<P> Scooter engine sizes range smaller than motorcycles, 50--850 cc (3.1--51.9 cu in), and have all - enclosing bodywork that makes them cleaner and quieter than motorcycles, as well as having more built - in storage space . Automatic clutches and continuously variable transmissions (CVT) make them easier to learn on and to ride . Scooters usually have smaller wheels than motorcycles . Scooters usually have the engine as part of the swingarm, so that their engines travel up and down with the suspension . </P> <P> Underbones are small - displacement motorcycles with a step - through frame, descendants of the original Honda Super Cub . They are differentiated from scooters by their larger wheels and their use of footpegs instead of a floorboard . They often have a gear shifter with an automatic clutch . </P> <P> The moped used to be a hybrid of the bicycle and the motorcycle, equipped with a small engine (usually a small two - stroke engine up to 50 cc, but occasionally an electric motor) and a bicycle drivetrain, and motive power can be supplied by the engine, the rider, or both . There is also Sport mopeds--a type of moped that resembles a sport bike . </P> <P> In many places, mopeds are subject to less stringent licensing than bikes with larger engines and are popular as very cheap motorbikes, with the pedals seeing next to no use . Mopeds were very popular in the United States during the fuel - crisis of the late 1970s and early 1980s, but their popularity has fallen off sharply since the mid-1980s . In response to rising fuel prices in the first decade of the 2000s, U.S. scooter and moped ridership saw a resurgence . Sales of motorcycles and scooters declined 43.2% in 2009, and continued to decrease in the first quarter of 2010, with scooter sales doing worst, down 13.3% compared to a 4.6% drop for all two - wheelers . </P>

How many different classes of moped are there
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