Speed Wbbles

[cadent]

Hi all, I was wondering who out there has experienced speed wobbles while on a high speed decent on a road bike.
I was doing a road race on sunday with the northern vets at Lancefield. I have just started racing with them. I've done a few crits and enjoyed them so
I thouhgt I would try a road race, my first. The coarse was 30km's and we did it twice. About 3/4 way around the coarse there is a really fast decent, as I got
half way down the bike started to get speed wobbles. I was doing about 70km/h I slowed down abit and safly got to the bottom. Second time around I tried to move my wheight further back and not hold the bars as tight, but again at around the same speed it started again. SCARY. I'm shore others have had this problem, and I would like to know if there is a fix for it. I have never been very confident at decending at high speed but this is the first time this has happened to me.

[Richard.L]

It is a misconception apparently to put your weight towards the back of the bike. You should be placing more weight to the front as it gives you more control and stability. Leaning back is only going to make you more aero? Did you have both your knees in contact with the top tube of the bike? This can solve speed wobbles

Its all in the bikes geometry that determines if you get speed wobbles or not. Getting the bike properly fitted to you is also another matter.

[hannos]

Cadent, have a read of this blog:

http://www.cyclingtipsblog.com/2011/03/ ... -its-head/

[rustychisel]

It is a misconception apparently to put your weight towards the back of the bike....

You say 'apparently' as though you are unsure of the advice you're giving, and so you should be. The advice you offer is wrong, potentially lethal, particularly for a rider trying to control a shimmy which has already begun, the significant point being a change in weight distribution, either fore or aft. All bikes are different, rider responses are different, and the complex causes underlying shimmy are difficult to quantify and isolate. What is certain is the oscillatory nature of the issue...

And riding downhill with your knees clamping the top tube to avoid the issue is no cure at all really...

this is one of the great debates in cycling forums etc and there is a vast amount of information available and many many opinions - try searching under 'shimmy'

see here for then old man's view, mostly correct in my opinion. http://www.sheldonbrown.com/brandt/shimmy.html

[ldrcycles]

A great tip in the comments on cyclingtipblog 'speed wobbles can smell fear'.
About 4 years ago i threw myself down a rough surfaced 22% road and got the wobbles at just over 95kph, got spat off and nearly lost my left leg, bloody lucky not to have been killed. Didn't ride for about 7 months afterwards then gradually got back into it, but found anytime i got some speed going down a hill, the wobbling would start. I tracked this to 2 things, firstly the fear from the crash (which gradually subsided) causing my hands to shiver on the bars, and secondly (and most importantly) because i had been scared, i was always holding onto the hoods. I now believe that riding on the hoods is a MAJOR cause of speed wobbles, my theory being that as you are more upright, the air against your chest will be unloading the front wheel somewhat, allowing it to move around.
Through trial and error i have found the most stable position to be tucked right down in the drops, knees against the top tube, cranks level, and putting my weight on the pedals not the seat. And of course, staying CALM.

I'm not saying this is the absolute 100% perfect scientific solution but it works for me.

[/\ |_ |]

I have experienced speed wobbles going down too Spit bridge (sydney) at peak hour! Was caused by two things grabbing a handfull of brakes and poorly adjusted front bearings. I think the wobbles was caused by the loose bearings and grabbing the brakes did not help. When I adjusted the bearings (cup and ball) never happened again. Check your your bearings for any play.

[cadent]

Head bearings are tight . Just had them done about 4 weeks ago. Could it be the wheels I am running. They are Zero 580 with bladed spokes. It wasn't really windy on the day, but I have heard that bladed spokes may cause this.

[rustychisel]

doubt it, bladed spokes and you not holding the handlebar might initiate it, I suppose... maybe.

I've considered many many possible causes over time, one of the more popular ideas (to me) being an unbalanced wheel caused by an imperfect tyre. Dunno. Can't prove a thing, but that;s the way it is with such issues, the bicycle is the sum of all those parts, and the rider, and the rider's behaviour (eg shivering). Put 'em all together AT the right speed ON the right road and you MIGHT get the right conditions to initiate shimmy.

The other thing to remember is that the cause and effect are two different things. The physical manifestation - harmonic vibration transmitted through the frame members to the head tube and inducing violent yaw - if untreated and unsuppressed, is the effect. Good luck. I solved my last shimmying bike by flipping the stem (dropped the bars 4mm).

[cadent]

doubt it, bladed spokes and you not holding the handlebar might initiate it, I suppose... maybe.

