How To: Choose the Correct Clutch

 

Want to get the most out of your transmission upgrade?  Then you need to select a clutch that will meet your driving style and performance needs.  This article will help you understand the purpose of the main components in your clutch and how they can affect the performance of your car.

There are several components to a clutch system. These include the pilot bearing or bushing, flywheel, clutch disc, pressure plate, throw out (clutch release) bearing, clutch release fork and the pivot ball.

 

Clutch System

 

The pilot bearing provides support to the transmission input shaft. It is typically made of a sintered bronze or brass material, but most modern transmissions require the use of a roller-bearing style pilot bearing. This is due to tighter manufacturing tolerances in modern transmissions.  Roller bearings also produce less drag and provide increased longevity. They do not become egg-shaped over time, which can happen in the sintered bronze or brass style pilot bushings.

The flywheel is the energy storage unit for the clutch system. It provides the mounting location for the ring gear, which the starter motor engages when you turn the key. It also provides the inertia to maintain rotation when the clutch pedal is depressed, and allows for a more even and controlled clutch engagement.

Most flywheels are made of billet steel, for longevity and easy clutch engagement.  Aluminum flywheels are used more in drag racing applications where smooth engagement is not of primary concern.

The pressure plate is the workhorse of the clutch system. It provides the clamping force necessary to prevent clutch slippage. When you depress the clutch pedal (disengage the clutch), springs in the pressure plate open, and relieve the pressure on the clutch disc, allowing it (and the transmission to coast, independent of the engine RPM.   As you raise the clutch pedal (engage the clutch) these springs close and clamp the clutch disc against the flywheel, transferring engine power to the input shaft.

Pressure plates come in 2 basic styles, diaphragm style in most modern clutch setups and 3-finger (also called "Long-style" or "Borg and Beck" style).  SST uses the diaphragm style, exclusively, to provide you with easier clutch action, smoother engagement and longer clutch life.

The clutch disc gets the most abuse of any component. It gets the most wear and is the reason for most clutch system issues. There are 2 basic types of clutch disc construction: sprung hub and solid hub.

The solid hub is used primarily in high horsepower all-out racing applications, while the sprung hub system is used in practically every other application. The sprung hub design helps to absorb the shock during initial engagement of the clutch, and allows that shock to be dissipated before it can cause damage to the drivetrain. Performance clutch discs use heavier-weight springs to absorb more of the load caused by higher horsepower engines, and extreme-performance discs can have a polyurethane cover over the spring for increased life and higher spring rates. 

Clutch Materials

Clutch discs are constructed of many different materials:

  • 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. 
     
  • 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 compatible with stock flywheels and pressure plates, making it a good upgrade choice.
  • Sintered Iron:  Sintered iron is well known for its ability to withstand some slippage and not lose its coefficient of friction.  It is the material choice for high horsepower clutch applications. 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.

No matter what your needs, SST has the products and the expertise to help you determine the best package to meet them.