In the early days motorcycles didn't have clutches; riders simply pedaled the machine into life and kept going. Planning a motorcycle ride in advance involved avoiding any hill starts for obvious reasons.
The earliest clutches were no more than a rudimentary belt tensioning system for the rear wheel drive. The first proper clutch (a leather cone design) was fitted to a 1913 500-cc Douglas.
By far the most popular clutch design is the multi-plate layout, a design consisting of a number of driven and drive plates; typically made of steel (driven) and cork inserted steel (drive). For most street applications the clutches are classified as wet, simply because they operate in an oil bath in the primary drive case in early machines or by sharing the engine/gearbox oil in later machines.
The majority of multi-plate clutches operate the same way: drive from the engine’s crankshaft turns a gear to an outer drum gear; when the clutch is engaged, drive goes through the clutch into the gearbox where ratios dictate the speed of rotation of the output shaft and final drive sprocket.
The multiple plates in the clutch are held together by a series of springs which apply pressure onto a pressure plate. Disengagement of the clutch is typically achieved by a rod passing through the gearbox shaft applying opposite pressure to the pressure plate. In other words, the rod counters the spring pressure, thereby disengaging the clutch.
On some motorcycles, the pressure of the pressure plate is reduced by a mechanism lifting the plate.
Depending on the rider and type of riding he or she does, most multi-plate clutches will last for many thousands of miles. However, purposely slipping the clutch (to increase the revs.) will wear the plates quickly. This is a common problem on racing machines in general, but especially on high performance 2-strokes.
In general clutch maintenance is due when the rider experiences one of two problems: slipping or dragging.
As mentioned above, purposely slipping a clutch will greatly increase its wear rate. However, it could be argued that setting off from a standstill requires the rider to slip the clutch to initiate forward momentum. Needless to say, motorcycles used in heavy stop go traffic will wear their clutches out much faster than a machine predominantly used on long highway rides. The first indication that the clutch is in need of maintenance is when it slips under heavy acceleration. However, the rider must check the adjustment of the center push-rod (where fitted), the running clearances, and the cable adjustment (where applicable).
In most cases, a slipping clutch will get progressively worse and the owner will have no other option than to inspect the plates, measure their thickness (driven plates) and flatness (drive plates) and replace as required. It is very rare for the plates to meet the manufacturer’s specifications for thickness and flatness yet still slip. Should the owner find this to be the case, he should inspect the springs which may not have the correct free length and therefore apply the required pressures. Another possibility is incorrect oil use. Modern oils have many additives which occasionally are not compatible with wet clutches.
If all of the above checks out, the rider should check the push rod. On some designs the push rod is a multi-piece item separated by ball bearings. Over time the inevitable differences in the surface hardness’ will cause the rod (typically) to mushroom which can cause stiction inside the gear shaft.
A dragging clutch is one where the engine and rear wheel are not completely disengaged when the clutch lever is pulled in. The most common cause of this problem is a badly adjusted clutch. However, modern oils can sometimes cause this problem.
The most likely scenario for a dragging clutch occurs when a machine has not been used for some time (winter storage, for example). In this case, the clutch plates can stick together causing only partial disengagement. To negate this problem, the rider should (before starting the engine) select first or second gear and rock the machine backwards and forwards until the clutch disengages. Failure to do this will result in the first gear crunching during engagement, and/or the possibility of the bike lurching forward until the clutch disengages.