You replace your rear tire because you can see the signs of wear, but the signs of clutch wear are trickier to spot. You can lay your bike over and pop off the clutch cover easily enough, but what do you look for then? Ron Hinson of Hinson Racing demystifies basic clutch inspection.
01 You should have 1/8 to 1/4 inch of freeplay at the lever. To check, pull the cable assembly away from the perch, then pull in the lever and watch the gap between the lever and perch before the cable engages the clutch. (If you use the quick-adjust when riding, reset the freeplay after the bike cools.)
02 Stock steel clutch plates are an unfinished aluminum/silver color. If your plates are blued, then these plates, the fiber plates and the springs have all been heat damaged and should all be replaced. If there's no heat damage, check the steel plates for flatness and measure the thickness of the friction plates (check your owner's manual for specs). Springs cannot be tested by length. Stock springs are stiff enough when new, but their rates go away easily when exposed to heat. Replace them often-every 10 hours for clutch abusers.
03 Check the clutch basket ears for notches. These will cause the clutch to drag and make engagement/disengagement sticky. Filing these smooth is a bad idea-you can create an edge that could break plates or uneven contact points between the different tangs, and you will be taking off any hard coating. (A telltale sign of a notched basket is if the bike won't shift into neutral easily.)
04 Check the inner hub splines for grooving. Same rules apply here as to notching/grooving on the clutch basket ears.
05 Reassembly: Petroleum oil is better than synthetic oil for the clutch. In a motor that shares trans oil, the oil will become contaminated from the clutch long before it degenerates, so the long-lasting benefits of synthetics are wasted. Use high-quality pure petroleum and change it frequently. (Do not use any oil with moly.) Install the steel plates with the rounded edges toward the inside of the motor. Soak new fiber plates, ideally overnight, in the same type of oil you will use in the transmission.