Speed wobbles

by Cooks   June 26, 2011  

I've just had an utter heart-in-my-mouth moment during a 35mph speed wobble. Are there any tricks to avoiding these wobble? why do they happen? Or am I wrong is it even a speed wobble that I had? All I know is that I was coming down a hill and my front end started to go. Managed to correct it by slowly braking, but bloody hell did it scare me.

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Blushing Had one a couple of weeks ago I had even picked which bit of the hedge I was going to try and land in as I did not want to fall in the road.
It was my own fault as I was pushing it and I knew I was on my own limit of control

Been going down the hills a bit slower for the past few weeks Whew!

posted by IOM Paddy [38 posts]
26th June 2011 - 13:25

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You shouldn't be getiing them at all, let alone at that speed.
What's the bike, tyre/pressures/wheels got the right length stem etc. And is your headset tight?
Some bikes wobble a bit at that speed when you take your hands off. And some are ultra stable i.e a cross bike with road tyres or a tourer.

posted by oldgit [27 posts]
26th June 2011 - 13:38

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Couldn't have been something caused by our less than perfect road surfaces could it? I slammed into a pothole in a descent at speed last year, no time or room to avoid it - okay there was no mistaking what that was (and luckily all I got was a puncture out of it) but a smaller blemish on the road could cause a wobble?

Simon_MacMichael's picture

posted by Simon_MacMichael [8517 posts]
26th June 2011 - 13:49

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could've been the road I suppose, some slight imperfections got exaggerated through the bike/poor handling by me. It probably didn't look that bad or anything, it may have felt much worse than uit actually was. Everything else is fine. It's a Carbon boardman, btw. I've gone much faster downhill on heavier bikes without a problem. Would that make a difference?

"I can't believe I ate the whole thing..."

Cooks's picture

posted by Cooks [492 posts]
26th June 2011 - 14:02

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A heavier bike might I supose be a little more stable, and if it's heavy because it's old it might have slacker angles.
That said Boardman bikes are well sorted. See if it occurs again, it shouldn't, but if it does then a liitle investigation work needs doing.

posted by oldgit [27 posts]
26th June 2011 - 14:21

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i've heard lots of theories as to why they happen over the years, and there's probably more than one reason for them. One i've experienced and been able to cure is an unbalanced wheel. check your front wheel to see if it's unbalanced, ie it always returns to the same position when you spin it. If it is, the difference in mass can set up a wobble if the movement of the mass corresponds with a resonant frequency in the front of the bike.

If you do have an unbalanced wheel, then you can balance it by weighting it like you would a car wheel. a bit of extra inner tube inside the tyre can be enough to balance things out.

Dave Atkinson's picture

posted by Dave Atkinson [7506 posts]
26th June 2011 - 15:02

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There is a certain descent going south from London - I forget the name of the hill, but it is the main road going from Wallingford. If you go left it takes you down to Westerham but if you go straight it crosses the M25.

The first and only time I went down it I was sure my bike was about to fall apart. There is just something in the corrugation of the road that plays hell with the bike. It isn't even visible, it's just something there.

Sounds like it might have been something similar.

abudhabiChris's picture

posted by abudhabiChris [569 posts]
26th June 2011 - 15:37

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I'm familiar with speed wobbles on 4 wheeled boards - longboards and mountainboards - but on 2 wheels it's a different phenomenon. It could be, as Dave suggested, a slightly misbalanced wheel, or something in the road surface resonating with some part of the bike geometry (this seems more likely - have you done that descent at that speed before?)

Stewie

posted by stewieatb [298 posts]
26th June 2011 - 16:57

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I have but on a different, heavier bike. Plus I had a bit of extra poundage myself. I'll buy the resonant frequency theory, sounds good and sciencey...

"I can't believe I ate the whole thing..."

Cooks's picture

posted by Cooks [492 posts]
26th June 2011 - 20:08

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dave_atkinson wrote:

If you do have an unbalanced wheel, then you can balance it by weighting it like you would a car wheel. a bit of extra inner tube inside the tyre can be enough to balance things out.

I never thought of using inner tube, i did an "old skool" scalextric trick that used to weight the back of the car down a bit, a lump of blue tack Smile

Gkam84's picture

posted by Gkam84 [9040 posts]
26th June 2011 - 21:05

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Hi, there are plenty of things that cause speed wobble. For a good article on the topic, read this; http://www.cyclingtipsblog.com/2011/03/speed-wobble-when-the-bike-shakes...

posted by The Weekly Cycle [1 posts]
28th June 2011 - 7:58

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as many say above, it should not happen at that speed!

