Courtesy the Manufacturers
July 18, 2011

Of course, all bets are off when it comes to pure competition clutches. Things like chatter, instant engagement, and high pedal pressure don't mean anything when you're not carrying groceries or passengers. The "mission statement" of it is that you want to engage the clutch as quickly as possible (without shocking the driveline on higher horsepower vehicles), and you want the clutch to hold whatever the engine's dealing out.

This author personally prefers to specify clutches for race applications, rather than high-horsepower street applications as you don't have to worry about the customer coming back a month later with a grimaced face talking about chatter. This may be a subject that we will want to cover in a future tech article, as there are many possibilities of combinations and strategies (sometimes you want the clutch to slip) when it comes to a race clutch.

Clutch Actuation

So what happens when you push your clutch pedal down? The gist of it is that when you shove the pedal in, you want the release bearing to press against the fingers of the pressure plate, which spring it out and away from the disc, allowing the disc to rotate freely and independently of the pressure plate. Your clutch pedal is basically connected to that release bearing by three different possibilities: mechanical actuation (mechanical linkages as used in vintage cars), cable-actuated clutches (such as those used in Fox body, SN-95 Mustangs, and so on), and hydraulic clutches (master cylinder and slave cylinder, or master cylinder and hydraulic release bearing, typical for S197 Mustangs).

Mechanical actuation is the simplest way of actuating a clutch system. If you look on the firewall of a vintage Mustang, for example, you will see that the clutch pedal is connected to a series of linkage rods and a "Z-bar" to push on a clutch fork. The clutch fork pivots on the driver side of the bellhousing and moves toward the rear of the vehicle when the clutch is engaged. The release bearing is clipped onto the clutch fork, so when you move the fork toward the rear of the car, the bearing slides forward on the input shaft bearing retainer and contacts the pressure plate fingers.

It's a very simple system and easy to work on. If your older vehicle is already equipped with this style of clutch, you're probably better off using it. However, in higher-performance applications, the linkage rods, bushings, and mounting attachments can all deflect and wear. Sometimes these components have to be replaced with larger rods, spherical bushings, and Heim joints. However, once upgraded, this setup is a bulletproof, reliable method of engaging your clutch.

Cable actuation is next in terms of mechanical simplicity. In this situation, when you push the clutch pedal down, you pull on a cable (much like a brake cable on a bicycle) that actuates the clutch fork. Since the cable has to "pull," you'll find that this type of setup uses a clutch fork that pivots on a ball stud located on the passenger-side of the input shaft. The driver-side end of the fork moves toward the front of the car, pulling the release bearing into the pressure plate fingers.

Like the mechanical method, this is a very simple and easy setup to work on. There are also lots of aftermarket parts available, including heavier cables, beefier cable quadrants, and firewall adjusters so that you can dial in the pedal travel. There exists the possibility, however, that the cables can weaken and stretch when dealing with higher pressure plate static pressures.

Lastly, we have actuation by hydraulic action. As was mentioned earlier, this can be done by combining the use of a master cylinder with either a slave cylinder or a hydraulic release bearing. When you push the clutch pedal down, you're actually pushing on a piston, which moves fluid and creates a hydraulic action. This hydraulic action can be used to extend/retract a slave cylinder (which is connected to a clutch fork) or it can be used to move a hydraulic release bearing, which slides itself along the input shaft bearing retainer.