speed wobble revisited

[Alan Dolid]

A number of weeks ago I posted a note about dreaded speed wobble to which many of you wrote and suggested that one "fix" was to lighten up weight over the front wheel. I read this at other sites as well. Relaxing as much as possible does work to a certain extent. I just read a post on Velonews by Nick Legan, a pro mechanic, that says "if you feel a speed wobble coming on, try to get more weight over the front wheel." I just had shoulder surgery this past Friday so I won't have an opportunity to test this out for about two weeks. Does anyone have any thoughts on this? Thanks as always everyone!

[Ken Robb]

The one thing we know for sure when we get a wobble is that we have to change SOMETHING. I believe wobbles are caused buy a resonance in the frame so I can believe that in some cases more weight on the front may stop it and in some cases less weight on the front may stop it so I don't think one technique is correct and the other wrong. I have had some success clamping the top tube with my knees but I'd rather be able to cure it by changing my weight distribution in case I ever ride a bike with a sloping top tube. :)

[William]

Alright, I'm about to give y'all a lesson in William Physics 101 so listen up (and take it with a grain of salt). :D

As a life long certified "Big Guy", I have ridden a lot of very large frames. One thing that the majority of these big frames have in common is that when you take your hands off the bars, or unweight the front end too much, the bike will begin to shimmy. Too much weight to the rear in a triangle that is very large and thus likely to oscillate. Now, I also like to descend like a banshee going Mach 7 with my hair on fire so dealing with a bike with this sort of characteristic is something I've become accustomed too. The thing that seems to work for me is a technique that keeps your weight low, and in essence moves your weight more forward or centered on the bike.

On a descent,, if I've spun out my gear, or I'm coasting straight or through corners, I keep my pedal position at 12 and 6 (changing depending on the turn), pushing down, or weighting the 6 o'clock pedal. That in theory shifts my weight from being high and on the saddle to a more forward and low position on the bike. I can still clamp the TT if needed, but really when I do that it's more to narrow my aerodynamic profile then it is to stop oscillation or shimmy. If you keep your pedal position at 3 and 9, and try to clamp the top tube, the majority of your weight is high, on the saddle toward the rear of the bike. Think back to when you were a kid. You may have done this or seen it happen, but when you were flying down a hill and you take your feet off the pedals, what happened? Your weight was high (above the axis) and to the rear...the shimmy started and got worse until the bike throws you and itself to the ground. :bike:

I'm not saying this is the magical cure. Just what works for me, and makes sense in my mind....though some folks think I'm crazy so take from it what you will. :)



William

[Willy]

The best advice given on this forum years ago was to put your knees together going down hill as the bike begins to shimmy - seems to stabilze everything.

[Ken Robb]

The best advice given on this forum years ago was to put your knees together going down hill as the bike begins to shimmy - seems to stabilze everything.

Usually it does but not always--I have the old stained shorts to prove it. :)
Of course I quickly sold that bike to a guy who had no problems with it. He was my height and about 35-40 lbs. lighter.

[RPS]

Does anyone have any thoughts on this? Thanks as always everyone!
One general thought on wobble – most all “fixes” are based on shooting in the dark; which has lead to a lot of different “rules of thumb” over the years. Those who happened to connect with the target mostly by luck now think they have the problem figured out and will swear by the solution.

In my opinion the problem is too technically complex to assign responsibility on one attribute. Without analyzing the involved fundamentals that lead to high-speed instability it’s mostly guessing – particularly when applying specific experiences to someone else.

[RPS]

....snipped.....
The thing that seems to work for me is a technique that keeps your weight low, and in essence moves your weight more forward or centered on the bike.

On a descent,, if I've spun out my gear, or I'm coasting straight or through corners, I keep my pedal position at 12 and 6 (changing depending on the turn), pushing down, or weighting the 6 o'clock pedal. That in theory shifts my weight from being high and on the saddle to a more forward and low position on the bike. I can still clamp the TT if needed, but really when I do that it's more to narrow my aerodynamic profile then it is to stop oscillation or shimmy. If you keep your pedal position at 3 and 9, and try to clamp the top tube, the majority of your weight is high, on the saddle toward the rear of the bike. Think back to when you were a kid. You may have done this or seen it happen, but when you were flying down a hill and you take your feet off the pedals, what happened? Your weight was high (above the axis) and to the rear...the shimmy started and got worse until the bike throws you and itself to the ground. :bike:

I'm not saying this is the magical cure. Just what works for me, and makes sense in my mind....though some folks think I'm crazy so take from it what you will. :)



William
Lowering the center of gravity makes the bike/rider dynamic system more stable. Whether it's enough for a particular rider is another issue.

Regarding your physics, how the rider loads the pedals doesn't actually change weight distribution or height of center of gravity very much at all unless he actually changes his center of gravity. IMO it works for you but for a different reason.

