A motor scooter—more usually abbreviated to scooter— is legally classed as a motorcycle in the UK if the engine displacement is above 49cc. Below that, and the machine is called a moped.
Irrespective of engine size, scooters tend to differ from true motorcycles in their physical layout, seating position and in the arrangement of gearing,and controls.
The most important of these is the gearing and controls. Scooters are almost universally twist-and-go machines. This means that to start from a stationary position, the driver simply opens the throttle and the scooter will start to accelerate forwards. The throttle, as on all bikes, is controlled by a twist grip on the right handlebar.
When the engine reaches some relatively low speed, a centrifugal clutch starts to bite, and transfers power from the engine to the rear wheel. As the machine speeds up, a simple, automatic gear change switches from a low-ratio start-up gear to a higher-ratio running gear.
A scooter clutch is not designed to slip, as on a real bike. This makes it impossible to maintain high engine revs during low-speed manoeuvering, as is almost essential on a bigger bike. This is one area where the 'real' bikers do have a valid argument for scooters being somewhat less safe than their larger machines.
To stop a scooter, the rider releases the throttle, and applies the brakes. Brakes are arranged like a bicycle, with the throttle and front brake under the control of the right hand, with the rear brake controlled by the left hand. There are no foot controls on most scooters. Compare a real bike which has throttle and front brake under the right hand; clutch under the left hand; gear change on the left foot and rear brake under the control of the right foot.
The physical layout of the scooter is also quite different from most motorcycles. A typical scooter layout has small wheels, a small, often 2-stroke engine mounted close to the rear wheel and off the centre line, with a small fuel tank and large storage capacity under the seat. Most modern motorcycles have 4-stroke engines mounted between the wheels and on the centre line, much larger fuel tanks, larger wheels and limited storage capacity.
Jasstrong says the rearward position of engine makes a scoot easier to wheelie: to wheelie a scooter, stop it, lean back a bit, hold the front brake, then wind on the throttle hard and a split second later drop the brake. it will wheelie. Don't ask me how I know, I just do, okay?
The biggest difference, however is in the chassis and riding position. Proper motorbikes have a frame which is nearly as high as the seat, and this is usually surmounted by a large fuel tank, forcing the rider to mount and dismount by swinging a leg over the rear of the bike. The riding position on most bikes is with knees bent through more than 90°. Sport bikes have the body leaning forward over the tank, with the back at a steep angle to the vertical. On a scooter, and many all-purpose bikes, the rider’s back is nearly vertical, and on a scooter, the legs are usually bent through less than 90°. Scooters also tend to adopt a step-through design, so that the rider simply steps through the bike to mount it.
Many serious motorcyclists regard scooters with some degree of disdain. The arguments range from the justifiable position that small-wheeled scooters are unsafe because the wheels do not give enough gyroscopic stability at low speeds, and can also pitch the bike forwards and out of control if the rider encounters a deep pothole, to the somewhat ludicrous--given the number of automatic gearboxes on the road--position that 'If you cannot handle gears you should not be on the road.'
Another valid argument is that many scooter riders do not take the biking thing seriously enough. It is obvious to anyone who has ridden in a city that many scooter riders wear minimal protective clothing and many of these ride in an unsafe manner, endangering the lives of more serious motorcyclists. While these complaints are certainly valid, they are not a reason to disdain the scooter as such.
A third argument is that many scooters are under-powered. Acceleration is a strange phenomenon. Once you have enjoyed serious acceleration, anything less is perceived as unsafe. I know racing drivers who have lightweight race vehicles with hundreds of horsepower. When they get in a Subaru Impreza turbo (thought by many to be a fine and powerful road car), they will liken it to an old cart-horse because of its lack of speed and acceleration. Sport bikes have better acceleration even than those race cars, and for someone used to such arm-wrenching torque, a 125cc scooter will certainly feel under-powered. Nevertheless, that scooter has a power to weight ratio comparable with many mid- and high-performance vehicles-—at least up to 40 or 50 mph, which is all that is needed in town. So, while the things may well feel under-powered by the standards of many bikers, the performance exceeds that of many cars on the road. I cannot call that unsafe. I have driven my share of high performance vehicles and I have to report that I have learned that effective brakes, good steering and anticipation offer a better chance of getting a driver out of trouble than the accelerator, no matter how responsive or gut-wrenching.