Pocket, cross-draw or appendix (if you have a slim enough build). I would think once they grab you by surprise you are down on the ground grappling and they are concentrating on your backpack and/or bike so practice. Practice (dry or better w a dummy gun) how you would draw while still on your bike and on the ground grappling. I think you'll find it is much quicker and less printed to draw from the front in either circumstance.
A man's life, liberty, and property are only safe when the legislature is NOT in session. Will Rogers