Racing Clutches – What did they ever do for us ? (part2)

In part 1, Tony Tewson explained the basic design of the clutch. He now covers materials.

Moving to materials, there are three general classes in mainstream use with all the clutch manufacturers. Direct metal to metal contact is pretty much the best way to conduct heat, and so naturally metallic friction material is very good at doing this. As in brake systems, heat is the main enemy of clutches, so metallic friction materials are most commonly used throughout motor sport. Confusing marketing names may be used, but it is simpler than it seems. All friction materials fall into the three general groups. Here are Tony's comments:

Organic - “This material is more usually associated with road car clutches. Organic is a bit of an outmoded term, going back to when asbestos was used. Technically, a more accurate description might be “Mineral”. Typical application is 280bhp road/track day Caterhams, or the Gardner Douglas Lola (GM 6/7l V8 powered) recreation. Any kind of normal road car clutch would either be too big and heavy, or simply be over-powered by the engine, and therefore a racing clutch with road manners is required.”

“Organic materials in racing clutches allow superb clutch control, much better than any other type of racing clutch. But because organic friction material does not conduct heat as well as metallic, it should be used with caution as it can be overheated and burn out. “Burn-out” will not occur with metallic friction material until sufficient heat is generated to melt metal, so for most motor sport metallic is the better choice for that reason.”

Metallic - “Metallic has more general advantages and wide-spread use in motor sport clutches than organic or carbon. Metallic friction materials are all sintered. This comes as a surprise to most people, but the reality is that so called cerametallic, ceramic and paddle are all just variations of sintered metallic materials – just think of brake pads being available in different compounds according to the manufacturer."

Sintered metallic clutch plates are easy to identify by their golden bronze colour. They are a mixture of materials – Copper, Bronze, Iron and Silicon are the usual ingredients together with one or two trade secrets – heated and pressed together to make blocks of friction material. Actually there are other variations including metal spraying, and silicon nitride treatments, but these are special cases not relevant to the vast majority of motor sport.

Because the clutch is operating metal-to-metal, there is better conductivity and so the heat is more easily led away. The coefficient of friction is lower than with an organic clutch when cold, but the operational temperature range is much wider and higher, and the friction value more stable. It will function well when an organic has long faded away or burnt out, or when a carbon is just warming up.

A traditional Rally clutch solution is to rivet small sintered blocks to a clutch disc centre in the form of a “squashed propeller” shape, which makes a “paddle” clutch. This is a long-established technology dating from the fifties, and often currently applied in Group N rallying where the clutch has to be of mass production origins.

However, there are much better solutions available as pioneered by Super Clutch. By directly bonding wafers of material to a disc, there is more complete contact than with a riveted surface and so the metallic's heat transmission properties are used to the full. Finish ground to size unlike a typical paddle block, there are no rivets to potentially damage contact surfaces, a significantly higher volume of friction material, and greater contact area allowing the use of a more progressive engagement characteristic friction material “compound”.

Tony continued: "Super Clutch uniquely offer three friction material thickness options according to “duty level”, or anticipated operating temperature. Developments in clutch design mean that often we can offer a single where previously the only option was a twin plate clutch, and sometimes even twin where previously a triple was required. The purpose is to reduce weight and inertia, and the bonus is reduced cost”. To prove this Tony runs only a single plate 5.5”/140mm clutch on his own 1600BDA powered Pilbeam F3 style hillclimb car instead of the traditional twin plate 7.25”/184mm solution, and a twin plate rally clutch on Shirley’s monster (4.5L) Rover V8 instead of the original triple plate.

Carbon – “Like the girl with the curl, when it’s good it is very very good, but when it’s bad, it’s awful. It's superb for big budget professional motor sport when it can be run in optimum conditions determined by continual testing to establish working parameters. But if you get it wrong it can cause many more problems than it solves and become very expensive cost-wise and in the time taken to perform the necessary maintenance. Best reserved for circumstances where operating temperatures are too high for metallic. Thus it ticks more boxes for brake use than clutches. For most applications metallic friction material is a better option, including many of those already using carbon. e.g. ex-BTCC saloons in private hands."

Finally, I asked Tony about clutchless changes and launch control. “Mixed views” he replied. “As a driver, enthusiast, and engineer, I love them, but as somebody whose business is selling clutches, I hate them!”.

The overall message from Super Clutch is that there's a lot of constantly developing technology out there, and with choice of cover, materials, diameter, material thickness and “duty levels”, not to mention different springs, there is a lot of combinations. The choice can be tailored to the requirements of the user, whether that's ultimate performance, driveability or durability, and according to whether the driver is “Mr Smooth” or his evil, brutal brother. Super Clutch's proposition is that “one size fits all” isn't always a great idea for tee shirts, and is an even worse idea for motor sport clutches. If any car-engined member is in the market for a new clutch, then giving Mono sponsor Super Clutch a call (01926 812136 or sales@superclutch.co.uk) could help to identify the ideal clutch spec for the job.

Tony Tewson was talking to Tony Cotton

3 sizes of Metallic Friction material

Organic and Metallic Friction Material

(Pics - Superclutch)

The competition:

A triple plate carbon clutch could cost about £4k new

Organic Astra Van clutch, £15 from Mono Ads

(Pics - Internet and Tony Cotton)