Friction Non-Fiction: The Truth About Clutches
In theory, the automotive clutch is a simple device – it acts as a coupler between the engine and manual transmission, to provide a smooth application of power between the two driveline components. In actuality, it’s a very complicated device to “get right”; that is, each of the parts and pieces that make up the clutch assembly need to be tailored to the specific combination if you are to ever get the expected results from your performance car.
As street vehicle performance has improved, both “out of the box” and in those vehicles that have been modified by their owners, so has the need for better performance clutches – they go hand in hand with one another. After all, what good would your newest modification be if your clutch can’t handle the increased power? Hammering the throttle and banging gears, only to have the clutch slip-slidin’ away, doesn’t really make for a fun drive or happy day at the races.
There are two main types of clutches for the purposes of our article – single-disc and twin-disc, both of which have their performance advantages and disadvantages. Single disc clutches are typically used in an original-equipment application, although some of today’s supercars like the brand-new 2014 Corvette C7 Stingray incorporate dual-disc clutch technology as standard equipment.
Both designs have their limitations, but the single disc presents more challenges when it comes to holding elevated levels of power effectively without slipping, as there is only so much clamping force that can be applied successfully through a single disc.
And that’s where the newest aftermarket twin-disc designs come into the picture; by effectively doubling the surface area, they also double the power-handling capability without substantially increasing the pedal effort, and in fact may drive just as easily as a stock clutch while harnessing 750 horsepower or more.
There are a number of different friction materials that are used in clutch disc creation, each of which have their own operating characteristics and must be selected properly depending on the end-use of the particular clutch.
From a full-face organic material single-disc clutch to a twin-disc, puck-style cerametallic design, most likely there will be a clutch available that will harness the power you’re looking to make, while retaining the driving nuances you prefer to experience, but only if you select the proper style at the time of purchase. Confused yet? We can help.
In an effort to better assist you in making a decision when it’s time to upgrade the clutch in your ride, we’ve assembled some of the industry’s top minds – Will Baty from Centerforce, ACT‘s Chris Bernal, David Norton from SPEC Clutch, and Mantic‘s Chris Astbury – to pick their collective intelligence centers on a variety of clutch-related questions.
LSXTV: Is there a particular clutch design that will work better with a power-adder car versus a non-adder car?
Chris Bernal/ACT: “This depends a lot more on driving style and planned use than on power. Performance street driving will have certain requirements, while weekends at the drag strip have others. Mistakes in clutch selection are often made if you’re only looking at torque output. The ACT web site has now has a configurator to help quickly narrow down various available options to the best one. Questions can also be answered on the live web chat. Of course, we have actual humans who answer phones as well…”
David Norton/SPEC: “Not necessarily. Each of our units are rated in foot-pounds of torque. The clutch can handle any setup within the torque capacity of the unit. Usage would more dictate a particular stage over another. For example, an engine turning 10,000 may call for lighter discs to help the transmission shift.”
Will Baty/Centerforce: “Naturally you want to make sure that the clutch you have is designed to hold the power/torque the engine will produce. There isn’t really a particular design for engines with power adders; it’s all about the torque the engine produces and having a clutch to withstand the power and intended use of the vehicle.”
Chris Astbury/Mantic: “A normally aspirated car can generally be handled with the Mantic Street Series ER2 clutch. Once forced induction is added, the power increase achieved may require a twin disc clutch to be fitted. All that is necessary is to match the power of the car to the correct clutch combination.”
LSXTV: How do different friction materials affect clutch operation and selection?
Will Baty/Centerforce: “There are basically three main types of friction materials used in clutches with many different grades in each type.”
“Kevlar typically has a lower coefficient of friction rating than most of the other types of friction material. Because of its lower friction, it needs a higher PSI load to work properly; it also works best against iron. Steel flywheels will reduce the holding capacity of this material. The nice thing about Kevlar is its wear characteristics.”
“Organic material is the most common material used today. Organic material works well against steel flywheels but an iron flywheel will provide more holding capacity. The drivability with organic material is good.”
Testing friction material in house is a vital part of developing a performance clutch. – Will Baty, Centerforce
“Cerametallic/iron material will offer the highest coefficient of friction, but at a cost. This material has excellent holding capacity and will take a lot of heat – but be careful, because too much heat and it will weld itself to the flywheel or pressure plate. The down side to this material is it tends to chatter, it doesn’t like to be slipped, and can cause excessive wear to the flywheel and pressure plate – it is basically a metal to metal clutch. The drivability on this material is not great.”
Chris Bernal/ACT: “The study of friction materials has a long and interesting history, and newer developments are constantly being tested. The organic materials of the past were often based on asbestos, which has been replaced by sophisticated blends of fibers, metals and binders. These offer the most comfort on the street, with some providing extended temperature ranges and good recovery properties.”
“For more aggressive use (racing, drifting, etc.), there are many sintered materials to choose from. These range from less friction than organic to almost twice as much. The higher friction levels tend to come with rather sudden engagement characteristics, which makes them far less suitable for the street. It is always best to use clutch discs with the same friction material on both sides, so that the thermal properties and coefficients of expansion are identical, reducing the chance of disc distortion at higher temperatures.”
