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The Critical Difference

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What's the best way to drive my manual transmission car?

Chapter 3

Clutch Discs

(click on any image to enlarge)


Typical sprung hub clutch disc	Typical solid hub clutch disc

CLUTCH DISC CONSTRUCTION

There are two types of clutch disc construction - solid hub and spring dampened hub.

Solid hub discs have the splined hub riveted directly to the disc carrier plate (the circular metal plate that carries the friction material). This construction is typical of all out racing discs.

Sprung, or dampened hub discs, do not have the splined hub attached directly to the carrier plate. The hub floats in a spring loaded assembly on the carrier. When the hub is loaded as when the clutch is engaged, the springs help absorb the load rather than transfer it directly to the carrier plate. This arrangement allows shock spikes from aggressive engagement to be dampened out to avoid possible damage to the drivetrain. This type of construction is used in factory replacement discs.

Performance discs feature stronger damper springs to absorb higher loads. Most RAM performance discs are equipped with RAM poly-coil springs, which are encapsulated in urethane providing a substantial increase in the spring rate over stock springs. The stronger spring prevents over travel of the hub and subsequent damage to the disc.

Selecting the correct disc construction is simple. Stock applications use the dampened hub. From there, as loads increase, the rigidity of the hub should increase right up to solid hubs used for all out racing.

TECH TIP - COEFFICIENT OF FRICTION - a simple explanation

Let's say we have a box sitting on a countertop. It takes 3 pounds of pressure to slide the box across the countertop. Now we place the box on a rubber mat. It takes 8 pounds of pressure to slide the box across the rubber mat. The rubber mat has more grip, or a higher coefficient of friction than the smooth countertop, thus taking more effort to slide the box.

In terms of the clutch disc material, a higher coefficient of friction means that it will take more load on the clutch to cause it to slip.

FRICTION MATERIALS

As performance requirements increase, it is necessary to step up the friction characteristics and durability of the clutch facing materials. We will review the full range of friction materials and their features.

ORGANIC - Organic material is common to all stock clutch discs, and offers the best drivability but has limitations as operating temperatures rise. Under high loads accompanied by slippage, they fade because their coefficient of friction drops off. In addition, at high RPM and high temperature they tend to fail structurally.

RAM 300 series discs use organic facing material that is bonded to a steel backing plate that increases heat dissipation and provides excellent structural strength that precludes disintegration at high RPM. This structural improvement does not compromise the excellent drivability of 300 series discs.

KEVLAR COMPOSITES - Kevlar material offers a higher coefficient of friction than organic material, but with some loss in drivability. As the coefficient of friction goes up in the disc material, so will the aggressiveness of the material on engagement. This means that some chatter can be expected with this material in low gear and reverse. Kevlar is compatibile with stock flywheels and pressure plates, making it a good upgrade choice for later model vehicles. We do not recommend Kevlar for competition applications.

BRONZE METALLIC - Bronze metallic (paddle discs) is the most aggressive materials in terms of engagement. It offers extended life using reduced static pressures, and a quick, clean engagement. Bronze metallic is aggressive on the flywheel surface and should be used only with steel or nodular iron friction surfaces. On the street, this material will cause chatter on engagement.

SINTERED IRON - Sintered iron is well known for its ability to withstand some slippage and not loose its coefficient of friction. It is the material choice for high horsepower clutch applications for drag racing and truck pulling. A street version of the sintered iron disc is the RAM 900 series, which utilizes a sprung hub. These discs should be used only with RAM steel or aluminum flywheels or excessive wear to the friction surface will occur. A key feature of the competition sintered iron material is that it is maintainable. If the clutch is slipped excessively, the disc can be resurfaced and reused.

TECH TIP – DRIVABILITY VERSUS PERFORMANCE

With disc choices ranging from organic to metallic to sintered iron, deciding which material to use is ultimately up to the end user, based on the type of performance and drivability is desirable. When deciding, one must think about what is most important for the vehicle. If on-the-track performance is the most important, select a disc material that is fairly aggressive. But realize that making this decision will effect how smoothly the vehicle will operate under normal driving conditions. If smooth and drivable is most important, select the disc based on this. Keep in mind though, that the life of the clutch disc under racing conditions will likely be reduced.

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