would wobble until I started doing this and focusing on staying loose, I've never had an issue since on any bike that I ride.






William

[MikeD]

This is what Dave Moulton said in bikeforums.net 10/17/04:

Shimmy is usually caused by not having enough trail. To explain trail for those who don’t know: If you draw a line through the center of your head tube and therefore the steering column, that straight line will reach the ground at a point (Point A.) ahead of the point where the wheel contacts the ground (Point B.)

I always built my bikes with at least 2 ½ inches of trail. Trail is common to all wheeled vehicles, cars and even a shopping cart will have it. If you make the head angle steeper it means less trail because you move point A closer to point B. Also if you increase the fork rake you make for less trail; in this case point B moves closer to point A. The worst scenario is a bike with a steep head angle and a long fork rake; trail can be reduced to almost zero.

Trail keeps the bike going in a straight line, and also assists a two wheeled vehicle in its self steering abilities. As you lean to the left, point A moves to the left and the wheel Pivoting on point B will turn to the left. The gyroscopic action of the spinning wheel also plays a big role in self steering, but this is another subject and I only mention it because the heavier the spinning wheel, the more it keeps straight. Road bikes with ultra light wheels and tires are more sensitive to small changes in the amount of trail.

What happens in a high speed downhill shimmy the wheel is turned one way or the other by a bump in the road or a gust of wind. (Like when swinging out of a pace line.) The caster action of the trail corrects this, but if there is not enough trail it will over correct and then correct again starting the wheel fluttering back and forth. You can see exactly the same thing on a shopping cart if you run with it across the parking lot the caster wheels will flutter back and forth in the same way.

Large frames are more prone to shimmy for two reasons. Large frames are taller and also should be proportionately longer, but there is a school of thought that believes a race bike should have a short wheelbase, so the builder makes the head angle steeper to shorten the wheelbase, but in doing so lessens the amount of trail. Large frames also tend to have shallower seat angles to accommodate the rider’s longer legs therefore the riders weight is more over the rear wheel.

Any vehicle that has its weight towards the rear is less stable; ask anyone who has driven an old VW bus in a cross wind. So if you are a tall person with a large bike frame, try to keep your weight forward when descending. Also keep your body in a low aerodynamic tuck; if you sit up wind pressure on you chest will push more weight towards the back wheel. Finally if you should get into a high speed shimmy; try not to panic, grip the top tube between your knees, and apply the rear brake first very gently and only apply the front brake after you have come out of the shimmy .

[benb]

Here's on example from youtube. Search "bicycle shimmy wobble" for more.

www.youtube.com/embed/xODNzyUbIHo

P.S. What's the trick to embed youtube videos in posts?

Heh.. how many people here would actually have the guts to do this and let the shimmy go on that long!

I don't think I do! :eek:

[William]

William, not invalidating your experience at all, but I think that whether your body weight is supported primarily at the pedals or on the saddle isn't affecting the COG of the rider/bike system. After reading Dave's explanation, it seems to me that the effect that lifting your butt off the saddle has is to change the mass and solidity the torsional spring of the frame is acting upon, which stops the harmonics quickly.

I'm not a physics professor so take what I say here with a grain of salt. If I weigh 250 pounds, my bike carries 250 pounds plus the weight of the bike itself. If I am sitting fully on the saddle, that is where the contact point is on the bike that carries the majority of my weight. If I stand up, I unweight the saddle and the pedals are where contact point is for the majority of my weight. If my pedals are at 12 & 6, 6 which is now the lowest point (besides the contact patch of the tires) is the contact point where the weight is concentrated. That move also moves my weight more to the center between the tires as opposed to being at the saddle and shifted more towards the back wheels. In that position, it's also very easy to lean the inside of the weighted leg against the top tube if needed.

How this effects harmonics? I'm not really sure, but I know it works and it makes sense to me at least. Sitting on the saddle and clamping your knees to the TT has always felt sketchy handling wise. As an aside, try riding your bike down a hill and unclip your feet from the pedals and let them hang. You will feel immediately how sketchy that feels from the saddle which will be your pivot point and where your weight is. Then try it standing on the pedals and unclip the 12 o'clock foot and see how stable it feels. Night and day difference.

