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Multiple Disc Clutch Systems Tech
There are two reasons why Mustang owners switch from the stock single disc clutch design to a multiple disc clutch. The most popular reason is for an increase in torque-holding capacity. The most important reason is to reduce the clutch assembly's moment of inertia (MOI). Lowering the MOI provides several benefits:

  • Quicker acceleration out of each corner on the track, resulting in lower lap times.
  • Quicker engine deceleration when the throttle is lifted. That improves braking at corner entry, often allowing more rear brake bias, leading to shorter stopping distances.
  • Quicker shifting because the transmission synchronizers will more easily match the speeds of the transmission gears during shifting. This also reduces wear of transmission parts.

Torque-Holding Capacity
While there are several ways to increase the torque-holding capacity of a single disc clutch, they all have limits.

  • Increasing the disc diameter: This is usually impossible due to packaging constraints.
  • Increasing the pressure plate's clamp force: Anything more than a small amount is impractical because the pedal effort will become too great.
  • Increasing the disc material's coefficient of friction: This also has practical limits, as undesirable side effects such as poor modulation and chatter can result.

To increase capacity with the least compromise: Increase the number of discs.

Simply put, with all else being equal, two discs have twice the holding capacity of one disc. The more complete explanation: The torque-holding capacity of a clutch is proportional to the mean diameter of the friction disc material, multiplied by the clamping force of the pressure plate, multiplied by the coefficient of friction of the friction material, multiplied by the number of friction discs.

Adding a second, and even a third disc, is the most effective way to increase torque-holding capacity. By increasing the number of discs, the torque-holding capacity can be raised without causing significant increases in pedal effort or in the clutch system's moment of inertia.

Torque-holding capacity: The Math
The torque-holding capacity can be quantified by this equation:

Torque-holding capacity = (mean diameter of friction material) x (pressure plate clamp force) x (coefficient of friction) x (number of friction disc faces)

MOI: That's What Really Counts
The term "moment of inertia" (abbreviated as MOI) refers to the rotational inertia of a rotating mass. When it comes to clutches and flywheels, the moment of inertia is a measure of the work required to accelerate and decelerate the rotation of the clutch and flywheel assembly. The higher the MOI of the clutch, the more power required to accelerate its rotation. The lower the MOI, the less power it takes to accelerate its rotation. For a given amount of power, a clutch and flywheel assembly with a lower MOI will accelerate up to speed more quickly, and decelerate more quickly once power is no longer applied. The MOI is sometimes referred to by the unscientific term, "the flywheel effect."

MOI: The Math
The MOI is proportional to the overall mass of the object multiplied by the square of the distance the mass is located from the center of rotation. In mathematical terms, this is expressed as:

      MOI=mr²

The distance (r) has a squared effect on MOI, while the mass (m) has a linear effect on MOI. This means that small changes in the distance of the mass from the center of rotation will have a large effect on the MOI. It is possible for two different clutch and flywheel assemblies to weigh the same, but have very different moments of inertia. For two flywheels of the same weight, and evenly distributed mass, the one with a smaller diameter will have a lower MOI.

The Bottom Line: A Lower MOI is Better
A clutch and flywheel assembly with a lower MOI will help a car accelerate more quickly, both on the road course and at the dragstrip. To create a very low MOI clutch, manufacturers must go to a smaller diameter clutch design. While designing a clutch with a smaller diameter will reduce its MOI, the friction material diameter will also be reduced, and therefore the torque-holding capacity will be less. That is why, to create a clutch with a very low MOI, it is necessary to have multiple discs. Increasing the number of discs, while also making the discs smaller in diameter, will increase the torque-holding capacity of the clutch, without much change in the MOI. The number of discs needed is determined by the amount of torque-holding capacity desired.

The Details of Multiple Disc Clutches
Multiple disc clutch systems are exactly what they sound like. The most common clutch design, the one that most Mustangs came off the assembly line with, has one clutch disc sandwiched between the flywheel and the pressure plate. Multiple disc clutches have two or more discs sandwiched between the flywheel and the pressure plate.

The main components are the flywheel, pressure plate, clutch cover, clutch discs, and floater(s). The flywheel, pressure plate, and clutch cover are very similar to single disc systems in their operation, while the discs can vary quite dramatically in both size and friction material. Between each pair of discs is a floater. The floater serves as a friction surface between the discs. Floaters are connected to the pressure plate/flywheel assembly, and therefore spin at the same speed as the flywheel and pressure plate. There are two basic designs for driving the floaters. Clutches intended for use on street-driven cars usually have straps connecting the floaters to the clutch cover. The second design has the floaters captured by pins located between the flywheel and clutch cover. While this design is stronger, it generates some noise when the clutch is disengaged, and is therefore less suitable for street-driven cars.