In my estimate, shimmy is something that is related to weight balance on the bike, frame/fork thickness/stiffness, and perhaps tires. Oh yeah – lets not forget velocity, either. If you enjoy stretching occasionally by riding no-handed, shimmy is a demon to be dealt with.
I’ve had some experience with shimmy on my A. Homer Hilsen, but it has been sporadic, and changing things like bar height, front and rear load, and tires seems to impact how bad it is. I leave out speed, as there’s no way I’m going to try to limit that.
When I first built the bike, it had Noodles at around seat height, and I was using Jack Brown tires. No shimmy was present with this 25 lb configuration. I went through a few variations, and initially noticed that when I moved the bars higher, I seemed to get some shimmy. This usually kicked in riding no-handed above 15 mph. I generally kept the load up front on the bike, and this seemed to dampen the shaking, so I was fairly happy. However, with my recent foray into low trail (Cycles Toussaint – no shimmy ever, BTW), I was interested in moving the load back on the Hilsen so it matched the geometry of this higher trail bike. Or so I thought…
It’s been said that a needle bearing headset can cure shimmy. I’ve never used one before, so I guess it was time to check this theory out. I overhauled the bike a month ago, and figured it was time to check the headset. Since it was out, I put in a Miche needle bearing headset, and at the same time added my former front rack to the back of the bike. It seemed like it needed a basket, so I put the medium Wald/Shopsack combo on it that worked so nicely in the past on the front of the bike.
S H I M M Y Y Y Y Y Y . . . . .
Wow – I took this in to work with perhaps 5 lbs of gear in the back, and the shimmy was so bad, it even shimmied with my hands on the bar several times. I experimented with holding my weight forward while I released my hands. This worked at first, but as I moved back to a more upright position, it released the shaking, shifting frame shimmy.
Hmmm – needle bearing not an answer here. After my ride home, I removed the basket/rack combo, and put my front handlebar bag back on. I tied off the tools in my roll bag under the seat. This cured the beast.
Solution for my bike – weight needs to go up front with only a bit of weight allowed under the seat. No weight out back or I get the shakes. It is interesting that my Hunqapillar has never shaken its wooly head. I generally carry weight on the front, however, and this bike has a long top tube that stretches me out putting the weight more forward. It also has beefy, stiff tubes that, like the Toussaint, seem to repel shimmy monsters.
I’ve read of several other light, road Rivendells having a similar tendency, so I will chalk this up to balancing weight properly for the bicycle. If you have shimmy, hopefully this will give you some things to mess around with.