| It's still really only one type - it's just that the later STM Evoluzione ones use a single central (larger) spring rather than a number of individual posts with associated springs.
The idea of the single central spring (STM call it a diaphragm spring) is to allow them to make the engagement of the clutch more progressive and to give better feel. It also reduces the weight of the clutch components.
All of them use ramps to disengage the drive under backwards torque, and some combine the ramps with ball bearings to make this action smoother - note that you would not normally "feel" the ramp action in everyday use (ie, when pulling away or changing gear) as it only kicks in to stop the back wheel locking up.
You've then got various designs of clutch basket, from the stock 12 tooth basket to the STM like 48 tooth basket. This spreads the load of the plates over 48 faces rather than the standard 12, which in effect gives you only a quarter of the wear rate on the basket.
Basket materials also make a difference. Most aftermarket ones are alloy, sometimes hardened or anodized (which is not always the same thing...) and the idea here is to reduce rotating mass of the clutch and help things spin up quicker. The only problem this creates is the mismatch between basket material and clutch plate material can lead to increase wear on the weaker material - ie, an aluminium clutch basket with oem steel plates means more wear in the basket. This is another reason to match the basket material with the clutch plate material, for example the ST alloy plates.  |