If you’re not a mechanic by trade or interest, you might not have spent a lot of time considering the individual components of your vehicle. As one of the three pedals of a manual car, a clutch is something many of us have heard of, but few of us understand how it actually works. The first step to reducing the likelihood you’ll need a new clutch is to know what’s going on beyond the pedal.
What is a clutch?
To start with the basics, the clutch is the left hand pedal in manual cars. This connects to the inner workings of the car, acting as a sort of bridge between the engine and the gearbox (which is itself connected to the wheels). Starting from the engine end, the clutch consists of:
- A flywheel
The flywheel is a disc that is attached to the engine’s crank on a permanent basis. This means that the flywheel is rotating whenever the engine is – they cannot move independently of each other. A flywheel has teeth around its edge which interact with a pinion – a small, round gear – when the car starts. The starter motor kickstarts the flywheel into motion using battery power, which starts the engine going. The engine then takes over and uses its own fuel.
- A clutch plate
The clutch plate is a disc that serves to create friction between itself, the flywheel on one side, and the pressure plate on the other.
- A pressure plate
Bolted to the flywheel, the pressure plate is slaved to spin when it and the engine spins. In between, the clutch plate isn’t attached to either the flywheel or the pressure plate. The pressure plate is made to clamp the clutch plate in place whenever the clutch pedal is engaged (raised).
When the clutch pedal is pushed down, the diaphragm spring at the centre of the pressure plate releases the pressure on the clutch plate, so it is no longer pressed against the flywheel. This is controlled by the release bearing and release fork which are activated by the driver depressing the clutch pedal. These parts all come together to form the clutch mechanism, which mechanics check over as part of your car servicing.
What does a clutch do?
Using the mechanism explained above, pushing down the clutch pedal disengages the engine from the gearbox and therefore the wheels of the car. This is essential to giving the driver greater control over the speed of the vehicle in a manual transmission set up, as it allows you to change gears according to driving conditions, speed requirements or anything else that may affect your driving.
If you try to change gear without properly disconnecting the engine and the gearbox, you risk grinding the gears, which can damage them over time. It’s important to get your clutch and gearbox checked during your car servicing, as these areas aren’t assessed as part of your MOT. Grinding the gears can also be quite jolting and uncomfortable for both you and your passengers.
Do automatic cars have a clutch?
Although we often think about the clutch as being something only manual cars have, automatic cars do have a similar system in place. It’s not a clutch in the traditional sense, as you don’t have a clutch pedal, but it performs roughly the same function.
In automatic cars, the mechanism you might call equivalent to a clutch is a torque converter. This interacts with what is known as a planetary gear set. Like the solar system, it has a main ‘sun’ gear in the centre, with various other gears orbiting it. A system of brakes and clutches – a different sort of clutch – engage and disengage these gears as necessary, all without the driver lifting a finger.
However, this mechanism is vastly different from the clutch of a manual car. A semi-automatic transmission system combines aspects of both manual and automatic transmissions to give the driver the opportunity to select gears manually without having to worry about a clutch pedal. Like automatic systems, these are quite different from manuals.
Do electric cars have a clutch?
Most electric cars use automatic transmission, so they don’t have a clutch. There are some electric cars on the market that do use manual gear selection – mainly to cater for those who don’t want to give it up – but it isn’t necessary in electric cars because they don’t use a combustion engine.
In a diesel or petrol powered engine, you (or the car, if it’s an automatic) change gear to compensate for the engine’s revs not matching what you need it to do. An engine must be at 1,000 revs per minute (rpm) to even be idling – but this has the power to make the car move at 75 mph. Naturally, you don’t want to be doing 75 mph when you first start the car, so you use a lower gear to push more of that power into supplying force than speed. This helps provide the huge amount of power needed to make a stationary car move.
Then, as you speed up, you go up in gears to compensate for the engine having reached peak torque. This is where the engine is spinning as fast as it can go. Increasing the gear allows more of the engine’s rotational power to go towards speed as opposed to force – which is why you use a higher gear on the motorways – and this allows the wheels to spin faster than the engine.
Electric cars don’t need to do this, as an electric motor can provide up to 20,000 rpm, so there’s no need to go up in gears, and no need to use a clutch.