Fun and fear

On Thu, 11 Oct 2007 20:06:59 -0700, Bob Nixon wrote:

> In a straight line headshake & wobble with steep steering geometry
> without a steering damper is pretty common.

> It's not really the frame but the front wheel coming off the ground
> misaligned with the rear tire particularly over a bumpy road. On a turn
> you're looking at more at a rear traction problem as the front will likely
> let you know it's about to loose traction by going wide in the turn or
> sliding a bit to the outside.

I've had a similar problem on three different Yamaha bikes but it showed
up as a whole bike wobble when accelerating around 65MPH.

It happens on a certain type of road surface where the idiots put 1/8"
deep grooves in the concrete road surface that run in the same direction
as the road, not across the road. It starts with the front wheel then
the rear wheel starts to act up. Time involved is about a 2-3 seconds.

Backing off the throttle immediately stops the problem. What really
scares me is that it's a special type of wobble like that which
destroyed the Tacoma Narrows bridge... (resonant, at the natural
frequency) - IOW I suspect not backing off would lead to a disaster.

Natural Frequency

Any oscillating object has a natural frequency, which is the frequency an
oscillating object tends to settle into if it is not disturbed. For
example, the natural frequency of a pendulum 1 m (39 in) long is 0.5 Hz,
which means the pendulum swings back and forth once every 2 seconds. If
the pendulum is struck lightly once every 2 seconds, the amplitude of the
swing increases gradually until the amplitude of oscillation is very
large. The phenomenon in which a relatively small, repeatedly applied
force causes the amplitude of an oscillating system to become very large
is called resonance. Many of the serious vibration problems in
engineering are caused by resonance. If, for example, the natural
frequency of the body of an automobile is the same as that of the series
of combustions in the engine when the car is traveling at a certain
speed, the body might begin to vibrate or shake roughly. Such vibration
can be avoided by mounting the engine on a damping material such as
rubber to isolate the body from the engine.


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On Thu, 11 Oct 2007 18:40:17 -0700, saddlebag wrote:

> Exactly. Feels a little uneasy when she comes back to the ground.
> Childsplay. You want uneasy, take a third shift job in a gas station...at
> night...anywhere in America...with a road name "Martin Luther King."

Oh yeah. I know that gas station. I go there at midnight the day before
a Bucs game and yells out in the station lot "I have $300 and need Bucs
tickets"



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