May 1, 2007

For many Mustang enthusiasts, there's nothing quite like ripping off the perfect powershift. We understand. Why wouldn't you want to drive with both hands and feet while being an integral part of the acceleration process? Rowing a stick is that much more fun. If it becomes a little routine after a while, you simply add more power, and the gear changing happens faster and likely with more fury. The fun is restored, until the next power upgrade is required. Eventually, the power upgrades become too much for the old clamp to hold, and out comes the death smoke from the clutch department. Too many of us know the smell of a burnt clutch-it's one you can't forget.

So you open the first clutch catalog you find, skip down to the bottom of the race clutches page, and install the full-competition setup in your street car. Now you have a heavy pedal, and every time you let up the pedal, it chatters your lug nuts loose-once again, the fun is gone. For a brief moment you consider installing an automatic trans, but then you wake up.

Maybe, with a little clutch science, your next clutch choice will hold the power without taking all the fun away. Thankfully, MM&FF is here to help.

Clutch BasicsMore than 100 years ago, those Germans who figured out how to adapt a crankshaft into a cannon-thus creating internal-combustion engines-realized the need to decouple the engine from the wheels in early vehicles so the engine could be started without having the car immediately run you over. Karl Benz is generally credited as the inventor of the automotive clutch we love (or hate, if we have the wrong one for the application).

An automotive clutch is a system that consists of a flywheel, a pressure-plate assembly, a clutch disc, and a mechanism to release the clutch (i.e., release bearing, fork, and clutch linkage-be it cable, hydraulic, or mechanical linkage). The engine is decoupled from the transmission when the clutch is released. When the clutch is engaged, the power from the engine directly drives the input shaft in the transmission. Naturally, different applications require different levels of grip for the clutch to retain power, resulting in no slip.

To accomplish no slip, the clutch disc has friction-material facings on both sides and is clamped tightly between a flywheel face and a pressure plate. As part of the pressure-plate assembly, springs are used to develop the force that clamps the disc tightly between the pressure plate and flywheel, thus preventing slip. Both the flywheel and pressure plate are attached to the engine crankshaft, so the torque from the engine is transmitted through the friction faces to the clutch disc. The disc is then splined to the transmission input shaft. When the clutch is released, the clamping force is withdrawn (the pressure plate and flywheel separate a small distance), and the disc is decoupled from the crankshaft rotation.

Here's the confusing part: When the clutch pedal is released, it engages the clutch (the disc is clamped tight and transmits torque), and the clutch is released when you step on the clutch pedal. Hey, we didn't name this stuff.

If the friction coefficient of the facings or the clamping force on the clutch disc is insufficient, the clutch will slip when it's asked to transmit torque, and a lot of engine power gets converted into heat (death smoke and the evil smell soon follow). Historically, clutch torque capacity was typically increased by using greater clamping spring forces, more aggressive friction materials, clutches with larger diameters (the larger the diameter of the friction ring, the greater the torque capacity of the clutch), or multidisc clutch setups (more on this later). With modern technology, we now have additional methods to increase clutch torque capacity, since all four of the previous techniques have downsides.

Pressure PlatesPressure-plate assemblies come in three basic configurations: diaphragm, Borg & Beck, and Long style. In the olden days, GM, Chrysler, and AMC (real olden days) used the B&B style clutches in any performance application, while Ford went with the much better Long-style arrangement. Each had its own specific bolt pattern, so they weren't easily interchangeable. There was, however, an aftermarket hybrid version that combined the desirable features of the Long style with the B&B bolt pattern, appropriately called the B&B/Long style. Today, just about every OEM uses a diaphragm-style pressure plate because it offers several advantages for street applications.

The B&B and Long-style pressure plates both use several coil springs (roughly the size of valvesprings) compressed between the cover and the pressure ring to develop the clamping force to hold the clutch disc firmly to the flywheel. Both use three levers that the release bearing pushes against to release the clutch disc when you step on the pedal. When you push inward on the levers, the levers pivot on the clutch cover to pull the pressure ring away from the clutch disc.

Besides the difference in bolt patterns, the other big difference between the two styles are the levers: The B&B uses wider, stamped-steel levers, while the Long style uses narrower cast or forged levers. Long-style release levers can include provisions for weights that can be attached to the outside of the levers (often called fingers). These weights are used to increase the pressure plate clamping force as rpm rises. This last difference is significant, so we'll expand on it.

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.

A major advantage of the diaphragm-style pressure plate is the pedal force. With the Belleville spring, pedal force reaches a maximum toward the middle versus travel, then pressure drops off. This makes it more streetable than the coil spring types, since you don't need to leg press the clutch pedal to the floor at a stoplight.

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.

While puck-style discs obviously lack the marcel spring, many full-circle race discs are also built without a marcel. The advantages of not having a marcel are reduced inertia, quicker engagement/disengagement, and a much stronger carrier for the friction material.

