Detailed Guide : The Motorcycle Clutch

Editor@Throttle|Updated: June 7, 2018 9:24
Motorcycle-Clutch

The clutch on a motorcycle engages and disengages the transmission and engine. When you squeeze the clutch lever, you're effectively putting the bike in neutral. When you let go, you are engaging the engine and transmission. 

 

 
What is a motorcyle clutch?
 
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A clutch is a mechanical device that permits the control of power transfer and motion between two assemblies. In a motorcycle, those two assemblies are the outer clutch basket and the input shaft of the transmission. The clutch basket is turned by the crankshaft, while the input shaft transmits power to the transmission and eventually the rear wheel.
 
 
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Components of a clutch
 
A clutch assembly typically consists of outer basket, inner hub, friction plates, steel/friction plates, pressure plates and springs.
 
 
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How does a clutch work?
 
The clutch pack is made up of a stack of alternating steel plates and friction plates. The steel plates have tangs along their inner diameter that engage with the clutch’s inner hub, which is fixed to the end of the input shaft via splines. The friction plates have lugs along their outer edge that slot into the outer clutch basket. Springs (usually of the coil type but occasionally a diaphragm design) press the steel plates and the friction plates together, coupling the outer clutch basket to the inner hub, thereby transferring the crankshaft’s rotation to the input shaft. More plates means a greater load capacity for the clutch.
 
The steel plates are locked to the releasing disc via their internals keys so they all spin together as one unit   
 
Once the drive disc is placed on top of the above stack of plates, spring loaded screws are used to hold the entire unit together. The screws pass through both the drive and releasing discs before threading into three clutch spring nut sectors. This design keeps everything aligned, and also allows for movement between the drive disc and the releasing disc. The clutch springs provide the friction between the friction plates and the steel plates by sandwiching them between the drive disc and the releasing disc. When the clutch is engaged, the spring pressure is strong enough to hold all the plates together without any slippage. The friction plates are not locked to anything, so without this pressure they could spin. The spring pressure is matched to the horsepower/torque of the motor, so if you upgrade the performance of your motor you often need to use stiffer springs to keep the plates from slipping. As the friction plates wear over time, the clutch spring screws can be tightened to take up slop and restore spring pressure. This entire unit is often referred to as a clutch pack.
 
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The last major component of the clutch assembly is the clutch basket which, as the name suggests, holds the clutch pack. On the inside of the clutch basket are keyways that match the keys on the outside of the friction plates.  These lock the friction plates to the clutch basket, but just like the keyways on the releasing disc, allow the friction plates to move up and down. A gear runs around the outside of the clutch basket which is connected to the engine typically by a chain or belt.  In this particular example there are two gears on the clutch basket as a double row chain is used to connect it to the engine. 
 
This entire assembly is mounted on a set of roller bearings that rides on the transmission's mainshaft. The mainshaft also has a keyway that locks the drive disc to the mainshaft. This is the typical state of the clutch when the lever or foot pedal is released and the clutch is engaged. With everything locked together, the engine turns the clutch basket, which turns the clutch pack inside of it, which turns the mainshaft on the transmission. 
 
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To disengage the clutch, the clutch plates need to be separated from one another. Since the friction plates are locked to the clutch basket and the steel plates are locked to the releasing disc, if the plates are pushed apart the clutch basket can continue to spin while the steel plates, releasing disc, and drive disc are all stationary. Once the drive disc stops spinning, the mainshaft also ceases to move and no more power is being transmitted from the engine to the transmission. To accomplish this, there is a rod which goes through the mainshaft, known as the clutch pull rod. On the clutch side of the transmission, it is attached to a three legged actuating plate whose legs pass through slots in the drive disc and press against the releasing disc. 
 
The clutch release lever is what actually does the work of disengaging the clutch. When the lever moves back towards the rear tire, it rotates the clutch release fork which pulls the clutch pull rod through the mainshaft. This in turn pushes the actuating plate against the releasing disc, moving it away from the drive disc and eliminating the spring pressure on the fiber and steel plates.  Once the spring pressure is released, the friction and steel plates are able to move apart and are no longer held together by friction.  
 
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Conclusion
 
  • Without clutch you can not virtually put your bike's engine into gear, specially when the bike is stand still without any momentum. However with precise timing and practice you can do it while moving but it is not recommended as wrong timing may result into broken gear or entire broken gear assembly.
  • Clutch allows you to stop at any speed by disengaging the engine
  • It may also prove to be life saving device in case of engine seizure as by depressing the clutch you can avoid the sudden arrest of the bike which would be highly fatal at high speed.