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.
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