Every engine on the starting grid is connected to a transmission. This connection usually consists of some sort of clutch mechanism. This clutch mechanism in your car has two basic functions. First, it has to be able to momentarily disengage the engine from the
transmission for gear changes and to stop the car. Next, it has to be able to transfer maximum engine torque without slipping when it is engaged. Sounds pretty simple. The problem is with all of the details.
The Flywheel
Let’s start with the flywheel. This is the disc that’s bolted to the end of your crankshaft. A flywheel can be made out of iron; steel or aluminum and it can come in a variety of diameters and weights. The flywheel has three main purposes.
The primary purpose of the flywheel is to store kinetic energy and maintain the revolving inertia of the engine as it passes through its various cycles. The engine’s power comes from the pistons going in a vertical or horizontal direction. These pistons are connected to the crankshaft in such a way that the vertical, or horizontal, motion is converted to rotational motion.
Keep in mind that the pistons only provide force 25 per cent of the time. Most of us have 4-stroke engines and only one of the four strokes actually produces power. The mass of the flywheel creates inertia to keep the crankshaft running between the power strokes. This is especially critical at idle.
Next, the flywheel serves as part of the engine starting mechanism. There are teeth all around the outer circumference of your flywheel. The teeth on the starter gear engage with the teeth on the flywheel to turn the engine over for starting. Finally, the machined face of the flywheel acts as part of the clutch mechanism. The face of your clutch disc interacts with the flywheel to transmit the engine forces to the rest of the drivetrain.
When you apply throttle you’re not only turning the crankshaft faster but you’re accelerating the flywheel as well. It takes more power and time to accelerate a heavy object than it does to accelerate a light object. A lightened flywheel will not increase horsepower overall but it will allow the engine to accelerate faster and will free up a small amount of horsepower your engine is already making. Slightly less power is needed to turn a lighter flywheel and this power can go to your wheels instead.
While you might want a big heavy flywheel to smooth out the power strokes on your streetcar you want a light and small flywheel for your racecar. Most engineers and crew chiefs will shop around for the smallest, lightest flywheel that will still allow for starter engagement. The goal is to have rapid acceleration and fast transitional throttle response. Smoothness of operation at idle is way down the list. In a dedicated racecar it’s almost impossible to have a flywheel that’s too light.
Flywheels are generally made from nodular iron, steel, or aluminum. Aluminum flywheels are better for maintenance since the friction surface is generally a steel insert that’s bolted in place. That makes replacement inexpensive. You only need to replace the friction surface, not the entire flywheel. It also makes it almost impossible to wear the flywheel out. When the surface that makes contact with the clutch face wears you only replace the insert. The rest of the flywheel is just fine.The Pressure Plate
The job of the pressure plate is to apply pressure and squeeze the clutch disc firmly between the pressure plate and the flywheel. Some people call this the clutch cover and it consists of the pressure plate and the springs. There are basically three types of pressure plates: the Long style, the Borg & Beck, and the Diaphragm.
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| Long Style |
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| Borg and Beck Type |
The late Carroll Smith felt the Borg and Beck pressure plate was the only one to use in road racing. He wrote that it was not only the lightest possible unit but it had the lowest moment of inertia. If everything was installed properly and maintained correctly a Borg and Beck unit should last forever.
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| Diaphragm Type |
The diaphragm pressure plate uses a single, large Bellville-style spring to load the pressure plate. There are several advantages to this style of spring. First, it loads the pressure plate evenly since the pressure is applied uniformly over the entire plate assembly. Secondly, as the Bellville spring is compressed (clutch released), there is a point where the pedal effort decreases because the spring over-centers. This makes holding the clutch pedal in on the grid, or a stoplight, much easier than a coil spring type pressure plate. The diaphragm may very well be the ideal pressure plate for driving on the street.
Clutch Face Material
The clutch disc is consists of a friction surface and a hub. The hub is what slides onto the input shaft of your transmission. Hubs are offered in sprung and non-sprung versions. These springs, which surround the center of the hub, act similar to a harmonic balancer. A coil-spring hub allows the disc to rotate a few degrees upon engagement. This softens the initial contact and reduces driveline torsional vibrations, and minimizes noise and vibration. Clutch discs with springs make clutch engagement easier on the drivetrain.
