How Clutches Work…

By Phillip Floria

With a few exceptions all motorcycles have a clutch, most are hand activated. You pull the lever and it disengages your engine from the transmission allowing you to stop the motorcycle or shift through the gears of the transmission. But just what happens when you pull that lever?

Early motorcycles had some rudimentary clutches, in most cases nothing more than a wheel which tensioned or de tensioned the leather belt running from the engine to the back wheel. No transmission, no shifting, speed was controlled by the engine rpm. The more gas you gave it the faster the engine ran and the faster the bike went.

Coming to a stop required use of the brakes and releasing the tension on the belt. Of course early motorcyclists had a lot more to do when out riding, on steep inclines it was time to use the pedals to give the engine an assist.

The idea of having a transmission with different gear ratios required a better way to put power to the rear wheel, something that wouldn’t slip under load, or when it was wet. The chain drive, which is common to most all motorcycles, was the answer, with that something more than a simple tension device was needed.

The modern motorcycle uses two types of clutches. Wet clutches are the most common on modern motorcycles, but some and racing motorcycles use a dry clutch.

The wet and dry clutches are very similar, with the exception of the oil which makes the wet clutch wet, both use a compact multi plate design.

A wet clutch consists of an inner drum connected to the transmission main shaft and an outer drum connected to the engine either by chain or gears. Both drums rotate in oil, independently of one another but on the same shaft. The two drums are linked together by two types of clutch plates. Fiber plates are keyed to turn with the outer drum while friction plates are keyed to turn with the inner drum. The plates are forced together by spring pressure. Pulling on the lever relieves this pressure, allowing the two drums to rotate independently.

The clutch lever located on the left handle bar may be cable operated or hydraulically operated. Cable operated clutches have a cable connected to the lever. When you pull the lever in the cable pulls a device to push the pressure plate away from the clutch plates releasing the spring tension. Hydraulic clutches operate the same way except that there is no cable.

The hydraulic system consists of a master cylinder with a fluid resevoir, tubing, hydraulic fluid and a slave cylinder. When you pull the lever a piston in the master cylinder forces the hydraulic fluid into the line. The line is connected to a slave cylinder, which moves according to the movement of the fluid. Just like hydraulic brakes. The slave cylinder is connected to the clutch through a rod that works on the pressure plate releasing the tension on the clutch springs.

The image on the right shows a typical clutch, the pressure plate is facing you and the springs, which apply the pressure, can also be seen. The lower image shows the bolts installed which apply tension to the springs that push the pressure plate against the pack of friction plates.

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