Help troubleshooting severe high speed wobble

[cmg]

Originally Posted by Cicli

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]

Originally Posted by bluesea

..... 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]

Originally Posted by bluesea

- 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]

Originally Posted by BdaGhisallo

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]

Originally Posted by 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.

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 .

Originally Posted by 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

[Ti Designs]

Originally Posted by David Kirk

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.