Surely someones knows, and before i start i know you guys are all for pedals and human power, but i am doing this to a spare crud bike.
i have all the skills, parts but no know-how.
Looking at putting a motor of some sort, probably whipersnipper or chainsaw onto an old mountain bike.
I am not sure which would be the correct/easiest method, which engine would be the best and easiest, also if anyone has pics of on they have built.
We thought about welding a bike peg onto the whippersnipper drive shaft and then placing the peg onto the tyre, when the peg turns so does the wheel, but this would wear out the tyre and become ineffecient.
so i though if i want to incorporate the pedals aswell, i could use an extended axel bolt and use two rear sprockets, effectively two chains, one for the pedals and the other onto the engine.
I saw a dude with kind of what you were describing about the whippersnipper, but don't know what at whippersnipper is, so I'm not sure. It was a chainsaw sized motor on a rack coming out behind his seat, and then a drive going down to a smaller wheel that turned to push the rear wheel. He'd gotten it from a kit. The smaller wheel was about 2" in diameter and rubber. I think there was a clutch mechanism so that either the wheel spun faster, or pushed further into bike tire for more engagement. It also disengaged when the motor was off.
He cruised by my at like 35mph, but had to stop to let the wheel cool down, and I caught him a little after, and checked it out. He had the normal chain/deraileur settup, just didn't need it with the motor.
I'd worry about the stopping power of regular brakes against the motor, you probably want disks at least on the rear, and should hook the brake up to something to disengage the motor.
I'd search for some of the kits on google or something - they'd probably give you a better idea than most people here would.... Maybe if you were at least using a low emissions motor (do they make them that small) you might get a little less heat...
That's why he had a clutch to bring the motor's wheel up from the rear wheel on the bike. But he'd have to hit that switch first before hitting the brakes, I think. You'd need to set it so that hitting either of the brakes on the bike lifted the engagement wheel from the motor as you hit them. I think that there are ways to set up brakes where when you pul on them you pull two cables - one would either stop the engine or remove the power wheel. You could probably modify one if you have some decent machining skills.
Honestly, though, it's a cool idea and all, but I'm one of those crazy riders that rides without brakes or coasting. I've just seen this once. There's a lot of tech involved that I don't know... Also, I wouldn't neccessarily do this on a crud bike. It's probably not going to have the proper components or fabrication to deal with the higher speeds.
Another thing I forgot in the first post - it changes the legalities of riding on the streets. That will probably vary depending on where you are.
And of course, wear a helmet, probably a motorcycle one if you're going that much faster.
Oh, and - do it on a fixie. In a really low gearing. I'd pay good money to see someone try to keep their feet on the pedals (even clipped in) without going down at 30mph on something like a 36/18 ratio....