yes indeed a great sample, it motivated me to work on it and i have been able to improve stability with faster speeds. i have put together a updated version (which is worse commented) but gives some hints how to add roads etc.
It's really more stable! :)
I didn't know about FiniteRotation routine )
But why you use sphere geometry, instead of cyllinder? With sphere it behaves strange, when i release acceleration and try to turn. IMHO, with cyllinder it more realistic.
And about translation - ok, but i do not think that it is necessary to you )))
I try to use fake method for moving the car forward. Now it like a jet engine.
I apply force directly to a body of the car, and it look more stable and realistic (especially on the high speed), in spite of the fact that it false.
Moving backward remains former.
Some cosmetic changes are made.
The track is added.
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And i made double comments - in Russian and in English