Chris Astbury/Mantic: “Mantic has settled on organic and cerametallic friction materials in various forms. For OEM drivability, organic facings are probably the preferred option. However, if the power requirement of the car cannot be contained by organic facings, then cerametallic facings are required. Mantic specializes in the “cushioned” button disc which has near OEM drivability, and will handle the power requirement. Puck ceramic discs are available, but are harsh for street use.”
David Norton/SPEC: “Friction materials are the most significant factor in engagement quality, especially from a stop. An organic or Kevlar fiber unit will be more manageable at take-off than a semi-metallic or metallic compound. The friction material has more to do with engagement quality on single disc units than twin disc units. Due to the increase in surface area, an aggressive material in a multidisc configuration can engage as smoothly as a softer street material in a single disc configuration.”
LSXTV: When do you find that a buyer will benefit going from a single disc to dual disc clutch?
David Norton/SPEC: “A consumer benefits from a multi-disc clutch when the capacity of a single becomes insufficient or when drivability of a sufficient single disc unit has deteriorated to the point that the car is no longer fun to drive. The benefits are two-fold: higher capacity and better drivability.”
“Multiple disc systems can hold more torque in the same amount of bellhousing space than their original equipment single or twin disc counterparts. More discs mean more surface area, and more surface area not only provides a higher capacity, but also adds progressiveness to the engagement and, thus, a more streetable unit.”
Chris Astbury/Mantic: “There are several significant advantages of a smaller diameter, multiple disc clutch over a larger diameter single disc unit. Multi-disc clutch advantages include the ability to increase the amount of torque able to be transmitted, a decrease in pedal effort, decreased clutch weight, and decreased moment of inertia.
The amount of torque that a clutch transmits is critical to the ultimate performance of a vehicle. – Chris Astbury, Mantic
“Multi-disc clutches increase the available torque capacity by two for a twin disc clutch. Torque capacity is affected by four factors. Decreasing the diameter reduces torque capacity. A smaller diameter clutch has a smaller diaphragm, which usually means a lower clamping force.”
“Adding a second clutch disc doubles the torque capacity. Increasing the coefficient of friction from the disc gives an increase in torque capacity. If the multi-disc clutch is correctly designed and engineered, the losses from the smaller diameter and lower clamp force are much less than the gain in coefficient of friction and extra disc surface area, giving a net torque capacity increase.”
Will Baty/Centerforce: “There are a lot of variables here – driving styles have a huge impact on how the clutch will perform and last. Today’s vehicles make power effortlessly, so in a lot of cases a dual disc is a natural decision because of the holding capacity and clutch life increases they offer over a single disc. If the consumer has found that their single disc clutch just isn’t holding the power or if they seem to go through single disc clutches often, then it may be a benefit to step up to the dual disc.”
Chris Bernal/ACT: “The crossover point can vary a bit depending on what type of driving is going to be done. Most often, one will do a large power upgrade that might require a very high clamp load (and corresponding high pedal effort) single disc clutch to hold the torque. Moving up to a twin disc clutch can bring the pedal back to a nearly stock feel since there are four friction surfaces available instead of just two.”
Watch SPEC’s David Norton discuss SPEC’s line of carbon-faced clutch products
LSXTV: What’s the difference between puck-style versus full-face discs?
David Norton/SPEC: “Rate of engagement. The clamp pressure on the material is higher when the material is pucked or segmented. On a full faced disc, clamping force from the pressure plate is distributed across more area, with a lower pressure per area of material. Therefore, manageability from a stop is not as good with a puck unit as a full faced disc, material being constant. The performance of the puck unit will be greater when the material used is designed to utilize the load.”
It is very important to fully consider how a car will be driven before selecting the disc type. – Chris Bernal, ACT
Will Baty/Centerforce: “There are many types and variations of puck disc. Most are typically a few segmented areas of friction, and what this does is increase the Pounds per Square Inch (PSI) load of the clutch. A puck-style disc will distribute the clamping force the pressure plate to the little friction points of the disc throughout the clutch surface. I like to describe it as if a 100 pound woman was standing on your chest with tennis shoes on (full face disc) – not too bad, right? Now have that same 100 pound woman stand on your chest with her high heels on (puck-style disc). That 100 pounds is now concentrated to the little heels on her shoe – that is PSI just like a puck-style disc.”
“The full-face disc is typically an organic facing on both side of a clutch disc. A full-face disc will distribute the clamping force the pressure plate provides evenly throughout the entire clutch surface.”
Chris Astbury/Mantic: “A puck style disc is primarily for competition use. Full face discs are generally organic for street use and the best drivability. Puck clutches usually do not have any cushioning between the surface areas, and as such have harsh, (sometimes called “sudden”) engagement characteristics. The Mantic kits with the cushioned button cerametallic facings are a good compromise, with very good drivability. Puck/Cerametallic style facings offer increased torque drive due to an increased coefficient of friction over organic facings.”
