I'm looking at building an electric-assisted bike. I plan on running a chain directly from the motor to the rear hub, and I will be keeping the crank also attached to the rear hub.

Now say the batteries go dead or something and I have to pedal home. This will spin the motor at a decent RPM without it ever actually being on. Will this ruin the motor? Nub question I know, but I don't want to fry it the first time the batteries go dead. If spinning it will fry the motor, I guess I could just keep a master link in the chain, and if the batteries die I just pop the chain off and continue on.

I realize there are much better ways out there of making an e-bike, but currently I have $30 invested and all I need to get still is a new sprocket for the motor that matches bicycle tooth spacing(~$15). If there's another way of making an e-bike for less than $50 let me know. I'm using 2 12v motorcycle batteries for power. Everything else is off of various electric scooters that I got on freecycle. It is NOT a brushless motor.

Thanks

Nah.. Spinningthe motor while off won't hurt it, however,since it won't have a "freewheel" on it, it's gonna be hard to pedal...
Also... The 2 - 12v motorcycle batts are NOT meant for deep cycling...
I'm betting the best you'll get from them is 10miles and then after a month, they'll be dead.

I work at a recycling yard so I can get used batteries for free as long as I turn in my old ones. They may only last a few cycles but this is basically just an experiment. If I decide that I like e-bikes then I will make a better quality one with a higher voltage. Thanks for the info.

A DC motor with magnets becomes a generator when it's spun, if you disconnect it from the battery the generator will not be 'loaded' so therefore it wont require much force to spin.