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.”
Double-Duty
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?
Lightweight Flywheels
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.
Massive Horsepower
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.
Image courtesy of Brett Becker
Clutches are one of those components that you probably don’t immediately think about when tuning a car, but which nevertheless perform an essential function in getting the power from your engine to the wheels.
The role of the clutch is to connect your engine to your gearbox. When you don’t have your foot on the clutch pedal, the clutch connects the engine to the gearbox, and in turn your wheels. This means that a clutch that can hold the torque your engine makes is essential for your car to go anywhere!
The main figure that determines whether a clutch is up to the job is how much torque it can hold. Torque is a measurement of force. There’s probably a whole blog article on the relationship between torque and horsepower, but for now let’s keep it simple and say that torque is the preferred figure for rating clutches because modern turbo-charged engines can pile on the torque very quickly at low revs. Any clutch you fit needs to be able to deal with the torque figure your engine is putting out.
Normally, on a standard engine, a standard factory-fitted clutch is up to the job. There are exceptions, and the way you drive the car can really make a difference. For instance, doing repeated hard-launches or kicking the clutch during drifting can have your clutch slipping in no time.
Once you start tuning the engine, particularly on torquey turbocharged cars, at some point you will probably need an uprated or performance clutch. Exactly when can be a bit of grey area as some vehicles come from the factory with very beefy clutches and transmissions, while others may be just adequate for the stock torque figure and quickly give up the ghost when the vehicle is remapped. It also depends how careful you are with the clutch.
Seeking out know-how on your specific vehicle is probably the best way to go here. If in doubt, we are here to help. Our sales team have a huge amount of experience of selling clutches for all kinds of vehicles and can steer you in the right direction!
If you’ve ever shopped for a clutch, you’ll know that there are three main components to a clutch – the drive plate, the cover assembly, and the bearing. There’s also the flywheel, which is bolted onto the clutch and comes with some performance clutch kits.
A flywheel is bolted onto the end of the crankshaft and does two jobs. Firstly, it’s a weight. As the engine spins, the flywheel spins. It smooths out any unevenness in how the engine works and keeps it turning. It’s also what the drive plate of the clutch sits against.
A drive plate, also called a friction plate, is the bit that transmits the torque from the flywheel to the rest of the transmission. These come in different materials which can deal with different levels of torque and heat. The material used effects the torque rating of the clutch.
A clutch cover clamps the drive plate onto the flywheel when the pedal is out. How hard the clutch cover clamps down on the drive plate also effects the torque rating of the clutch. The springs in the clutch cover assembly dictate pedal feel to a large extent.
A bearing simply keeps the other components in the clutch turning smoothly. There are a few different types of these. It’s worth checking whether a clutch kit includes this, as not all do.
Depending on how the car is being driven and how much torque the clutch needs to hold, you will have a few different types of clutches to choose from.
An “organic” clutch is similar to the clutch your car came with from the factory. The drive plate is a continuous disc, and the material used is designed to be slipped a bit. While you should avoid unnecessarily slipping your clutch (particularly under power), if you’re in stop-start traffic or parking some slippage is probably inevitable.
Fitting an uprated organic clutch will preserve this ability to slip the clutch. By contrast a paddle clutch will probably chatter a bit and have much more of an on/off-switch feel.
Generally, on a daily driven road car, it’s hard to argue with the ease of use of an organic clutch. The lifespan on organic clutches tends to be better than other types if the car is getting driven in traffic. It may not be possible to fit one because of the torque output of the engine however. This would often be when you’d start looking at paddle clutches or multi-plate clutches.
You may see these referred to as both Cerametallic Clutches and Paddle Clutches. Cerametallic refers to the material used and, as you’ve probably guessed, these are a blend of ceramics and metals.
Paddle (or puck) refers to the shape where instead of a continuous disc, the bits of the drive plate intended to make contact with the flywheel are in separate segments, known as “Paddles” or “Pucks”.
This segmented design means the clutch connects more aggressively when the pedal is let out giving the clutch a more off/on feel. This is probably ideal on track, but less so if a car has to be driven in traffic or reversed into a parking space. The more paddles used, normally the smoother the engagement of the clutch, although smooth is a relative thing here. You should probably expect some “chatter” which will feel like a sudden shudder as you let the clutch out.
The combination of material and shape normally gives these clutches a higher torque capacity than organic. Because of the different driving characteristics, we generally recommend that you know what you’re getting into before fitting one of these. You may not mind the more severe feel of the clutch pedal, or you may not like it at all.
