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physics of bicycle rollers information?
QUESTION:When riding straight ahead on rollers the wheels are essentially
perpedicular to the rollers. When one zig-zags or otherwise make large
corrections such that the wheels are no longer perpendicular to the
rollers I wonder how this affects rear steering? The spacing of the
rear patches changes and the location of the patches seems to me that
it would introduce some yaw forces. Gravity is also trying to pull the
wheel down between the rear rollers and when the wheel is no longer
perpendicular I think the force of gravity and the offset patches
would server to further tend to apply a steering force the likes of
which do not occur on the road. I wonder how great these forces are.
ANSWER: If the rear wheel is not perpendicular to the rollers (as seen from
above) you will get two patches with shearing forces caused by the
forward rotation of the drums, accompanied by the somewhat screechy
sound of rubber slipping on the metal drum (I have never ridden rollers
with PVC drums, so I don't know what that sounds like).
If the front wheel is not perpendicular to the roller (as seen from
above) it starts to try to slip down the roller because the steering
trail pulls the contact patch backwards and gravity does its thing, and
the shearing force tries to drag the tire back up the slope. Of course,
if the roller is set too far back, the wheel may try to slide down the
front of the roller pulling the bike forward and possibly off the rear
rollers.
The narrow width of rollers limits the angle that can be achieved,
however, which in turn limits these adverse effects.
Here's a thought experiment: it seems obvious that one could not ride
rollers if the front drum was disengaged and the front wheel therefore
did not turn, because you couldn't steer the contact patch under your
center of gravity. Is it possible to "ride" rollers if only the front
drum is turning (e.g. motorized) and the rear drums are stationary? My
guess would be "yes," at least somewhat ridable.
About as easy as a skid. While a skid requires more concentration to
controll that regular riding, it can be maintained more or less
indefinitely as long as you keep the rear wheel from washing out,
which wouldn't be a problem in the set-up you suggest.
Another thought experiment (or a real one for anyone out there with
the equipment!) would be to try to ride a unicycle on the back rollers
to see whether the shearing forces on the back wheel would amount to
anything with any effect.
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