You should check your bellhousing/flywheel for alignment/runout anytime you install a new clutch, but it's especially critical if you have a clutch disc without a marcel, since any misalignment in the system will cause the disc to grab first in some areas, rather than equally all around the disc, and clutch engagement will be anything but smooth (i.e., it will chatter, and with more aggressive friction materials, or a solid hub disc, the chattering will shake your teeth out).

To get around the marcel problem with a puck-style disc, several clutch manufacturers now offer full-circle discs that use several small friction pucks spaced evenly around the disc instead of a full circle of friction material. Obviously, the reduction in inertia is not as great as it is with a true puck disc, but it allows you to use heavier metallic friction materials on street setups without ending up with a disc that has excess inertia, or a horrible chattering problem.

Another method of clutch disc inertia reduction is the use of a solid hub. Typically, the factory clutches are designed to be as smooth as possible, so your grandmother could drive the car without complaint. As a result, the clutch disc is not directly splined onto the transmission input shaft, but rather uses a damper arrangement on the hub. The damper transfers the torque from the clutch disc to the splined hub through a bunch of coil springs mounted around the central hub. The stiffness of the springs in the damper are tuned to the level of isolation (shock and vibration reduction) desired. In the case of a factory clutch on a non-performance vehicle, the springs can be very soft.

Soft damper springs present the possibility of fully compressing the springs ("stacking" them) when asked to transmit large amounts of torque or during shock loading (i.e., the 6,000-rpm clutch dump). When compression springs are stacked, they become damaged and can fail. So we again have the possibility of a clutch flying apart at sonic velocities, reminding us why scattershields are necessary.

You might then think that the solid hub is the way to go for all performance applications. Well, maybe. The problem with the solid hub is the level of shock forces going into the rest of your drivetrain when the clutch "hits." If you're into high-rpm clutch dumps, a solid-hub disc will murder your trans, U-joints, rearend gears, and axles in short order. Trust me-been there, broke that.

To still allow a damped hub on a performance clutch application, manufacturers use much stiffer springs made from stronger materials and also encapsulate the entire spring in plastic to prevent the spring from ever stacking or coming apart in extreme cases (for drag-race setups).

As far as friction materials go, there are plenty out there for clutch discs, but they can generally be separated into two camps-organic and metallic, with things like Kevlar and carbon confusing the categories somewhat.

Way back when, asbestos was the common organic of choice, and it did an outstanding job balancing friction, wear, heat resistance, and cost. But as a result of the banning of asbestos in modern times, many other recipes have been developed. In general, the organics combine low cost with smooth engagement characteristics, minimal wear to flywheels and pressure plates, and reasonably low inertia. For stock and mildly modified applications, they are the best choice.

There are problems with organics, however, when it comes to high-performance applications. First, the friction coefficients are generally low, which limits torque capacity. Second, the slip characteristics are not good for two reasons: 1. Slip creates heat, and the organics can cook out the resins when they get hot, so the friction facings get irreparably damaged once they're overheated (under extreme heat, they can also come apart); 2. The friction coefficient drops with increasing temperature, so if the disc slips a bit, gets hot, and loses friction coefficient, it will slip even more, get even hotter, and so on, as it quickly falls into a death spiral.

Bronze metallics used for clutch facings provide significantly more grip than organics and are therefore typically used in puck-style discs. Since metallics are much heavier than organics, the puck style keeps the disc inertia reasonable. Built without marcel springs, these discs have quick engagement characteristics and a quick, clean release, so these work well for quick shifts.

The downside to bronze metallics is additional wear on the flywheel and pressure plate due to the aggressive nature of the material. A puck-style disc without a marcel also means chattering is likely upon slow engagement (i.e., street driving). Like the organics, friction coefficient in metallics drops off with heat, so once the disc starts slipping and gets hot, it's only going to get worse.

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In-between the organics and bronze metallics, you'll find Kevlar facings. With a greater friction coefficient than organics but a lower level of abrasiveness compared to metallics, it's not a bad choice for a high-performance street application. Like the others, however, excessive slip and the resultant heat will kill the disc in short order, so it's not always the best choice for all-out race applications.

At the top of the heap are the sintered-iron disc facings. Unlike all the previous facings, the sintered-iron facing material does not significantly lose friction coefficient at high temperatures, so it can withstand major amounts of slip and heat. And like flywheels, the sintered-iron discs can be refaced when worn. Pay attention to disc thickness, though, since it will affect base clamping pressure.

Of course, as always, there's a price to pay for the advantages of the sintered iron. Besides the wear issues on the pressure plate and flywheel, the sintered-iron discs are heavy. Hence, they do not always shift well at high rpm with synchronized transmissions. Drag racers using sintered-iron discs must either have clutchless transmissions (i.e., shifting occurs without releasing the clutch), or have trans-missions specifically designed for quick shifting with heavy discs, such as face-tooth engage-ment systems.

