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Re: That jerk again!



David Bowman says:

As I have followed this discussion unfolding I think that Brian has come
closest to unraveling the mystery so far. Now that I have a few moments of
time I thought that I would contribute my take on the situation.

As, I think, Brian noted, the suspension system of the car is somewhat
loosely coupled to the rigid car frame by springs, struts, and shock
absorbers that all have a significant range of motion. In different states of
motion and acceleration the equilibrium position of these springs differs.
When the car is in a freely accelerating roll (due to the road surface not
being perpendicular to the Earth's gravitational field) the equilibrium set
point of the suspension system is different than its equilibrium point when
the car is at fixed rest on the same tilted hill. From a frame in which the
car remains at rest the direction of the effective gravitational field is
different when the car is in a free roll than the direction of this field
when the car is at rest on the tilted slope. The discontinuous change in the
direction of this force field when the brakes are applied at the 'stop point'
causes a reequilibrating response of the suspension system which causes a
temporary motion of the rigid car compartment.

That's the same conclusion I come to, but the effect seems too large to
me. What I'm trying to do, of course, is to effect the smoothest braking
possible. I would be surprised if everyone else hasn't tried to do that.

One thing is clear from the discussion so far. It is easier to think in
the accelerated frame when considering such a problem, especially if you
are sitting in that frame.

Leigh