[ODE] Simpler Suspension Geometries

[Russ Smith]

> What are other people using for simplified geometries? ipion and 
> havok have special classes just for building cars. That fastcars guy 
> too. What are they doing? Special fancy joints help out, but the 
> parts still add up quickly.

hi,

one way to simplify is, like you say, use special joints - then you
only need one body and four wheels. in the simple case the joint can
be like ODE's hinge2 (see the docs) - basically a shopping trolley
joint with suspension. adding more complicated kinematics should
not be too hard and may actually improve the vehicle behavior.

a further simplification is to recognize that the wheel rigid bodies
dont contribute much to the variable component of the chassis-
relatve inertia, so you can basically just treat them as constants and 
use a single rigid body for the chassis. once you do this you can 
optimize the entire engine to support this single-rigid-body case. the 
disadvantage of this approach is that it's hard to get realistic 
collision response in the wheels if they don't have associated rigid 
bodies. but you can kinematically fake it. this MAY be what havok and 
fastcar do (i'm not really sure).

the lesson here is that for specific problem domains, you can always
optimize the physics model and solver for that case. i would expect
an optimized car engine to go 10-50 times faster that ODE - but it
will only be able to simulate cars. another example: i have implemented 
an ODE-like solver for vascular catheters, and it's WAY faster when you
only have one fixed system structure to think about. 

russ.

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