Ford Clutch Guide - Coming Through In The Clutch

As previously mentioned, the clutch torque capacity is dependent on the clamping force of the pressure plate (combined with the coefficient of the disc), which is developed by the springs in the pressure-plate assembly. Increasing the spring pressure thus increases the torque capacity of the clutch setup. But there's a downside. When you press the clutch pedal to release the clutch, you're pushing against those pressure-plate springs. Consequently, pedal effort goes up with the pressure-plate spring pressure. Pedal effort also increases the further you push the pedal. Old-school musclecars often had clutch-pedal efforts that would make even the strongest left leg shake at a long red light (and forget about letting your wife drive it, unless she was an Olympic weight lifter).

To combat the pedal-effort problem of performance pressure plates, centrifugal boost techniques were developed. On the Long-style clutch, the release levers could have extra weights added, which protrude from the "windows" of the pressure plate cover. As the clutch rotates, the weights are located on the side of the pivots such that centrifugal forces on the weights increase the clamp load on the pressure plate. Therefore, the pressure plate can be built with softer springs to reduce the "base" pressure (and more importantly, pedal effort), but as the rpm increases, clamp load and clutch torque capacity also increase. It's like having your cake and eating it, too.

Long-style pressure plates built for racing applications often have spring adjusters on the coil springs, so base clutch pressure can also be tuned by twisting a few screws. This is important since most Long clutches are used with a sintered-iron clutch disc, which can slip and then be tightened with no ill effects. Additionally, the centrifugal boost weights are removable, so clamp pressure versus rpm can also be fine-tuned by swapping on different weights. This makes for an infinitely adjustable clutch.

Lastly, the Long-style pressure plates have a cover design that allows excellent ventilation to remove heat from the pressure plate ring better than the B&B and diaphragm styles. This becomes important if there's significant clutch slip.

Despite the performance of Long-style clutches, diaphragm pressure-plate assemblies are all the rage now. Instead of several individual coil springs, the diaphragm types use a single "Belleville" spring that has a conical/diaphragm shape with a hole in the middle. The outer diameter edge of the spring acts against the pressure plate ring, which clamps the clutch disc. At the cone end of the Belleville spring, slots are cut radially around the central hole, which creates many individual "fingers." At the bottom of each slot, rivets fasten the spring to the pressure plate cover, which allow the fingers to work as release levers (i.e., as you push down on the fingers, the spring pivots about the rivets and lifts the outer edge, thus pulling the pressure plate ring away from the flywheel and releasing the clutch).

A quick tip about release bearings: For the self-adjusting cable clutches used in modern Fords, with their diaphragm pressure plates, the release bearing actually rides full time on the release fingers. Therefore, if you buy a cheap replacement unit that's not specifically designed for constant running, you'll soon be hearing the characteristic squirrel noises coming from your bellhousing that only a cooked release bearing makes. Now, back to our regularly scheduled program.

Besides the simplicity and cost advantages of the diaphragm type, it has another major advantage over the B&B and Long-style pressure plates: pedal effort. Unlike the coil-spring pressure plates, where pedal effort increases the further you push the pedal (because the spring rate increases), with the diaphragm types, pedal effort increases at first, reaches a peak, and then actually decreases as you push the pedal further due to the "overcentering" ability of the Belleville spring. So now you can hold the pedal to the floor at a red light, and you (or your wife) do not have to fight against maximum spring forces.

Centrifugal boost is also available with diaphragm pressure plates, such as the units from Centerforce, by adding weights into the slots on the release fingers. Then, as rpm increases and the weights are pulled outward by centrifugal forces, the clamping pressure of the assembly increases. This allows reduced base pressure, which reduces pedal effort at low rpm.

Regardless of the type of spring system used to develop clutch clamping force, all pressure-plate assemblies have a pressure ring that clamps against the clutch disc. The pressure ring has to dissipate the heat developed from slipping the clutch during engagement (which can sometimes be considerable), as well as be strong and stiff enough to deal with the clamping and centrifugal forces. For most applications, the stock pressure ring is made of gray cast iron. It's simple to make and cheap, but gray cast iron doesn't have the strength to endure high-performance abuse (e.g., dumping the clutch at 6,000 rpm with sticky slicks). The scary thing is, when the pressure ring fails, it's usually at high rpm, so parts will fly apart in all directions at considerable velocity. Basically, it becomes a bomb, thus the need for an SFI-approved blowproof scattershield for racing or general high-performance applications. To prevent a nasty pressure-plate failure when racing, always use an SFI-approved pressure plate, made from nodular iron, steel, high-strength aluminum, or if you have really deep pockets, titanium. Ditto for the flywheel. It's like the American Express commercials: SFI clutch, $500; SFI bellhousing, $300; your legs, priceless.

Aluminum pressure plates are actually good at two other things: increased heat tolerance and lowered inertia. If your application generates a great deal of clutch heat, the aluminum does a better job of transferring the heat out and away from the clutch disc than steel or cast iron. Having a lower-density aluminum also reduces the rotational inertia of the pressure plate. Since the pressure plate rotates with the flywheel, any reduction of inertia in the pressure-plate assembly has the same effect as using a lighter flywheel. This can be a good or bad thing, depending on your application.

Clutch DiscsClutch discs come in many sizes, shapes, materials, and constructions. As previously mentioned, simply increasing the diameter of the clutch disc increases its torque capacity, but there's a limit (and downside) to that. A clutch with a large diameter not only reduces ground clearance (or forces a higher engine location), but it also increases the rotational inertia of the clutch disc. Unlike the inertia of the pressure plate (where sometimes more inertia is a good thing), high inertia in the clutch disc is almost always a bad thing.

Consider shifting gears for a moment. Say you're shifting from First gear to Second gear at 6,000 rpm, and the gear ratio spread from First to Second drops the rpm to 4,000 after the shift. When you release the clutch at the top of First gear, the clutch disc, and therefore transmission input shaft, are spinning at 6,000 rpm. To engage Second gear, the trans needs to spin down to 4,000 rpm, but the inertia of the clutch disc on the input shaft is preventing that deceleration. If you have a synchronized transmission, all that clutch inertia is torturing your synchros and slowing down the shifts. Anything that reduces the disc inertia will make shifting quicker and easier on your trans.

One method of reducing disc inertia is to use a "button" or "puck" design. In other words, the friction facing and carrier are not full circle; there are only three, four, or six individual pads, with nothing but air in-between. While this is a great way to reduce inertia, it comes at a price for streetability: no "marcel" spring.

On full-circle clutch discs, the friction material is typically attached to a carrier that is formed into a wave shape, called a "marcel" spring. This allows the disc to compress a bit as the pressure plate puts the clamp on. Without the marcel, the clutch has more of an on/off behavior rather than a smooth, slow, predictable engagement.