William Physics 101 is now concluded. :)





William

[NHAero]

I'm not disagreeing at all with your conclusions about how you deal with the shimmy. However, the center of mass depends where the mass is, not where that mass is supported. Think of it this way:
Picture yourself on a bike, perfectly vertical, not moving, weight on the saddle. Now picture that you slowly lean to one side, such that you are leaning over at a 45 degree angle and a vertical cable attached to the top of your head holds you from falling over further. Now imagine that you don't move your body at all but your saddle magically disappears so that your weight is now supported on the pedals. The force on the cable hasn't changed at all, except by the absence of the saddle's weight.

I'm not a physics professor so take what I say here with a grain of salt. If I weigh 250 pounds, my bike carries 250 pounds plus the weight of the bike itself. If I am sitting fully on the saddle, that is where the contact point is on the bike that carries the majority of my weight. If I stand up, I unweight the saddle and the pedals are where contact point is for the majority of my weight. If my pedals are at 12 & 6, 6 which is now the lowest point (besides the contact patch of the tires) is the contact point where the weight is concentrated. That move also moves my weight more to the center between the tires as opposed to being at the saddle and shifted more towards the back wheels. In that position, it's also very easy to lean the inside of the weighted leg against the top tube if needed.

How this effects harmonics? I'm not really sure, but I know it works and it makes sense to me at least. Sitting on the saddle and clamping your knees to the TT has always felt sketchy handling wise. As an aside, try riding your bike down a hill and unclip your feet from the pedals and let them hang. You will feel immediately how sketchy that feels from the saddle which will be your pivot point and where your weight is. Then try it standing on the pedals and unclip the 12 o'clock foot and see how stable it feels. Night and day difference.

William Physics 101 is now concluded. :)





William

[William]

I'm not disagreeing at all with your conclusions about how you deal with the shimmy. However, the center of mass depends where the mass is, not where that mass is supported. Think of it this way:
Picture yourself on a bike, perfectly vertical, not moving, weight on the saddle. Now picture that you slowly lean to one side, such that you are leaning over at a 45 degree angle and a vertical cable attached to the top of your head holds you from falling over further. Now imagine that you don't move your body at all but your saddle magically disappears so that your weight is now supported on the pedals. The force on the cable hasn't changed at all, except by the absence of the saddle's weight.

It wouldn't at the cable thinking of this in a straight vertical line, but the pivot point would change on the bike and your weight would shift from closer to the rear wheel to being more centered in-between.. Remove the cable and view from the back of the bike using my example of sitting on the seat with your feet hanging vs standing on the pedals. The bike leans the same no matter which way you do it, but the transition will be more abrupt and less controllable with your weight planted higher then it will be with your weight on the pedals. Again, your weight transfer would shift as well to being more centered as opposed to being weighted toward the rear wheel. Again, inherently more controllable.



William

[NHAero]

It may well be more controllable, but not due to a shift in center of mass

It wouldn't at the cable thinking of this in a straight vertical line, but the pivot point would change on the bike and your weight would shift from closer to the rear wheel to being more centered in-between.. Remove the cable and view from the back of the bike using my example of sitting on the seat with your feet hanging vs standing on the pedals. The bike leans the same no matter which way you do it, but the transition will be more abrupt and less controllable with your weight planted higher then it will be with your weight on the pedals. Again, your weight transfer would shift as well to being more centered as opposed to being weighted toward the rear wheel. Again, inherently more controllable.



William

[William]

It may well be more controllable, but not due to a shift in center of mass

I may not be using the correct terminology to describe to describe the forces involved but I know it works. If you try the example of being on the saddle with your feet free vs standing on the pedal and maneuvering your bike you'll feel immediately which is more controllable...weight perched high or perched low? Weight toward the back or weight more centered. True that the overall weight won't change, but where it's located on the bike has an effect on control and handling.




William

[benb]

If you put the pedals at 12 and 6 and load the pedal at 6 you're putting a sideways load on the frame.

Given what Dave said before about frames that are out of alignment being resistant to shimmy it would make sense for weighting one pedal to work.. you're mostly likely twisting the frame slightly out of alignment and changing how it loads/unloads.

