Clutch Technology Part Two, The Clutch Disc Friction Material

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semi metallic clutch disc
A typical semi-metallic disc has strands of metal wire woven through the organic material to help high temperature friction characteristics as well as to improve burst strength and wear.

The next level of friction material in the progression of performance is the semi-metallic disc. Semi-metallics are usually the best compromise for your typical modified streetcar because they have a nice balance between smoothness, long life, good grip and low wear. For this reason some cars have semi-metallic stock discs. Semi-metallics describe a wide range of materials. Your typical semi-metallic is an organic disc with strands of copper or brass woven through it. The metal strands help improve burst strength and allow for more consistent grip under higher temperatures. The metal helps dissipate heat through the material as well. Often fiberglass or Kevlar is added to the mix for strength and temperate resistance. The metal strands could be anything from brass or copper to steel fibers.  Brass and copper have the properties of a metal with some lubrication properties.  Steel has more grip but is less smooth and more abrasive.

More aggressive semi-metallics designed for heavier abuse have powdered ceramics, copper, bronze, carbon or iron added to the organic based mixture to help improve grip at higher temperatures. Most semi-metallics have low abrasion and good wear characteristics but the ones that have a lot of iron and ceramic friction modifiers can be somewhat harsh on the flywheel and pressure ring. The more aggressive of the semi-metallic discs tend to be grabby and harsh.

metal disc
A multi-puck full metal clutch disc with a sprung hub and Marcel is the next most aggressive disc. Some people can tolerate this in daily use.

For extreme applications there are metal discs. These use a sintered copper or bronze (or a mixture of both) friction material that usually contains particles of ceramic and perhaps iron or carbon. It is made by compressing the mixture of powdered copper, bronze, iron, carbon and ceramics at high heat and pressure. Copper and bronze are excellent conductors of heat and have self-lubricating properties that make them ideal as a base friction materials. The iron, carbon and ceramics are friction modifiers that help give the copper bite. It is kind of like the grit in Lava soap or your girlfriend’s apricot facial scrub (not that I stand in the shower reading the ingredients to know this). This sort of friction material is usually called copper ceramic or carbon metallic since carbon is a sexy marketing term. These materials are very resistant to heat and abuse, and actually bite harder with more heat but they can still be overheated in extreme cases and the copper can smear and melt over the friction modifier particles ruining the disc.

metal disc
A four-puck full metal disc with a sprung hub is the most aggressive disc that a sane person can take on the street. This would be unstreetable for many people

The metallic materials can last a long time but are much harsher on the flywheel and pressure ring than semi-metallics. These discs are good general racing discs and work well for just about all motorsports use, from drag racing to road racing. There is no way that a copper ceramic disc is going to be very smooth, it will be grabby and chattery, especially when cold but they can be on the verge of being streetable.

6 puck metal disc
A six-puck solid hub copper ceramic disc is nearly the most aggressive disc you can get.

Sintered iron discs are the ultimate in grip. Sintered iron is powdered iron that has been compressed under very high pressure and heat to form a porous soft material. Sintered iron works better when hot and lasts the longest of any of the friction materials. You almost cannot overheat sintered iron. Being very hard, iron does not have a very smooth engagement and is more like an on-off switch, and as such are poor for street use. They also wear the friction face of the flywheel and the pressure ring severely. When you replace one of these clutches, you have to replace everything. At the minimum your flywheel will need a heavy re-surfacing. Sintered iron is so harsh; its use should be relegated towards drag racing. Although metal discs are long lasting under tough racing conditions, they tend to wear quickly at low temperatures on the street.

4 puck metal disc
A four-puck iron solid hub disc is really for racing use only unless you are a sucker for punishment and can tolerate a short life when used on the street.

 

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