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CVT Continuously Variable Transmission


Gears (cogs) can only give a fixed set of ratios between input and output shafts. Continuously Variable Transmissions (CVT's) rely on pulleys, wheels, and/or cones that can transmit drive at any radius thus offering infinitely variable "gear" ratios, within limits.

The motor scooter drive (right, c2001) shows a drive pulley, left, a driven pulley, right, and a belt between them. (The "teeth" on the belt are only to improve grip; there are no teeth on the pulleys.) The drive pulley is in fact made of two cones which can be moved closer together or further apart. Such movement varies the radius at which drive is taken off the drive pulley. The driven pulley is similarly constructed and the two operate in cooperation to keep the drive belt taut.

The motor scooter system only needs to handle low power and torque. A similar system was used on small DAF cars of the 1970s; two systems were used, one for the left rear wheel, one for the right rear wheel, thus obviating the need for a differential because they could deliver different ratios to the left and right of the car.

Belts are all very well for low power systems, although they do need to be changed regularly. Some more modern, higher powered, CVT's use steel belts which are more durable. Others use conical drive and driven members with a steel intermediate wheel (not toothed) which can move to take, and give, drive at different radii.

The advantage of CVT's is that they enable the engine to operate at exactly the optimum revs for given power demands. Early CVTs were limited in power handling ability; as of 2001 they developed enough to be offered with 2.5 litre V6's of up to 130kW, as in the 2001 Audi A4 - © L. A11ison, 4/2001

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