This article first appeared in the August, 2015 edition of SportsCar Magazine. SCCA members can read the current and past editions of SportCar digitally here after logging into their account; To become an SCCA member and get SportsCar mailed to your home address monthly in addition to the digital editions, click here.
It’s Complicated
From ingredients to assembly to heat management and beyond: everything you needn't know about performance clutches
Chances are your competition vehicle puts power down via a clutch, which takes abuse with nary a complaint. But while there are a variety of clutch types and materials available, a clutch’s ultimate ability to perform comes down to how it handles heat. But even that, in and of itself, is a complicated topic.
Managing clutch heat effectively begins with carefully chosen ingredients. “For years we’ve said the main factors in the clutch are pressure, area, and friction,” explains K.C. Payne, Engineering Manager for Centerforce Clutch. “Pressure is straightforward – everyone knows what a pound is. Area is pretty straightforward – the area of a circle and calculating the surface area of that circle. But when you get into friction material ingredients, that’s where the black magic comes in.”
The specific ingredients are often proprietary information held close by the clutch manufacturer. There are, however, similarities. One common ingredient in organic clutches, says Payne, is carbon black, which is produced through a process of incomplete combustion of heavy petroleum products. That material works in conjunction with a woven rope or yarn, often fiberglass in non-carbon-fiber-based clutches. “The weave gives the friction material its burst characteristics, its strength characteristics, and also adds to the wear characteristics,” Payne notes. A series of fillers and bonders are added to the mix, and a resin is used to hold everything in place.
“Friction materials consist of two basic components: a structural matrix, and modifiers to improve the friction performance,” explains David Norton, owner of Spec Clutches & Flywheels. “The additives help increase the coefficient, make that coefficient more consistent, help improve wear, improve wear of the opposing surface, make it smoother, and help make it quieter.”
There are clutches made of exotic materials on the market, like full carbon fiber, but many SCCA competition classes specifically prohibit the use of these high-end materials – and for good reason. “Full carbon friction materials are very expensive to manufacture,” Payne explains. “This is why you really only see them on very high-end applications.” Consequently, our conversation focused on organic, semi-metallic, and full metallic construction.
So, what are some of the other ingredients in clutch friction materials? Surprisingly, cotton is not uncommon; graphite is also fairly common, thanks to its lubricity, Payne explains. “Back in the old days, they’d use lead,” he says. “Lead was used to keep the friction material from sticking. Now, in the metallic compounds, there’s graphite in the mix.”
“Friction materials are defined by the agglutinant, or the substance that holds the particles together,” says Norton. “For example, in sintered metal – metallic or semi-metallic puck and full-faced materials – the agglutinant is the molten metal used in coalescence [the formation of particles into a mass without liquefaction]. Carbon metals, graphite, limited amounts of iron, and non-ferrous metals like bronze and copper are some of the ingredients.
“In the case of organic facing, coalescence results from the chemical reactions between organic compounds, such as phenolic-modified resin and modified polymers,” Norton continues. “The polymers are modified to better resist the heat and wear. Kelvar and bronze strands are also used in some organics.”
Getting the friction material to stick to the steel clutch backing plate is the next step. “They take the core, or the backing steel plate, and coat that with copper or brass, which are fairly low melting point temperature metals,” Payne explains, “and then that’s compressed with the friction material. The friction material also has copper in it, and with pressure and temperature, it basically melts the two materials together.” Some clutch manufacturers, he notes, will add holes or barbs in the clutch core for strength.
That fairly low melting point is still hot, however – upwards of 2,500 degrees F in the case of metallic-based clutches, according to Norton – but if a clutch is extensively overheated during use, it is possible to have the friction material delaminate from the disc core. “It is possible, but it’s not common,” says Payne. “That can happen above 1,000 degrees F on the metallic clutches. Obviously, that temperature is lower on organic clutches.”
According to Payne, the average bell housing temperature for street and mild performance applications is around 200 degrees F, with much of that heat being generated from clutch slip when the clutch is engaged and disengaged. “Organic materials are really not much use over 500 degrees F, as they begin to outgas,” says Payne, noting that clutch outgassing is akin to brake fade.
Serious competition vehicles undoubtedly operate with bell housing temperatures above 200 degree F, so for those applications, there are clutches with more metallic ingredients. “Where the drop-off temperature in organic clutches begins is really where the metallic clutches come to life,” says Payne.
For metallic clutches, “The temperature can easily top 1,000 degrees F for sintered materials without causing lasting damage, as long as it is for a very short duration,” says Norton. “The metallic clutches for street use can operate at sustained temperatures up to 550 degrees F before wear rates start to increase.”
Norton notes that carbon fiber clutches offer an exponentially greater ability to handle heat, exceeding 5,000 decrees F.
Like with selecting brake pads, it’s important to choose a clutch that operates in the right temperature range – but even then, extenuating circumstances can lead to overheating. That said, overheating does not necessarily mean the end of life.
“It’s one thing to overheat the unit with flash heat, as long as you’ve got a good recovery, which is what the flywheel and pressure plate can help do between shifts,” explains Payne.
While lightweight aluminum flywheels are a good way to decrease rotating mass, they also affect a clutch’s ability to cool. “A good cast iron flywheel will draw a lot of heat out of the clutch,” Payne notes. “If you put an aluminum flywheel in there, while it gives up the heat quickly, that heat has to have somewhere to go.”
A potential solution to removing heat is moving air through the bell housing. “Properly vented bell housings that can produce a cross-flow of air could be better for cooling and expulsion of particles,” Norton says. “However, it does present the opportunity for debris to enter the bell, and the effectiveness of the modification will depend solely on air-flow characteristics under the car.”
While extreme heat probably won’t delaminate the clutch’s friction material from the backing plate (“The bond for both the metallic [and] semi-metallic [clutches] are more durable than the materials themselves,” Norton explains), too much heat can warp components. You might be able to diagnose warped parts from inside the car in the form of clutch slip, chatter, or poor release characteristics. That said, a clutch disc rarely warps. “Warping occurs in the pressure plate or sometimes in the flywheel,” says Payne.
If your clutch might have overheated, there are ways to evaluate. “If a clutch overheats, but not to the point that the properties of the material or surfaces are altered, cooling the unit will restore its performance,” Norton says. “However, properly cooling it requires operation; not allowing a still state where it is susceptible to heat soak.”
Once cooled, inspecting the clutch material, pressure plate, and flywheel should tell the tale. “We look at the heat in the flywheel and pressure plate,” says Payne. “You’ll normally get small heat checks that show it has had localized heat, not sustained heat.” But if the clutch friction material is shiny or the metal components show massive discoloration, that’s a sign of extraordinary, extended heat. “We’ve seen some clutches that got so hot it transferred some of the clutch friction material to the mating surfaces,” Payne says, noting that this is, indeed, rare.
So, what does this mean for you? With the correct clutch installed, nothing, really. Failure is rare without extreme circumstances, and even then it still might be possible to minimize the damage and have the clutch live to race again. To find the right clutch, you needn’t research complicated manufacturing processes or proprietary ingredients; rather, clutch shopping is as easy as surfing the web or picking up the phone. In other words, choosing wisely from the start means you’ll never need to know anything you just read.
Words by Philip Royle
Image by iStockPhoto.com/Lizalica
