The process of taking a production car and converting it for use in racing or high-performance street driving, or perhaps something that can do double-duty, sooner or later, is going to involve choosing a clutch setup.
Horsepower and torque figures, tires, goals and expectations, and how the car will be used primarily all factor in to your choice. There are a lot of options for any of the most-preferred cars used in NASA road racing, HPDE and Time Trial. A lot.
Variables include power adders that you might have installed on the engine, the clamping force of the pressure plate, sprung or unsprung hubs, organic or cerametallic friction materials and puck-type or full- circumference discs. We could break it down by designs and materials, but that might become impenetrable and make your eyes glaze over and roll back in your head.
Instead, this story will attempt to guide you in your choices of clutch setups based on how you will be using the car. We’ll break it down into three most common types of use we see on any given weekend at a NASA event: high-performance street with occasional track use, double-duty street and track, and, of course, full-boogey racecar or Time Trial machine.
To help guide us in guiding you, we spoke with Chris Bernal, vice president of engineering for Advanced Clutch Technology in Lancaster, Calif. In fact, it was Bernal who suggested we approach the story based on how your car is going to be used. He ought to know. He’s seen peoples’ eyes glaze over and roll back into their heads when he has tried to explain it in more technical ways.
“In general, the mindset should be incremental. In other words, if you’ve got a street performance car that you’ve done either no upgrades to, or some mild upgrades to, I would try to resist a temptation to jump too far ahead,” Bernal said. “It’s usually best to stay away from the cerametallic materials and kind of things that would make drivability suffer for really no reason.”
High-Performance Street Car
One of the first things Bernal pointed out is that clutches only “see” torque. They don’t see horsepower. The torsional force is what clutches are trying to grab onto and hold for transfer through the gearbox and to the wheels.
The second thing he pointed out is that what makes a racing clutch “grabby” is not the presence or absence of a sprung hub, but rather the friction material used. The organic fibrous materials encapsulated in resin are more comfortable for street use because they slip more initially than the cerametallics, which are sintered under high pressure in a preform, then put in an oven at high temperatures.
“The cerametallic stuff will take heat all day long, but the wear rate will go up. It’s really weird if you treat them too gently and don’t heat them up at all, the wear rate goes up,” he said. “If you’re kind of aggressive with them to a point, the wear rate’s actually pretty low. And then if you overheat them, they’ll still hang on, whereas an organic will fade off. You’re going to fade it like cheap brake pads. They fade with too much temperature.
“Generically, on the materials, the organic disc is going to be made out of woven fibers, and there are multiple fibers that can be used for that,” Bernal said. “And then it’s resin infused, so it it’s glued together. And in a performance disc, you’re going to add things like brass or copper. You add some metal strands in there to help cool, and oddly enough, lubricate the friction surface. But keep in mind, it’s resin infused, so it works beautifully for smooth engagement and long life, but there’s a thermal limit.”
Anyone who has been around cars long enough has smelled that thermal limit: burnt clutch disc. Organic materials can handle a little of that if you back off as soon as you smell it. But if taken too far, the resin melts and gets smeared onto the pressure plate and flywheel and forms a glaze. Then it’s time for a new clutch.
So, for street cars that only see infrequent track use or only have mild power-adders — cold-air intake, headers, exhaust, and maybe cams — you want to stick with organic materials. At most extreme for street use, you could go with an unsprung full-circumference organic disc, which is lighter than a sprung disc and allows for quicker shifting and in some applications, more precision. The quickest-shifting clutch disc is the lightest one. If you do go with unsprung, you can expect higher-than-stock drivetrain noise.
Bernal highlighted that springs are placed in disc hubs to combat noise, vibration and harshness, not for engagement quality. For example, if you look at the 3-Series BMW from the late 1980s to the present day, they came fitted with dual-mass flywheels and a “guibo” between the gearbox and the driveshaft, all to quell drivetrain noise.
“The primary reason those are used on OE cars is to shut the drive line up, to quiet gear rattle and NVH in the drive line,” Bernal said, adding that dual-mass flywheels are the most effective at quieting things down. “Those springs act as a cushion to help make that car quiet. Everybody wants to drive a fast BMW, but no one wants to hear racecar sounds on the street, the buzziness or any of that rattle. Everybody wants it quiet.”
The double-duty car, the car you drive to the track, drive on track, then drive to work on Monday is the most difficult application for finding just the right clutch, because whatever clutch you choose typically ends up being better for one type of use than the other. If it’s great on track, it might not be the best for your commute and vice-versa.
How many times have you known someone who put a racing clutch in his street car, then ended up turning it into a track-only car?
“For the street/track guy, that that’s more difficult to handle from engineering side. It can get convoluted for a customer to make the right decision, because there’s a lot of information out there that’s not helpful,” Bernal said. “Some companies rate clutches in stages. You see a lot of stage one, stage two, stage three, as if there’s a linear progression.
“There is not,” he said. “At some point down the road in choosing your clutch, it’s a fork. You have to say, I’m going to stay on the street, and I’ll deal with some of the consequences if I go far enough with my horsepower. The other direction is saying, ‘This is a track car. I may drive it to the track, but I primarily use this as a track car.’ That’s a fork in the road that eventually, as someone increases more and more performance in their car and takes it to the track more, eventually they get to that fork. And a stage approach doesn’t allow for that.”
