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CLUTCH GUIDE: EVERYTHING YOU NEED TO KNOW

CLUTCH GUIDE: EVERYTHING YOU NEED TO KNOW

Posted by Glenn Rowswell on 8th June 2021

It’s rare to see a tuned car without an uprated clutch, so we spoke to Competition Clutch to find out why in our performance clutch guide.

The main purpose of an uprated clutch is to replace the standard item which can’t handle the level of torque the car is producing. It generally depends on the car and driver, as a hard used, but less powerful engine will wear its clutch out far faster than a more powerful engine driven gently. Clutch wear can be as extreme as slipping instantly under load, or it can occur when you use full power for short periods including drag racing, drifting, or during fast road use.

Standard clutches are designed so that even the least talented motorist can drive smoothly. But the payoff is a shorter life and inability to handle extra power. Many supercars for example struggle to launch hard more than a few times without destroying the clutch for this exact reason. But if the manufacturer had installed a clutch that could effectively handle the power under hard use, it would be too difficult for most owners to use.

Read on through our clutch guide and you’ll become an expert in the field!

What is a clutch and what does it do?

In simple terms, your clutch is the component that connects the engine to the transmission to drive the wheels. It works by using a friction disc (connected to the gearbox) that is pressed by a sprung pressure plate against a flywheel (which is connected to the engine) to transfer drive between the two. Disengaging the clutch, by pressing the clutch pedal, separates the friction discs and the flywheel, stopping that transfer, allowing the engine to continue turning without the connection to the wheels, this in turn allows you to change gear or stop altogether.

Clutch guide: Are they all the same basic design?

Most automotive clutches use the same principle of some form of spinning friction disc clamped to a flywheel by a pressure plate to transfer power, but the types of friction materials used, the size and design of the discs themselves and the number of friction discs used in any platform or application can vary. You start with an OE replacement clutch for non-modified vehicles. However, once you begin to increase power, you will need a clutch with stronger pressure plate and more aggressive and hard-wearing friction materials. These generally come in stages, such as 2, 3 and 4, and allow good street driveability. Next is multiplate clutches, which increases the torque capacity without the need to increase spring strength or clutch diameter. This uses multiple (usually two or three) clutch discs, with friction materials on both sides of each and makes for a lighter clutch pedal than an equally-strong single plate application. The final option is what is known as a ‘paddle’ clutch, which instead of the clutch plate being a circular disc, it is made up of a number of fiction ‘paddles’ or ‘pucks’. Known for their snatchy nature, but also an ability to hold high power without slipping, they commonly come in 3, 4 and 6 puck configurations.

What are clutches made of?

While the outer pressure plates are generally always a sprung metal design, the clutch discs themselves will use various friction materials depending on the application and power handling. Here are the most common materials and their qualities:

Organic

Smooth, durable (and usually quieter) and easy on the opposing mating surface, organic materials are common but suffer in high performance applications due to their intolerance to heat build-up and shorter lifespan.

Carbon/Kevlar

Similar clamping properties to organic materials with smooth operation, but able to withstand heat more vigourously when mated to a highly-rated pressure plate.

Ceramic

Offering the highest friction co-efficient, ceramic offers a lower necessity for high clamping force as the material provides more grip. The flipside is that ceramic materials wear the flywheel’s surface more quickly, sometimes causing shudder and a harsher feel. Mainly for competition use and applications where you are doubling or tripling your car’s stock output.

How do aftermarket clutches differ to OEM ones?

OEM clutch kits are designed to work with the factory power output of the vehicle and most will begin to fail if you substantially increase the power through tuning. Aftermarket clutches are available in different ratings to be able to work at much higher power and torque levels, meaning whatever your car’s power or driving style, there will be a clutch available to suit. Also, many aftermarket performance clutch options are lighter than stock clutches too (especially when combined with a lightweight flywheel) meaning less rotational mass for the engine to turn, equating to freer revving and lower transmission losses.

Why, or when, would you need to upgrade your clutch?

It is worthwhile upgrading your clutch if you are planning a considerable increase in power and torque – for example, boost increases, re-mapping and engine builds – as this will place more strain on the whole drivetrain and wear out a stock clutch more quickly. This is when you need a performance clutch.

The options available to you will depend on your level of power upgrade and intended usage.

How important is it to get the right clutch?

Aside from making sure you get a clutch that physically fits your car’s transmission, you also need to consider your car’s power and torque, plus your driving style and how you intend to use the car. While there is no point simply buying the strongest paddle clutch available if you are not going to have the power to warrant it, under specifying your clutch can lead to it wearing out prematurely and not being fit for purpose. Just bear in mind that for street driving, pedal weight and ease of modulation are also a concern if you want your car to drive well and be useable in traffic etc., whereas in racing conditions an aggressive, snatchy clutch is less of a problem as long as it can take the abuse.

Clutch guide: What causes clutches to fail?

Clutches are fairly durable, but heavy abuse or increases in power will mean they wear much more quickly and need replacing more often. If the clutch slips (doesn’t fully engage), the linkage could be out of adjustment, or the clutch disc could need replacing. You can tell if your clutch is slipping by flooring the pedal in a high gear at low revs and seeing if the revs rise without a corresponding increase in speed. If this is the case, your clutch is on its way out.

What other mods should you consider when uprating your clutch?

You should consider upgrading the flywheel, if it is available for the vehicle, and always replace the throw-out bearing and flywheel bolts when installing a new clutch. If you are upping the power considerably, you may also want to consider fitting a limited slip differential, uprated driveshafts or even an uprated gear-set.

Are there any downsides to uprated clutches?

