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# How to rig a car for a 3d game

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Recently I modeled a car for my game and made a physics rig for it, only to find that it didn't work at all.

This is the first car game I am working on and I would like some advice.

Is there a difference between a rig used in a game like GTA to one used in NFS ?

Where can I learn how to rig a car?

What physics is involved in a car game?

I am especially concerned with the wheel axis, my own rig's wheels would turn witch ever why thy wanted.

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I think it depends on how deep you want to go. If your working on a realistic physics system, you'll need to model torque at the engine, torque through the transmission, torque to the diff, and torque to the wheels. you will need to deal with friction at the wheels, and how that is handled by the diff. Springs from the axles to the car body, and sway bars or similar to stop rolling during turns.

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If you want to roll your own, you still may want to look at some (free source) physics engines (no pun intended) that are out there. Specifically for rigging vehicles, I'm only familiar with ODE. I'm sure there are other examples for Bullet, etc.

What physics is involved in a car game?

If you're not already familiar with the basic application of forces (f=ma, force-at-position=torque+force-at-CG, springs/dampers, etc), you'll want to brush up on them. Burnt_Fyr provides a pretty good list of the parts of the system you may want to model. You may also want to google for information on "how a car works."

You can start with a basic model, e.g.:

1. rear-end-drive means wheels fixed with position to the CG. Forces where the tire meets the road - gravity, rolling friction, side forces, etc.

2. front-end-steering means the user controls the angle the wheels make with the CG. Friction perpendicular to the rolling direction causes the car to rotate (i.e., turn).

Go from there:

a. Rear-end-drive - frictional force from wheel rotation, through the suspension to the CG provides one of the forces on the CG. Also, work your way from the wheels, through the drive axle, the diff, the engine, etc., as Burnt_Fyr describes (in reverse) to get the source of that wheel rotation.

b. Front-end - work your way from the tire against the ground, back up the suspension to the center of CG.

In general, you'll start with all the forces that act on each of the four wheels. For each of those, calculate the force and torque on the CG for it. Sum all the forces and apply F=Ma. Sum all the torques and calculate the angular velocity of the CG.

Edited by Buckeye
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