Ed Burgy at Fidanza feels that “each type of clutch has it’s own merits and brings different benefits. While solid hubs are great for positive engagement they do transmit more shock to the driveline. This can lead to wheel spin and increased wear to the driveline components. A solid hub could also mean harsher shifting and increased driveline vibration.” Moving out from the hub is the friction surface. This friction material is similar to the material on your brake pads. Just as you can purchase brake pads with different coefficients of friction you can do the same with your clutch disc.
We really have three broad categories of friction materials to select from.
Organic: This is actually an outdated term since it goes back to a time when asbestos was used in the clutch. Asbestos was a really good friction material because of its high heat resistance, good strength and a high coefficient of friction when it was subjected to high temperatures. Unfortunately it was also a carcinogenic.
Today organic friction material is generally created from cellulose reinforced with materials such as fiberglass and mineral wool. It’s then encased in a phenolic resin base with a thermoset process. The cellulose provides the initial bite while the mineral wool and fiberglass provide burst strength. Some add copper or brass materials to increase strength, improve wear and better dissipate heat.
These organic friction materials provide smooth engagement and excellent initial bite. The problem is they’re not very effective in high-temperature applications. It is very easy to overheat the organic material under high torque loads, which make them impractical for racing use except with small-capacity, low-torque engines. The good part is that if you overheat an organic clutch it will return to normal operation once it cools down.
Some variation of the organic clutch remains as the most popular clutch material on the market. You can maintain all the stock driving characteristics of your car and they’re easy on the flywheel and pressure plate surfaces. Even better they’re usually inexpensive.
If we checked on every car in the paddock some version of the organic clutch would be the norm. First, it’s inexpensive. Next it does everything well. Not great mind you but well. Dirk Starkesen of Advanced Clutch Technology Inc. points out that “single disc organic clutches produce the least amount of compromises.”
Semi-metallic clutch materials look similar to a regular organic clutch but can withstand much higher levels of heat. The exact mix will vary from one manufacturer to another but its generally some combination of powdered ceramic material, copper, bronze, carbon or even iron mixed in with the organic material to further enhance friction at elevated temperatures.
All of this makes them suitable for high-torque applications. They feature a woven structure and strands of brass or copper improve the burst strength of the material. Semi-metallic discs that contain high levels of iron or ceramic material can have somewhat reduced feel though and the clutch disc tend to bite suddenly.
Engagement is extremely positive. This is good for racing but generally too harsh on the street. This material tends to accelerate flywheel and pressure plate surface wear. They can also be a lot heavier. A double-sided, full-face, semi metallic disc can weigh twice as much as a comparably sized organic disc. Some metallic discs designed for racing use three to six separate pucks on a winged disc as a way of to lower overall weight.
Composite The third basic family is the composite. This includes things such as Kevlar, carbon fiber and even more exotic materials. I’m going to include sintered metallic in this
group since it it’s usually made up from sintered iron or brass. This family of material has the highest friction coefficient and is extremely aggressive.
For clutch discs it is usually a mixture of metallic compounds designed to provide the optimum coefficient of friction and wear resistance. It might include (but not limited to) copper, bronze, iron and carbon. Mixing carbon and ceramic into a compound allows the self-lubricating benefits of a copper or bronze base material – providing smooth engagement – as well as the high bite and temperature resistance of carbon and ceramic.
In applications where extreme temperature is an issue, such as an 8,000 bhp Top Fuel dragster sintered iron is the best option. This material is produced from a powdered base stock and can withstand very high temperatures. In fact the friction increases with higher temperatures.
Due to the very aggressive nature of the material engagement is very harsh and sintered iron discs tend to be used only in drag racing applications. Sintered metallic turns your clutch into an on/off device. You’ll also need a special flywheel since a sintered metallic clutch will quickly destroy a normal flywheel. This is serious stuff.
Kevlar offers good burst strength and has great wear characteristics but it has a relatively low coefficient of friction. This allows for smooth engagement characteristics but it requires the use of very high clamping pressures to provide sufficient friction to prevent slippage.
