Under Pressure: How does a clutch work?

For the majority of car enthusiasts, a vehicle with a manual gearbox is something we will have at least experienced, if not owned several times in our years of driving. Side by side with the opportunity to change gears whenever you feel, is the 3rd pedal on the floor.  Ever asked how a clutch works? Well here is what happens when your left foot goes down...

What is a clutch?

We have established you need it to change gears, but what exactly is a clutch? In short, there are three parts, a pressure plate, a friction plate and a release bearing mounted on a clutch fork / clutch arm. The pressure plate is attached to the flywheel and rotates with the speed of the engine. The clutch disc friction plate sits sandwiched between the pressure plate and the flywheel and spins on the gearbox input shaft, transmitting the rotation of the engine through to the road wheels.

How does a clutch work?

The clutch relies on friction and pressure to operate. When your pedal is up, the two plates are connected and forced together. When the clutch pedal is pressed down the clutch arm is operated, and the release bearing forces them apart, allowing a new gear to be selected without the engine spinning everything. With the correct cog picked out the pedal is brought back up and the drive is re-engaged, and you're back in business.

Where does my clutch live?

Unless you have terrible misfortune, for example, you run over a big rock and smash the gearbox case to pieces, the only time you'll ever see your VW or Porsche clutch is when you buy a new one and take it out the box, or when you (or your garage) remove the old one and put it back in the box to prove the work has been done.

Manual or Hydraulic Clutch?

Connecting your pedal to the clutch itself is either a clutch cable or a pipe full of hydraulic fluid joining the master cylinder and slave cylinder together. Let's start with the simpler version first, as found in an early Golf, Type 2 or even a VW Beetle clutch system. As mentioned above, as the pedal is pressed down the clutch cable pulls the clutch lever, which in turn pushes the release bearing into the pressure plate,  separating the two friction surfaces from each other. The hydraulic version, as the diagram illustrates, isn't vastly different, but relies on the hydraulic fluid to activate the clutch arm (or clutch fork) and operate the clutch release bearing.

Hydraulic or Mechanical Clutch: Which is better to drive with?

In short, vehicles with cable operated clutches tend to be older and naturally feel a little bit more connected to the driver. They typically have a heavier pedal, offering more feedback and are somewhat simpler to work on. However, cables stretch and need adjustment over time, and will eventually snap! The more modern hydraulic clutch option offers a smoother engagement and a lighter pedal but with this refinement comes complications. Hydraulic systems require bleeding and fluid changes (these systems use brake fluid, which is hygroscopic and absorbs atmospheric moisture over time) and there is the potential for both the clutch slave cylinder and the clutch master cylinder to fail, which would leave you stranded with no way to quick-fix it on the roadside.

Is it expensive to change a clutch?

If you own a simple classic car you might give this a shot yourself, either at home or at a mates workshop. However, if you own a Porsche or a more modern vehicle, a clutch change might be best left to the professionals. Purchasing the parts could range from £100 for a VW Golf clutch kit through to £700 for a Porsche 911 clutch kit, but the biggest expense with changing a clutch is likely to be the time or labour cost at your chosen workshop. To access the clutch, the gearbox must be separated from the engine, and that can take a good few hours in some cases.

Why do I need to change the clutch on my car?

The successful operation of your clutch relies on friction. Just as other friction components like brake pads and discs are subject to wear, so is your clutch. As your clutch wears, so you will experience the clutch slip and a change in 'biting point' at the pedal. This will be most noticeable when the vehicle is under load; accelerating hard, carrying weight or towing, and driving up steep hills. You may have heard people refer to a clutch smelling, or burning out - quite often aimed at a pensioner who is leaving a supermarket at 10mph with their engine screaming at 5,000rpm. This smell is the clutch friction surfaces slipping against each other, and getting hot - burning. If you get this smell, you need to change your driving habits, or change your clutch! There is another reason that you may have a slipping clutch, and that is because of an oil leak from the gearbox or engine sabotaging the friction qualities of the clutch disc. If this happens, you need to renew the clutch, but also fix the oil leak too.

How to make your clutch last longer.

Let's wrap things up by looking at how to improve the life of your clutch. The big one is by ensuring you don't drive with the clutch partially engaged, typically done by drivers who rest their foot on the pedal. When waiting on a hill, use the handbrake to hold the vehicle, rather than rocking back and forth on the clutch biting point. Finally, pull away gently from junctions and traffic lights; it might not be so much fun, but it will reduce the stress on the friction surfaces.

I hope that helps.

Andy