[ODE] "fixed" joint with ERP and CFM and limits?

[Graham Fyffe]

Hi, this may be an odd question.  I want to modify the JointFixed type
so that I can set ERP and CFM and set limits.  Basically I'm trying to
simulate the way the hub of a tire attaches to the body of the tire. 
My idea was to treat the connection as some kind of omnidirectional
spring with limits, and if I could, I would have separate CFM / ERP
values for latitudinal and longitudinal displacement.

This sounds like it could work, though I can't quite work out how to
do the mod.  As an alternative, I would settle for some kind of
Contact type that could have a hard limit in addition to the CFM /
ERP.  The CFM / ERP gives me nice springiness, but the wheel falls
right through the ground if it's going fast enough, before bouncing
out :P

- Graham Fyffe