Let’s create a scenario that could cause any car enthusiast to break out in a cold sweat. You are sitting in your ride at a crowded intersection and your foot is firmly holding the clutch pedal to the floor. When the light turns green, you plan to annihilate the rear tires on your car. You know your car can do it, you’ve got enough horsepower to immediately turn your tires into asphalt crayons. When the light does turn green, you slide your left foot off of the clutch pedal and smash the throttle. But for some reason, something doesn’t seem right. You see smoke billowing from under your car, but the smell is different. It doesn’t smell like tire smoke. At that moment, you see a guy taking video of you smoking your car’s stock clutch instead of the tires. You, my friend, are about to go viral on YouTube as an epic fail.
While the OE clutch in your car worked great when your car was all OE, upgrades tend to put more stress on parts — some of us learn that the hard way, but I digress. Replacing the clutch with another OE unit will probably heed the same results, so it’s time to step up. But, stepping up requires a little knowledge about choosing the right parts.
How do you know what you need if you don’t know what’s available? That’s where we come in. We spoke with clutch professionals from Spec, Mantic, and Quarter Master, to get some insight into different clutch materials and designs, and to find out how you can properly choose the system that is perfect for your car.
Clutch Terms You Should Know
- Friction disc — Made of varying types of friction materials that come into contact with both the flywheel and the pressure plate to create the force required to turn the transmission.
- Floater Discs — Used in multi-disc clutches, they act as an intermediate surface between the friction discs.
- Flywheel — This is the steel wheel that connects the clutch to the engine. The pressure plate bolts to the flywheel.
- Bellhousing — Connects the engine to the transmission and houses the clutch assembly.
- Pressure Plate — uses spring pressure to squeeze the clutch disc against the flywheel and transmit power through the assembly.
Jeff Neal at Quarter Master told us, “The right clutch choice depends on the usage the clutch will see. Our Pro Series is robust, and has a high-torque capacity which is great for road racing.” He continued with, “Our V-Drive clutch is great for those on a tight budget, and is great for short track, road, and rally racing. Then we have our Optimum series clutches, which are designed for high horsepower racing applications.”
When we asked Geoff Gerko of Mantic Clutch, he recommended their organic disc clutch for daily driver cars that see only occasional passes down the dragstrip. If looking for a clutch that is at home on the street or the race track, Geoff says, “Our Sprung Hub, Cushioned Ceremetalic clutch features smooth operation, great drivability, and increased torque capacity containment. This is our most versatile and popular clutch material for the average enthusiast.” Geoff continued, “Our Solid Hub Ceremetalic also features smooth operation, but is best suited for use in purpose-built cars that see frequent track days or consistent bracket racing.”
Last but not least, Shelly Norton of Spec Clutch told us, “Generally speaking, all of our units are built to survive abuse when used within their specified torque ratings. Most of our “staged” clutches are also classified as multipurpose.” Shelly continues, “Our Stage I through Stage III kits are considered street-friendly, but can still be used for many types of racing. Our Stage IV is good for road racing, drag racing, and drifting; and our Stage V is primarily a drag race unit.”
As the performance of your car increases, so should the characteristics and durability of the material used to make your clutch disc. The following is a brief synopsis of clutch materials available, and when each should be employed. When it comes to a clutch material for racing, Neal says, “We typically recommend a sintered-metallic (bronze) material. This has a very high coefficient of friction with manageable driving characteristics. For street-performance clutches, we offer organic (rag type) materials for a more street-friendly driving experience that delivers race-type friction capability.”
Organic material is primarily used to make stock-style clutch discs. Organic material is acceptable for normal driving conditions and usage, but as operating temperatures rise, or you place the clutch under heavy loads (which is usually accompanied by slippage), the clamping ability will fade because the coefficient of friction drops off. In addition, at high RPM and/or when the disc gets hot, they tend to fail structurally.
The use of Kevlar material offers a much higher coefficient of friction than organic material, but with some loss in drivability (i.e. it gets grabby when releasing the pedal). This occurs, because, as the coefficient of friction goes up in the disc material, so will the aggressiveness of the material when the clutch is engaged. This usually results in clutch “chatter.” Since Kevlar is compatible with stock flywheels and pressure plates, it makes a good upgrade choice, but using it takes some getting used to in regards to driveability.
Bronze-metallic materials are the most aggressive materials in regards to clutch friction. Since it is very aggressive, it offers an extended life over Kevlar and organic materials. According to Gerko, “Our Ceremetalic material is our friction material of choice. It engages smoothly, wears well, tolerates the heat, and has a consistent coefficient of friction. This enables us to create clutch kits that accomplish everything for our customers.”
By reducing static pedal pressures, usage of metallic materials can result in a quick and clean clutch engagement. Since metallic materials are the most aggressive materials available, it will also wear the flywheel and pressure plate surfaces much faster. Therefore, they should only be used with a steel or nodular-iron pressure plate and flywheel. If used on the street, this material will cause chatter when the clutch is being engaged.
