[ODE] test_buggy strangeness

[Nate W]

I just finished modifying the 'buggy' test app to include a closed loop of
joints and bodies.  It's behaving a bit strangely, and I'm wondering if I
have overlooked something or introduced an instability or what.  The
modified test_buggy.cpp can be downloaded from
http://diesel.whatever.net/tmp/test_buggy.cpp

I've attached two long thin boxes alongside the buggy, one attached to the
"chassis" near the front, and the other attached to the edge of a wheel on
the same side.  The boxes are attached with hinges in the same axis as the
wheel rotation.  The two boxes are then joined with a slider.  That much
works as expected, with the boxes telescoping into each other and changing
angles as the wheel rotates.

But, the buggy bounces vertically as it rolls, and it's very difficult to
steer.  I was hoping the buggy would continue to drive normally, just with
this new linkage hanging off the side.

It does behave normally if you leave out the slider joint and just let the
new boxes flop around.  They flop around nicely and the buggy drives
normally.  But, constrain the new boxes with a prismatic joint, and things
get odd.

The odd behavior can be demonstrated in the modified test app by just
pressing z a few times and following the buggy as it lurches away.  
Notice the lurching.  Then hit "." a few times to turn the wheel, and
follow it some more.  Notice the sideways sliding.

Anyone got ideas as to how to fix the buggy?  

Thanks,


Nate Waddoups
Redmond WA USA
http://www.natew.com