(1) Although the clutch in your bike may seem like a complicated mechanical device, nothing could be further from the truth. A motorcycle clutch is a “deadman switch.” When it is pulled in, the engine is disconnected from the drivetrain. When it is released, the engine and the drivetrain are hooked up.
(2) The working mechanism on a motocross bike’s “deadman switch” is a spring-loaded coupling that has enough spring pressure to keep the side geared to the crankshaft and the side geared to the transmission engaged. The trick is to have enough spring pressure to ensure that the coupling will stay engaged under the load of 50 horsepower, but not be so strong that the human hand can’t disengage the coupling.
(3) The spring-loaded coupling is comprised of two parts that are joined together by a pack of clutch plates designed to slip against each other—before locking themselves together. Why do they have to slip? Because if the plates didn’t slip, the bike would lurch into gear so suddenly that the engine would die from the sudden load. Because the plates are spring loaded and controlled by the clutch lever, the rider can release the clutch slowly or quickly depending on the circumstances.
(4) For maximum hookup, the stiffest possible clutch springs are desirable, but with several caveats. First, the stiffer the clutch springs, the harder they are to pull in. Given that during a race a rider could be forced to pull the clutch in more than 300 times, a clutch that is too stiff could wear the rider’s left hand out. Second, stiff clutch springs ramp up quickly compared to softer springs, thus stiff springs are harder to modulate when trying to feed in clutch slippage. Of course, while softer clutch springs are easier to modulate, they also lack the clamping pressure required to keep the drivetrain engaged under heavy loads.
(5) Clutch designers can fine-tune their clutches by altering the number of springs and the number of plates. For example, in 2009, Honda was interested in producing a very lightweight clutch that had a feathery feel. They elected to go with four clutch springs instead of the six they used the year before (2008). The problem with using fewer springs is that each of the four springs has to be stiffer than each of the six springs they replaced to provide the clamping pressure that the clutch needs; thus Honda should have gone to four stiffer springs on their four-spring unit. But, when the CRF450 engineers did this, the clutch pull was too hard. So, they compromised and put in softer springs. In the end, the 2009-2012 Honda had approximately 35 pounds less clamping pressure, plate to plate, than the 2008 clutch. The result was clutch slippage.
(6) Clutch plates get worn down by rubbing against each other during the engagement process. Additionally, the flat surfaces of the inner hub and pressure plate (the parts that compress the stack of clutch plates) also get worn down from constant rubbing. Next, the clutch springs take a set and lose some of their spring tension. Weak springs cannot hold the clutch plates together firmly enough and slippage occurs—especially as the plates wear and the tolerances change inside the clutch. If your clutch slips, you need to think about new plates, new oil, new springs, and inspecting the pressure plate and inner hub surfaces.
(7) The three telltale signs of a worn clutch are slippage, odor and heat. Slipping is the first sign. If you are revving your engine at 10 grand but are only doing 10 mph, your clutch is probably slipping. The second sign is that burnt clutch plates emit a foul odor that is easily recognizable. The third sign is visible when the clutch is taken apart. The steel drive plates will be blue, the fiber plates will stink, and the pressure plate will look galled.
(8) A clutch gets hot because the rider abuses it (slips it too much). Additionally, as the clutch heats up, the fiber clutch plates expand, which changes the clutch-lever end play. In the course of overheating, a clutch will go from the perfect end play to no end play to too much end play. All of this takes place in a single moto and creates a myriad of clutch problems, none of them good.
(9) The fiber plates are designed to both slip and grip. They slip when the rider feeds the clutch lever out, and they grip once the spring tension is maximized. Between each fiber plate is a bare-metal plate called the “drive plate.” The metal drive plates are connected to the transmission via the inner hub, while the fiber plates are connected to the engine via the clutch basket. The interplay between the two types of clutch plates is what determines motive power.
(10) Drive plates can be made from steel or aluminum. Aluminum drive plates reduce rotating mass and increase throttle response. Aluminum plates, however, tend to wear quicker than steel plates, and the aluminum particles contaminate the oil. Steel drive plates rarely wear out, but can still be damaged by excessive heat.