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

In a message dated 01/16/2004 11:43:36 AM Pacific Standard Time, gillies@cs.ubc.ca writes:
> I don't understand this comment
>
> >I don't believe that the adjustment of the headset is key to shimmy. It is
>
> >as Brandt says, a product of the entire system, you and the bike. I do
> believe
> >that a bike with less trail (less than, say, 50mm) will shimmy easier than
> a
> >bike with more trail. It makes a squirrelier handling bike (hence my
> comments
> >on my old Raleigh International that started this thread) which causes you
> to
> >not only overcorrect but to grip the bars tighter in an attempt to control
> >the shimmy, thereby propagating it. We have deraked the forks on a number
> of
> >Internationals ( and Treks, and other bikes) over the years to minimize
> their
> >shimmy.
>
> increased rake = increased trail.
> infinite rake == infinite trail, think about it.
>
> by de-raking the fork, you have decreased trail. if this solution
> fixed the shimmy, then it was probably not the frameset causing the
> shimmy but the tire profile that caused the shimmy, because you are
> making the bike less likely to go straight ahead by deraking the fork.
> however, you have reduced the "laydown" from turning by deraking the
> fork, which increases stability.
>
> this "laydown" could perhaps have been more easily reduced by tires
> with a different profile.
>
> I tend to think Richard Jow (who is on topic) had the right answer on
> this issue, and not Jobst Brandt (who is off-topic, his FAQ is too
> recent.) :-)
>
> Sorry Don but you are incorrect. Increased rake = decreased trail, etc. They are in inverse proportion. Here is a web page that explains it very well: http://www.phred.org/~josh/bike/trail.html Deraking the fork makes the bike more apt to go straight, and less easily made to shimmy. I speak from experience, not from conjecture.

Trail is never off topic. It is thoroughly discussed in Archibald Sharp's Bicycles and Tricycles: A Classic Treatise on their Design and Construction, first published in 1896 and recently reprinted. Even so, it is little understood today by some bike designers who should know better.

Bob Freeman Seattle, WA Been pondering trail, rake and shimmy for nigh on 30 years.