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Billet Clutch Tech Talk:
Pedal or 'Glide?

Black Star Billet ClutchThe automotive clutch was originally designed to disengage the driver (engine) from the driven (transmission). Drag racers learned that the clutch could be manually feathered — i.e., slipped — by a skilled driver's foot against the clutch pedal. This allowed the drive tires to spin less, yielding quicker elapsed times. By the mid-Sixties, Top Fuel teams were starting to experiment with clutches that had a certain amount of slip designed in, enabling a car to run quicker and repeat more consistently.

The long-style pedal clutch, which uses a combination of spring pressure plus centrifugal clamping to make the unit lock up, was used initially because it was a simple matter to make the springs adjustable and the counterweight adjustable. Eventually clutches appeared with their springs located behind the cover, instead of between the cover and the pressure plate (drive shoe). These became known as "'Glide" clutches, and allowed a driver to remove his/her foot from the clutch pedal before leaving the starting line. As engine rpm increases, the counterweighted levers overcome the static springs, and the clutch locks up. This system allows the tuner to put a fixed amount of load on the engine at idle (also referred to as "stall").

Both pedal- and 'Glide-type clutches can be operated in multiple stages. In these applications, pressure is applied to the drive shoe by a few of the many levers available. As the throwout bearing is allowed by the clutch controller to move back, more and more of the levers engage, adding squeeze to the clutch pack. Most Top Fuelers and AA/Funny Cars use multistage, 'Glide-type clutches. Most Top Alcohol Dragsters and Funny Cars, Pro Modifieds and Pro Stockers, plus many nostalgia cars, run single-stage, pedal-type clutches.

Clutch science is as complex as fuel or ignition technology. In fact, both the fuel and induction systems must be in synch with the clutch for a car to achieve that final two- or three-tenths of performance attainable. Pedal clutches have some advantages, as do the 'Glides. With a pedal-type, you'll typically experience less clutch-pack wear per run. You can better control 60-foot time, because you can leave at a much wider rpm range (from 1500 to 8000 rpm). You can adjust how violently the counterweights come in by adding or subtracting static spring pressure.

You can vary total clamping force in either a pedal or a 'Glide clutch through adjusting centrifugal counterweight (by changing mass or geometry). The main advantage to a 'Glide-type clutch is the ability to put a load on the motor at the starting line. This can also be accomplished with a pedal clutch manually, by using the driver's foot to load the motor.

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