Algodoo

Select everything and click on "no self collision". Also add 3 hinges on each part. I noticed that you have fixed them with around 1-2 hinges and some parts don't have hinges at all. Hope I helped

Thanks, but when I check no self collision parts just randomly drop off and spin in mid air.

Because they don't have enough hinges. If a part has only one hinge it will spin, if it has no hinges it will fall.

But I put 3 hinges on all parts?

On the scene you have uploaded there aren't 3 hinges per part. If the same happens idk. Try making breakable cars as I do them. My strategy isn't brilliant but with a few tweaks you can make a great car on that way

Oops, turns out that I'm such a retard that I didn't save it, for some reason this happens with most of my smashable/crashable stuff. It just flys away/disappears/has a massive spaz attack. I'm pretty pissed off, because I wanted to make this one really good. Thanks very much for your help, though.