For my 2-penny-worth:
Although the two parts should obviously be designed to last more or less the
same time, it is not an exact relationship.
Wear on the carbon throw-out has a direct relationship with the total amount
of time the clutch is depressed (somewhat like a generator carbon brush), as
it is designed to wear, as and only when, it provides friction.
Thus, Jan's comment about holding the clutch depressed (for example at
traffic lights) has much more relevance for a carbon system than for a
ball-race.
However, clutch plate wear is related more to driving style. More aggressive
driving, and clutch 'dragging' will wear out the plate relatively faster.
Also the carbon throw-outs do not tend to suffer from random failure to the
extent that ball-races do, at least in my experience.
Julian - UK
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