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RBR Newsletter

Preventing Bike Shimmy, Part 2

We started this series with Part 1 last week, detailing my worst crash, which came early in my cycling career and was caused by a serious shimmy on a hairy descent. I had no idea what was happening, or how to control it.

In sharing some of the possible causes of shimmy, how to fix them, and how to react if you experience shimmy, we hope to equip you with the tools to avoid my fate. We’ll continue with Part 2 today with that in mind.

Check the load.  Carrying a load on a bike that’s not designed for touring can often make a previously well-behaved bike wobble all over the road. In fact, this sometimes happens on so-called “touring bikes” that don’t have the beefed-up top tube necessary to stabilize a load carried in rear panniers.

Example: I have a cyclocross/light touring bike that’s extremely stable in normal conditions. I rode it on a 2-day tour, equipped with a rear rack. The first day was with a group, and its sag vehicle carried my 2 panniers containing overnight gear. So the only thing on the rack was my rain gear. I hit 49.5 mph (couldn’t quite crack 50 no matter how small I tucked) for several miles going down Poncha Pass into Salida, Colorado, without a hint of a wobble.

The next morning the tour headed north, but I went west with the panniers on. That extra 15 pounds made the bike nearly uncontrollable on the 8-mile descent of Monarch Pass. It shook, shimmied and scared me to death. I rode the brakes all the way down, spoiling a wonderful downhill. This bike's lightweight tubeset wasn’t designed for even a light load -- and it rebelled.

So make sure your bike is up to the task. Then consider distributing the load between front and rear panniers and pack heavy items low. However, it’s worth noting that some touring cyclists like to keep heavy items high and close to the rear of the saddle. Their argument is that a bike is designed to carry the bulk of the rider’s weight on the saddle, so it stands to reason that additional weight should be placed as close to that location as possible. Experiment to learn what your bike likes.

Check the environment.  Crosswinds can make a bike shimmy if your front rim has a deep-V shape giving it more side area. That’s why time trialists usually don’t use front disk wheels if there’s even a hint of wind. Handling problems would outweigh the aerodynamic benefit.

How to Handle Shimmy on a Ride

What if your bike starts to shimmy? Don’t panic and tense up. As we discussed in Part 1, tension is a major component of shimmy.

Clamp the top tube.  At the first sign of frame shimmy, pin the top tube between your knees. Some riders automatically do this when they begin coasting downhill, so shimmy won't have a chance to start. Even resting one leg against the top tube can prevent it.

Feather the brakes.  Panic also induces riders to grab hard at the brakes, which can make shimmy worse. If you feel frame tremors, feather the brakes, apply the rear brake more than the front, and slow gradually.

Pedal.  If you aren't going so fast that you're spun out, resuming pedaling in a high gear can put an end to shimmy. This is a hard remedy for many riders to use because it means speeding up when on the verge of losing control. In the same vein, if you can pedal all the way down a hill applying even a small amount of power to the bike, shimmy is very unlikely.

Distribute your weight evenly.  Don’t adopt extreme descending positions. Balance your body weight on the bike so the frame geometry can do its work.

Look for safe landing places.  If you’re really on the ragged edge of control, you won’t have the luxury of scoping out the roadside for a soft patch of grass. But if you have any segment of your attention span left, it’s worth a look. Landing in grass is far preferable to hitting a rocky shoulder or guardrail -- or an oncoming car.

Share your own shaky stories and solutions on the Comments page.

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