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March 11, 2011
Image by Endless Pedal
One of the most frightening experiences a cyclist can encounter is getting up to 70km/hr and experiencing speed wobble (a.k.a “death wobble” or “speed shimmy”) while approaching a corner. A couple weeks ago when I was chatting to Darren Baum about the geometry of a bike and how it affects handling, the topic of speed wobble came up. Darren taught me some interesting things about this that I thought I’d share with you.
Speed wobble is the term used to describe a quick oscillation of the handlebars while riding at high speeds. Any vehicle with a single steering pivot is capable of experiencing speed wobble.
Your immediate instincts are to think that something wrong with the bike (loose headset, flat tyre, etc). Panic can often occur and the first thing many people do is straighten their arms, lean back and put more weight on the saddle. Unfortunately these are all the wrong things to be doing. Speed wobble can be attributed to many different factors:
1. Sometimes speed wobble can result simply because of a poorly manufactured and designed frame. The wheels may be misaligned (front wheel and back wheel aren’t tracking in the same spot) and the bike is fighting itself. There are very few frames this poorly made these days, but you’ll still find them.
2 A scenario seen relatively often is that the top tube of the bike is under-built. If you put a lot of weight on the saddle the front end of the bike can pivot around the seat tube and create oscillation. An under-built top tube isn’t stiff enough to stop that from happening. Another thing that can happen is that the trail (the product of head angle and fork rake) of the bike is too large. What happens is that the bike becomes too stable at speed and the large trail over corrects itself and brings the wheel past center, and the wheel moves back and recorrects itself again which is where the wobble comes from. This is relatively common problem that Baum tells me he’s seen in the past.
3. Incorrect weight distribution is a very common cause of speed wobble. Quite often, speed wobble has just as much to do with the rider as it does with the bike. If speed wobble starts occurring, many people will intuatively put their weigh towards the back of the bike instead of putting their weight towards the front to actually stop it and dampen it out. The best thing to do to get out of a wobble is to weight one of your legs down at the 6/12 o’clock position, put some weight on the front end by bending your elbows (use soft hands – don’t grip handlebars firmly!), and take some weight off your seat (to take the pivot point away) which puts more weight back on the handlebars, which puts more weight on the front wheel. This will usually bring stability back to the bike and correct the oscillation. The reason it’s suggested that the pedals be in the 6/12 o’clock position (rather than the 3/9 o’clock position) is because this will bring your body weight into a better balanced position which will calm the bike down.
Different riders may experience speed wobble on different bikes. A bike can descend like a dream for one rider,while another will get the fright of his life on the very same bike. Things like stem length on a particular geometry and weight distribution of the rider can have a lot to do with it.
If you’re brave you can induce speed wobble quite easily by giving the handlebars a good slap and doing all the mistakes mentioned above. I strongly, strongly discourage it however. DO NOT TRY THIS!
Have a great long weekend. All the people doing the 3 Peaks Challenge will be doing a heap of descending so ride carefully and remember some of the tips above if you experience the death wobble.
A few more tips for the big weekend ahead:
“Eat before you are hungry.
Drink before you are thirsty.
Rest before you are tired.
Cover up before you are cold.
Peel off before you are hot.
Don’t drink or smoke on tour.
Never ride just to prove yourself.”
Paul de Vive – Velocio