Scooper

Jan Heine wrote an interesting article on shimmy in a recent issue of Bicycle Quarterly. He thinks imperfections in the facing on head tubes is responsible in many cases, and suggests that headsets with needle bearings (like the Stronglight A9) instead of ball bearings often resolves shimmy problems as the needle bearings align automatically and compensate for the imperfections in facing.

Bicycle Quarterly, Volume 6, Number 3

Curing Shimmy on a Bike

by Jan Heine and Mark Vande Kamp

Some bikes shimmy, others don’t, even if they are made from the same tubes. Some believe that poor frame alignment is the cause for shimmy, but we have ridden modern custom frames from very reputable builders that shimmied, even though they appeared to be aligned very well. Yet many older frames with unknown histories do not shimmy, despite being obviously misaligned.

Looking at the variables involved, Jan noticed that he never had ridden a bike with a Stronglight needle-bearing headset that shimmied, yet similar bikes (from the same maker, with the same tubing and geometry) with ball bearing headsets often shimmied.

Stronglight headsets are different from all others in that the bearings align automatically, and thus compensate for imperfections in the facing of the head tube. On a standard headset, the balls on one side tend to run looser than those on the other, because the top and bottom edges of the head tube never are perfectly parallel. Furthermore, the needle bearings of the Stronglight headset may add a little resistance to the headset, perhaps enough to dampen shimmy, but not enough to change the steering characteristics of the bike.

To see whether a different headset could make a difference in shimmy behavior, we replaced the Chris King headset on Mark’s Ti Cycles, a bike with a very strong shimmy under certain conditions, with a Stronglight needle-bearing headset. Mark rode the bike to the workshop with a load in the handlebar bag, and it was easy to provoke shimmy.

After we replaced the headset, Mark tried to induce shimmy as he had before, but the bike no longer shimmied. After numerous attempts, he finally got the bike to shimmy briefly, but instead of continuing until Mark put his knee on the top tube, the shimmy now attenuated on its own within a few oscillations. We then loaded the handlebar bag with about 4 kg (8.8 lbs.). Still, the bike’s shimmy was gone in most speed ranges. Only when coasting downhill no hands at 40 km/h (25 mph), a violent shimmy occurred that was beyond the capacity of the headset’s dampening. Placing one hand on the handlebars immediately stopped the shimmy. Mark rarely rides no-hands at speeds this high, so the shimmy problem on this bike has been mostly eliminated with a simple headset change.

In conclusion, we still do not know what causes shimmy on a bike. From Mark’s and my experience, it is unrelated to top tube diameter. A handlebar bag does increase the likelihood of shimmy on Mark’s Ti Cycles, but many other bikes with handlebar bags do not shimmy. We found that we could eliminate most of the shimmy on one of the worst bikes we experienced, by replacing the Chris King headset with a needle-bearing Stronglight model. If your bike shimmies, it is worth a try.