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Modern scooters, now appearing in their thousands on our roads, have some very interesting features which make them much easier to ride than the scooters of former years.
They don't have a clutch, to begin with, and you never have to change gears. In fact, they don't seem to have gears at all. You get on, start it, twist the throttle and it goes. When you want to stop, you close the throttle, apply the brakes, and it stops, the engine idling, ready to take off again. How do they manage this?
On these scooters the engine crankshaft is connected, via a centrifugal clutch, to a pulley situated roughly under the rider's seat. This is the driver pulley. From it a notched V-belt, similar to the fan belt still used on many cars, goes to another pulley which is joined to the rear wheel. This is the driven pulley.
When the crankshaft is spinning fast enough, the clutch engages by centrifugal action, as explained below. The driver pulley must then rotate along with the crankshaft, and this rotation is transferred to the driven pulley, and hence to the rear wheel, by the V-belt.
The entire drive-train is enclosed by a removable cover, visible on the one side of the scooter. So far the principle is the same as on an ordinary bicycle, where the drive is taken by chain from a front cogged wheel to a rear one.
The real ingenuity on the modern scooters comes in providing the varying gear ratios needed on any petrol engine. When pulling away, the engine needs to be spinning much faster than the drive wheel. At cruising speed, on the other hand, the ratio of engine revs to wheel revs has to be far lower.
On cars, this has traditionally been accomplished by a gearbox providing 4, 5 or 6 distinct ratios. On scooters, it's now done by means of a continuously variable transmission. It works like this: Both driver and driven pulleys are split into two halves, so that each pulley consists of two shallow cones facing each other. The distance between the two sections can be varied on each pulley.
When the sections are moved closer together, it causes the V-belt to ride higher on the pulley, thus increasing the effective diameter of the pulley.
Novice riders will enjoy the story about the macho biker who turned up at a meeting on his 750cm³ road rocket and decided to try out a little 50cm³ buzzbike equipped with a centrifugal clutch.
On leaving, he had to stop at a set of traffic lights. He felt the urge, naturally, to blip the throttle a few times while waiting for the lights to change. This time, however, the lil' 50 bucked out from under him at the first blip, dumping him unceremoniously on the road.
Needless to say, this was not viewed in silence by the assembled fraternity.