Downhill Wobble

Peddle Up!

On a ride this morning I was coming down a long hill when my front wheel went into a wobble. Because the hill is on a very busy road I try to keep the speed down by feathering the back and front brakes and I was doing around 26 mph. It was as if my wheel was tracking in a "S"-shaped grove, and for a moment I thought I was a goner!

I managed to slow the bike enough to get into the kerb and I was thinking of unclipping and jumping off into the roadside when the wobbling stopped. I carried on with the rest of the ride, but it really shook me up and I have no taste for hills after that.

What I'd like to know is:

1. What might have caused it?

2. How can I stop it happening again?

3. How can I get my courage back!?

I guess the answer to 3. Is to keep on cycling, but an understanding of the cause would go a long way too.

Thanks for any help you can give.

sungod

sounds like speed wobble, aka shimmy

it is due to a natural resonance of the bike between the fixed ground, and a large mass (you)

once it starts it can build to cause a crash

once started, braking can make it worse

holding the bars too tight can make it worse

the effect can occur with any bike

one way to prevent it, or stop it once it begins, is press a knee hard against the top tube, or take weight off the saddle (stand on the pedals)

when doing high speed descents i keep a knee on the tube whenever i'm not pedaling

that covers 1 and 2, and now you know how to prevent/manage it you should be able to ride in confidence

more here...

http://www.sheldonbrown.com/brandt/shimmy.html

pbt150

It's known as speed-wobble, and has something to do with the resonant frequency of your frame/fork/wheel setup. From the bike point of view, just check everything is done up properly.

Never happened to me, but the advice normally given here is to RELAX your grip of the handlebars, and hold on to the top-tube with your knees to dampen the vibrations.

Good luck getting back out, it sounds pretty terrifying! I'm not looking forward to it inevitably happening to me

pbt150

Just to add:

http://www.sheldonbrown.com/brandt/shimmy.html

datpat64

Same happened here to me !

Uncrontrollable front wheel shake when doing nearly 40 on Cat and Fiddle

Really shook me up and I pulled over thinking I'd had a puncture.....nothing ! Front wheel still firm, no play in bearings or wheel, all the spokes OK and the tyre still at full pressure. I honestly thought the thing was falling to pieces it shook so bad.

Worse thought is that I'd just paid well over a grand for my bike second hand. So once I got it home and double checked evrything again, I got into this forum and at least my issues about the bike being faulty were put to bed. Have rode same stretch since at (almost) same speed and had no issue (although the knee across the tubing for damping was used). It's the confidence thing I now worry with !

Hope yours gets better

Peddle Up!

Thanks for the replies guys. It's worrying that it happens for no simple reason, but at least I've got an insight into how to tackle it should it recurr. For the record, do I go for clamping the top tube between my thighs (oo, er), or wedge one leg against it?

For the ulimate backuip plan - what's the best way to fall off? :?

eltonioni

Cause as above, there are two ways of solving it that I know of; slow down (back brake, front brake shouldn't matter even if it seems counter intuitive) and unload the front wheel, ie shift your arse backwards. Don't try to push through it, there might be a bigger shimmy waiting for you.

Don't fight it since you're likely to set up a rider induced oscillation that really will have you off as you won't be able to keep up with the shimmy and you'll just get a huge wobble that you caused - face meet road.

If it's a regular happening you need to do something to change the geometry or frequency of the front wheel, maybe a shorter or longer stem, different forks, different saddle position, etc.

sungod

Peddle Up! wrote:

Thanks for the replies guys. It's worrying that it happens for no simple reason, but at least I've got an insight into how to tackle it should it recurr. For the record, do I go for clamping the top tube between my thighs (oo, er), or wedge one leg against it?

For the ulimate backuip plan - what's the best way to fall off? :?

just press one knee/leg hard against the top tube, don't need to try any clamping

Peddle Up!

Poscript to the previous posts. I still feel nervous on a descent. Any tips on how to get my mojo back? Thanks!

furrag

I was thinking of unclipping and jumping off into the roadside

Be persistent in attacking the descents. Confidence breeds confidence after all.

antfly

26mph doesn`t seem fast enough to have a speed wobble. Check your wheels out.

Peddle Up!

antfly wrote:

26mph doesn`t seem fast enough to have a speed wobble. Check your wheels out.

What should I look for? They seem to run true.

markos1963

+1 for thinking 26mph is a bit slow to be too worried. I think that dragiing the brakes is a no-no, its probably amplifying a problem with the wheels or if the wheels have a poor joint where the rim comes together causing an imbalance.

As for descending with confidence its down to you relaxing and enjoying it. Being tense at speed makes it even more difficult. Saying it and doing it are very different things and really you need to get out there and get the experience.

On the limited hills we have around here I set myself speed goals for each one to see how fast I can get down them. So far 46mph is the best but I felt comfortable as I had worked my way up to that speed over some months.

antfly

Peddle Up! wrote:

antfly wrote:

26mph doesn`t seem fast enough to have a speed wobble. Check your wheels out.

What should I look for? They seem to run true.

I would have the bike checked out by a good bike shop, if I were you.
Also, when descending put most of your weight on your pedals.

IShaggy

Agree that 26mph is too slow for resonance-induced speed wobble. Usually happens at 40mph+.

Check for movement in the head-set.

morstar

I hate to say it but, aren't all these cases of speed wobble a sign of poor engineering? Seems to be ever more common these days in very new ultra stiff bikes.
My suspicion is that the new materials have brought frames to a point where resonant frequencies are more in the scope of normal riding than maybe they used to be. I bet a lot of the manufacturers are only designing bikes with geometry and stiffness in mind with little consideration of harmonics.
Maybe this needs to change.
The Tacoma Narrows bridge is worth googling to understand resonant frequency and its impact. They don't make bridges like that anymore but I find very little material from bike manufacturers about the speed wobble topic. Easy to brush away as freak occurrences but arguably they aren't.

Pretre

I'm unsure how a bike designer could take into account harmonics, as you suggest, given the extreme varialbility of conditions you ride in? The Tacoma Bridge disaster was because of the way the wind came up the valley wasn't taken into account by the designers IIRC - if a bike was stuck in one place then yes, harmonics would be a valid design consideration, but they aren't...

sherer

Peddle Up! wrote:

Poscript to the previous posts. I still feel nervous on a descent. Any tips on how to get my mojo back? Thanks!

I had a crash and this affected me a bit. Best advice is to find a hill that you know well and that has very little traffic. Not always easy. Then you can start going down at 15 and then just work your up and up until you are back where you were again.

skyd0g

Peddle Up! wrote:

Poscript to the previous posts. I still feel nervous on a descent. Any tips on how to get my mojo back? Thanks!

Try to un-clench your buttocks. Relax and absorb the bumps ~ being tensed-up will make it worse.

Mossrider

I had one a few months ago on a new bike at the top of Fleet moss. Utterley terrifying. I think that it is due to a unique combination of many factors all at once

Bike geometry (which doesn't mean it is wrong)
Your position in relation to that
tyre pressure
road surface
crosswinds

In my case the road surface was very poor, steep and with a severe cross wind.

just try and change your weighting on fast descents (I now stand slightly off the seat on the pedals and if nervous simply lay a knee on the bar). Because its such a combination of factors, I suspect (hope) it doesn't mean your bike is particularly prone to this.

From the experience I'm now aware of this on fast (especially windy / poorly surfaced) descents and consciously adjust my position. since it hasn't happened again, I seem to have got being just as bad a descender as I was before the wobble!