Auxillary power to the back wheel of a pedal bike with a pressure contact rotating wheel/gear affair has been around for the last 70 years, so for those that are musing about whether it will have enough traction on the tyre in the wet, it would be productive to search for the previous generations of the type and see how they used to fare. The info should already be available. Regards, TL
The kids around here do the same thing with old weed whacker motors they pick up at lawn sales.
A wooden spool wrapped with adhesive rubber tape on the crankshaft gives their friction.
A 40cc 2 stroke weed eater motor will propel a 70 pound 10 year old on a BMX at light speed!
We used to use a similar thing to the weed whacker idea that MB's kids used. We used to get old 50cc engines and try to convert them. After a friend had a nasty crash we all stopped.
Simple enough, get a 50cc that runs, but the bike is bust up, then move the engine over. They used to be transferable onto a go cart as well. They were awesome fun. Kind of thinking I have a new project idea to be made.....when I have time.