As I have made cars, I came across problems that I found solutions to. So I put down my discoveries in this Forum (other people may already know how to fix many of these issues as well.)

Spinning out of control:
If you are having trouble with keeping you car from spinning out, check out the suspension, best way to do this is loosen your suspension, or if you have a custom made system, see if any of the blocks are connected to other blocks that limit movement.
(If I worded anything in a way you cannot understand, let me know and I'll correct it, and tag you if you want.)

Weight issues:
Simple, just use XML modding to made parts weigh nothing, if you do not have XML modding, then ask someone to do it for you or just balance out the weight.