In a sense, that ultimately binary choice boils down to percentage of use. If you use your car more than 51 percent of the time for street use, then stick with street-friendly clutch setups. You know, full circumference disc made of organic materials, sprung or unsprung depending on your tolerance for the NVH that Bernal alluded to earlier. Even if you end up using your car for mostly track use, but still drive it on the street, bear in mind that cerametallic disc materials are going to be noisier, with harsher engagement.
“Now you find out how good your motor mounts and your trans mounts are. You just start getting shuddering and whatnot, and that’s the engine moving around,” Bernal said. “When people say my clutch has a lot of judder, that’s what they’re saying, that the engine is rocking around. So now you have to look at motor mounts. As a car transitions into a track car, these are the things you have to address.”
Track Use Only
In some sense, a racecar is a much easier application for which to choose a clutch setup because the goal is so singular. Sure, a racecar or dedicated Time Trial car might be a bit clumsy in the paddock, but that matters little.
For a racing application, which would include sprint race cars, enduro cars and Time Trial cars, you are going to be looking at the sintered metal clutch discs in either sprung or unsprung configuration.
“As you get to all-out racing where you’re competing, and every 10th counts and shift times count, then you want the lightest disc possible because anything, any weight in the disc contributes to inertia, which means that the synchros have to work harder,” Bernal said. “So that’s when you would consider a solid disc to get rid of the weight of the springs.”
With cerametallic discs with a solid hub, the cushion is gone, so the engagement is much shorter, and that means you don’t have to push the pedal as far down to shift. You could use a pedal-stop to ensure less pedal travel, which also would be helpful to faster shifting. Here again, with the unsprung, racing-only applications, you are going get more NVH from the drivetrain, but you’ve got earplugs in anyway, right?
With any discussion of clutches comes a discussion on flywheels, specifically lightweight flywheels. As sexy as aluminum is, it might not be the best choice for high-rpm racecars, and according to Bernal, they might not have any lower inertia than a steel flywheel even though they weigh less.
“Inertia is a weird thing because there’s the weight of the clutch and flywheel, which is a static thing. Put it on a scale. That’s how much it weighs,” he said. “Then there’s inertia, which is the amount of energy it takes to spin it, and they are related, but they’re not exactly the same.
“The engine’s job is to spin these things up. Every bit of inertia that’s there is stored energy. So the engine loses some its power, spinning things up and that’s what you have to slow back down, after it’s at speed. And that’s lost acceleration, any bit of energy you put into your rotating things, you don’t get back.”
For street applications, you can go too light, which makes it easier to stall.
“I designed a lot of aluminum flywheels in my younger days,” Bernal said. “I design all steel flywheels now. But the aluminum flywheels, even though they’re typically lighter, don’t always have either much less inertia, or sometimes any less inertia, than a steel one, because on a steel one, I can be more intricate out by the ring gear in the design. I can take a lot of material out, whereas with an aluminum flywheel, you can’t because you have to support the ring gear out there. You end up with a lot of mass underneath the ring gear. So, you may have two pound lighter aluminum flywheel, but the engine may not see it that way.”
Bernal also noted that aluminum expands thermally at three times the rate of steel. If an aluminum flywheel is designed too thin, it can actually stretch the ring gear, which must be joined to the flywheel mechanically. He also doesn’t recommend aluminum flywheels for sustained use above 6,000 rpm. He said with forged steel, engineers have a lot more design freedom to put the weight where they want it. Ultimately, it might weigh more than an aluminum flywheel, but it makes no difference to the engine because of its inertia.
Of course, we haven’t talked much about big-power applications simply because there aren’t many of them used in amateur sports car racing. However, for those who do run higher horsepower, many high-performance pressure plates do offer increased clamping force, but as horsepower gets up to psychotic levels, there’s only so much clamping power you can add before the clutch pedal becomes too heavy. For those applications, you can use a twin-disc setup, which have become more popular. Here again, material and design choices boil down to how you use it.
“The only way to make more torque capacity, with the same friction material, if you leave that the same, you have to add clamp load. We’re going to add more and more and more clamping force to keep that disc from slipping. At some point you end up with a very heavy pedal and the driver complaining that he’s got to use two feet on the clutch pedal,” said Bernal. “What you can do then is add another disc, so instead of two surfaces, now you have four surfaces. Of course, you need a floater plate in between. With a twin disc, you’re doubling your amount of torque capacity for a given clamp load. So you can reduce the clamp load. You can reduce the spring down and make it much easier to drive.”
The takeaway of this whole story is not to get caught up in whiz-bang materials, but to be realistic about how you intend to use your car. You won’t miss the comforts of a quiet street car with smooth clutch engagement until you’re trying to get to work in stop-and-go traffic, and your clutch is all herky-jerky. By the same token, it’s important to make the most of your full-on racecar by choosing equipment that will allow it make the most horsepower and allow for the slickest, quickest shifts. A realistic assessment of how you plan to use a car dictates the clutch type you choose.