Application and TuningAs with just about everything else, you'll be rewarded with the best performance and driveability if you choose the "right" clutch for your application. If you get it right the first time, you can also save a bunch of money and anguish.

For street driving, it's all about being smooth, affordable, and low-maintenance, but still having enough holding capacity to ensure the clutch does not slip when maximum power is required (without having a monumental pedal effort). Organic discs and diaphragm pressure plates are your best bet. To get more capacity without higher pedal effort, use centrifugal-boost pressure plates to reduce base pressures. For crazy torque levels, consider a dual-disc clutch, where two clutch discs are used between the flywheel and pressure plate (separated by a "floating" plate indexed to the pressure plate cover bolts). This effectively doubles the torque capacity while still using the same clamp pressure. Such a clutch is standard in the '07 Shelby GT500.

In addition to capacity, size and inertia are key requirements for road race, stock car, or autocross applications. Small-diameter clutch assemblies allow you to set the engine/trans lower in the chassis, which gets the center of mass down low for better handling. They also allow the engine to rev up quicker since less mass needs to be accelerated. While quick shifts are nice, they're not as critical in road racing as for drag racing, so the inertia of the disc itself isn't as important as the inertia of the entire flywheel/clutch/pressure plate assembly. Keeping the rotating inertia to a minimum allows greater acceleration potential, so small-diameter, multi-disc setups made from whatever exotic lightweight material your bank account supports are ideal.

For drag-race applications, it gets more complicated. For synchronized transmissions, low-disc inertia is important for quick, clean shifts. For face-tooth or clutchless transmissions, greater disc inertia can be tolerated. For heavy cars without much power, you need plenty of flywheel to get the car off the line, so low pressure plate inertia is not important (since it combines with the flywheel inertia), and vice versa for light, powerful cars. To save inertia, sprung hubs can be dumped if your driveline is up to the abuse or if your clamp pressure is tuned to give a softer initial engagement. Marcel spring discs are all wrong if you want quick, clean shifts. Without the disc compression from the marcel, clutch pedal travel can be reduced for even quicker shifting.

Managing clutch slip in drag racing is critically important. Now, don't simply think that clutch slip won't be a problem if you choose a stiff enough clutch. Slip is something any clutch has to deal with in-between being engaged and released, but for drag racing, it gets much worse. Think about what happens when you dump the clutch at 6,000 rpm on the last yellow. At the instant you engage the clutch, you have a flywheel and pressure plate spinning at 6,000 rpm, but a clutch disc at zero rpm. If the clutch were able to lock up with no slip (besides the fact that the forces and torques generated would be infinite), you'd instantly blow the tires off (not a good scenario unless you're into the ballet of drifting). Realistically, to get the best launch possible, you want the clutch to slip and not the tires. Ever wonder how those Top Fuel cars can run without transmissions? It's all clutch slip, but there's a fine line between toasting the clutch and roasting the tires. See why the slip characteristics are so important?

Managing the rate of slip is everything for the "slider clutches" used in clutchless or transmissionless drag race applications (and sintered-iron discs are mandatory). But for us regular folk who have to shift gears, overkill on the clutch is almost as bad as not enough clutch. Too much clutch can not only spin the tires on launch, but can also spin the tires on the shifts if it hits too hard on engagement. Poor e.t.'s resulting from tire slip can be almost as embarrassing as smoking your inadequate clutch.

For drag racing, when the clutch is on the verge of slipping at maximum torque, usually right after shifting into high gear, it has reached the ideal clutch capacity. Tunable clutches are there-fore mandatory, so we look to the Long-style pressure plate for serious drag action.

Previously, we covered the tuning advantages of Long-style pressure plates, both for base pressure and centrifugal assist. If you have to use the clutch to shift at high rpm, centrifugal boost is a bad thing, since all that extra clamp force kicking in at high rpm makes the clutch engagement too harsh after the shift, and tire spin usually results (not to mention you'll have to push the clutch pedal against all that extra force). Using little, if any, centrifugal assist and tuning the base pressure is the better bet. You may be surprised by the low base pressure many dedicated drag cars run when combined with aggressive discs. Less than 1,000 pounds is typical.

The choice of friction material for drag racing depends on the power (torque, actually), the weight of the vehicle, and the gearing available. Consider the worst case of a heavy car without much First gear in the transmission (like a 2.66:1) nor in the rearend (say 3.27:1), that is equipped with a supercharged mountain motor on huge, sticky slicks. Dumping the clutch at 8,000 rpm will generate a ton of clutch slip before the car starts accelerating to the point at which the flywheel speed matches the input shaft speed on the transmission. In this application, a sintered-iron disc would likely be best.

At the other end of the spectrum, a light-weight car with lots of gear, limited traction, limited power, and a lower launch rpm won't put anywhere near as much energy into the clutch disc, so an organic disc could live a long and happy life in that application.

For applications in-between the two extremes, some choices have to be made based on all the information presented above.