[ODE] slider trouble

[Russ Smith]

> I've noticed that when I connect two boxes with a slider, and set
> stops on the slider so it can only move between +1 and -1, the
> box-slider-box system spins off into infinity if I toss it into the
> air.  Without the limits, the system behaves just fine.

this one took a while to figure out, but it's now fixed (joint.cpp
updated in CVS, with a long comment explaining the problem).

the problem was that the constraint forces being applied to a powered
or limited slider joint resulted in a torque couple if the body centers
of mass did not line up along the slider axis (as they do in all my
slider tests!). that caused the joined bodies to build up angular
velocity.

slider joints between objects that weren't spinning freely (e.g. sitting
on the ground) would not have been affected much by this problem.

nate, can you verify the fix?

russ.

-- 
Russell Smith
http://www.q12.org