Wheel wobble at high speed?

70\ʹspenguin

Not put this in the workshop forum as im not sure this is a technical issue.

On my ride home from work i go down two steep hills, both of about 14% on tarmac. I usually get between 30-35mph freewheeling on the way down and have been doing this for for ages with no issue.

Tonight i decide to see how fast i can get down, so i pedal as hard as i can and begin the decent when all of a sudden the front wheel starts wobbling uncontrollably. Im in full on free wheel, arse off the back of the seat, head down to the stem mode at 30 mph and accelerating. I was too afraid to brake so i just rode it out to the bottom. Needless to say i was a bit freaked out.

Got home and checked the wheel - all good. Checked the hub and QR - all good. No movement in the stem or headset or fork bushings.

Anyone else had this happen to them? Im not sure if this is a technical issue, a balance issue or just a freak one off?

Chunkers1980

It's possible.

Just like my car, at 87 mph the steering wheel feels different to say 75mph. No need to comment you speed limit warriors

Also, another thought. When you pedal fast with your bike upside down you'll notice different levels of shakes depending on speed with the fastest not actually being the most.

BrainDent

Yep, that is the old nasty lady called High Speed Wobble. It comes from a cumulative resonance of the individual components. Everything gets a little chaotic at certain vibration frequencies. The wobble is usually no problem except when all of the components have similar frequency ranges. Then they all start to sing at once, add to the effect in each other once the synchronize and things get a little crazy.

The key is to remember that it happens within a fairly narrow range of frequencies (speed). The speed and the weight distribution have large effects. To stop the wobble:

Slow down or speed up. Get back outside of the problem speed in either direction and the wobble will cease. Smooth is the key of course. No slamming the brakes one or sprinting. Smooth and calm, smooth and calm.

Clamp the toptube between your knees. Tends to minimize the toptube vibration and therefore lessen the headtube vibration.

Change either your fork, tires, wheels, pressure, stem, bars, position and on and on. The wobble comes from all of those occurring that narrow range. Change on of those parts and it tends to have an immediate effect.

As in all issues with handling, smooth and control are where your safest and most effective reactions will be. No slamming the brakes, no sticking your rib cage on the saddle or experimenting with the idea that letting go of the bars will stop the vibrations. (Yep, knew a guy. I can attest that the way you imagine it ended is accurate.)

70\ʹspenguin

Ooh sounds interesting BD. Thank you for that.

Murphybarnes

Had this occurrence once before when descending the Rockies on an old Trek. Admittedly I had heavy panniers on the back and speeds reached 43 mph but the whole back end started snaking like a caravan about to break loose. Best to just hold on and let fate take it's course.