[CR]When shimmy isn't good...

(Example: Framebuilding:Tubing)

Date: Mon, 24 Mar 2003 07:13:45 -0500
From: "HM & SS Sachs" <sachs@erols.com>
To: gregb51@lycos.com, classicrendezvous@bikelist.org
Subject: [CR]When shimmy isn't good...

"Greg Brooks" wrote

Hey all, If this question isn't on topic, I guess just ignore it or send me your thoughts off list. O.K.- Can someone explain the mechanics/physics involved in high speed,hands-off shimy experienced in cycling? I assume the root cause is a slightly misaligned frame. But why does the front wheel "vibrate" back & forth rapidly instead of just veering off to the right or left? (on topic material- My '72 Paramount has a tendency to do this on steep descents when I take my hands off the bars, which I don't do too often.)

Counterintuitively, this sounds like a REAR wheel problem. First bet is poor dynamic balance. In the Good Old Days, I even had one buddy true his tire instead of his wheel to get rid of the problem. I would also check for incorrect dishing.

After the shock of the crazy idea wears off, think about the rider/butt as a more-or-less immobile fulcrum. Oscillation of the rear wheel is then transmitted forward...

"When all other hypotheses have been excluded, whatever remains, however improbable, just might be the cause." (modified from Arthur Conan Doyle)

harvey sachs
mcLean va