Understanding car clutch systems

What is a clutch?

A car clutch provides a mechanical connection between the engine and the rest of the driveline, allowing power from the engine to be sent through the gearbox and to the wheels which drive the car. When the clutch pedal is depressed, an arm (clutch fork) presses on a release bearing which releases the clamping pressure, and the mechanical connection is broken. In this interruption of power through the gearbox, higher or lower gears can be selected, or the \vehicle can remain at a stand-still without stalling.

Your clutch system contains a few parts that allow for smooth engagement, reliable operation and the ability to deal with various applications and driving styles. While the main parts are the clutch plate and pressure plate, there are several other often overlooked pieces which can be a make or break when it comes to installing your new clutch kit.

Critical components of a clutch system include:

• Clutch Cover - A spring-loaded diaphragm which clamps the friction plate to the flywheel. Varying spring tension levels between different clutch models allow for higher clamping forces, higher torque capability, higher power resistance and a more aggressive pedal feel.
• Friction Plate - The link between the engine and the transmission. Its surface area and material type vary in accordance with the power and torque levels associated with the clutch kit. Some clutches feature twin or triple plate arrangements which use two (often smaller diameter) friction plates for extremely high torque ratings and compatibility with seriously high-performing engines. This piece also features springs which dampen engagement forces; however, with higher-end clutches designed for excessive power and aggressive driving, these springs become more aggressive or even non-existent.
• Flywheel - The flywheel is connected directly to the engine's crankshaft. The flywheel acts as a mounting surface for the clutch cover and also acts as a ring gear for engagement with the starter motor. The flywheel also helps to maintain rotating mass during driving which makes for smoother gear shifts and more comfortable driving. High-performance vehicles may call for lightweight flywheels, which can help to increase engine performance by decreasing the engine's rotating mass. Some vehicles use a DMF (dual mass flywheel) to dampen engine and driveline vibrations. Unlike standard flywheels, DMFs cannot be re-surfaced when replacing the clutch, which means a new DMF must be fitted. There is also the option to fit a DMR (dual mass replacement), a conversion kit to replace the dual mass flywheel with a solid unit.
• Pilot Bearing (also referred to as spigot bearing, spigot bush or pilot bush) - Not a necessary component in all applications, most commonly found on rear-wheel drive layouts. The job of the pilot bearing is to support the engine side of the gearbox input shaft. The bearing is located in the back of the crankshaft or the centre of the flywheel and helps to locate the input shaft when the clutch is disengaged.
• Clutch Release Bearing (also referred to as a throwout bearing) - This is attached to the clutch fork and presses against the clutch cover's spring diaphragm in order to release the clutch. While pressing on the clutch cover, the release bearing spins to match the speed of the engine/clutch assembly.
• Concentric Slave Cylinder - Only applicable to certain vehicles, a concentric slave cylinder is effectively a combo of a release bearing and a slave cylinder. A concentric slave cylinder does away with the need for a clutch fork, and instead, hydraulic pressure is sent directly to the concentric slave cylinder, which presses on the clutch cover to release the clutch.
• Clutch Fork- This arm is connected to the gearbox and is actuated by the clutch slave cylinder. The slave cylinder presses on the clutch fork, which presses on the clutch cover using the release bearing previously mentioned to disengage the clutch.
• Master Cylinder - This is one of the two main parts of the hydraulic system in the clutch. The clutch master cylinder is attached to your clutch pedal and transfers pedal action into hydraulic pressure, which is then sent via the clutch line to the slave cylinder.
• Slave Cylinder - The clutch slave cylinder receives input from the master cylinder at the other end of the clutch line. When the slave cylinder is actuated, it uses a piston to push the clutch fork/release bearing assembly to disengage the clutch.

These work in harmony to provide drive from your engine to the transmission input shaft and ultimately to the wheels. Your clutch connects both your engine and gearbox, and gear changes happen when you disengage the clutch to enable you to change gears.

Internal combustion engine

A common theme in most things automotive is the internal combustion engine. Pistons move up and down and work together to spin the crank.

This spinning of the crankshaft delivers the torque to the driving wheels. Torque originally meant "to twist" and is not defined as a force that tends to cause rotation.

