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Good stuff

Motorcycle Wobble
Selden Demeer, Atlanta, Georgia [email protected] writes:
I'm forwarding, from the Hawk List, the most detailed explanation I got re MC chassis oscillations.


From: Mike Dornheim
Subject: Re: Low Speed Oscillation

There are about 4 gross oscillatory modes of motorcycles and the two ones of interest are front castor wobble and rear castor weave. Both are like the overspeeding tea cart. Front castor wobble involves primarily the front wheel, with the rest of the bike considered quasi-fixed. Rear castor weave is looking at the entire bike aft of the steering head weaving back and forth. Through the steering geometry this makes the front wheel wobble and it appears to the rider as a front-end problem, but the root cause is the weaving of the entire bike. If and how these exhibit themselves depends mainly on the geometry, suspension, mass & balance, and tires.

What the VFR (and apparently the PC) has is front castor wobble. It usually occurs at medium speeds, e.g. peaks at ~45 mph on the VFR, and has a ~10 Hz. frequency. It usually can be cured by a steering damper, but why bother since it is also cured by keeping your hands on the bars.

Some bikes just do it and it is no big deal with your hands on the bars in the critical speed range. The VFR is pretty much immune above 60 mph. and you can drive hands-off with a throttle lock above 60 without problem.

The Hawk can be on the edge of rear castor weave, depending upon tires and inflation (lower pressure is good). This is more dangerous than front wobble as it can occur at ~115 mph. and is not cured by having the hands on handlebars, and not clear if steering damper helps.

Cracked frame, loose steering head or swingarm, etc. can bring oscillations to bikes that didn't have them before. But tires and inflation pressures are also worth a check. It's hard for bike engineers to thread a design through the full speed range, under different loading conditions, without having some sort of oscillation. The Hawk is one of the success stories.

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