I had success with connecting a convex shaped-wheel entity with a chassis using a 6D-joint. All you have to do is to enable linear motion on the y-axis (up) and angular motion around the x-axis (forward) and y-axis (up) and set velocity as drive type, as well as constraints. If that doesn't work out for you, make two 6D joints. Don't use regular joints, like hinges or so, they are only intended for what they are designed for. Then, instead of applying forces on the wheels, I had good results with just adding a velocity to the chassis - the wheels will react then properly.