You will need to add an invisible entity. Such an entity would also be a physics entity constrained via a hinge joint to your motorbike. Or, you can try to fake the bike effect by having 4 wheels in close proximity with 2 invisible. I have gotten a motorbike working with the first method and it works quite well. The only problem is that the bike leans the opposite way that it does in real life. I've gotta try figure that part of the problem out tonight...