Nearly crashed on a descent - wheel wobble - what happened?

Bi50N

I was in Richmond Park this weekend, descending one of the hills (anti-clockwise, south-eastern section, steep right hander - Broomfield?) when my bike started to wobble uncontrollably.

Not sure if it was the front or rear wheel, or both. I was doing about 30 and thought I might have to bail on the grass rather than take a fall on the tarmac. Luckily managed to get my speed down and get back under control.

My first experience of this, and it was really quite unpleasant - any ideas what might have caused it? My theories are -

I hit a crosswind early on - could that set it off?

Found a broken spoke on my rear wheel (now out of true and being repaired). Could that be the culprit?

Any comments appreciated!

nochekmate

Yes

Yes

30 mph should not be enough to induce a 'speed wobble' otherwise. Suspect the wheel was the bigger issue here.

Monty Dog

Try searching this forum or googling wheel-wobble or wheel-shimmy and you'll find plenty of discussion, hypothesis and proposed cures. The phenomena is due to a harmonic vibration of the whole bike system, including the rider and can be initiated due to all sorts of factors.

crankycrank

One technique used to prevent this from happening is to press your knees against the top tube while descending.

unixnerd

I had this once and it scared the sh*t out of me. Increased my tyre pressure and it never came back. If you push into the top tube with your knee I'm told it can absorb the vibration, luckily I've not had to find out again.

Bi50N

Thanks for the replies. I had no idea it was a common(ish) phenomenon. It really shook my confidence, but having read upon it, reassuring to know there is a remedy, of sorts...

klep

Probably its because of ur wheelproblem. Was it your first time riding at this velocity? It could also have to do with the stiffness of your frame/forks...

Monty Dog

Attributable factors include flexy frame, forks, loose headset, flexy wheels, tyre pressures, road surfaces. I had it once on an Alan Carbonio, a reknown flexy frame - swapped the forks and never had it again.

Crankbrother

It will def be the wheel ... But the advice about steadying top tube wjmith your knees is the best advice in all conditions ...

Although it's a bit like a parachutist whose parachute fails and they ask if it's the way they fell ...

Bobbinogs

klep wrote:

Probably its because of ur wheelproblem. Was it your first time riding at this velocity? It could also have to do with the stiffness of your frame/forks...

...and not forgetting the right degree of stiffness by the rider. By which I don't mean "Fnarr, Fnarr", but I find that keeping a firm grip whilst trying to keep relaxed everywhere else helps a lot. Not always easy to achieve when hitting a blind corner with an adverse cam at 40mph in the wet...so I try to slow down a bit every now and then too

Peddle Up!

Bobbinogs wrote:

...but I find that keeping a firm grip whilst trying to keep relaxed everywhere else helps a lot.

That's interesting. The advice I was given was to relax my grip ("as one would hold a pen").

FWIW, my experience

Bi50N

It's not the stiffest bike - steel, an On-One Pompetamine (see signature). I've been faster before, 38 at most. The hub gear puts the weight at the back, but with rider too I suspect that has a negligible effect. It's always felt like a very assured, cruise-y ride before, not the skittish beast it turned into.

In any case, think I will be taking it a little easier for a bit!

Bobbinogs

Peddle Up! wrote:

Bobbinogs wrote:

...but I find that keeping a firm grip whilst trying to keep relaxed everywhere else helps a lot.

That's interesting. The advice I was given was to relax my grip ("as one would hold a pen").

FWIW, my experience

You could well be right Peddle Up, I descend like a big girl's blouse anyway

Peddle Up!

Bobbinogs wrote:

You could well be right Peddle Up, I descend like a big girl's blouse anyway

That must make me a full party frock.