How to Make Key Adjustments to a Drag Racing Clutch

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.

Drag Racing Clutch

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.

Drag Racing Clutch

On the left rests a 10-inch billet flywheel with floater, two clutch discs and six titanium stands. On the right stands the cover assembly, showing three machined slots in its steel friction rings. The slots, which are also found on the flywheel, allow the spent friction material to escape, enabling the adjacent wearing surfaces on the discs to remain flat. The friction rings are secured to the cover assembly and to the flywheel by thirty 12-point screws.

 

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.

Clutch plate

Floater plates reside between the clutch plates and are available with friction surfaces of steel or bronze. Bronze is currently popular because of its excellent coefficient of friction and impressive dissipation of heat. If the clutch slips significantly while leaving the starting line, the steel floater plate doesn’t dissipate the heat as well as its bronze counterpart. As a result, the clutch disc temperature increases and the coefficient of friction decreases. In other words, it is harder for the counterweights to lock the clutch when a steel floater becomes hot.

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.

copper-alloy bushing

Placing a copper-alloy bushing between the clutch fingers and the clutch pivot pins eliminates friction and contributes lubricity. The bushings make disengagement smoother and add longevity to the levers, the pins and the bushings.

 

Text by Sam Logan Photos by Moore Good Ink

Source

Ram Clutches

201 Business Park Blvd.

Columbia, SC 29203

803.788.6034

Ramclutches.com

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