Kevlar provides greater temperature resistance than organic clutches and a lower wear rate. At the same time it can be burnt out if subjected to excess heat. This is because the friction material does not return to its original state after exposure to high temperatures.
Often times Kevlar clutches are offered in a segmented version. In other words the material has segments missing. This allows for better heat dissipation. The only real problem is that Kevlar too often wears out faster than an organic clutch.
Carbon-carbon: The most recent development in clutch material has been the introduction of carbon-carbon. In this type of clutch all the friction surfaces, including the flywheel mating surface and the discs are made from amorphous carbon material. The material is made by heating preformed discs of white polyacrylonitrile (PAN) fibres until they turn to a black, pre-oxidized state. PAN is a synthetic, semi-crystalline organic polymer resin, which is used as the basis for high-quality carbon fibers. Once pre-oxidized, the fibers are layered together before being oxidized and then cut to a rough shape.
These rough blanks are subjected to two densification heat cycles at more than 1000 C before being machined to a finished shape. It is these densification cycles that make the manufacturing process so lengthy, with each cycle taking several hundred hours. During the process, hydrocarbon-rich gases are injected into the ovens used to heat the blanks, allowing the layers of material to fuse together and form a solid disc.
Friction modifiers can be added to the mix to alter the material’s characteristics, the result being a clutch that is highly resistant to temperature. In fact friction increases as the clutch heats up. These discs are also very light which reduces drivetrain inertia. The very lengthy manufacturing process though means that carbon-carbon clutches are very expensive.
Manufacturers are working to produce cheaper varieties of the material, and no doubt these will filter down from the upper reaches of the sport over time. Until then most clutch manufacturers will continue to improve the performance of their organic and metallic compounds.
Multidisc Clutches
Clutch makers have traditionally used a brute-force method to handle high power demands. They would build a stiffer pressure plate, increase the friction coefficient of the disc, go to a larger clutch disc and pressure plate, and finally move up to a coil-spring pressure plate.
Multi disc clutches first became popular for oval-track racers who didn’t have to worry about standing starts or rapid power shifting. By using smaller-diameter multidisc clutches the engine will accelerate and decelerate quicker. This means you can drive the car deeper into the turns and have the engine rpm drop quickly. When you accelerate out of the turn, the engine is able to reach peak rpm quicker than possible with a heavier clutch. With a multi-plate design, the total friction area is greatly increased because you have multiple clutch plates. The added friction surfaces provided by the multiple discs compensate in terms of holding power for the reduction in disc diameter.
Some feel that the real beauty of multi disc systems is that you have smoother engagement with even greater holding power. At the same time you have reduced pedal effort. You can use a low rated pressure plate that gives lighter pedal weight. The trade-offs are the possibility of shudder and noise during operation and increased mass.
The important point is the large number of variables you’re dealing with. We have several types of flywheels, three different types of clutch covers and a huge number of friction materials. You really need an expert to help here. You can start by asking around the paddock. Then ask whom they used as a supplier. Just keep expanding your circle for information and you’ll end up with a clutch system that works for you.
Types of Clutch Faces
Full Circle Driven Plate: This is what was most originally installed in your car. It’s also the most common replacement clutch. These are generally very substantial items. There’s a lot of material and engagement is really smooth. The downside is that they weigh a lot.
Hardware
At some point you’re going to have to bolt all your parts together. Ace Hardware may not be your best source for pressure plate and flywheel bolts. ARP pressure plate bolts have a tensile strength of 200,000 psi and only cost roughly $15.00 for a set of 6 complete with washers. A set of 200,000-psi flywheel bolts for a Chevrolet is usually less than $15.00 too. Even Porsche flywheel bolts cost less than $40.00. Good hardware is critical for safety; don’t skimp here.
Hydraulics
A lot of our cars use hydraulics for clutch actuation. If you pull the slave cylinder boot back and fluid comes out, your slave cylinder is dead. If the slave cylinder is dead, the master cylinder will also be worn. You really should replace them both at the same time.
I know there are rebuild kits available for these units but if your slave or master clutch cylinder is leaking you generally have wear issues on the internal bore. Just buy new units. Some people have had good luck with the honing the bore and installing new O-rings. It’s just never worked out for me.
Originally published in
Originally published in