The Velosolex was very popular in the 1950's. It fastened to the handlebars of a standard bike, and propelled the front tyre using a knurled roller. You engaged the motor with a handle, then pedalled, thus starting the engine. A lawn-mower type throttle was fixed to the handlebars. The engine itself was registered, complete with a road legal number plate, and could be affixed to any bike. Still a few around today.