Next time to accelerator and the brake, this third pedal might seem insignificant, but that couldn’t be further from the truth.
Anyone who can drive knows roughly what the clutch is or how we use it. The third pedal along with the accelerator and brake is our clutch pedal and we use it to change gears, to perform an emergency brake, even to help get the car moving. What we often don’t know, however, is what actually happens when we press the clutch pedal down and how the clutch actually works. That all ends today with our comprehensive guide to what a clutch is and how it works, along with the different types of clutch you might have in your car.
The basics of gears and clutches
Put simply, the clutch is essentially a mechanical device that transfers power from the engine to the gearbox and then in turn to the wheels on the car. As part of this, it has the power to stop power being transferred from engine to the wheels and makes it possible to change gears when driving.
The clutch is hugely important. The engine generates power all of the time and has parts which are constantly moving, but the wheels cannot be constantly spinning. Taking away the power from the wheels makes it possible to start the car safely and to change gear.
To understand what is happening with the clutch, it’s best that we get to grips with gears to start with. The transmission system in manual transmission cars, of which the clutch is a key part, changes the gear ratio to make sure you are getting optimum power from your engine to the wheels.
The transmission is made up of an engine shaft or input shaft linked to the engine and an output shaft, connected to the wheels. Underneath these two shafts is the counter shaft and gears are on all three of these different shafts. When the input shaft rotates thanks to the power from the engine, the gears on the counter shaft are turned, and that as a result turns the gears on the output shaft, moving the wheels.
All gears move,but they move at different speeds. When they aren’t engaged the gears spin freely and independently from the output shaft. There are also hubs on the output shaft, with sleeves over them. These rubber sleeves have teeth that lock into the gears and anchor them, with the help of synchroniser discs, to the hub to drive the output shaft. Only one gear at a time can be engaged and thus the output shaft (and therefore wheels) rotate at the speed determined by this gear. So we have a situation where the input shaft is now spinning the output shaft, the engine is making the wheels rotate, with the help of the counter shaft and the gears.
So where does the clutch come into this
When we want to change gears, the connection between the wheels and the engine needs to be temporarily broken. One way of doing this is to turn off the engine, but this isn’t exactly ideal for seamless driving, and this is where the clutch comes in. If we try to change gears without breaking the connection, the output shaft will want to try and spin the counter shaft at a different speed to the engine, which is already spinning it. This is where you get a grinding sound if you try and change gears while driving without engaging the clutch. What the clutch essentially does is to take away all of this spinning energy and allow a smooth changing of gears.
There are two main parts to the most common basic fiction clutch: the clutch disc and the flywheel. The flywheel is being driven by the engine and the input shaft is mounted to the clutch disc. If your foot is not pressing down on the clutch pedal, there are a set of springs which keep a pressure plate pushed up against the clutch disc. This pressure also keeps the disc attached to the flywheel and so the power from the engine gets transferred from the flywheel to the input shaft via the clutch disc.
There is also a diaphragm spring between pressure plate and cover. When the clutch pedal is depressed, the diaphragm spring underneath the plate disconnects the clutch disc from the flywheel. This breaks the connection between the rotating engine and the wheels, meaning that the wheels continue to spin but under their own momentum, not through the power of the engine. This leaves the gear free to be changed without the risk of coming up against force from engine powered countershaft gears. The gear stick selects the gear and the cogs line up to one another with the help of the hub and the synchroniser, as soon as they are connected, the pedal can be released and the engine can begin to turn the countershaft again, this time spinning the output shaft based on the new gear selected. The speed the car can achieve depends on the literal size of the gear and the gear ratio. Without being too specific, if the gear ratio is high (so the gear on the countershaft is smaller than that on the output shaft) the car won’t go as fast, but it will be easier to make it go (lower gears) and if the gear ratio is low (the gear on the countershaft is bigger) the car will go faster (5th gear).
Different Types of Clutch
What has been outlined so far is the most common form of basic friction clutch, but there are many different types of clutch. Here are a few of the most regularly encountered clutches.
With multiple friction plates stacked on top of each other, one of the obvious benefits of multi-plate clutches is that the amount of friction generated within the clutch can be greatly amplified and so it can deal with a much higher torque input. Used in many racing cars including Formula 1 and WRC, the amount of friction needed to stop the clutch slipping can be fitted into the same diameter as a single-plate clutch due to the neat stacking.
Wet and Dry clutches
Wet clutches most often have multiple clutch plates and have a supply of oil to lubricate and cool the components. They are used in high torque situations where friction levels would be high and therefore clutch temperatures would soar without some form of coolant. A wet clutch should really be used for any powertrain producing over 250N-m of torque so that any wear produced through overheating is avoided.
Dry clutches have no lubrication on the other hand and are generally single-plate. This means they can be more efficient – lubrication might sound good, but it can lead to a lack of friction between the plates. The reduced amount of friction in a wet clutch system is therefore the reason for having multiple plates for effective clutch performance.
Dual Clutch transmission
Dual-clutch transmissions now dominate the premium car market. Using one large clutch for odd gears and a smaller clutch for even gears, this form of transmission is known for its ability to produce quick, seamless changes and if you’re driving a supercar, the chances are it is using this system. It’s also frequently found now on many hot hatches and saloons.
Used in automatic and semi-automatic setups, DCTs use two wet multiplate clutches which eradicate the need for a torque converter. The shifts are smoother due to the fact that the torque output to the driven wheels is not broken as it can be applied to one clutch while the other is disengaging, meaning no break in output.