Sintered iron is an optimal choice for street use, as it has a greater ability to withstand slippage and not lose its friction coefficient. Sintered iron is also preferable for high-horsepower applications and drag racing. When it comes to choosing the clutch material for your car, Norton explains, “We actually have particular materials for particular setups. For instance, we do not have one, two, or even three preferred materials. However, our Stage II-Plus carbon/Kevlar hybrid materials and our Stage III-Plus materials are the most recommended in high-horsepower environments. Likewise, our organic and pure Kevlar materials are the most recommended for mild-power applications.”
Checking On Twins
Not only do you have to choose what material your clutch disc is comprised of, but you also need to decide if you need one, two, or more discs. Clutches are generally rated by their torque-holding capacity. A single-disc clutch will inherently have less holding capacity than a twin disc when based solely on their surface area. According to Norton, “We recommend installing a multi-disc unit when using a single-disc clutch is insufficient. If the clutch works but experiences significantly-reduced wear life, it is insufficient. There are also cases where a single-disc might seem sufficient, but the overall setup can benefit from a smaller-diameter, multi-disc unit to deliver better shifting capabilities or a lower inertia flywheel package for significant rate of RPM and horsepower gains.”
Twin-disc clutches are designed to have a lower inertia, but will have higher torque-holding capabilities since they spread the load across more surface area. Twin-disc clutches also tend to be noisier in comparison to their single-disc counterparts. This is because there are more plates and separator discs in the package. Gerko tells us, “The main goal of a multi-disc clutch is to increase clutch surface area without substantially increasing pedal effort.”
A good rule of thumb is that a single-disc clutch is a good all-around performance clutch to use with a stock or mildly-modified engine. Depending on the clutch material used, it will have OE-like engagement and shifting qualities. Twin discs are designed to handle a lot more torque than a stock or aftermarket single disc, and therefore are a better fit in higher horsepower applications. To make the right choice, you will need to know the torque capability of your engine, and consult the clutch manufacturer.
When talking clutches, you have to factor in the flywheel. The flywheel not only has the teeth for starter engagement, it is also an energy-storing device. A heavy flywheel causes the engine’s RPM to climb at a slower rate than a lighter (aluminum) flywheel, but since it also stores more energy because of its mass, the engine’s RPM will not drop as dramatically (like between shifts) as if using an aluminum flywheel.
Also, a lighter flywheel will allow the engine to spin quicker, but a more dramatic drop in RPM will be noticed (like between shifts), and the lighter weight could also cause an issue if using your car for daily transportation. This is because the lower inertial mass of the lighter flywheel means the car will be harder to get moving from a stop sign or traffic light. Aluminum flywheels are generally used in road or drag race applications where the engine is continually kept at a higher RPM.
What Is Moment of Inertia
The moment of inertia (MOI) is the measurement of an object’s resistance to changes in its rate of rotation. MOI is a huge factor when designing clutches. This is because in order to accelerate a vehicle, you first need to overcome the vehicle’s resistance to acceleration — its moment of inertia. Reducing the MOI has the same effect as adding power to the engine. A small change in the diameter of a clutch has a dramatic effect on MOI. For example, an increase in diameter from 7.75 inches to 11 inches, equates to approximately doubling the MOI. This difference requires almost doubling the power required to accelerate this clutch.
The pressure plate of your clutch system is what applies the clamping force that squeezes the clutch disc against the flywheel. But just like with the flywheel and clutch disc, you have a choice to make. For this article, we’ll focus on the three main types of pressure plates: the long style, the Borg and Beck, and the diaphragm. According to Gerko, “The diaphragm-spring clutch really is a superior design, because the clutch can be designed to steadily increase its clamping load as the disc(s) wear. This is tremendously beneficial in multi-disc applications. It helps the customer get the maximum performance from the clutch before it needs replaced.”
The long-style pressure plate is identified by the three thin fingers that contact the throw out bearing. The long-style pressure plate is typically used in drag race applications, and will deliver a considerably-hard pedal feel. The Borg and Beck style is similar to the long style in that it functions via three fingers, but the difference is it has wider fingers that release plate pressure. According to Neal, “The long-style design is great for high-horsepower applications like drag racing.”
The Borg and Beck style uses rollers under the pressure plate cover. These rollers are forced outward under centrifugal force as the engine spins. This increases the clamping pressure as the engine RPM increases. Finally, a diaphragm pressure plate uses a series of “fingers” (also called a Belleville spring), that completely encompass the center opening of the pressure plate. The main advantage to this style pressure plate is that holding the clutch pedal down at a stoplight is much easier than with a long or Borg and Beck-type pressure plate. “We almost always prefer a diaphragm unit.” says Norton, “Diaphragm clutch technology is superior for the majority of uses. Diaphragm units produce more clamp load, more of the time, with friendly pedal requirements and high RPM capability. Diaphragm pressure plates are also a no-maintenance option.”
The Choice Is Yours
So which flywheel, pressure plate, and clutch material do you think you need now that you’ve smoked your clutch and become a YouTube favorite?
At this point, we have hopefully shed some light on choosing the correct clutch. Now that you have done your homework, know your vehicle, and know the professionals you can trust, all that is left is for you to get the right clutch for your ride.