The clutch enables the engine to be disconnected from the transmission in order to engage or disengage the gears. It consists of two friction plates. When the clutch pedal is pressed down the two friction plates move apart. This disconnects the engine from the wheels meaning none of the engines power can be transmitted to the wheels.
When you release the clutch the two friction (clutch) plates move together and power starts to be transmitted to the wheels. This is known and the 'biting point'.
The more you release the clutch pedal the more the plates touch and the more power can be transmitted from engine to wheels. Once the clutch pedal is fully released the clutch plates lock together and all the power from the engine can be sent to the wheels.
When you hold the clutch pedal so the clutch is only half engaged it is called clutch control. This can be helpful when needing to drive very slowly such as when carrying out manoeuvres.
Slipping The Clutch
This is holding the clutch pedal partially down so that the clutch is not fully engaged. This lets the engine rotate faster than it would if fully engaged. When manoeuvring at slow speeds it is often necessary to 'slip the clutch'. This technique is also useful when driving through shallow flood water, as it allows you to rev the engine higher at low speeds, forcing more exhaust fumes through the exhaust pipe which helps prevent water from travelling up the exhaust and into the car's engine and causing expensive damage.
Slipping the clutch can cause excessive wear to the clutch, however, so use only when necessary.
Driving with the clutch pedal pressed down is known as coasting. Obviously every time you change gear you will 'coast', but you should always try and keep coasting to a minimum, as it reduces the control you have over the car because:
engine braking will be eliminated
vehicle speed downhill will increase rapidly
steering response will be affected especially on bends and corners
it may be more difficult to select the appropriate gear when required
Allowing the car to travel in neural gear is also coasting.
Riding The Clutch
Driving at a steady speed with your left foot resting on the clutch pedal is known as 'riding the clutch'. This may seem harmless, as the pedal doesn't seem to be in use, however, clutches contain a clutch release bearing which applies pressure to the pressure plates to aid releasing the clutch plates. Resting your foot on the clutch can cause wear on the release bearing and too much pressure can cause the plates themselves to slip, causing significant wear to the clutch plate friction material.
Press The Clutch Pedal All The Way Down?
You don't have to press the clutch pedal all the way down in order to change gear. In fact, if you do this, you will cause unnecessary stress and wear on the diaphragm spring in the release mechanism. To change gear, you only have to clear the drive to free the gearbox, which is achieved as soon as you have passed the 'biting point'.