Rear Bike Tire Lasting

Good lord. Please stop what ever you are trying to tinker together. I say this because your description implies to me that you are trying to make a motorized vehicle capable of being driven at 50 MPH on a level road using a bicycle's rear axle that has a Bendix style coaster brake. I'm not saying that these parts cannot be put used to do something approaching your fantasy, but you come to us with such a poor project description that it is obvious you have no idea what kind of problems you can have in this project. You also quickly slip in the idea that you think that this can be done safely if we answer your future questions. An adult human quickly decelerating from 50 MPH (by impact) will easily die. Decelerating from 50 MPH with a braking system meant to work on lesser masses and lower velocities will work for some scenarios but I can guarantee that the internal steel on steel drum braking surfaces internal to the hub can and easily will liquefy surfaces in a truly spectacular brake fading condition.

Having stated my critical alarm, I fully admit that many an adolescent tinkerer has fabricated a motorized dirt bike from a lawn mower engine and an old rusting bicycle. Many of these contraptions did eventually exceed 50 MPH. None of these aspiring mechanical engineers ever thought about safety. Where's the fun in that? Beside that, parents worry about their kid's health and safety, the kid doesn't. A tinkerer also never worries about how long the bike parts will last. It was an old bike in the first place. When it breaks, they just fix it or junk it.

So if you really are the tinkerer of this thrill ride, stop. You have the completely wrong perspective to do this project. You will get somebody severely injured from your efforts. If you are instead the worried parent of a future mechanical engineer, I again say stop. But I'm not sure what or who you should stop. If you are coming to us because you don't understand what you child is doing, stop asking us and ask them. I do recommend that you make sure that this project goes through some reasonable intermediate low power & velocity testing. This way when the suspension or brake failures occur (and they likely will) riding skill and/or the magical healing capabilities of youth will make the moment in hindsight a great memory. If your not the parent, well stop, you will only give yourself grief interfering with another person's child.