Explanation of a Racing Clutch

Advanced friction materials are just part of what makes the racing clutch what it is.

Clutch technology has come a long way in the last century or so, but for all of their improvements, they're still basically the same as they ever were. Newer friction materials and multi-disc assemblies allow for next-generation clutches with all the holding power of their racing predecessors and the driveability of a street clutch.

Basic Assembly

A clutch assembly consists of either two or three basic parts, depending on what you consider part of the assembly. They are the flywheel, pressure plate and friction disc. The assembly starts at the flywheel, which bolts to the crankshaft. The friction disc sits on top of the flywheel, and splines to the transmission input shaft. The pressure plate bolts to the flywheel, sandwiching the friction disc against the flywheel and pressing the friction disc against it with spring-metal "fingers" pointed inward at the center. The throwout bearing pulls the friction disc away from the flywheel when you press the clutch pedal.

Flywheel

While not technically part of the clutch assembly, the flywheel plays an important role in the mechanism. The flywheel is a heavy, metal disc that bolts to the engine's crankshaft and acts as a friction surface for the friction disc to press against. The flywheel also does double-duty as a kinetic energy storage device; its inertia helps to keep the engine spinning in between combustion events. Racers tend to prefer lighter-than-stock and smaller-diameter flywheels, which allow engine rpm to rise or drop more quickly. While this does help with acceleration slightly, the racing flywheel's primary purpose is to allow for quicker up- and down-shifts.

Standard Friction Materials

Clutch friction materials are very much like brake pads, and use similar materials. Original equipment clutches usually use some sort of organic resin to bind together Kevlar, Aramid and glass fibers. Organic clutches engage smoothly but are limited in their ability to handle heat and power. Kevlar clutches can handle more power and higher temps, but glaze over and fail easily if overheated. Segmented Kevlar clutches have missing sections for better heat dissipation, but they often require stronger pressure plate springs.

Race Friction Materials

Carbon graphite and ceramic clutches can accommodate very high temperatures and horsepower levels, but they're not as progressive as organics or Kevlar. Carbon clutches don't wear out as quickly as ceramic, and don't impose as much wear on the flywheel, but ceramics work at higher temperatures and grip a bit harder. Sintered iron clutches are the most powerful and least forgiving of all. They're essentially a mixture of iron and other metal powders pressed together, and work by temporarily "welding" themselves to the flywheel. Kevlar and carbon/ceramic brakes will work for 90 percent of applications, but you may want to look into sintered iron if you're making 700 horsepower or more.

Multi-Disc Clutch Packs

A multi-disc clutch pack works similarly to the clutches in an automatic transmission. The friction disc in a multi-disc pack is two-sided; one side presses against the flywheel and the other presses against a plate behind the disc and splined onto the inside of the pressure plate. When the pressure plate springs push down, they push on the backing plate and the friction disc presses against both it and the flywheel. A multi-disc -- two or three disc -- clutch has multiple clutch frictions and plates. The multi-disc clutch effectively doubles the clutch surface area, allowing racers to use a more progressive friction material and lighter springs to reduce pedal effort and travel without sacrificing holding power or driveability.

Hybrid Clutches

Clutch technology is steadily moving toward a hybrid approach to building multi-disc packs in an attempt to offer the best of all possible worlds. The hybrid multi-plate clutch uses several different types of friction material, often some combination of organic, Kevlar and carbon/ceramic. The organic portion grabs first, reducing clutch judder and helping to tame some of the carbon/ceramic's more evil tendencies. Once the pressure plate fully engages, the more powerful carbon/ceramic takes over and keeps the transmission shaft locked to the crankshaft.

References

• Today's Technician: Manual Transmissions and Transaxles
• UUC Motorworks: How to Choose a Clutch