Chris Bernal/ACT: “Full-face discs offer the least temperature rise, most engagement comfort, and longest wear. Puck-style or segmented discs will grab more suddenly and usually allow quicker shifts, yet will wear out sooner, depending on the material being used. There is no difference in how much torque capacity each can handle, aside from their various friction properties, as this is a function of clamp load and friction ring diameters, not segment configuration.”
LSXTV: How does a sprung hub affect clutch operation?
Will Baty/Centerforce: “The purpose for a sprung hub disc is to dampen the vibrations and shock being transmitted from the engine to the transmission. Speed shifting is hard on a transmission; a sprung hub disc will dampen the blow and help prevent transmission, driveshaft, or rear end breakage. There are only a few clutch manufacturers that offer a spring hub dual disc; one of them is Centerforce’s DYAD clutch.”
Chris Astbury/Mantic: “The advantage of a sprung disc hub is that it absorbs some of the shock transmitted to the vehicle transmission during shifting, drive, and overrun. A solid hub will allow all of the shock to go to the transmission. That system is best suited to competition application. Mantic takes this sprung hub principle a step further. In certain applications, Mantic offers a “multi-rate clutch disc hub”, which is further refined in removing driveline shock and noise from the vehicle.”
Chris Bernal/ACT: “Sprung hubs were originally developed to reduce gear rattle noises in the transmission. Many vehicles now come with a dual-mass flywheel, which is often more effective in mitigating, if not completely eliminating, drivetrain noises. Unfortunately, dual-mass flywheels are not service- or modification-friendly and are unsuitable for any type of racing. Most single disc clutch kits are offered with a sprung hub option, which is a big help in reducing gear rattle. Designing a dual disc with a sprung hub can be accomplished in a variety of ways, most of which have compromised durability. The most reliable is to use two discs each with a sprung hub, although this is also the heaviest and slowest shifting option. There are some newer developments on the horizon.”
David Norton/SPEC: “We use dampened hubs in multidisc clutches when possible or when drivability is a concern.”
LSXTV: What do you suggest to a customer when they are looking for a clutch to “grow into”?
Will Baty/Centerforce: “Good question – that is why we always like to know the intended use of the vehicle. We say ‘buy for what you are planning to do with the vehicle.’ Since clutches and labor are not cheap there is no sense in doing it twice.”
Chris Astbury/Mantic: “Mantic believes that after the initial drive, apart from noticing how good the clutch feels, the clutch should go ‘unnoticed’ straight away. The driving experience is not enhanced if the driver has to re-learn how to drive their car. As pointed out earlier, Mantic tries to match the clutch to the consumer’s requirements, so that ‘growing into’ is not necessary.”
Chris Bernal/ACT: “While this concept sounds pretty reasonable, it can get costly to achieve when talking about a wear item that takes some time to install. A step in that direction would be to install a kit with street performance disc and replacing it with a puck-style disc later when deciding to go racing, since they are a direct swap.”
David Norton/SPEC: “Overclutching your application in an effort to plan for future increases in power or changes in driving environment is smart, as long as the desired short term drivability characteristics are maintained. You can save a lot in labor charges and parts cost by addressing future power plans when choosing your clutch.”
ACT’s Dirk Starksen discusses ACT’s clutch lines at SEMA
LSXTV: Are there specific break-in procedures for each type of clutch?
A sprung hub is strictly a low speed drivability tool. – David Norton, SPEC
Chris Astbury/Mantic: “The friction area of a clutch is designed to have a certain surface area in contact with the pressure plate and flywheel. The area is calculated to give a certain wear life and heat absorption capacity. More friction area means lower temperature and less wear.”
“The issue with new clutch plates is that the friction material is not in full contact with the pressure plate and flywheel as it has high spots – the surface is not fully flat. So it makes sense to not drive aggressively with a new clutch until the surface of the friction material has full contact with the pressure plate and flywheel. This takes about 300 to 500 miles of city-type driving.”
Will Baty/Centerforce: “Organic material requires a 450-500 mile break-in regardless if it is a single or twin disc. Cerametallic/iron material does not require the same amount of break-in as the organic, but we like to see about 100 miles of break-in on the cerametallic material.”
Chris Bernal/ACT: “For either the single or twin street-type clutches, we recommend about 500 miles of normal street driving before attempting any hard launches. This is to completely mate all friction surfaces to each other, lapping them to get 100% contact, much like a camshaft. Unfortunately, freeway miles don’t count, as the clutch is fully clamped.”
David Norton/SPEC: “It depends on the material. Like brake pads, all clutches will wear longer and perform better when seated under normal conditions. One can get away with not usage-seating (industry standard break-in seems to be 300-500 miles for most units) some performance clutches, but it is always good to do so to maximize capacity, drivability, and wear life. Only a few extremely high coefficient-of-friction materials will not benefit from a slow seating process. Theoretically, the discs in singles and twins see the same engagement process and would utilize the same seating time, but the extra surface area and capacity in the twins provides a more controlled environment and, thus, would less likely be overcome by premature loads.”
As you can see, there are varying theories on some topics, while our experts agree on others. Clutch selection and operation is a sticky subject, but hopefully along with the input of our panelists we’ve given you some insight into how you can select your next one for smooth engagement, excellent power-holding ability, and long life, no matter the application.