SF Cyclotouring

Ride reports and other ramblings from a San Francisco cyclist.


Shimmy is as Shimmy Does...Unfortunately

From Jobst Brandt:

Emphasis in bold is mine.

"Bicycle shimmy is the lateral oscillation of the head tube about the
road contact point of the front wheel and depends largely on frame
geometry and the elasticity of the top and down tubes. It is driven by
gyroscopic forces of the front wheel, making it largely speed
dependent. It cannot be fixed by adjustments because it is inherent
to the geometry and elasticity of the bicycle frame
. The longer the
frame and the higher the saddle, the greater the tendency to shimmy,
other things being equal. Weight distribution also has no effect on
shimmy although where that weight contacts the frame does. Bicycle
shimmy is unchanged when riding no-hands, whether leaning forward or
backward. "

In addition...

"Shimmy is caused by the gyroscopic force of the front wheel whose
tilt is roughly at right angles to the steering axis, making the wheel
steer to the left when it leans to the left. This steering action
twists the toptube and downtube, storing energy that both limits
travel and causes a return swing. Trail (caster) of the fork acts
on the wheel to limit these excursions and return them toward center.

So it's generally non-correctable (unless I decide to ride with my
saddle impractically low) and the 2nd paragraph would seem to imply
that it's more prone in a low-trail bike, where the trail is reduced
and thus the steering-limiting mechanism that works against toptube
and downtube twisting is weakened.

Good grief!

-Jim G


Blogger Dodger said...

Tom Kellogg at Spectrum wrote a nice piece on shimmy here:


Might not help with your problem, but it is a pretty good summary in layman's terms.

2:43 PM  
Blogger Dodger said...

And here's one from the wonderful Dave Moulton archives:

Shimmy Re-visited

4:32 PM  

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