Vehicle setup

I’m having nasty troubles with setting up from the Double Suspension tutorial.
I think the captions on the video explain it, but in brief – physics and collision aren’t being friendly. I’d like to know what I’ve done wrong.

Video : - YouTube
Project (rar 11mb on Mega) : MEGA

If anyone wants to diagnose the 3dsMax scene I can send it – I noticed that exporting the FBX out from UE4 doesn’t import back in again without crashing the editor – maybe something to do with vert welding or scale… anyway.