Car clutches form the bond between getting the energy from all that internal combustion in the engine through to the transmission and then finally out to the driven wheels. And with countless combinations of engine and transmission types scattered all over the automotive world, many different denominations of clutch exist to adhere to the job required. Whether they are dealing with 90bhp or 900bhp, there is a clutch out there that – once engaged – will be able to help transmit as much torque as possible through to any transmission. Previously we discussed the complete clutch assembly, and we will drive into the car clutch disc component of that assembly.
Each type of clutch has its 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 like 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.
Wet And Dry Car Clutches
Wet clutches in general have multiple clutch plates (in cars) and have a supply of oil to lubricate and cool the components. They are used in high torque situations where friction levels would be high and therefore clutch temperatures would soar without some form of coolant. Any powertrain over 250lb ft of torque should really be employing a wet clutch to avoid excessive wear on the rest of the transmission through overheating.
Dry car clutches on the other hand have no oil supply and are generally single-plate. This means they can be more efficient as lubrication can lead to a lack of friction between the plates in a wet clutch as well as producing parasitic losses from the drivetrain as a pump is needed to supply the lubricating oil. The small coefficient of friction in a wet system is therefore the reason for having multiple plates for effective clutch performance.
Organic Car Clutches
This is an outdated term since it goes back to a time when asbestos was used in the clutch. Asbestos was a 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 carcinogenic.
Today organic friction material is generally created from cellulose reinforced with materials such as fiberglass and mineral wool. It is 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 are not very effective in high-temperature applications. It is quite 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 are easy on the flywheel and pressure plate surfaces. Even better they are usually inexpensive.
Ceramic/Ceramic-Metallic Car Clutches
The second basic family is the ceramic or ceramic-metallic clutch. This normally consists of ceramic material that is mixed with metals to provide superior grip and heat resistance.
Semi-metallic clutch materials look like a regular organic clutch but can withstand much higher levels of heat. The exact mix will vary from one manufacturer to another, but it is 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 elevated levels of iron or ceramic material can have 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 Car Clutches
The third basic family is the composite car clutches. This includes things such as Kevlar, carbon fiber and even more exotic materials. I am going to include sintered metallic in this group since it has 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 extremely high temperatures. In fact, the friction increases with higher temperatures.
Due to the extremely 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 will 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 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 hot temperatures.
Often 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 Clutch Disc
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) fibers 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 exceptionally light which reduces drivetrain inertia. The very lengthy manufacturing process though means that carbon-carbon clutches are 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 Car Clutches
Car 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 car clutches first became popular for oval-track racers who did not 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 can reach peak rpm quicker than possible with a heavier clutch. With a multi-plate design, the total friction area is 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 crucial point is the considerable number of variables with which you are dealing. 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 will end up with a clutch system that works for you.
Types of Car Clutch Faces
Full Circle Driven Plate: This is what was most originally installed in your car. It is also the most common replacement clutch. These are very substantial items. There is a lot of material and engagement is really smooth. The downside is that they weigh a lot.
Paddle Driven Plate: These are also called puck or button clutches. They have lesser amounts of material on each paddle. When you only put the clutch material on the winged pucks you save a lot of weight. This means that the clamping load is concentrated into a smaller area though which can make the clutch grabbier.
General Car Clutch Replacement Tips and Suggestions
- When replacing a clutch always replace both the pressure plate and disc assembly, never use a new disc with a worn pressure plate.
- General installation tip: if oil is present on the old clutch assembly, the leak must be corrected before installing a new clutch assembly. Often, it will be your rear main seal, be sure to check this as now would be a great time to replace it.
- If you are not replacing your flywheel, it should be resurfaced to the original specifications, this will insure proper, trouble-free clutch performance and operation. Before assembly, make sure the flywheel is clean of oil and grease. Brake cleaner works well.
- Make sure and replace the throw out bearing and pilot bushing/bearing to ensure trouble free performance of your new clutch.
- Always use a clutch alignment tool during installation to insure correct positioning of the pressure plate assembly, disc, and pilot bushing/bearing.
- Torque all pressure plate and flywheel assembly bolts to manufacturers specifications using a progressive crisscross pattern.
Finally
Choosing the correct car clutch for your application requires knowing the estimated torque your engine will produce. Once you have an estimate of the torque your engine is producing, we recommend choosing a clutch that will handle at least 10 percent over the torque output of the engine.
