If you are an avid cyclist, and like to rocket down hills and live to tell the story, you should read some more. There are also some good lessons for motorcyclists too.
I bought a 2005 61cm Fuji Team Super Lite bicycle on Craigslist two years ago and decided to use it on a regular one hour lunch time loop in Brisbane, northern California. The ride circled San Bruno Mountain, and at its peak on Guadalupe Canyon Parkway reached 700 ft above sea level. Riding eastward, the road dives to 40 feet above sea level while only covering an extra mile of distance. It is one of the windiest roads in the Bay Area with foggy spring gusts from the Pacific Ocean often reaching 50 Mph. The road is as smooth as silk, and you can easily hit 40 Mph with little effort while getting a picturesque view of Oakland, South San Francisco and the shallow bay in between.
One day I was riding with my group and very near the bottom of this steep descent, at a speed of about 35 Mph, I was hit by a sudden burst of wind. It rocked me and I noticed that the bike began to shimmy. I tried to control the to and fro vibration of the handle bars by stiffening my arms, but it only got worse. Soon it became a very bad wobble and with each swing of the bars, the situation deteriorated. I desperately tried to brake to slow down, but the danger only increased, even at increasingly lower speeds. When I reached close to jogging speed, I jumped off the bike and began to run like a man possessed. Only divine intervention prevented me from stumbling and meeting certain injury on the asphalt. As you can see from this video it can be very violent.
Previously I had a similar experience on my 63cm Trek Madone while igniting the western descent of Hicks Road in the Almaden Valley of San Jose, but at 25 Mph, not close to 35 like in Brisbane. I braked and the bike only began to behave itself when its speed reached a complete stop. I thought nothing about it till the Fuji incident.
Both bikes, though having different frame sizes had very similar dimensions when you compared the distances between the seat, bottom bracket and handle bars. I had used a heavier Trek 7.3 FX on many occasions before without incident on the same route and I decided to do my research.
There are many terms for this characteristic wobble or vibration. In biking forums they call it a speed wobble, or death wobble. In motor cycle forums they call it a tank slapper. At certain speeds the vehicle not only becomes unstable, but a sudden jolt close to the natural frequency of the frame can create a dangerous situation for the rider.
I replaced my wheel set with a heavier pair, the general instability was still there, but it was less. I adjusted the seat, added a longer stem to create a more relaxed stance and added thicker tires. I also trued the wheels, and that helped very noticeably with general stability improving too but even so, the problem was still there at very high speeds. I added heavier wheels, without much further gains. The Fuji, unlike the Trek, has much more responsive, lighter steering which may have contributed to the problem more than any other factor. Even on slight descents, the light weight Fuji was more fidgety.
Reading more and more I eventually discovered this close shave video on YouTube with motorbikes:
Notice how the rider with the wobble hits a bump, the front wheel hits the ground at a slight angle and then the tank slapper wobble starts. The rider loosens up and then regains control of the motorcycle. In this other YouTube video the rider fights the wobble and pays the price.
My strategy for the lunchtime ride changed. I was much looser when gripping the handlebars and firmly rested my thigh against the top tube of the frame to dampen the vibration before it started. I also slide my sitting position back till my hip bones aren’t resting on the saddle at all. This makes my thigh placement easier and also makes my center of gravity lower and shifted to the rear in a way that makes me feel more secure. Since developing this little technique in all downhill rides, my experiences have been much better. I have since discovered an article on this wobbling tendency on Wikipedia which mentions five contributors to the shimmy including the center of gravity, and dampening factors. I also mentions the stiffness of the front wheel and firm tracking by making sure your front tire isn’t under inflated. Some people say they rise off the seat a little to get a similar dampening effect, but I’m not so sure as raising your body off the seat makes the bike more unstable in general.
The result of trying this method was a much more pleasant ride and as my confidence rose I was reaching the high speeds again, though I tried not to break cycling’s Usain Bolt world record like before. It seems so counter intuitive that at high speeds you get more stability by loosening your grip on the handlebars, and especially in the biceps of your arms, but it works for me. I eventually sold the Fuji bike as I felt it was too light with unstable steering that was too loose. My Madone and custom cyclocross are much better bets.
If you have had a similar situation, let me know how you feel.
