Vehicle clutche

There are many diferent vehicle clutch designs but most are based on one or more friction discs, pressed tightly together or against a flywheel using springs. The friction material is very similar to the material used in brake shoes and pads and contained asbestos in the past. Also, clutches found in heavy duty applications such as trucks and competition cars use ceramic clutches that have a greatly increased friction coefficient, however these have a "grabby" action and are unsuitable for road cars. The spring pressure is released when the clutch pedal is depressed thus either pushing or pulling the diaphragm of the pressure plate, depending on type, and the friction plate is released and allowed to rotate freely.

While engaging the clutch, the engine speed may need to be increased from idle, using the manual throttle, so that the engine does not stall (although in most cars, especially diesels, there is enough power at idling speed that the car can move although fine movements with the clutch are needed). However, raising the engine speed too high will cause excessive clutch plate wear and cause a harsh, jerky start. This kind of start is desired in drag racing and other competitions, however.


Wet and dry clutches

A 'wet clutch' is immersed in a cooling lubricating fluid, which also keeps the surfaces clean and gives smoother performance and longer life. A 'dry clutch', as the name implies, is not bathed in fluid that robs it of some energy. Since the surfaces of a wet clutch can be slippery (as with a motorcycle clutch bathed in engine oil), stacking multiple clutch disks can compensate for slippage. Most Moto Guzzi and BMW motorcycles use a triple plate clutch like a car.


Clutch operation in automobiles

In a car it is operated by the left-most pedal using hydraulics or a cable connection from the pedal to the clutch mechanism. Even though the clutch may physically be located very close to the pedal, such remote means of actuation are necessary to eliminate the effect of slight engine movement, engine mountings being flexible by design. With a rigid mechanical linkage, smooth engagement would be near-impossible, because engine movement inevitably occurs as the drive is "taken up". No pressure on the pedal means that the clutch plates are engaged (driving), while depressing the pedal will disengage the clutch plates, allowing the driver to shift gears.

A manual transmission contains cogs for selecting gears. These cogs have matching teeth, called dog teeth, which means that the rotation speeds of the two parts have to match for engagement. This speed matching is achieved by a secondary clutch called a synchronizer, a device that uses frictional contact to bring the two parts to the same speed, and a locking mechanism called a blocker ring to prevent engagement of the teeth (full movement of the shift lever into gear) until the speeds are synchronized.


Non-powertrain clutches in automobiles

There are other clutches found in a car. For example, the radiator fan may have a clutch that is heat-activated. The driving and driven elements are separated by a silicone-based fluid. When the temperature is low, the fluid is thin and so the clutch slips. When the temperature is high, the fluid thickens, causing the fan to spin.


Clutch operation in motorcycles

On most motorcycles, the clutch is operated by the clutch lever, located on the left handlebar. No pressure on the lever means that the clutch plates are engaged (driving), while pulling the lever back towards the rider will disengage the clutch plates, allowing the rider to shift gears. Some cars and mopeds have a centrifugal clutch, using centrifugal forces to engage the clutch above certain rpm, see Saxomat. Racing motorcycles often use slipper clutches to eliminate the effects of engine braking.


Other clutches

A clutch may also be a device on a shaft that will "slip" when higher than normal resistance is encountered on a machine. An example of a clutch such as this may be mounted on the driving shaft of a large grass mower. The clutch will "slip" or "give" if the blades were to hit a rock, stump, or other immobile object.