[David Kirk]

The center of mass is more-or-less at about your belly button and there is no practical way to get it to be lower on any given bike than it is when sitting in the saddle.

dave

[William]

So as an aside, can anyone explain how they think this will effect handling?

If I put a scale on the saddle and the pedals...

If I'm sitting on the saddle scale it will register the majority of my body weight there, very little at the pedals. If I stand up the weight on the saddle will go to zero and all of my body weight will register at the pedals. One high above the axle axis of the wheels, one below the axle axis of the wheels and more centered.


Seriously, shifting the weight has to have an effect.

If you put the pedals at 12 and 6 and load the pedal at 6 you're putting a sideways load on the frame.

Given what Dave said before about frames that are out of alignment being resistant to shimmy it would make sense for weighting one pedal to work.. you're mostly likely twisting the frame slightly out of alignment and changing how it loads/unloads.

It makes sense that it could work as a deflection that counter acts/nullifies the oscillation. Now, how do we verify it other than "It works for me"? :)





William

PS: Sorry if this bike belongs to a member, I just pulled it off the inter web.

[marciero]

...
I'm not saying that speed wobble is caused by the rider, it's still the sum of the harmonics between the top tube and down tube. As Dave found, wobble can be induced, either his way or something as simple as hitting a small rock which moves the front wheel over a fraction of an inch. Some riders have all kinds of problems with speed wobble, others on the same bike don't.

My solution is rollers - learn how to ride them. If you can ride rollers as relaxed as you ride down the road, and you take up maybe 3" of the roller (not wobbling around from side to side), and your bike has a speed wobble, blame the bike. If not, you have two possible causes, and one of them is you.

Top and down tube contribute, for sure, but you cant attribute wobble to any one or two things-the dynamics are too complex. The front end geometry clearly plays a role. Modeling as a spring-mass system (or more accurately as a system of coupled spring-mass systems) helps to explain, but does not capture everything that is going on, and you also have to include the rider in this. The physics is classical in the sense that it's all F=ma, but not simple. For the most part what has been written about wobble on the internet, including what Jobst Brandt, Sheldon Brown, Zinn, and others have said, has to be considered folklore. Not that there is not truth there, but none of it captures the whole phenomenon.


There are entire scientific journals (at least one) dedicated to bicycle dynamics. You'd be surprised what is not known!

For example, it was long thought that the ability of a rider-less bicycle to stay upright was due to a combination of gyroscopic stability and trail. But recently a bicycle was built that had negative trail and gyroscopic effect cancelled, but that could self-steer and stay upright. In fact, I think this was posted in a Paceline thread.

http://www.nature.com/news/the-bicycle-problem-that-nearly-broke-mathematics-1.20281

[jamesau]

So as an aside, can anyone explain how they think this will effect handling?


I can't offer an explanation but can offer some intuition.

First, I'm sure most here have experienced the transformation in handling and traction that occurs between the fully seated to fully standing position on the bike when traversing rough terrain.

The fact: As others have stated, simply weighting the pedals/unweighting the saddle does nothing to change the center of gravity of the bike/rider composite (other than the slight vertical displacement needed to get your butt off the saddle). The physics of this is well-understood.

The intuition: Changing your weight-support location from the saddle to the pedals creates a sophisticated bogie-type suspension system (bogie suspensions are used on trucks, airplane landing gear, and trains). The bicycle becomes a 2-wheeled bogie with the frame as rocker arm and the pivot effected by the bottom bracket/pedal spindle/ankles. The rider is the main load whose arms and legs become the spring and dampening elements of the suspension. Add to that the anticipation the rider uses to preact on obstacles and you have serious technical complexity.

So, fully-seated with feet off the pedals results in no suspension effects whereas standing on the pedals with weight off the seat (or railing a corner with your outside foot supporting all your weight) results in a complicated and effective suspension bike/rider system that smooths the ride and maximizes traction.

This question is all part of the charm, beauty, simplicity, and mystery that makes riding so compelling.

Back to the OP. If nothing seems to alleviate the handling difficulties with that bike, I'd pass it along and get a different one. There are far too many beautiful and well-behaved bikes out there; don't put up with one that you can't trust.