@lazybitgames: boxbp is included in the zip, in box directory “box\boxbp.uasset”. In short the skeletal mesh have two bones, main and child. Main is for kinematic part and child is for simulated part.
@lastcoingames: hard to find it somewhere but you can look here, it is used for this: *Holbra VLOG* #56 Tatra 148 a Man - problémy na skládce - YouTube
Here is demo project with this things, use car under number 5: 155.4 MB file on MEGA