This article was first published in 1999.
The clutch is one of the most frequently replaced components on a high-performance car. So it helps if you can understand the basic concepts yourself without relying on other people's advice. Read this and you'll be able to talk about clutch configurations, their applications and how you might possibly be able to improve your own car's wavering clutch!
The clutch disc is probably the most frequently abused bit of hardware on a high-performance car. It's the poor little bugger that gets squeezed in between the engine's flywheel and the pressure plate (clutch cover). In this position, it can often be subjected to massive loads as well as intense heat. Sometimes on even just a slightly modified car, the clutch can prove to be the weak link in the car's mechanical armory.
Here are the possible solutions to the problem, and the low-down on current clutch technology:
To find an effective solution to a clutch problem, you firstly need to consider the application of the replacement clutch package. Factors that need to be considered are available tyre traction, gearing, driving style, engine torque at certain rpm, and the amount of load frequently placed on the engine. That's sort of a fancy way of saying how hard the car gets driven! All of these contributing values will have a bearing on what is the right combination for you. But it is important not to just focus solely on the clutch plate, as the flywheel and pressure plate also play a major role. It's a good overall package that we're looking for.
Types of Clutch Plates
Clutches come in a variety of sizes - the limiting factor in their diameter is the space available within the bellhousing. As a guide, most Japanese cars (even the high-performance ones) come with an 8 to 9-inch clutch, while a Commodore V8 comes with a larger plate diameter of around 10 to 11-inches.
These clutches are sometimes quite unusually shaped and have relatively small areas of friction lining on each paddle. This serves to reducing the overall contact area acting upon the flywheel when compared to a full-face clutch. With no other accompanying changes, fitting one of these will give a higher maximum clamping pressure (pressure = force / area) and will therefore deliver more resistance to slip. There are, however, some problems with simply installing a paddle clutch with no other changes. Because the total contact area of the clutch is reduced, the rate of lining wear is increased, clutch engagement is more sudden and shudder can also be encountered on take-off.
The vast majority of factory cars come equipped with a full-face clutch plate. These have a friction lining that extends around the entire periphery of the clutch plate with no large gaps. Full-face clutches offer smooth engagement and a long service life but, depending on the chosen friction material, require a highly rated pressure plate in a performance application.
There are combination full-face/paddle clutches now appearing in the form of full (or near full) circle clutch plates, which are fitted with small areas of friction material. These come with a varying amount of lining area and are a great compromise for someone after some of the advantages of both full-face and paddle clutches. The appropriate pressure plate rating and engagement smoothness depends on the area of the lining material of the individual clutch plate. The clutch plate itself is often more substantial in construction when compared to the more sculpted paddle types - the trade-off being increased mass.
Multi-Plate clutches are becoming increasingly popular in the aftermarket, especially amongst high-performance Japanese vehicles. But theoretically, they should only be used where there isn't enough room for a larger diameter clutch. In the case of a multi-plate design, the total friction area is greatly increased, as there are multiple clutch plates. The advantages include an ability to get away with low rated pressure plate, which gives lighter pedal weight. The trade-offs are the possibility of shudder and noise during operation. Only the more expensive high-performance factory vehicles, such as the twin-turbo V8 Lotus Esprit, run a twin-plate clutch as standard.
Clutch Centres and Cushioning
Most factory cars come fitted with a sprung centre to aid engagement smoothness. These units have a series of small springs located radially around the hub that allow the clutch assembly to rotate slightly upon engagement to the flywheel.
This helps to smooth out any torsional fluctuations and vibration that would otherwise be passed on through the driveline and cabin. Damping the movement of these springs are small friction washers fitted between the hub, retainer and adaptor plate.
Not surprisingly, a solid centre clutch plate doesn't have any springs fitted as the cushioning media. And because there is no "give" upon engagement, its operation is much more ragged - making it less suitable for road use. The primary goal of a solid centre is to achieve the highest possible strength and durability under extreme conditions.
However, solid centres can sometimes be used conjunction with a "marcel backing" around the outer edge of full-face clutches. The marcel is a wavy material-lined backing on the clutch plate, which gets progressively compressed and released against the flywheel and pressure plate in relation to pedal movement. This design is sometimes used when some form of softening is required on the take-up of the pedal, and when space limitations within the hub prevent bigger hub springs from being fitted. The bigger hub springs are used when backing a high-torque engine.
The aforementioned Lotus Esprit uses this design to soften the operation of its factory multi-plate clutch arrangement. For a road car, this has a positive effect on clutch pedal feel between the engaged and disengaged positions.
There are three main types of clutch lining material available, with combinations of any two able to be bonded and/or riveted on either side of the clutch plate. Note that for each type of lining, alumimium or steel backings can be specified. This reduces the likelihood of high rpm clutch explosions, while the aluminium type backings have the advantage of reduced mass when compared to steel. The thickness of the clutch lining is also important. Thick linings offer longer service life and smoother operation, but aren't as suitable for race or competition applications. This is because their thickness doesn't offer quite as fast engagement, which equates to slower gear-change capabilities.
