as it applies to full CVT transmissions ie: Honda Hobbits, Vespas/Piaggios and Derbis, not swinging engine types ie: Peugeots and Motobecanes
The driven pulley, otherwise known as the contra pulley, is an important part of a CVT transmission. It is often misunderstood and misused.
Driven Pulley Faces
There are but a few major dimensions when considering the driven pulley's faces/cheeks. The first is the angle of the faces. This angle should ideally be the same as the variator's and belt's. A steeper angle will allow for more variation for a given horizontal push or pull but will cause more parasitic drag. The second dimension is face diameter. The diameter should be large enough for the belt's friction surfaces to be totally covered by the faces at rest. The third dimension is driven pulley offset. The driven pulley offset refers to the center line of the driven pulley being in-line with the center line of the variator. If these are not in line parasitic drag will increase.
Scooter Driven Pulleys with integrated clutch function
Scooter driven pulleys often times integrate a clutch function. This allows for you to tune your clutch's lock up rpm above that of your variator's variating rpm, however this should never be done. A clutch tuned with a lock up rpm above the variator's variating rpm takes away from the variator's effective variating range. A scooter driven pulley with integrated clutch function, however, will still fully engage the clutch even if it's lock up rpm is slightly above that of the variator's variating rpm this is unlike a CVT with the clutch integrated into the variator like on Honda Hobbits for example. On a CVT with a clutch integrated into the variator, like on a Honda Hobbit, if one were to tune the clutch's lock up rpm above that of the variator's variating rpm the clutch would always slip and never fully lock up, this would cause severe premature clutch wear. A scooter driven pulley, however, can be useful for getting the clutch's lock up rpm extremely close the variator's variating rpm as having the clutch on the driven pulley allows for a over lap that's not terribly detrimental to clutch life. While still taking away from the variator's variating range this forgives clutch tuning inaccuracy which can be ideal when tuning for high rpm clutch lock up for drag racing. However you must beware trying to put a scooter driven pulley on a moped in most situations. Because of the integrated clutch function of most scooter's driven pulleys the actuation of the pulley is opposite of that of a driven pulley without a clutch function. Meaning that on most driven pulleys, a Hobbit's for example, the inner driven pulley cheek moves more inward the more the variator variates. This is in contrast with most scooter driven pulleys with a integrated clutch function. On a scooter driven pulley with a integrated clutch function the driven pulley's outer cheek moves more outward the more the variator variates. When a scooter driven pulley, with integrated clutch function, is used on a CVT that had originally had a driven pulley without a clutch function it causes the CVT's belt to ride cocked at rest, straight at mid variation, and cocked the other direction at full variation. This causes more parasitic drag. To use a scooter driven pulley with integrated clutch function one should also use a scooter variator that doesn't have an integrated clutch function, this lets the CVT's belt ride straight throughout the variation.