As others have indicated its all about the total gear reduction. The target overall reduction seems to fall in the range of 18:1 - 22:1 depending upon motor type and its powerband. The Chinese motor you mention has a primary reduction of I believe 4.2:1 built into it. The older/early motorcycles achieved this total reduction in one shot, hince the need for a large dia. rear sheave/pulley. The Whizzer that you mention uses V-belts and use a jackshaft mounted on the frame so there is a primary reduction to the jackshaft and then final reduction to the rear wheel. The V type belt won't wrap around a very small pulley so to get the same amount of total reduction larger pulleys are needed. The biggest draw back to using belts is the fact that they don't come apart like a chain and there for comming up with a driveline that dosen't hit the chainstays and seatstays of the bicycle can be difficult. This problem is really challengeing with a frame mounted motor, less so with a rack mounted motor. The other area you will find challenging is converting the output driver of the chinese motor from its 11t sprocket to a driver for a belt. If the Gilmer Style Belt Drive is what interests you you might have better luck looking into the world of Metric Timing Belts. Pfeifer Industries :: Metric Timing Belts and Pulleys Pfeifer Industries in Naperville Ill. Is a leader in the field of metric Timing Belt Drive Trains and their web site has lots of tech information on the various parts of this type of drive train. Another source of parts for this type of drivtrain is Viza Motors in Scottsdale AZ. V2 Belt Drive Parts Viza Motors - Push Scooters - Gas Motorized scooters - Electric Generators - Water Pumps - Viza Motors LLC They have 3 different sizes of 5mm metric timing belt pulleys. They have a 20t driver and two different driven pulleys a 70t and a 102t.

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