Continuously variable transmissions occupy sort of an odd place in the automotive universe. From a technical standpoint, engineers regard the CTV as something of a Holy Grail amongst mechanical achievements. On the other hand, they've been around at least since the days of Leonardo DaVinci. And on the third hand, people just don't seem to like them. Regardless, the belt-type CVT that scooters and other low-power applications use has been around for decades, reliably -- if not always efficiently -- transferring power from engine to drive wheels.
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A continuously variable transmission is one that can vary output speed relative to input speed without using distinct gear ratios. A CVT can allow an engine to remain at its peak power, torque or fuel economy rpm regardless of wheel speed. There are many different types of CVT, with the V-belt type being the most common.
A V-belt CVT consists of a pair of pulleys: one on the engine and another on the transmission. Rather than using two fixed sides, each of the two sides of each pulley can move closer together or further away, making the pulley as a whole wider or narrower. The inside walls of the pulleys are conical, closer together at the centre than at the edges.
A thick V-shaped belt connects the drive and wheel pulleys; the angle of its V matches the angle of the pulleys' inward slope. When the pulley sides get further apart, the V-belt drops down lower between them and closer to the base. As the pulleys squeeze together, they force the belt upward and closer to the edges of the pulley. Thus, the pulley can effectively change diameter by moving the belt closer to its centre or further out toward the edges.
The scooter CVT can have a variable pulley on both the engine and wheel, or a fixed pulley on one and a variable pulley on the other. Both utilise a centrifugal clutch inside; as the pulley spins faster, the weights on the clutch spin outward, pulling the pulleys closer together or pushing them further apart via a linkage between the centrifugal weights and the pulley. The wheel pulley is set up so that it gets narrower as wheel speed increases, thus increasing the ratio between its diameter and that of the engine pulley. The engine pulley does the opposite: getting smaller when the wheel pulley gets larger.
Pros and Cons
The belt-driven CVT functions well as long as you don't ask it to transfer too much power, but its ability to transfer power is fairly limited. Additionally, water, grease or oil on the belt can cause it to slip and fail to transfer power. Such CVT systems operate in an almost constant state of slight slippage, which will eventually cause the belt to wear out, stretch and require replacement.
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