In The Clutch
I was talking with Will Baty of Centerforce the other day, and he gave me some great information on seating a brand-new clutch and why this is so important. If you think about it, a clutch operates in many ways like a disc brake. We have a friction material (the clutch disc or brake pad) that is pushed in direct contact (pressure plate clamp load or disc brake clamp force) with a rotating friction surface (a flywheel or disc brake rotor). New brake pads should be properly bedded to obtain the maximum coefficient of friction performance, and the same is true with a clutch disc. Will used the example of a typical Centerforce 101/2-inch-diaphragm clutch for a small-block Chevy. Centerforce has a machine that will measure the actual clamp load of a given clutch in lb-ft of load. Using this machine, the company has determined that a brand-new 101/2-inch clutch assembly right out of the box will hold 400 lb-ft of torque. If the clutch disc friction material is properly seated (bedded) to the flywheel and the pressure plate, the load capacity increases to 520 lb-ft. That's an impressive increase of 30 percent. If excessive heat is applied to the clutch and the disc becomes glazed, then clamp load capacity drops to barely 310 lb-ft, which is less than its out-of-the-box capacity. This illustrates why it's so important that the clutch be properly seated.
Centerforce recommends a typical street clutch be driven for 500 miles before it is used in a drag race-style load application. This mileage figure might be a bit high, but the idea is to use the clutch in normal street operation without applying full load until the clutch has a chance to transfer some of the friction material from the disc to both the flywheel and the pressure plate. This is exactly the same idea behind bedding brake pads in which a portion of the friction material is transferred to the brake rotor to improve the overall coefficient of friction. According to Will, even 200 to 250 in-town miles will do a good job of transferring material to the friction surfaces and allow the clutch to properly seat.
Earlier, we mentioned glazing, which is what happens to a clutch disc (or a brake pad) when excessive heat is applied to the friction material. Will says most street Centerforce clutches like to operate in the 180- to 220-degree-F heat range. He says the friction material is stable up to around 350 degrees, but its ability to hold torque is reduced at higher temperatures. The clutch's friction material is happiest and enjoys its highest coefficient of friction in the 200-degree-F range. This figure will change with different friction materials. As the temperature increases, the resins that help bond together the different compounds begin to melt, causing the resins to leach to the surface of the clutch disc. You can actually see the result of the resin melting on a clutch disc because the friction surface will become shiny and much more reflective. At that point, the disc is ruined and must be replaced.
The friction material is the key to how well a clutch works. The best street clutch uses a friction material that increases the torque capacity while still allowing smooth operation with minimal chatter or harsh feedback during normal street use. The other variable when designing a clutch (or a disc brake) is surface area and/or clamp load. One way to increase torque capacity is with a twin-disc clutch, which has become very popular lately as an easy way to increase torque capacity while maintaining a comfortable pedal effort. Adding a second clutch disc while maintaining the same friction and clamp load effectively doubles the torque capacity. So this is what allows Centerforce to claim that the company's dual-disc clutch can hold up to 1,200 lb-ft of torque. In an abbreviated format, that is the essence of clutch operation and why properly breaking in a clutch will pay off in terms of far better performance. That's why you spend the money for good parts.