With single-disc clutches, the ‘in traffic’ drivability tends to suffer if you move past organic and segmented ceramic setups and onto paddle clutch setups. This is because of the snatchy, all-or-nothing nature of high performance paddle clutches meaning useful clutch slip (to aid modulation when engaging) is all but non-existent.  The solution to the downsides of a heavier paddle-style clutch are solved by using a multi-plate organic clutch such as the MPC from Competition Clutch. They have the same power holding capabilities as a racing twin-disc clutch, but the ‘in traffic’ drivability of a Stage 2 street clutch.

Other than ensuring you get the right clutch for your application, what are the most important things to look out for when buying uprated clutch?

Look for a reputable brand (such as Competition Clutch) that you know or have seen in many magazine articles and that is used on race cars and enjoyed by fellow enthusiasts across the globe.  That way you know you will have great customer service and back-up should you have any problems. As far as price is concerned, you really do get what you pay for, and that unbranded Ebay-special may end up costing you a lot more in the long run if it fails to live up to expectations. We’d also recommended a professional installation because clutch installs (especially the alignment) can be more a difficult job than you may think. The right tools really make all the difference in the world.

FAST CAR CLUTCH GUIDE

Clutch guide: What makes up a clutch? 

A clutch is fairly simple with three main components in a single plate example. It basically consists of two surfaces; one connected to the engine and one connected to the gearbox, and a central friction disc that transmits torque between the two.

Flywheel

This is the part that is bolted to the crankshaft of the engine and therefore spins at engine speed at all times. With the clutch engaged, the flywheel friction plate and the clutch plate are pressed together, sending torque from the flywheel to the gearbox. Different weights of flywheel are available, and although these are mostly used for other performance reasons, a lighter flywheel will also have an increased clamping effect. We’ll explain the effects of flywheel weight later on in this feature.

Clutch plate

This is the part connected to the gearbox. When the clutch is engaged the clutch plate is pushed against the flywheel to transmit power between them. There are various materials and styles of clutch plate friction pads depending on the use of the car and the torque the clutch is expected to transmit, and this will be talked about in a separate section.

Clutch cover

This is the part that pushes the clutch plate on to the flywheel when you engage the clutch. It has sprung metal fingers arranged in a circular fashion facing inwards that give the clamping force that helps stop the clutch plate and flywheel from slipping. An uprated clutch usually has an uprated cover which increases the clamping force of the clutch cover fingers. The downside of more clamping force is a stiffer clutch pedal, which can range from slightly stiffer to something barely useable on the road.

FAST CAR CLUTCH GUIDE

Paddle clutches

The majority of uprated clutches are actually what is known as a paddle clutch. A paddle clutch, rather than having 360 degrees of friction material like a standard clutch, is segmented into sections, usually four or six, but other combinations are available. These are commonly known as a four/six paddle clutch, or sometimes four/six puck in the USA. The reason for this segmented design is to help with heat dissipation. With increased power you are more likely to overheat the clutch, potentially warping it or simply ruining the friction material. Making it less effective.

Sprung plate or not?

Almost all uprated clutch plates for road cars, even ones intended for enormous power levels, are ‘sprung’. This means they have springs between the centre plate and the friction pads to dampen the shock load as the clutch is engaged. This makes driving it on the road, where the clutch is constantly being engaged and disengaged, much smoother than without springs. On race cars smooth clutch operation isn’t so important so these are generally unsprung to give the most positive action possible, less weight, and less potential failure points.

Multi-plate clutches

Multi-plate clutches are commonplace these days on uned cars. They have more than one clutch plate with metal plates sandwiched between them. Using multiple plates spreads the load and increases heat dissipation, giving increased grip for the overall diameter of the clutch. This means a twin plate using friction materials that are more road-friendly can handle similar levels of torque to a single plate race-only sintered clutch for example. In race cars they use multi-plate clutches to save weight and rotating mass by using the smallest diameter clutch possible with a huge number of plates. Conversely, cars with very large diameter clutches as standard, V8s for example, tend to not need multi-plate clutches until a much higher power level than most four cylinder engines with smaller clutch diameters.

FAST CAR CLUTCH GUIDE

Flywheel weight

There’s a lot of talk about the effects of a lightweight flywheel, but to put it simply, on a high performance car there are many benefits. The first thing to remember is as it’s a rotating mass you are not simply removing 5kg. The effect is magnified by the speed of rotation and the gear ratios, meaning it can be the equivalent of a 150kg weight saving in a low gear, though in high gears it can be less than 10%. From a performance point of view it is why cars tend to rev faster and more freely with a lighter flywheel, especially in lower gears and of course out of gear. From a clutch point of view this lighter rotating mass makes it easier for the clutch plate to grip it, further increasing the clutches’ performance.

As ever with tuning, there is a downside, and that is a slight drop in drivability due to this decreased clutch mass as the revs drop more easily, making it harder to drive smoothly and easier to stall. Sometimes a slightly higher idle speed is used to stop a car with a very light flywheel from stalling when the clutch is disengaged.

Hydraulic or cable operated clutch?

The vast majority of cars these days come with a hydraulically operated clutch. But there are still many popular cars with a cable clutch, Cosworths and Mk2 Golf GTIs for example, and this can cause problems when going for a high performance clutch. In a similar way to brakes without servo assistance need to be pushed harder to operate, a clutch with cable operation is much harder to use than the equivalent clutch with hydraulic assistance. With organic clutches and standard covers it isn’t an issue, but once the cover is stiffer you need a strong leg to operate the clutch. And once the plate material is uprated you need extra care to pull off smoothly. Single plate clutches can just about be driven on a cable clutch, but anything more will need a hydraulic clutch conversion to maintain any hope of road drivability.

FAST CAR CLUTCH GUIDE