The Physics Asset is incredibly important for vehicles, and cannot be overlooked or skipped. When you first have Unreal Engine 4 create a Physics Asset for you, it will
probably end up looking much like this:
That is because the Physics Asset Tool (PhAT) attempts to wrap the vertices skinned to a joint the best it can. And honestly, you are going to delete and re-create them all.
Why? The PhAT does not currently have a way to gracefully handle the re-creation of the constraints that hold all the Physics Bodies together. So if you delete an intermediate
Physics Body, it does not create a new constraint up the hierarchy. However, if you delete all the Physics Bodies and start building them from the root joint out, all your constraints
will get created correctly.
In the Hierarchy pane, Shift Select everything, and then press the Delete key. This will remove all Physics Bodies from the asset.
Now, starting with your root joint, start creating physics bodies on your vehicles joints. Keeping in mind, you only need a physics body on a joint that either needs to be physically
simulated or affects the bounds of the vehicle.
In the case of our buggy here, a box for the root and spheres for the wheels will be just fine to start, but it will require a number of other Physics Bodies to get the desired