There are several issues in the setup. The main one is with the mesh setup. For best performance and setup the bones should end in the middle of the wheel, the center of the rotation, also all should points +x and if you are usign 3dsmax reset rotation and transformation before exporting.
Also remove the “root” bone, the first bone in the tree should be the mainvehicle mesh itself.
Take a look to this guide, under Joints.
https://docs.unrealengine.com/latest/INT/Engine/Physics/Vehicles/VehcileContentCreation/