Friction drive vs standard drive motors
We often hear people ask, "What's the difference between a regular bike engine kit and a friction drive engine? And how will I know what's going to work for me?" To answer that question there are a few things to consider: the mounting, sizing, and running of these engines. Through building and running each style, here's what we've come up with:
Friction drive motor installed on a beach cruiser
Mounting: Mounting an engine to a bicycle can be tricky, especially if you're the only one working on the bike. Standard drive motors require you to mount the engine in the center of your frame, which means loosening and tightening motor mounts, possibly drilling through your frame, and the assembly of a sprocket and adapter to your rear wheel. Friction drive motors mount over the front or rear wheel of your bike and drive directly on the tire, so there's no difficult sprocket assembly. However, there is a special mount the engine assembles on to, and that mounting hardware can be a little tough to install. Mounting ease advantage: Friction Drive motors.
Mounting advantages of a friction drive motor
Sizing: One of the hardest things about motorizing a bike is figuring out if it'll fit on your bike. Friction drive engines have a wide range of bikes they'll fit on, from 16" up to 29" wheels, so it can mount to nearly any bike you can imagine. But that may not be such a good thing. While standard drives are limited to 26" and 29" wheels, their limited compatibility plays to the safety of you as a rider. 26" and 29" wheels provide a wide riding room for your tires, allowing you a more secure ride. If you have a big engine riding over a 16" wheel you'll be replacing that wheel, tube, and tire a lot more often than you'd like. Remember, just because you can do something doesn't always mean you should. Sizing advantage: Standard Drive motors. Running: When I say "running" I mean the overall operation and life of these engines. Standard engines are a little slow on acceleration because you have to pedal, engage a clutch, then throttle while pedaling for a second; whereas friction drives only need you to pull the pull start and hit the throttle.
4-stroke motor on a beach cruiser
But if speed's a concern for you, consider that the friction drive has only been made up to 49cc's. Standard engines are made in all types of displacements (37cc, 38cc, 48cc, 49cc, 60cc, 80cc) and have tons of performance parts to add to their speed and torque. You also want to think about how much work you want to do on your engine. All engine kits will require some sort of maintenance, so you'll want to know what kind of maintenance you want to do. Standard engines require carburetor, clutch, electrical, and compression maintenance- the toughest to do would probably the clutch.
Friction drive on a beach cruiser
Friction drive engines don't have that many possibilities for maintenance, so it is required less often. However, the parts for friction drive engines are a little hard to come by. While you might not need to do more intense work on your bike, the parts for friction drives may not be as available than another style of engine. In terms of running, I'd say the friction drive and standard engine are matched. So what'll work best for you? If you're concerned with an engine fitting on to your bike (wheels smaller than 26" or women's frames) a friction drive is the way to go. But if you've got a cruiser or mountain bike and you want to run FAST go with a standard drive motor.

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