My first suspicion is always spoke tension, you would be amazed what a difference it can make. many shops are obsessed with the wheel being perfectly true (which is a huge step above not giving a sh@t if its less than 2mm out!) rather than even tension, fortunately my latest LBS has a physics genius in the workshop and understands that even tension is the key, even if that means the wheel being slightly out, to avoid a myriad of sins, including wobble.

anyway, technical probs aside, which you no doubt have to get a wobble at that speed, to be able to help get yourself out of these and other problems when this happens, in mountainbike world we put a bigger/grippier tyre and/or run less pressure on the front, so the rear should be telling you that you are on the edge before the front does, and is easier to correct a rear slipping than the front. In road world this translates to running 5-10 psi less in the front and or 5/10psi more out back (all other things being equal. ie if you are already running way too little or way too high a pressure in your tyres then 5psi is not going to make a diffierence)

the biggest problem is that in road world a lot of people do the following:

1) run way too much pressure in both tyres: no way out of a wobble: 5/10 psi meams sod all if you are already 20/30psi too high!

2) put a thinner tyre on the front, less grippy tyre at the front, swap a skidded out rear for the front, etc

3) are obsessed with slicks and weight alone, a bit of tread is a good thing!

4) never ride a mountain bike - there is no better way to learn wobble recovery than hacking it on a mountain bike and managing squirm, wobble, traction loss, etc at least once a week/month

sermon over, get your wheels/hubs checked Smile

roadie come mountain biker come single speeder and back again

posted by cborrman [84 posts]
28th June 2011 - 10:55

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dave_atkinson wrote:
If you do have an unbalanced wheel, then you can balance it by weighting it like you would a car wheel. a bit of extra inner tube inside the tyre can be enough to balance things out.

On my front wheel I find that the lightest point is the valve. ie: If I leave the wheel to spin freely, it stops with the valve at the top. I tried screwing some extra nuts onto the valve, but they weren't heavy enough. Would be nice to have some lead doughnuts that fit over the valve to balance the wheel.

two wheels good; four wheels bad

posted by cat1commuter [1380 posts]
28th June 2011 - 12:58

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Having read a couple of the links, getting more weight over the front wheel sounds like a good idea.

two wheels good; four wheels bad

posted by cat1commuter [1380 posts]
28th June 2011 - 13:27

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maybe you could attach fishing leads to the spokes Smile

Dave Atkinson's picture

posted by Dave Atkinson [7506 posts]
28th June 2011 - 13:30

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Only time I got a death shimmy ( at 75kph ) it turned out later that the bike had a crack around the seat tube/top tube join

I guess this helped it to be less rigid and so more prone to vibration

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posted by vorsprung [295 posts]
28th June 2011 - 13:54

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One theory is front end stiffness Surprise I know people in the past have swapped forks to a stiffer one and this has solved things. I havent read the link but presume you know about the gripping the top tube with your legs thing?

arrieredupeleton

posted by arrieredupeleton [566 posts]
28th June 2011 - 15:24

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The only corrective measure when it happens is to grip the top tube with your legs. Helps dampen the oscillations.

I assume you weren't braking were you? I've suffered severe judder when braking at about 30mph. Scares the c--p out of you. The cause was a build up of brake rubber on the rim. Good clean of the rim and blocks - no further problems.

posted by Posh [46 posts]
29th June 2011 - 11:42

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No I wasn't braking, just descending. Having read some of these links, putting my knees on the toptube seems to be the way to go, whereas stiffening up, gripping tighter and making my peace with God like I did is prob no the best way to correct it. I think I'll prob be able to sort it next time, I'll give my hubs a thorough going over in the meantime.

"I can't believe I ate the whole thing..."

Cooks's picture

posted by Cooks [492 posts]
29th June 2011 - 14:12

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Swapping my forks for a stiffer, heavier, straight-bladed model worked for me. But I was sure there were no defective or misaligned parts, and doing the same may have no effect for you.

posted by handlebarcam [533 posts]
29th June 2011 - 19:04

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Just remembered. I had it once very badly and all it was was two loose spokes. When they were about to come out the whole frame started to fishtail.

posted by oldgit [27 posts]
30th June 2011 - 14:04

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cborrman wrote:

3) are obsessed with slicks and weight alone, a bit of tread is a good thing!

Confused tread may be good for off road but with a contact area about the size of a penny (ish) the last thing I would want is bits missing from my road tyre, even in the wet there is enough capillary action from the road surface to remove water - and that's the only reason tread is needed, moto gp and f1 prove this. perhaps a compound with more grip would be better advice in this situation? imho

posted by 37monkey [143 posts]
5th July 2011 - 0:42

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