Organic linings are most frequently used on factory bog-stock cars as they are durable, smooth, easy on mating surfaces and cheap to manufacture. In a performance application though, they frequently show their intolerance to heat build up and clutch slip inevitably occurs. With a low co-efficient of friction of around 0.32, they also require a fairly high clamping pressure.
Kevlar, like organic linings, needs a fairly high clamping pressure to maintain grip. This is because Kevlar's co-efficient of friction is similar, at around 0.30 and 0.35.
Its biggest advantage over organics is its ability to withstand large amounts of heat. As a guide, up to 40-50% more can be endured, so long as it is being used in conjunction with a highly rated pressure plate. It also places virtually no wear on its mating surfaces and it quite a has smooth operation.
Of all types of linings, ceramics offer the highest co-efficient of friction at around 0.48-0.55. This means they can be used with a lower clamping force pressure plate as the lining provides the grip. But because of this attribute, they cause a lot of wear on mating surfaces and shudder can be commonly encountered. Ceramics can take more heat than an organic material, but deliberately slipping the clutch tends to burn up the flywheel and pressure plate more than the lining. Made up from sintered bronze with ceramic material mixed in, there are a few different brands available that vary in their carbon content, but they are all mainly suitable for competition use.
Pressure Plates/Clutch Covers
Pressure plates squeeze the clutch plate against the flywheel, so they're obviously largely responsible for the total amount of holding force generated. Car manufacturers usually aim to achieve a light clutch pedal weight to make driving easier, and they do this by using a fairly marginal pressure plate load rating.
The good news is the clamping force of an OE pressure plate can usually be increased by 50-100% by altering the fulcrum point and swapping or re-shaping the diaphragm. However, this amount depends on the location of the factory fulcrum point and the quality of the OE part can also be a concern. Some cars, such as Toyota GT-4 turbos, only have scope for another 10% pressure in which case an aftermarket pressure plate is usually required.
The maximum tolerable pedal weight is merely an individual opinion, but note that clutch cables (where fitted) can be stretched or overloaded if you go too far. On the other hand, cars fitted with hydraulic clutch actuation can be fitted with a pressure plate of substantially more load rating without excessive pedal pressure. Note that with a too highly rated pressure plate, it can also be possible to bend or break clutch forks and strain engine crankshaft bearings.
The humble flywheel can be of significant importance in the overall clutch equation. The lighter a flywheel is the less moment of inertia it has. Lightened OE, light steel, or aluminium flywheels offer faster engine acceleration at the expense of idle quality, ease of take-off, steady state cruising and light-throttle smoothness. The good news for the clutch is whenever the pedal is engaged, the rotational difference in speed between the clutch and the lightweight flywheel can be quickly equalised. And the faster this speed stabilisation occurs, the less accompanying clutch slip there is.
But problems can arise. For example, when aluminium flywheels are combined with ceramic clutches, it is possible for some microscopic aluminium particles to be lost to the harsh clutch lining. To combat this, some companies now heat-treat their flywheels prior to installation, and some have a replaceable steel heat shield inset in the mating faces.
On the other hand, really hard-launching cars might need to opt for an extra-strong flywheel such as a hardened billet steel or chromoly item - as OE and some lightweight flywheels have been known to disintegrate off the starting line. And factory cast flywheels that have been lightened are notorious for failing, so be careful.
Some Typical Clutch Problem Solutions
With the above information in mind, here are some examples of some typical clutch upgrade scenarios and what would be the recommended solution:
Scenario 1 - A mildly modified 4WD turbo owner wants to spend as little money as possible and is wanting a long clutch life, with near-factory smoothness but is able to withstand occasional hard launches.
Solution - Retain standard cast flywheel, re-line standard full-face clutch with Kevlar lining and fit a more serious aftermarket pressure plate.
Scenario 2 - A Suzuki Swift GTi owner wants to achieve a more positive clutch pedal engagement and less chance of slip.
Solution - Fit a Kevlar or organic lined hybrid-type sprung centre clutch plate, along with a high clamping pressure plate and standard flywheel.
Scenario 3 - Owner of a regularly drag-raced 400-500hp Holden Commodore wants to eliminate any chance of slip, maintain maximum service life and isn't too concerned about engagement smoothness or cost.
Solution - Option one would be a single plate clutch in a large-as-can-fit 10½ -inch diameter with a heavy-duty pressure plate. A choice of linings would be available to suit. The second option would be a twin-plate clutch, also with a choice of linings but a medium-duty pressure plate will suffice.
You should now all be able to speak authoritatively on the topic of clutches, and know what to do if your own car's clutch packs it in. Of course, not that the readers of AutoSpeed would be the sort of folk that chew through clutches...
Adelaide Clutch Service
+61 8 8234 2222