I think your car keeps falling for another reason. I used to have this problem very often when I played with newton for the firs times, but now I can make it work almost all the time in my project. There´s something mistical with it lol.

Are you working on a demo ? Do you have a Newton car demo ? I started loading a demo level , studing the newton parameters and then deleting the car and tyres and inserting mine ones, and made it a thousand times till my car finally got his tyres and fall in the ground with his wonderfull working suspension. Someone told you that you have to group the tyres before group them with the car model ? Are you placing your car far from the ground ? Arent you by mistake grouping something else with the car or tires ?

Cya