Transmission technology continues to move forward at a rapid pace, especially in terms of automatics, but manual gearboxes haven't changed drastically since the idea of multiple speeds for a rotating machine surfaced in the early 1900s. Modern cars have synchromesh and some even have rev-matching facilities, but all of them without fail use a clutch
of some description and it's a serviceable item so it's good to know when it needs attention.
What does a clutch do?
In simple terms, the clutch transmits power from the engine to the gearbox, but more importantly, it can isolate the engine from the gearbox to allow changing of gears within the gearbox. The clutch plate itself is usually made mostly of metal trimmed in a friction material on both faces. It has a splined centre section which enables it to slide along the input shaft to the gearbox. In its default position (i.e. without the clutch pedal pressed), it is clamped against the flywheel by the clutch pressure plate. There is no slip in this position.
When you press the clutch pedal, it disengages the clutch so power is interrupted. Some cars use a clutch cable to pull a lever within the gearbox (called the release bearing fork), which in turn pushes the release bearing into the middle of the pressure plate. There, a diaphragm spring releases the clamping pressure, meaning the clutch plate is no longer held against the flywheel, allowing it to rotate at a different speed depending on the gear selected etc. The clutch can be 'slipped' depending on how far the clutch pedal is pressed.
While the principle is the same, other cars use hydraulic operation instead of a clutch cable, where the pedal movement controls a master cylinder that alters hydraulic pressure to a slave cylinder. The latter then controls the movement of the release bearing fork.
How do you know if your clutch needs to be replaced?
Over time the friction material on your clutch will wear out and when that happens the clutch will start to slip. It should be quite obvious when your car's clutch is slipping when it shouldn't be. With the clutch pedal fully out (i.e. not pressed at all), there should be no slip whatsoever. Slip can be identified by an unexpected increase in engine revs without any accompanying acceleration when your car is in gear, the clutch pedal is not pressed and you push the throttle pedal. It will also be obvious when you attempt to accelerate up a steep hill. Though the degradation of a clutch takes place slowly over time (depending on your driving style and conditions - stop-start traffic can take its toll on clutches faster than, say, loads of motorway driving), if it is slipping in this manner then it really is time to have it replaced. There are other symptoms to watch out for, such as strong smells from the engine bay when you pull away from a standstill or a higher 'bite point' on the clutch pedal than before.
The latter can also signify something is up with the clutch hydraulics (if you car has a hydraulically operated clutch). First thing to do is check that the master cylinder reservoir (in the engine bay - looks like the brake fluid reservoir) is topped up with clutch fluid. If it looks ok then it's time to bring the car to a garage to diagnose if the slave cylinder needs replacing. In cars that use a clutch cable instead of hydraulic actuation, the cable itself can stretch and it's not an expensive or difficult component to replace.
Clutch judder is most noticeable when setting off from a standstill. It manifests itself as a strong vibration
from the engine/transmission when you release the clutch to get the car moving. It is usually something like oil or hydraulic fluid that has somehow found its way on to the surface of the clutch itself and causing it to not grip properly or smoothly when engaging causing the juddering effect as it grips in part, then slips, then grips again and so on. It can also be caused by misalignment of the clutch or the flywheel being slightly warped or just being plain worn out or glazed to the point it's not getting consistent grip. Clutch judder can actually be quite severe, so much so that it can make the car very difficult to drive, especially in traffic. It's also extremely annoying to live with! The only real solution to clutch judder is to get it replaced.
Worn Release Bearings & Dual Mass Flywheels:
I've grouped these 2 component together because while very different, when they begin to fail they can sound very similar. If you can hear a low rumbling sound coming from the gearbox that goes away when you press the clutch pedal then it's possible you have an issue with either the release bearing or the dual mass flywheel (if your car has one)
There are also a number of clutch related issues you might encounter that may not be the fault of the clutch itself. Sometimes the problem is not with slipping, but with sticking. If your clutch won't release properly, it will continue to turn the input shaft. This can cause grinding, or may completely prevent your car from going into gear. Some common reasons a clutch may stick are:
- Broken or stretched clutch cable - The cable needs the right amount of tension to push and pull effectively.
- Leaky or defective slave and/or master clutch cylinders - Leaks keep the cylinders from building the necessary amount of pressure.
- Air in the hydraulic line - Air affects the hydraulics by taking up space the fluid needs to build pressure.
- Misadjusted linkage - When your foot hits the pedal, the linkage transmits the wrong amount of force.
If you're experiencing any of the above symptoms you may be lucky and not have to replace the clutch at all, it might be something much more simple and cheap to fix!
Replacing the clutch in your car.
It's necessary in virtually all cars (there are a few exceptions) to remove the gearbox from the car in order to replace the clutch, which is why it's such an expensive and time-consuming piece of work to get done - regardless of the price of the parts themselves. Hence, even though it's often just the clutch plate that needs replacing, it's worth buying the whole kit, including the pressure plate and release bearing.
If you're going to undertake to replace the clutch for yourself, make sure you know what you're doing, consult a workshop manual if necessary and stay safe.
A note on dual-mass flywheels.
A lot is said about dual-mass flywheels. They effectively dampen out vibration in cars that produce high torque at low speeds, but can cause problems in all models depending on the type of driving the car is mostly used for. The mechanism weakens and breaks up over time, which can lead to poor starting and running. If you're replacing the dual-mass flywheel it's well-advised to replace the rest of the clutch, as the gearbox will be removed in any case. And a word of warning: it's a popular conversion on some cars to change from a dual mass flywheel to a solid flywheel setup. Often because they're cheaper but often because dual mass flywheels get a lot of bad press about reliability - but they're there for a reason and replacing yours with a solid conversion will result in a noticeable increase in vibration through the clutch pedal and the cabin in general.