[ODE] 2 simple questions

[Góral]

Hello everybody,

I'm trying to make a car sim. Suppose my car is built from 2 bodies: lower
part (chassis) and uperr part (windows and the roof). How should I correctly
connetc those 2 bodies?
That's what I do:
-create body for lower chassis and set mass parameters for it
- create geometry for lower chassis (box) and attach body to this gemetry
- add lower chassis geometry to car geometry group
-create body for upper chassis and set mass parameters for it
- create geometry for upper chassis (box) and attach body to this gemetry
- add upper chassis geometry to car geometry group

Should I use some kind of a jonit to connect 2 parts of the chassis, or
there is another method for this? Is there any method to disable collision
between 2 parts of a chassis, or it's not needed?

Question 2 is about collision with terrain. My terrain is generated form
heightmap, so i have some number of 'ground' trinagles. As for now, my idea
is to put 4 thin boxes (each for each wheel) at the height of a terrain, but
it won;t work if car is upside down. How should I properly handle collision
with it. Any sources, examples, suggestions, anything please? :)

Thanks in advance
Goral