Re: [CR]Elementary Control-Theory Mistake in Jobst's article on Shimmy

(Example: Racing:Beryl Burton)

From: <RDF1249@aol.com>
Date: Fri, 16 Jan 2004 20:16:02 EST
Subject: Re: [CR]Elementary Control-Theory Mistake in Jobst's article on Shimmy
To: Bikerdaver@aol.com, classicrendezvous@bikelist.org


In a message dated 01/16/2004 4:41:53 PM Pacific Standard Time, Bikerdaver writes:
> Ok-
> I guess I get to give my two cents on the matter of "shimmy". In all the
> cases, that I have encountered "shimmy" since riding lightweights for the
> past thirty plus years, it has revolved around some part of the bike being
> "loose". Basically this means some part that is normally tightened--more often
> than not--like bearings, need proper tightening.
> Others have suggested the headset as the most obvious place to begin to
> look for shimmy. Bob, writes below that short wheelbased bikes tend to shimmy
> more often than longer wheelbased bikes--and that I have found to be true as
> well.
> But two other bearings that can become loose that will also contribute to
> "shimmy"--and please forgive me if someone has already mentioned this--are
> the BB axle and the wheel/hub bearings. This first one, shimmy caused by a
> loose BB axle, is perhaps so obvious to the rider that it may not be worth
> mentioning. However, I have found that with hub bearings being ever so slightly
> loose, will contribute GREATLY to nasty shimmy when going down hills at 30+mph.
> It gets worst the faster you go, and if you ever want to get the p_ _p
> scared out of you---and actually I don't reccommend this--try loosening your hub
> bearings and test this out yourself.
> Just be advised, that having read this email, you have agreed to my
> disclaimer of just assuming it's a valid test, and will hold me harmless just in
> case you are the "Doubting Thomas" type. Cheers
> dave anderson
> cut bank mt
> Dave - Certainly all manner of mechanical deficiencies can cause handling errors. We are not talking about those. All of my comments on this subject so far are considered in a "all other things being equal" or 'best case scenario" light. In other words, assuming all your bearings etc are tight, wheels are true, and you still get shimmy, which I have seen on many many bikes of all types over the 26 years I have been in the bike business, then you have to look elsewhere for the cause. I think Jobst Brandts analysis (http://yarchive.net/bike/shimmy.html) is the most level-headed and spot-on explanation of shimmy. The bike and you comprise a mechanical system which is subject to oscillation under the right circumstances. Find those exact circumstances and you have the problem solved. Therein lies the rub. You might be able to experience the problem but not nail down what particular features of your and the bike's setup/geometry/karma add up to produce it. But as I have said, best bet is to relax your grip, and if that does not damp out the oscillation, then touch your knee to the top tube, and that will do it for sure. My experience has led me to believe that larger, more flexible frames with low trail (50 mm or less) are more prone to shimmy than smaller, more rigid frames with adequate trail of 55 to 62 mm.

Moving on... Bob Freeman Seattle where we sometimes shimmy and sometimes shake.