When we first take an interest in drag racing we soon realize that nothing in its mechanical history is more absorbing than the racing clutch and its operation.
Multi-disc drag racing clutches are constructed in four or five different diameters. They range mainly from 6.25 inches (500-ci Pro Stock) to 10 inches (Mountain Motor Pro Stock). Comp Eliminator and Sport Compact classes often run 7-inch twin-disc clutches, while other engine sizes use 8 inches. Mountain Motor clutches comprise two discs, while Pro Mod and Pro Nitrous use three.
Obviously, the object of these clutches is to transmit formidable engine power to the manual gearboxes and to the rear wheels, but they also must provide predictability and adjustability. Modern billet drag racing clutches feature five principal adjustment mechanisms. These include provisions to address the following: clutch disc wear; static or base spring pressure; centrifugal pressure generated by counterweights, which are positioned on the clutch levers; throw-out bearing distance from the clutch levers; and the air gap, which is the distance that the cover assembly separates itself from the clutch plates. Of course, launch rpm could also be considered an adjustment mechanism.
Clutch disc wear causes the clutch levers (the fingers) to move from their optimum operating position toward the release bearing. Adjustment is made by inserting a pin punch in the small holes and rotating the upper barrels of the titanium stands. This adjustment, which is made each time the clutch is installed in the car and sometimes between runs, returns the clutch levers to their proper operating position.
Static or base pressure is altered by increasing or decreasing the pressure on the springs located inside the red aluminum housings. Data acquisition software is used to monitor driveshaft speeds. It calculates the amount of clutch slippage that occurs during launch and at intervals as the car is traveling along the race track. From this data the most appropriate base pressure springs are determined.
Centrifugal force or counterweights compress the clutch pack and cause the clutch to lock fully. The centrifugal force is used to bring the clutch and engine speeds together. It also provides the additional clamping force that enables the next higher gear to be held. A racing clutch could be constructed using only spring pressure, much like the diaphragm clutch used on the street car. However, under these circumstances the clutch would perform adequately in first gear but it would slip severely in second gear because of insufficient clamping force.
On the other hand, if you provided the clutch with sufficient base pressure clamping force to hold second gear, it would have too much clamping force in first gear and would therefore promote too much wheel spin—overpowering the tire. Thus, by adding centrifugal weights to the fingers (in the form of small bolts, nuts and washers), the clutch has the ability to add the perfect progressive clamping force in each gear, unlike a street-strip diaphragm clutch. The street-strip assembly operates with a fixed and unalterable clamping force. As a result, it doesn’t have the benefit of the centrifugal action of the drag racing clutch, which smooths the vehicle’s acceleration at all engine speeds.
Throw-out bearing clearance refers to the distance between the bearing and the clutch levers, which is usually set to around .300-inch. The air gap is determined by the distance the cover assembly separates itself from the clutch pack and is usually measured by feeler gauge. Air gaps, which affect reaction times, are set for every run and typically vary from .030-.075-inch.
Text by Sam Logan Photos by Moore Good Ink
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