I've considered many many possible causes over time, one of the more popular ideas (to me) being an unbalanced wheel caused by an imperfect tyre. Dunno. Can't prove a thing, but that;s the way it is with such issues, the bicycle is the sum of all those parts, and the rider, and the rider's behaviour (eg shivering). Put 'em all together AT the right speed ON the right road and you MIGHT get the right conditions to initiate shimmy.

The other thing to remember is that the cause and effect are two different things. The physical manifestation - harmonic vibration transmitted through the frame members to the head tube and inducing violent yaw - if untreated and unsuppressed, is the effect. Good luck. I solved my last shimmying bike by flipping the stem (dropped the bars 4mm).

Yeh I was also thinking of maybe changing the stem length. Not sure if I go longer or shorter. I suppose I will have to try both and see what happens

[Ken Ho]

The steep steering head angle of a bike with race geometry lends itself to speed wobble. Go fast enough on a rough road, and you are inviting it. Wheel balance probably becomes important at high speed too.
Motor-bikes get it, but they tend to be light in the front end at very high speed because of the power they are putting down at the back wheel. Most sport bikes have a steering damper to counter it. It's a freaky experience to have a big shimmy at 200km+ as you go over a small bump.
I agree with the firm grip in the drops and weighting the pedals to lower your centre of gravity. A death grip is going to make it worse, not better. A light touch on the rear brake will settle the bike. Grabbing a big handful of front brake not a good idea. Nor is locking the rear.
Be a good passenger on your bike if you want it to stay composed.

[cadent]

The steep steering head angle of a bike with race geometry lends itself to speed wobble. Go fast enough on a rough road, and you are inviting it. Wheel balance probably becomes important at high speed too.
Motor-bikes get it, but they tend to be light in the front end at very high speed because of the power they are putting down at the back wheel. Most sport bikes have a steering damper to counter it. It's a freaky experience to have a big shimmy at 200km+ as you go over a small bump.
I agree with the firm grip in the drops and weighting the pedals to lower your centre of gravity. A death grip is going to make it worse, not better. A light touch on the rear brake will settle the bike. Grabbing a big handful of front brake not a good idea. Nor is locking the rear.
Be a good passenger on your bike if you want it to stay composed.

My bike is an Avanti cadent team, 2009 model. This is a more relaxed geometry. Does anybody have the same bike and have they had the same problem?

[Robdog]

My bike is an Avanti cadent team, 2009 model. This is a more relaxed geometry. Does anybody have the same bike and have they had the same problem?

Exactly the same frame in an XL (600mm ETT) and no, never experienced them at up to 80km/h

[Nobody]

see here for then old man's view, mostly correct in my opinion. http://www.sheldonbrown.com/brandt/shimmy.html

Thanks RC. Haven't see that one before. I've often wondered why I've never experienced this (to a degree I've noticed anyway) and then came across the bit below.

The longer the frame and the higher the saddle, the greater the tendency to shimmy, other things being equal

Since I have neither, I may be safe.

I believe geometry would play a factor, in that a fast steering bike should have more trouble. I used to race skateboards many years ago (just for fun) on standard length boards. We found by using the hardest truck rubbers available and bolting them down real hard, we could eliminate the death wobbles even at speeds higher than most people can get to on a bike. By then I think the average car had a smaller turning circle.

[nickj_d]

What is certain is the oscillatory nature of the issue...

And riding downhill with your knees clamping the top tube to avoid the issue is no cure at all really...

Don't know about preventing death wobbles, but clamping the top tube with both knees after the wobbles start definately gets things back under control. As you've said, there's an harmonic issue going on when death wobbles occur, and clamping the top tube effectively dampens the whole setup. Others here will better understand/disprove the physics here, but I know from personal riding experience that clamping the knees has gotten me out of trouble on three occasions.

I now ensure i am down in the drops, over the front wheel, with a light grip on the bars whenever i'm descending.

[trailgumby]

Never had the shimmy or speed wobbles. But then my bikes have much slacker head angles than you guys on roadies (68~69deg versus about 73deg on most roadies).

[Cycotic]

Speed wobbles can be caused by an unbalanced wheel. Try slowly spinning the front wheel while off the ground. When it slows to a stop, does it begin turning the other way until a 'heavy' part of the wheel is at the bottom? This can be caused by something simple like the computer magnet. If this is the case, try mounting the magnet closer to the hub and opposite the tyre valve.

I've also heard of a case of speed wobbles caused by having slime in the tubes.

[chill]

Yeh I was also thinking of maybe changing the stem length. Not sure if I go longer or shorter. I suppose I will have to try both and see what happens

I'm not sure I'd go shorter. A longer stem and a lower position would be better.

[discrepancy]

I have never been very confident at decending at high speed but this is the first time this has happened to me. [/quote]
Yep, that could be the problem.