Hi Folks, A couple weeks ago Rick Hammond put out a paper on motorcycle
steering wobble & weave, and I sent it to my Ardun friend Gary Tefft,
who happens to be employed by Harley-Davidson in Milwaukee as an
engineer. Gary has many years experience with riding and factory
development of big Harleys, and he composed his own personal thoughts on
the subject of wobble and weave, which are presented briefly below.
Note that in no way does this represent any policy of The Motor Company.
Gary gave me permission to send this out to you, but no further, unless
you get permission from him. I believe "design limit speed" below in
general refers to legal highway speeds, or a little more. In other
words, if you modify your bike and/or run it beyond those said speeds,
from a manufacturer's point of view, you're on your own. Ardun Bill
HIGH SPEED WEAVE
By Gary Tefft
The phenomena of low speed wobble and high speed weave are well known.
My own knowledge of them is mostly awareness, rather than being schooled
enough to be truly conversant in the physics or mathematics describing
them. High speed weave, of course, is the more dangerous of the two.
As the article described, it is a harmonic oscillation of the chassis
and suspension. I don't believe that it is ncessary for the bike to be
leaned-over in a turn for it to initiate.
Preventing it from occurring within the operating range of the vehicle
can usually be achieved without mastering the complex equations
describing it, as I'll explain.
In the physics of vibration, a force or torque or vibration applied to
an object or system is called "an exciting force" (or torque or
frequency). Objects or systems, such as motorcycle chassis, have
numerous "natural frequencies of vibration" in various planes or for
different components or subsystems. If an exciting force or torque
causes a deflection in a component or moves a system in stability
"off-center", but not out of the bounds of stability, the component or
system then rebounds. If it rebounds at a rate near the natural
frequency of the component or system, a "harmonic oscillation" begins.
This can be demonstrated by pushing a child on a swing. A small force,
applied in sync with the natural frequency can, over time, result in a
large amplitude of oscillation.
The rotating wheels have powerful gyroscopic effects imbued by their
rotation at high speed and the law of conservation of angular momentum.
A torque applied to the steering stem(which is perpendicular to the
axial direction of the angular momentum) causes a "precession" torque
perpendicular to both. This can cause the deflection noted above. Also,
the stiffer the chassis, the higher will be the natural frequency and
the lower the amplitude of oscillation for the same level of energy.
Resistance to high speed weave is something we have standards and test
procedures for. Believe it or not, the testing involves having a
skilled test rider achieve a steady speed, then sit upright in the
saddle, take his hands off the bars and give one handgrip a whack! The
bike must return to stability within a specified number of oscillations;
the test is repeated at intervals up to the design limit speed. The way
to recover from a high speed weave is to lean forward and hold a steady
throttle. Slowing usually makes it worse, before it gets better. A
stiffer chassis will have a higher natural frequency and(in general) a
reduced amplitude of oscillation.
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