Speed wobble on a bicycle?

[Dave W]

Is it possible? One of our group who normally sits near the back uphill due to lack of fitness has discovered he can't go fast downhill because his bike gets a violent "tank slapper" it's a fairly upright Specialized Sirrus. He's had the headset torqued by a mobile service chap. Last week he pulled in and declared his headset was undone again but had actually undone the pinch bolts by the time I got to him.
I personally don't believe he has anything wrong with his headset but I do wonder if it's possible for a bicycle to go into a wobble around 30mph?

[Brucey]

Is it possible? .... but I do wonder if it's possible for a bicycle to go into a wobble around 30mph?

the speed will vary but any castor can be made to oscillate, or shimmy, or speed wobble. This will (undamped) occur at exactly one speed. Add damping and the maximum amplitude will be reduced, but the range of speeds over which it will occur can increase.

In reality additional damping is usually a good thing. Altering the weight distribution on the bike can usually sort things out but

- a flexible rear carrier with a load on
- relatively little weight on the bars
- a flexible frame, fork or wheels
- wheels/tyres that are not true or balanced
- any patterning in the road surface

are all things that can exacerbate shimmy.

I test my bikes for shimmy by riding no-hands down one particular hill near me, which allows a gradual acceleration. All of my bikes will shimmy, it is just a question of how badly they do it, at what speed, and how easily it may be stopped. Normally one hand on the bars is enough to stop it; if it needs more than that then there is likely one or more of the things listed above that is helping to 'drive' the oscillation.

Roller bearing headsets work for some people by adding damping to the system. I don't like the way the bike steers the rest of the time, so each to his own...

cheers

[pete75]

Is it possible? One of our group who normally sits near the back uphill due to lack of fitness has discovered he can't go fast downhill because his bike gets a violent "tank slapper" it's a fairly upright Specialized Sirrus. He's had the headset torqued by a mobile service chap. Last week he pulled in and declared his headset was undone again but had actually undone the pinch bolts by the time I got to him.
I personally don't believe he has anything wrong with his headset but I do wonder if it's possible for a bicycle to go into a wobble around 30mph?

Had one bike that did this at about 27 mph but not when loaded with panniers front and rear. Touching the crossbar with my knee stopped the wobble

[robgul]

My old Galaxy has a BIG 25.5" frame and when touring with rear panniers and bar bag it shakes a bit going downhill fast ... I assume there's some flex in the frame (headset is Stronglight old style with a quill stem) - all I do is grip the crossbar (top tube in current parlance) with my knees and all is well!

Rob

[pwa]

+1 for gripping the top tube with knees. That worked on a steel framed bike I had which wobbled at about 40mph. I suspected the problem might be due to (or made worse by) a Carradice SQR bag that was clamped to the seat post. Its clamping apparatus had a bit of flex that allowed it to wobble sideways a little, and I thought that maybe this sent pulses (technical term not readily to hand) down the frame. I plead guilty to overloading the bag.

[fastpedaller]

Had a bike that used to wobble at 70MPH, sorted it by pedalling faster!

[pwa]

At 70mph it would be me wobbling, not the bike.

[drossall]

It's a fairly rare effect, but nonetheless well-known amongst keen cyclists - check for previous threads. Causes can be very mysterious, but Brucey has set out some known factors.

[tim-b]

Hi

I had a flat handlebar hybrid that would do this at a tad over 30mph. I found that leaning a little further forward and clamping the top tube between my knees helped dampen the effect

I sold the bike

Regards
tim-b

[Gearoidmuar]

I've had a wobble from hell, twice, on a carbon framed road bike in a triatholon descending on a roughish road at something like 35mph. It was terribly frightening the first time I got it and I had to stop, convinced the bike was fractured. Gripping the bars with my knees and moving forward did not stop it.
A couple of years later on the same hill, I got it again, but this time discovered that moving my hands from the drops to the hoods stopped it. I moved them back, it came back, back on the hoods, gone. So that CAN help.

Another bike I have, a Thorn Raven tour was inclined to wobble under speed and with a load. I discovered that changing the tyres from the Specialized Fatboys which I had used for years (26 x 1.5) to fatter tyres 26x2) stopped it completely and made the bike ultra-stable.

[drossall]

It must be a resonance effect, so changing almost anything might help, but not necessarily the same thing for everyone - such as your hand position above.

I've had wobble on my normally very-stable (Mercian) tourer. Must have been the way I packed the panniers that time. However, it was not as severe as some here are describing.

[Dave W]

My friend gets in such a wobble he can't apply the brakes and is convinced the bars are undone. However when he pulls up eventually everything is pointing in the right direction. He has fitted 23mm tyres believing the bike will be faster (it isn't) so possibly that doesn't help either, I've not witnessed it myself but it scares the life out of him.

[james01]

It must be a resonance effect, so changing almost anything might help, but not necessarily the same thing for everyone - such as your hand position above.

.

It can be very subtle, even tread pattern can cause it. The first easy checks before delving deeper are: wheel bearing play,headset adjustment,luggage distribution etc. The quickest fix I heard of was removing the front wheel and replacing it the other way round.

[pjclinch]

Don't discount the rider. I've seen cases where one rider is fine on a bike and another suffers from wobbles.

There's a lot of subtle steering input to control a bike, and especially if a rider tenses up significantly this might mean a problem for one person but not the next, and all interacting in very subtle ways with the geometry/setup of a bike.

Sort of related, I ride recumbents and there's often a choice of handlebar style. There is never any significant weight or pull on any of them, but put me on a bike with underseat or "mantis" bars and I'm fine and on an otherwise identical bike but with "hamster" bars I'm never entirely happy. And I know people the other way around, much happier with the "hamsters", others who use any of them completely interchangeably.

Pete.

[jimlews]

Is it possible? .... but I do wonder if it's possible for a bicycle to go into a wobble around 30mph?

the speed will vary but any castor can be made to oscillate, or shimmy, or speed wobble. This will (undamped) occur at exactly one speed. Add damping and the maximum amplitude will be reduced, but the range of speeds over which it will occur can increase.

In reality additional damping is usually a good thing. Altering the weight distribution on the bike can usually sort things out but

- a flexible rear carrier with a load on
- relatively little weight on the bars
- a flexible frame, fork or wheels
- wheels/tyres that are not true or balanced
- any patterning in the road surface

are all things that can exacerbate shimmy.

I test my bikes for shimmy by riding no-hands down one particular hill near me, which allows a gradual acceleration. All of my bikes will shimmy, it is just a question of how badly they do it, at what speed, and how easily it may be stopped. Normally one hand on the bars is enough to stop it; if it needs more than that then there is likely one or more of the things listed above that is helping to 'drive' the oscillation.

Roller bearing headsets work for some people by adding damping to the system. I don't like the way the bike steers the rest of the time, so each to his own...

cheers

Brucey, I am intrigued - what is it about the way a bike with roller headset steers that you don't like? Serious question.