rec.autos.simulators

Physics: Rigid Bodies

Alex Smit

Physics: Rigid Bodies

by Alex Smit » Fri, 06 Feb 2004 07:46:56

Hello!

Having been re-inspired by Ash :) I've decided to have another go at my sim
which desperately needs a re-write, starting from the beginning with rigid
bodies as the base for every moving thing.

Now I've done a bit of research into it before and failed MISERABLY :), but
a while ago I bought a book entitled "Physics for Game Developers" which
seems to have lots of good stuff in it so I've decided to attack it again.
I'm going to start with the basics and have a block falling from the sky and
hopefully make it bounce properly (I say hopefully, I've tried several times
to get even that going properly!) and am a bit more confident of my
understanding this time!

But before I start on my quest, my question is about multiple rigid bodies.
Does a car count as a single rigid-body, or multiple connected bodies? If
it's multiple, then what constrains the bodies to move together if they are
connected (e.g. all the wheels will be connected to the chassis, even if
they are experiencing different forces). I can get my head around single
bodies, but multiple bodies affecting a parent 'car' body is hard to get my
head around. For instance, when integrating the position/rotation of e.g. a
wheel, the wheel will not move itself, but will exert a force on the parent
chassis which will move. Does this then mean that each different type of
body (the wheel, chassis) have its own way of integrating? Or maybe a wheel
body has a flag which says 'do not integrate its position' ?

Just wondering how other people out there have handled this!

Regards.

Byron John Forbe

Physics: Rigid Bodies

by Byron John Forbe » Fri, 06 Feb 2004 22:28:56

   "Integrating"?


Ruud van Ga

Physics: Rigid Bodies

by Ruud van Ga » Mon, 23 Feb 2004 10:54:11

On Wed, 4 Feb 2004 22:46:56 +0000 (UTC), "Alex Smith"


>But before I start on my quest, my question is about multiple rigid bodies.
>Does a car count as a single rigid-body, or multiple connected bodies?

You can view it as multibody, or simplify and use 1 body for the
chassis.

Ehm the wheel is actually a separate body that moves by itself. It's
just connected by the springs and a couple of rods. Those restrain it
(not easy to implement btw). The spring force acts on both the wheel
(downwards mostly) and the chassis. That is why the chassis at some
point starts moving. It's not a direct result of a moving wheel, only
indirectly through the springs and rods.

If you really do multibody, just add forces and constraints, and it
will all work out (jacking forces, camber etc). In Racer, I have a
simpler system where the wheel moves purely vertical. Racing Legends,
F12k2 and IIRC Netkar use the full suspension multibody method.
As I said though, that is not simple to do (and slower, but handles
the physics more gracefully if it works).

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/


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