Regardless of the racing series, almost every racecar in the world will feature a clutch. However, the friction materials used in these clutches will differ radically depending on the torque and power delivery characteristics of the engine. So what are the materials available and how to they vary?
So-called ‘organic’ clutches have been around for the best part of 50 years. Organic is something of an outmoded term now, as it harks back to when asbestos was used in the clutches’ construction; a more accurate description now might be ‘mineral’. Asbestos made a good friction material due to its high heat resistance, good strength and high Mµ (coefficient of friction) when subjected to high temperatures. Unfortunately it is also carcinogenic and is therefore now banned in most industrial applications.
Organic friction material is now generally made from cellulose reinforced with materials such as fibreglass and mineral wool encased in a thermosetting phenolic resin base. The cellulose provides the initial bite while the mineral wool and fibreglass provide burst strength. While organic friction materials provide very good ‘feel’ and excellent initial bite though, they are not very effective in high-temperature applications. It is very easy to overheat the organic material under high torque loads, making them impractical for racing use except with small-capacity, low-torque engines, such as a bike-engined sportscar.
The next stage up from an organic friction material is one that uses chopped Kevlar fibres. Kevlar offers good burst strength and wearing characteristics but has a relatively low coefficient of friction. That makes for smooth engagement characteristics but requires the use of very high clamping pressures to provide sufficient friction to prevent slippage.
Kevlar provides far greater temperature resistance than organic clutches, and much lower wear rates, but it can still be ‘burnt out’ if subjected to excess heat. This is because the friction material does not return to its original state after exposure to high temperatures, greatly reducing its frictional properties.
Semi-metallic clutch materials look similar in construction to a regular organic clutch material but can withstand much higher levels of heat and are thus more suitable for high-torque applications. The clutches can still feature a woven structure, but instead of organic materials they will contain stands of brass or copper to improve the burst strength of the material while also increasing the resistance to high temperatures.
Semi-metallic clutch compounds can also contain powdered ceramic, copper, bronze, carbon or even iron mixed in with the organic material to further increase friction at elevated temperatures. Semi-metallic discs that contain high levels of iron or ceramic material can have somewhat reduced ‘feel’ though, with the discs tending to bite suddenly.
Sintered items are made by filling a mould with powdered material and then fusing it under heat and pressure. For clutch discs, it is usually a mixture of metallic compounds designed to provide the optimum coefficient of friction and wear resistance, and these ingredients include (but are not limited to) copper, bronze, iron and carbon.
This is where so-called carbon-metallic materials come into play. Mixing carbon and ceramic into the compound gives the self-lubricating benefits of a copper or bronze base material – providing smooth engagement – as well as the high bite and temperature resistance of carbon and ceramic. Under extreme conditions though, the copper base material can melt and coat the friction-modifying materials, massively reducing the friction level.
In applications where extreme temperature is an issue, for example an 8000 hp Top Fuel dragster, the favoured option is sintered iron. These are also produced from a powdered base stock and can withstand very high temperatures – in fact the friction increases with temperature. Due to the very aggressive nature of the material, however, engagement is very harsh and sintered iron discs tend to be used only in drag racing applications.
The most recent development in clutch material has been the introduction of carbon-carbon. In this type of clutch all the friction surfaces, including the flywheel mating and floater discs, are made from amorphous carbon material. These parts are made by heating preformed discs of white polyacrylonitrile (PAN) fibres until they turn to a black, pre-oxidised state. PAN is a synthetic, semi-crystalline organic polymer resin, which is used as the basis for high-quality carbon fibres. Once pre-oxidised, the fibres are layered together before being oxidised and then cut to a rough shape.
These rough blanks are subjected to two densification heat cycles at more than 1000 C before being machined to a finished shape. It is these densification cycles that make the manufacturing process so lengthy, with each cycle taking several hundred hours. During the process, hydrocarbon-rich gases are injected into the ovens used to heat the blanks, allowing the layers of material to fuse together and form a solid disc.
Friction modifiers can be added to the mix to alter the material’s characteristics, the result being a clutch that is highly resistant to temperature, and with friction increasing as the clutch heats up; the discs are also very light, reducing drivetrain inertia. The very long manufacturing process means carbon-carbon clutches are very expensive though, and thus limited to the top echelons of motorsport, although lower cost carbon-steel options are available. These feature carbon friction discs combined with steel floaters and flywheel mating surfaces.
Ultimately, carbon-carbon is the ideal solution for a racing clutch friction material. Unfortunately its high cost makes it inaccessible to most racers. Manufacturers are working to produce cheaper varieties of the material, and no doubt these will filter down from the upper reaches of the sport over time. Until then, however, other clutch manufacturers will continue to improve the performance of their organic and metallic compounds.
Written by Lawrence Butcher