There is no surprise that the engine torque (the rotation or spinning of the crank) is the force that rotates the wheels, but if an engine were connected directly to the wheels, they would turn all the time. Drivers couldn't sit at idle, wait at lights or change gears comfortably.

Drivers need to be able to connect and disconnect the spinning motion of the crankshaft without turning off the engine, and that is where the clutch system comes in.

Why is it called a clutch?

To clutch something is to grasp something tightly – grab hold of it.

A good way to look at it is that a clutch is a way of grabbing hold of the torque of the engine (and letting go of it as well).

What is the principle?

A clutch is just like a vice. You have two sides to the vice that are designed to clamp down onto something, in this case, a clutch plate. When the vice is tightened, the disc is held; when the vice is loosened, the clutch plate is not held.

Components of a clutch:

Flywheel

The flywheel is one side of the vice. It is a big metal disc that bold directly to the crankshaft. Its role is to:

• Provide a flat clamping surface for the clutch plate to match up to.
• Keep momentum; it's a heavy mass that keeps turning the crank.
• Take heat away from the clutch plate.

Because the flywheel is bolted directly to the crank, the flywheel will always spin at the same rate as the engine's crankshaft. (Around the outside of the flywheel, you will find a ring gear. Although it is part of the flywheel, ring gears have nothing to do with the clutch itself. They work with the starter motor).

Gearbox input shaft

A small but very strong shaft. At one end is a locating pin that sits in the centre of the flywheel/crank. A spigot bearing sits in the centre of the flywheel on the locating pin or the shaft. Because the input shaft is not bolted to the crank, it does not turn with the crank; it just uses it as a locating point.

The opposite end of the shaft is connected to the gearbox. You'll find gear teeth along the shaft, called the spline; these match the centre hole of the clutch plate.

The gearbox input shaft's purpose is to:

• Line up the gearbox with the centre of the crankshaft, keeping the rest of the drive train straight and true to the crank.
• Give the clutch plate something to locate onto and position itself.

Clutch Plate

The clutch plate is a disc that slides over the gearbox input shaft. The spline or teeth at the centre of the clutch plate match the teeth of the gearbox input shaft. When the clutch plate slides over the shaft, the matching spline teeth mean the clutch plate is keyed or locked into the gearbox input shaft and is positioned flat to the flywheel.

Around the outside of the clutch plate, there is a hoop of friction material on both sides. The friction material riveted to the disc is designed to grip the flywheel. Because of the interlocking spline, the clutch plate and gearbox input shaft will then turn together.

"When things rub together, friction comes into play. In some cases, you don't want friction, so a lubricant is used. In other cases, you want the friction and use materials to maximise it to achieve grip. Examples of friction material include brake pads, tyres and the material on clutch plates."

Clutch assembly

The cover assembly bolts to the flywheel and OVER the clutch plate. The purpose of the cover assembly is to carry the matching surface (a heavy metal casting) for the flywheel and a spring-loaded system that pushes the casting towards the flywheel. The cover assembly and flywheel work just like a vice and work together to clamp down onto the friction material of the clutch plate. Its default position is clamped when the cover is bolted to the flywheel. The vice is tightly secured onto the clutch plate.

Actuation system

To disconnect the clutch or let go of the torque, you have an actuation system that starts at the pedal and ends at the clutch cover assembly. The thrust bearing in a 3-piece clutch kit is one component of the actuation system.

Putting your foot on to the clutch pedal is the same as winding back one side of the vice. When you push onto the pedal, it works a series of connected parts, which end with a push onto the spring-loaded part of the assembly. The spring load is taken off, and because the clutch plate is no longer clamped to the flywheel, it stops spinning at the same rate as the crank. It means you have a disconnected drive.

Remember, the locating pin of the gearbox input shaft is just a locating pin. It will sit in the flywheel but is not bolted to it, so it won't spin at the same rate.

Upgrades

An OE vehicle comes fitted with a standard clutch. The largest category of clutch sold is standard replacement kits, but we are not all standard drivers. An upgraded clutch might be needed if an engine is modified from standard to improve performance or if the vehicle is under a higher strain than normal (carrying or towing heavy weight).