Your experience as a kid is an excellent example of how to make the system less stable. Again, if we stick to fundamentals of dynamics it's easy to see why this is not a good idea at all. In fact, it has the opposite effect as clamping the top tube with knees. And for what it’s worth, clamping top tube with knees IMHO may or may not be enough to prevent wobble, but it's moving in the right direction. And IMO it has nothing to do with damping as frequently claimed.

[false_Aest]

Hommes,

I know what the name implies but I have no idea what the sensation is like.

How fast are ya'll going to experience this thing?
What is your position on the bike when it's happening?
How much of a "wobble" are you talking about (vibration or 2" of fighting wobble?)

Seriously. I've ridden a Trek 1000, a 1986 Fuji Team, a serotta Classique and a Cannondale CAAD9 and I'm really not sure that I've experience this.

[Ken Robb]

rider behind me said all of a sudden it looked like your frame was made of noodles. It was big at 40 mph +/- I was in the drops.

[William]

Lowering the center of gravity makes the bike/rider dynamic system more stable. Whether it's enough for a particular rider is another issue.

Regarding your physics, how the rider loads the pedals doesn't actually change weight distribution or height of center of gravity very much at all unless he actually changes his center of gravity. IMO it works for you but for a different reason.



If I may clarify and explain maybe a little better then I did previously, if I shift my weight off my saddle and more to the leg at 6 o'clock, then I believe I am shifting my weight from a high point (the saddle), to a lower point (the pedal spindle - lower then the BB) that's slightly forward.

As I remember it, Ron Keifel talked about this same technique to the folks at the 05 Open House.



William

[Peter P.]

Weighting the front end is almost impossible once the shimmy starts. The forces causing the shimmy are stronger than your body weight or your arms' strength to absorb it. The key word in your post is "try". You can try it, but I doubt it'll be useful. Clamping your knees on the top tube (sometimes just one knee against the tube is enough) is about the most effective, besides reducing your speed.

[dave thompson]

I used to have a bike that would begin the wobbles at 19mph, exactly. I could relax my arms to absorb the wobble or put my knees against the top tube, both of which would stop it. I could do nothing mechanically to ameliorate the problem, therefore concluded it was a prolem of the physics of the bike and me. It was a large bike and I'm a borderline Clydesdale.

[KevinPak]

Loosening death grip also helps

[oliver1850]

.

[KevinPak]

Didn't catch that either..

19mph & speedwobbles
I can't picture that.

[fogrider]

Loosening death grip also helps
but at 55 mph, I was not going to loosen my grip! about 3 years ago, my carbon bike started to wobble and pull side to side, if my grip loosened, I would have been on the ground. I was able to grab the brakes and hung on! since I had to work to get up to 55 mph, I was not too concerned but I did strip the bike down and put the parts on a different frame. I have since rebuilt the bike with different parts and should be ready to ride next week.

[rustychisel]

I'm with William on this, get down low and get some weight forward, but really no one knows what causes shimmy from one bike to the next, from one occasion to the next. There are tons of theories and almost as many ideas on how to dampen it.

One of my bikes gets a little squirrelly at 38~39kmh if I lean back and lighten the front. I know it, I can catch it, and it doesn't happen at slower of faster speeds. Just the way it is; and it's not a big bike (55cm TT) though I agree its more prevalent with bigger framed cycles. All things being equal bigger bridges sway more.

[gone]

Didn't catch that either..

19mph & speedwobbles
I can't picture that.
I had a bike (60 cm) that was prone to shimmy. I was riding it and had just crested a small hill and was going down a slight downhill when I sneezed. The bike started shimmying and I was going 12 MPH.

Shimmys are only partly about speed, they're also about oscillations and I started one when I sneezed. I must have gripped the bars tight and given 'em a good shake and that's all it took. It was definitely weird.

[RPS]

If I may clarify and explain maybe a little better then I did previously, if I shift my weight off my saddle and more to the leg at 6 o'clock, then I believe I am shifting my weight from a high point (the saddle), to a lower point (the pedal spindle - lower then the BB) that's slightly forward.

As I remember it, Ron Keifel talked about this same technique to the folks at the 05 Open House.



William
William, let me also clarify my response. Shifting weight is not the same as shifting center of mass. If you shift weight from saddle to pedals it doesn't necessarily affect weight distribution on the wheels unless you also shift your center of mass.

In your example, lifting "weight" off the saddle and placing more weight on the lower pedal can't happen unless you also either move your torso around or weigh the bars differently (exception is that unless your saddle is directly over the BBKT). Regarding elevation; a rider isn’t all of a sudden relocating the “mass” from over 3 feet off the ground at the saddle to less than 6 inches off the ground at the lower pedal. My point is that relocating placement/location of forces to hold you up isn’t the same as relocating your body’s mass.

