In the Clutch - Everything You Need to Know About Clutches

Comparing a 10.5-inch to an 11-inch clutch, the larger clutch disc offers roughly 16 percent more surface area. This allows the same clutch facing to hold additional torque without resorting to higher static pressure or a more aggressive metallic material.

Comparing a 10.5-inch to an 11-inch clutch, the larger clutch disc offers roughly 16 perce

Discs

All street clutches are designed to provide easy engagement for normal street driving. From a dead stop with the engine running, the disc is required to slip slightly as the transmission and everything downstream accelerates up to engine speed. There are lots of things happening when this occurs. A typical street clutch disc is made up of a hub, a drive plate (where the facings are attached), two friction material facings, several springs, and four drive limit pins. The springs are located in the hub, which rides on the input shaft. Inside the hub face are six to eight small coil springs. These springs are designed to cushion the engagement by absorbing a portion of the impact load as the disc is squeezed between the flywheel and the pressure plate. If these springs are too weak, they can bottom out, which allows the hub to hammer the limit pins. If this occurs, it can cause clutch chatter. The evidence of this is found when you inspect the pins and find flat spots. The fix is generally a disc with stiffer springs.

An additional cushion, if you will, is something called the Marcel spring, which is a very thin wafer-like spring that is placed between the clutch facings and the disc. It also acts to cushion the engagement as the disc is loaded by the pressure plate. This may not sound like much, but if you've ever driven a car with a race clutch without the Marcel spring, you've noticed they grab very quickly and can chatter like crazy. Clutch chatter problems with street clutches are usually related to the disc (rather than the pressure plate) and can sometimes be traced to a flat Marcel spring that allows the disc to skip across the flywheel creating the chatter.

Clutch face material is perhaps the most important factor in choosing a clutch disc. Aftermarket companies like Centerforce, Hays, McLeod, and Ram all offer several choices of clutch discs based on the clutch facing coefficient of friction. Over a decade ago, Centerforce made the Dual Friction concept popular, with a metal matrix material placed in segmented pucks on the flywheel side of the disc. This idea has spread to many aftermarket clutch companies because it combines the more aggressive metallic facing that has a higher coefficient of friction with a more conservative organic facing on the pressure plate side of the disc. The combination of the two facings can hold increased power levels without slippage.

The Marcel spring is that thin, wafer-looking spring placed between the two clutch facings that helps cushion the engagement of a typical street clutch. Race clutches eliminate this spring, which makes the disc engagement much harsher--that's fine for racing but not good for the street.

The Marcel spring is that thin, wafer-looking spring placed between the two clutch facings

The advantage to the metallic facing is directly related to its temperature capabilities. In most cases, as a friction material temperature rises in an organic compound, the coefficient of friction decreases. This is also true of metallic linings, but they do offer higher temperature capacity. It is possible to mount segmented metallic facings on both sides of a street clutch, but this also creates a much more aggressive connection between the flywheel and the transmission.

Heat is by far the biggest enemy of any clutch material. Heat is generated anytime you have two surfaces sliding past one another, so there is always heat in a clutch system. The key is to not exceed the clutch disc's temperature ceiling. As a clutch slips, much of the heat is transferred to the flywheel, which acts as a giant heat sink. But if you abuse the clutch by creating more heat than the flywheel can absorb, the temperature quickly spikes in the clutch disc lining. This excessive heat then melts the resins that bond the clutch lining material. These resins rise to the surface of the disc and create a glaze that drastically reduces the coefficient of friction. This creates even more slippage, and pretty soon you have a dead clutch disc that will barely move the car. That's when it's time for a new clutch disc and pressure plate.