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Dynamics of Wheel Wobble; Might Need To Check Carbon Forks



 
 
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Old September 27th 20, 08:13 PM posted to uk.rec.cycling
Bret Cahill
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Default Dynamics of Wheel Wobble; Might Need To Check Carbon Forks

Take an extreme case. To keep the frequency low and the speed safe use a large unbalancing mass and the low spring constant of a MTB shock.

Drain the fluid out of one front fork shock leaving the damping to the other side. Unbalance the front wheel with a 200 + gms of plumbers putty or other weight taped to the spokes near the rim.

The undamped spring - mass on one side results in different forces/displacements on each fork each revolution. The caster of the headset then converts the unequal cyclical displacements to cyclical torques on the front forks, twisting the wheel and handlebars each revolution.

Different spring and damping constants on each side of a carbon fork would result in different forces much the same as a bad shock but it will happen at a higher speed than the test rig above.

Composite materials may be strong and light but it could be tricky guaranteeing uniformity of structural parameters into symmetrical items, no matter how finished. These differences would not be readily visible to the eye.

The manufacturer needs to check spring and damping parameters directly then toss all forks that have more than safe % variations between left and right. An adjustable carbon fiber test fork will determine what is safe at the low cost. They will eventually figure out how to manufacture uniform spring and damping characteristics into each fork to reduce wastage.

In the mean time cyclists can check and balance the front wheel before descents -- even cheaper and easier than checking for spring and damping constants.


Bret Cahill
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