I know what you describe helps reduce tendency to wobble but not for the reason stated. :beer:

[Steve in SLO]

Two thoughts:
We have five contact points on our bikes (two hands and feet and our butt). Under most circumstances our bodies act as elastomers, damping oscillations and keeping them under control. Clamping the top tube adds two more contact points, which should help at most times.
Under certain circumstances, either the bikes tendency to oscillate overcomes our bodies' dampening properties or our bodies input aggravates the process. It seems that under each episode of wobbles there is a unique relationship between the bike and rider which has a unique solution. This is probably why the opinions differ on the solution to the problem i.e. there are many solutions, all of which work but only one of which WILL work.

Second thought: Has anyone ever tried to pop a wheelie or do a bunny hop during a wobble? A scary thought, but what a quick way to 'reset' the system.

[christian]

If I may clarify and explain maybe a little better then I did previously, if I shift my weight off my saddle and more to the leg at 6 o'clock, then I believe I am shifting my weight from a high point (the saddle), to a lower point (the pedal spindle - lower then the BB) that's slightly forward.
No. Center of gravity is based on the center of mass (obviously). All you've really done in that scenario is move the weight forward.

[RPS]

Second thought: Has anyone ever tried to pop a wheelie or do a bunny hop during a wobble? A scary thought, but what a quick way to 'reset' the system.
You first -- and let us know how it works out. :cool:

[William]

... All you've really done in that scenario is move the weight forward.

Yes, moved it forward, shifting weight toward the front wheel.

Now help me understand the rest...

If i am sitting on the saddle, the majority of my weight is on the saddle. If I shift my weight to the low pedal (practically standing on it), doesn't the weight lighten on the saddle and increase on the pedal? Less weight resting high to increased weight resting low? I know the overall weight doesn't change.





William

[VoyTirando]

I was getting consistent speed wobbles on my racer (87 pinarello)... scared the daylights out of me. Turns out my headset needed a half-turn tighter. there was the slightest movement in the bike's front end (between the head tube and fork steerer) which was unnoticable at low speeds but at 40 mph caused a some serious shimmy.

[gasman]

Yes, moved it forward, shifting weight toward the front wheel.

Now help me understand the rest...

If i am sitting on the saddle, the majority of my weight is on the saddle. If I shift my weight to the low pedal (practically standing on it), doesn't the weight lighten on the saddle and increase on the pedal? Less weight resting high to increased weight resting low? I know the overall weight doesn't change.





William

Your weight is shifted on the bike and where it rests on the frame by weighting the pedals but the center of gravity won't change unless you move from sitting upright to a tuck to sitting on the top tube.
At least that's how my small brain figures it.

[William]

Interesting thought by Jobst. Not saying it's right or wrong....

Found on the Sheldon Brown website:

Subject: Shimmy or speed wobble
http://www.sheldonbrown.com/brandt/shimmy.html




William

[ultraman6970]

In more than 30 years using big bikes, neither using POS 20 inches bikes when I was a kid, never gotten this problem ever. Even using wheels with truing problems never gotten this problem riding no hands fast or slow either.

The only thing i can think off is that the rider's position and center of mass of/in the bike is simply wrong or something. Some people can't ride no hands no matter what bike they use, so would not surprise me that they have problems controlling the bikes due to bad set up or something.

[Ken Robb]

While musing on the "lower center of gravity" debate I wonder: if we transfer weight from the saddle to the pedal(s) we may not change the center of gravity but don't we change the point where weight/leverage is applied to the frame? Is weight applied at the end of a seat post more leveraged than the weight when it is applied to a bottom bracket and so more likely to flex the frame more and incur resonance at a different frequency?

All engineers and wannabe engineers please don't criticize my vocabulary here because I have no training in the field but I have to wonder if this is the explanation as to how two bright posters above can have such different opinions. :beer:

[William]

In more than 30 years using big bikes, neither using POS 20 inches bikes when I was a kid, never gotten this problem ever. Even using wheels with truing problems never gotten this problem riding no hands fast or slow either.

The only thing i can think off is that the rider's position and center of mass of/in the bike is simply wrong or something. Some people can't ride no hands no matter what bike they use, so would not surprise me that they have problems controlling the bikes due to bad set up or something.

They were all off the shelf road bikes/frames. Customs including Serotta and my current Zanc are rock solid. And yes, I can ride no handed, even on the rollers.



William

[jlwdm]

I have problems with the dold weather shimmy. If I ride in cold weather I get the shakes going downhill. As soon as my body warms up I am okay. I find I just need to start with a little up hill riding to warm me up.

I had some shimmy problems related to grabbing the bars too tight. On the hoods I have no problems if I brake using my left hand, but there is something in how I brake with the right hand that can cause a shimmy. It seems that my right forearm rises higher than my left when braking from the hoods.

All of my frames are 63.5cm and up.

Jeff

[ultraman6970]

Ex tracker her.. i used to ride on the rollers no hands also, even in the track. I always wonderer about this problem because is one of the things i never gotten problems with ever. Knocking wood.