These aren’t the only materials on offer for clutch drive plates. Depending on what car you have, you may see different materials like Kevlar Clutches or Carbon Fibre Clutches on offer. These typically blend higher torque capacity with organic clutch like drivability. If in any doubt about the materials on offer, it’s best to get in touch and talk to our sales team.
Multi-plate clutches are a technology that has filtered down from very specialist applications like supercars onto road cars.
As the name suggests, instead of one drive plate, the clutch will have several, arranged in a stack. The total torque it can hold is that of all the plates combined. This means that you can have an organic, multi-plate clutch which offers a similar pedal feel to a standard clutch, but which can hold a much higher torque load.
You can also get clutches using multiple plates of other materials to give a higher torque capacity.
While not technically part of the clutch, you may have the option of upgrading your flywheel at the same time. There are a couple of reasons you’d want to do this.
Firstly, a flywheel can wear out. A lot of cars come with a dual mass flywheel now. This is where the bit that connects directly to the engine is separated from the bit the drive plate touches by some sort of buffer. This can be springs or bushings. This is done to further smooth out the workings of the engine. As the springs or bushings soak up vibrations, they can wear out. By contrast, a solid cast-iron or chromoly steel flywheel has no components to wear out. It will probably need resurfacing when you next fit a clutch, but solid metal lasts a long time.
This is doubly true if you’re swapping in a harsher performance clutch which will place more load on a dual mass flywheel. Replacing a worn (or at least wear-prone) dual mass flywheel with a tough single-mass flywheel makes a lot of sense as part of a transmission upgrade.
Some higher spec clutches require an uprated flywheel to mount. Even where they don’t, we’d normally recommend you upgrade your flywheel when fitting a significantly uprated clutch.
Secondly, your factory flywheel will have been primarily designed to give a smooth, comfortable, quiet drive. This probably translates to it being on the heavier side. This has pros and cons. It means that once the engine is turning it wants to keep turning, which is good for fuel economy. On the downside, it also means that the engine is going to be a little reluctant to gain or lose revs. That reluctance can make shifting gear slower than it would be otherwise.
Replacing this with a lighter flywheel can make the engine more eager to respond to being “blipped” for a downshift. It also gives the engine a more eager, free-revving feel. Some manufacturers offer a choice of how lightweight you want to go.
A “lightweight” flywheel will shave a reasonable amount of weight off, whilst still being pleasant to drive on the road. You may see some sort of trade-off in fuel economy, and vibration felt (particularly when replacing a Dual-Mass Flywheel), but the car should stay very driveable. These are normally fine for road use.
Some manufacturers go even lighter and offer an ultra-light flywheel. These have a significant amount of weight removed from them. You will probably notice a lot more vibration coming from the engine into the cabin. You may also find the car less pleasant to drive in traffic or on hills. These are normally the sort of thing you’d fit on a very tuned weekend car, or a track car.
The right choice of clutch for you is going to depend on a mix of how much torque your vehicle makes, and what you’re using it for. For a lot of people, either an OE replacement or an uprated organic clutch are going to be the preferred choice on a road car. Some of the performance organic clutches on offer can hold a surprisingly high amount of torque.
Once these aren’t up to the job, you need to give some careful thought to how the car is going to be used and how you want the clutch pedal to feel. In some ways this one is a bit chicken and egg. If you want a car with a nice easy to drive clutch, it might be worth considering that when planning your future upgrade plan.
Most of our sales team have been here for over three decades, and have experience of supplying clutches for everything from humble hot-hatches, to full fledged race-cars, so if you need any advice please get in touch.
As you browse our site, you will probably notice we offer both clutch kits and individual clutch components.
A clutch kit is usually intended to be a complete upgrade, containing everything you need in one box, including fitting tool and appropriate bolts are usually provided also. It will have been listed as fitting a specific vehicle as a kit.
Separate components allow more technical customers to mix and match the components they need.
Generally, unless you know exactly what combination of separate parts you need, selecting the clutch kit that best fits your requirements is the way to go. The components in the kit will have been carefully selected to work together. The combination of clutch cover and drive plate will give you a torque figure to work with. It also avoids you replacing some of the components and then discovering afterwards that others are worn a similar amount.
We carry an extensive range from AP Racing, Black Diamond, Competition Clutch, DKM Clutch, Exedy, Helix, Sachs, and Xtreme Clutch.
This is a complicated topic which we’ve lightly breezed over here. If you’re at all unsure, please get in touch with our sales team. They can advise on everything from road use all the way up to dedicated motorsport clutches.