Try the simple things first: move the saddle slightly forward or
back, maybe 1/2", to change the weight distribution of you on the
bike. Change the distribution of stuff you carry on the bike (or
consider carrying less stuff). Raise or lower the bars to change
your position. Make sure the headset is adjusted. Make sure the
wheels are true and properly aligned in the frame. Swap the front
tire to the back and vice versa; look for casing defects in the
tires. Try less inflation pressure (or more) in the tires. Try a
different size tire. If your bike starts to shimmy with your hands
on the bars, stand up slightly and ease your weight off the saddle-
this will usually stop a shimmy instantly.
The cause of shimmy is not exactly known. Since it tends to be speed-
dependent, it is likely that vertical and gyroscopic forces play a
part. There appear to be at least two fulcrum points, the rear tire
contact patch and the saddle. The bike frame appears to act as a
spring, whipping back and forth; nutation can be observed with the
steerer turning slightly in the head bearings as the front part of
the frame travels laterally. Interestingly the front tire contact
patch usually continues to track in a straight line but, as the
shimmy worsens, the contact patch can begin to arc back and forth on
the road. This is what is often meant when someone talks about a
death wobble, because the vehicle becomes unstable and will often
crash unless the wobble can be stopped. Wobbles can be started by
road surface irregularities
*Great* video by the much-missed-in-rec.bikes.tech Damon Rinard of a
deliberately induced no-handed shimmy:
http://www.youtube.com/watch?v=xODNzyUbIHo
I wish that was in slo-mo.
This is Rob English crashing at the Battle Mountain IHPVA event on
level terrain at about 70 mph (I bet he appreciated his fairing very
much). It appears to me that his pedaling effort caused the problem
and that as the bike started to wobble, his pedaling amplified it.
http://www.youtube.com/watch?v=i5Dapy1xUq0
My observations with my bikes (a 1996 Riv A/R [26 x 1.25], a 1995
Ritchey Road custom [700 x 25], a 1998 Gunnar Crosshairs [700 x 28]
and a home-built road bike [700 x 25] all with Panaracer Pasela tires
at the rated maximum pressure) is that:
1. none of them currently shimmy with my hands on the bars.
2. the Gunnar shimmied when I mounted a Nitto mini-rack on the front
fork, even with no load on the rack, and stopped as soon as I removed
the rack- I could feel the pulsation in my hands when they were on
the bars. The Gunnar sometimes shimmies if I ride no handed and lean
back. Reynolds 853 with Waterford fork upgrade. Oddly it did not
shimmy with a really old Eclipse bar bag (one of the ones from the
late 70s with a rack that slipped over the bars and under the stem
and with elastic cords going down to the dropouts).
3. the Riv doesn't, with or without the same Nitto mini rack, with
or without a load. Reynolds 753 frame with 531 fork. No shimmy that
I can ever recall having on this bike.
4. the Ritchey shimmies at certain speeds no-handed (19-20 mph and
up) if coasting but not if pedaling; worse if I lean back. Tange
Prestige Ritchey Logic tubes and fork tubes.
5. the home built bike shimmies occasionally no-handed. Reynolds
501 frame and fork tubes.
5. the scariest shimmy I ever had was descending on a Bianchi
Reparto Corsa-built bike when I tried the aero trick of sliding off
the back of the saddle. It felt like the rear wheel was shimmying,
very disconcerting. The Ritchey replaced that bike and handles so
much better than the Italian job.
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