The clutch, an often overlooked device, is an integral part of motorcycles. While its concept is simple, allowing the rider to disengage power from the rear wheel while the engine spins, early motorcycles did not always feature clutches. Riding such motorcycles, which lacked clutches, offered a unique experience. Every start required a bump start, and stopping meant either stalling or lifting one’s weight off the saddle to allow the rear tire to gently spin, as tire traction was not a priority in those days. Furthermore, without a clutch, motorcycles couldn’t have gears.
Most motorcycle clutches are manual, found outside of automatic gearboxes, and they are typically wet clutches, operating in oil. These clutches are multi-plate, utilizing multiple friction surfaces. To understand the mechanism, imagine a disc-shaped plate resembling a large washer with a central hole. This is a clutch plate. When the rider wants to transfer power to the back wheel, they release the clutch lever. This action allows springs to push the clutch plate onto another plate connected to the rear wheel via the gearbox and chain. As friction between the two plates increases, more power is transmitted to the rear wheel until the plates lock, delivering maximum power from the engine to the tire.
Motorcycles present a challenge when it comes to clutch design due to limited space, especially across the frames of 4-cylinder engines. If only two clutch plates were used, they would have to be massive to provide enough surface area for the required friction. Consequently, most motorcycles employ multi-plate clutches. The principle remains the same, but instead of two large plates, there are usually over 10 smaller ones. Half of these plates are connected to the crankshaft, either directly at the end of the crank or turned by a primary drive gear. The other half are linked to the gearbox input shaft. The crankshaft plates have friction material attached to their faces and use tabs on their rims to lock into the clutch housing, the part that is typically driven by the crank. The gearbox plates are plain steel with teeth on their inner circumference that slot onto the gearbox shaft.
The two types of plates are alternated to ensure that both sides of all plates, except the end ones, contribute to creating friction. Releasing the clutch lever disengages the clutch springs, which exert pressure on a plate known as the pressure plate, evenly forcing the clutch plates together.
The number of clutch plates used depends on various factors. It is determined by the amount of power that needs to be transmitted (more power requires more plates), the size of the plates (smaller plates necessitate more of them), the characteristics of the friction material (slippier material necessitates more plates), and the strength of the clutch springs (weaker springs require more plates). In compact motorcycles with substantial power, a heavy clutch may be required.
Despite the importance of friction, most clutches operate within a lubricant, typically engine oil. This is because friction generates heat, which can easily damage the friction material. Engine oil helps prevent the clutch from burning out. Wet clutches can endure significant abuse and last a long time. They also facilitate smooth movement of other components such as bushes, bearings, and the plates themselves. However, even with specially designed friction material compounds, efficiency in oil is limited, resulting in the need for more plates, thus adding weight. This leads us to…
Dry clutches experience less power loss and provide greater friction than wet clutches. As a result, they can be smaller and lighter. This is why they are favored in MotoGP bikes and have been used in Ducati motorcycles for decades. Dry clutches produce a distinctive clatter, resembling an engine about to explode, at idle (the sound is caused by the rattling of the plates, which disappears when the clutch is released). BMW twins also often feature dry clutches, primarily because their crankshafts run from the front to the rear, allowing more space for a larger dry clutch similar to those found in cars.
However, it’s worth noting that dry clutches are becoming less common in road bikes. They are less durable, noisier, and can be more finicky to use when they become hot. As a result, their usage is declining in favor of wet clutches.
In conclusion, the clutch is a crucial component of motorcycles that allows the rider to disengage power from the rear wheel. Most motorcycle clutches are manual, wet, and multi-plate. The friction between the plates transmits power to the rear wheel, and the number of plates depends on factors such as power requirements and friction material characteristics. Wet clutches operate in oil, offering durability and smooth operation, while dry clutches provide higher friction but are less common in road bikes due to their drawbacks. Understanding the function and characteristics of clutches enhances our appreciation of these essential devices in motorcycle mechanics.
