Q&A: Speed wobble

By Jez Loftus | Tuesday, March 4, 2008 12.08pm

Q: I have recently upgraded to a Cannondale 2008 Synapse 105 after riding a flatbar Trek 1000 for the last four years, and have found my riding experience greatly enhanced by the new, lighter bike.

Last weekend I decided to test out my new bike on one of the few big hills around here and found the ascent much easier than before. However, on the long descent (a 10% slope over about 1.2 miles), on reaching about 37mph, the handlebars began to wobble quite violently and for a time I seriously thought I was going to come off.

After managing to brake to reduce my speed, the wobble stopped and I regained ‘control’ of the bike, but the experience had quite literally shaken me as coming off at that speed could have caused serious injury to both me and my bike.

Now I am nervous about attempting another descent at such a speed in case this recurs. Is it due to the stiffer carbon frame, the set-up of the bike or just that having ridden a flat bar for years I still need to acquaint myself properly with the new riding position?

Damon Taylor

A: Unsurprisingly, what you’ve described is called a speed wobble, Damon. It’s something that all bikes exhibit but thankfully few of us experience. If you’re interested in the physics behind the speed wobble there’s a wealth of debate on the internet – just type ‘bicycle speed wobble’ into a search engine.

Speed wobbles are a complex phenomenon and any vehicle with a single steering pivot is capable of exhibiting one. All things have a resonant frequency and when the resonance frequency of the front wheel interacts with that of the frame you get a speed wobble.

Strangely enough, you’re most likely to get a speed wobble on smooth roads. Mountain bikes rarely exhibit speed wobbles because of the varying terrain, and knobbly tyres make it hard to reach the resonant frequency. It’s even been said that the shivering of a rider can cause a speed wobble as it is the same as the resonant frequency of the frame.

We know the Cannondale Synapse well and it’s a good bike. The trouble is, we’ve never experienced any wobble on it, and it’s not the sort of thing you can go out and create – nor one that we’d actually want to practise curing! – which doesn’t help your situation.

Suggestions of ways to combat the problem include, when in a wobble, unweighting your seat by standing up on the pedals, the theory being that your weight acts to maintain the wobble, so taking the weight off should dissipate the oscillation; another is to squeeze your legs tightly against the frame’s top-tube to stop the vibrations.

There are so many variables it’s hard to pinpoint one cause, and you may never experience the wobble again. It’s unlikely that there’s anything wrong with your bike, though you shouldn’t overlook doing some safety checks on your headset, hubs and fork alignment. It could be as simple as swapping the tyre, moving your body position or even just loosening your grip on the bars.

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