Clamp load

Increasing the clamp load on the cover assembly is a common way to get more torque capacity from your clutch system. It involves increasing the load exerted by the diaphragm to clamp the clutch disc between the pressure plate and the flywheel. The clamp increase is limited as this can make the clutch pedal feel heavy and strain the clutch actuation system (hydraulics or cable etc.)

Friction material

Changing the friction material on a clutch disc can increase the torque capacity and improve heat properties.

Common clutch disc materials are:

Organic – most commonly used on OE applications, this material is great for drivability but is not suited for high-performance applications due to poor torque capacity when hot.

Aramid – a general mixture of organic and aramid; this compound has the drivability of standard organic material with much higher torque capacity and better heat resistance. Found in 4Terrain Ultimate, this material is perfectly suited to 4WD applications where higher performance is needed without any compromise to drivability.

Cerametallic/Ceramic – suited to high performance or race applications, this material can handle a high level of heat while also giving a massive increase in torque capacity. Unfortunately, the driveability is significantly decreased as the material provides a more aggressive engagement than organic material. This material can be found in the Mantic performance range.

Increasing surface area

Increasing the size or the surface area of a clutch increases the torque capacity of the system; however, unlike brakes, you are generally not able to simply install a large clutch. There are two methods that are used by performance clutch manufacturers to increase the surface area.

Multiple plates

In most cases, installing twin or triple plates can be done without modifying the bell housing and can double or triple the torque capacity without detriment to the drivability. The Mantic Track series has a range of twin and triple plate kits which feature several different plate options to suit various applications from street to track.

ER2 groove

This groove is a patented system that has been specifically designed to increase the Mean Effective Radius of the pressure plate and assist in heat removal. The increase in the Mean Effective Radius of the cover assembly gives a significant increase in torque capacity.

By adding the groove to the pressure plate, the inside radius of the friction face has effectively been increased. The first 5/16 inches (7.9375mm) of the pressure plate has all but been removed and added to this; there is progressively less material removed as the radius increases. The net effect of this is to move the effective inside radius further out.

The Mean Effective Radius of the clutch is directly proportional to the torque capacity of the system. Therefore, as the Mean Effective Radius increases, so does the Torque Capacity of the clutch. This system can be found in both the Mantic and 4Terrain performance clutch range.

6 signs your clutch needs replacing

• Spongy or soft clutch pedal – this is a very common sign that your clutch needs replacing, usually one of the first signs you may notice. You will most likely notice this when you are driving along; you will notice the difference between what your clutch pedal usual feels like compared to if it is spongy and soft.
• Burning clutch smell – this goes hand-in-hand if your clutch is on its way out. This is caused when the fiction from the clutch slip; this will cause an unpleasant smell that will fill your cabin filter.
• Unusual noises – if parts of your clutch are not functioning correctly, this can cause a variety of unusual noises. The noise can be caused when your vehicle is driving or when you engage your clutch. This can be caused by a variety of factors, including lost components within your vehicle which can grind against each other, interfering with other components within your clutch system. If parts are loose within the clutch, this can cause damage and pull apart different components of your clutch system.
• Pedal getting stuck or sticky clutch pedal – if your clutch is sticking or stuck, this can prevent your vehicle from going into gear or your vehicle will become undrivable. This will be one of the last signs you notice that your clutch needs replacing.
• Higher biting point – over time, you may notice the biting point of your clutch becomes higher than usual. This will happen over a period of time; it will become higher and higher to the point where you will need a replacement clutch.
• Manual car clutch slipping is basically when you are accelerating from lower gears quickly or under heaving load; your clutch can slip, causing your gear to jump out of gear. If you notice this symptom, it is a very common sign your vehicle needs a replacement clutch.

How to choose the right clutch?

Now that you know how the clutch works and all the parts that go into the system, it's time to figure out what type of clutch is best suited to your unique application, driving style and vehicle requirements. Whether you need to replace the clutch in your eco hatch-back daily driver, fully built 1000HP race car or anything in between, there's a clutch at Repco for you.

Sourced from reputable manufactures such as Clutch Industries, and Mantic Clutch you can rely on Repco and our range.

How to install a clutch kit in your car