When is Centrifugal Clutch Pressure Effective?

Feb 2, 2022 | Clutches, Racing

Centrifugal clutch pressure, or ‘counterweights,’ can be a tremendous aid to achieving the optimum clutch combination for high power drag racing applications. In racing situations, some ‘slip’ is required on the initial launch, and the clutch needs to be progressively locked up as the car accelerates down the track. But is it effective in a streetcar application?

First, we need to understand the method for increased holding power in a clutch. This can be done three ways – additional static or clamp pressure, increased coefficient friction clutch disc materials (with higher heat and holding capacities), or using centrifugal pressure. 

The first two methods – increased clamp load and more aggressive friction material, are constant. If you have this increase in holding capacity, it doesn’t matter if the engine is turned off or spinning 7000 RPM.  

Centrifugal pressure, on the other hand, is dependent on engine RPM. So if you look at a graph on centrifugal pressure, the curve starts off very shallow at around 5500 RPM. Then, as the engine RPM increases, this increase in pressure goes up by the square of the engine RPM. Around 7500 RPM, the curve would be roughly 45 degrees upward, and at about 9000 RPM the curve is shooting virtually straight up.

Herein lies the problem with centrifugal pressure in a street application – most street engines make the bulk of their torque at RPMs much lower than 7000, some as low as 2500! 

Centrifugal pressure cannot address this high torque in a low RPM situation. The only way to handle it is with a higher clamp load, more aggressive friction material, or a combination of these two. 

If you look at a pressure chart for a RAM 11 inch diaphragm pressure plate like those found in an Musclecar, HDX, Powergrip, or Powergrip HD set, and compare it to the pressure curve of an aftermarket centrifugally assisted pressure plate, the centrifugal plate application will have to operate up to 9000 RPM to reach the equivalent holding capacity of the RAM pressure plate. 

How many street engines are going to see this kind of use?

A fourth method of increasing holding capacity has emerged in the last ten years – the street dual-disc clutch. The beauty of these clutches, such as the RAM Concept 10.5, is that you can have your cake and eat it too – a high capacity clutch system that does not require extreme static pressure or too aggressive of friction materials to achieve the same holding power.

What is the takeaway? 

Be knowledgeable about your application and what you expect from your clutch system, and don’t fall for sales hype for your street or street/strip vehicle’s clutch system.