Have you ever thought about the myriad complex actions a clutch system performs with one push of a pedal? With a simple movement of your left leg, your Corvette C5/C6’s clutch can help you make a partial or direct connection with the driveline, change gears up or down, creep or rocket away from a dead stop or a roll, and get amazing mileage with far less slippage than an auto trans. Oh, and it can enable power shifts and wicked burnouts, too—but you already knew that. For simply pressing on a pedal, that’s pretty amazing technology.
So amazing, in fact, that clutches are sometimes seen as too complex to understand—people don’t know how they work, as long as they work! But clutches can be actually be easy to comprehend, once you identify the parts involved and have a basic concept of their operation. This article was written to help you achieve that understanding.
First, we’ll cover the components used in clutch systems, as well as the material variations that help separate stock, performance, and race clutches. Next, we’ll review common formulas and calculations used in the design and testing of clutches. We’ll then hear from two of the industry’s clutch experts, who will answer some of the commonly asked Corvette C5/C6 clutch questions.
And finally, we’ll provide the latest aftermarket clutch recommendations for you C5/C6 owners. Whether you’re a granny-shifting cruiser or the kind of speed freak who refers to halfshafts as “fuses,” we’ve got you covered.
The disc is the heart of a clutch system: The attached friction material supplies the friction needed to pass the engine’s torque through the drivetrain. Clutch discs can be a full-face (round) or puck-type design.
Friction materials attached to the middle and outer parts of the disc will affect how the clutch engages. There are several different types:
Organic compounds are usually used for street or street/’strip duty. They engage smoothly, yet can handle heat better than stock clutches. Materials include fiberglass, ceramic, and carbon, although various metals can be added to the blend to increase friction.
Sintered metal compounds are normally formulated for racing applications. A higher friction coefficient means a more positive engagement—they grab harder—but in most cases these compounds are not street friendly. Materials include bronze, copper, iron, and full ceramic.
Exotic compounds such as carbon are more popular than ever, and for good reason: Revolutionary carbon-carbon discs offer smooth engagement, great friction, low weight, and low wear. Once prohibitively expensive for street applications, carbon clutch systems are much more affordable now—and they’re fully rebuildable, too.
At the disc’s center is the hub, which also includes the spline. The disc and hub are connected, but the type of connection determines how smoothly or harshly the clutch engages and disengages. Racing discs are usually riveted to the hubs for solid (but harsh) engagement. Many street discs use hubs with built-in coil springs that allow a small amount of rotation when engaged, which softens the engagement and results in better driveability.
Speaking of driveability, a marcel is a thin, wavy metal plate attached to the clutch disc in some street applications. This wavy plate helps reduce clutch chatter by facilitating a smooth engagement. This plate differs from the ones in racing clutches; as chatter isn’t a concern, they use a flat plate.
The pressure plate actually engages the clutch by clamping the disc against the flywheel. There are several different types:
The ubiquitous diaphragm pressure plate uses many “fingers” that the throwout bearing presses against to disengage the clutch. This type is known for good driveability and pedal effort, and it is found in many late-model vehicles, including Corvettes.
The Borg & Beck is a coil-spring design that uses three levers to engage and disengage the clutch disc; the coil springs apply pressure to the disc. This type was common in classic muscle cars.
Finally, there’s the Long-style pressure plate, which is a Ford version of the Borg & Beck that is used in racing applications.
In addition to standard metals such as ductile iron, pressure plates can also be had in aluminum—they’re lighter, which helps with responsiveness.
SFI-approved pressure plates are the best choice for performance and safety, particularly in cars that see use on the road course or dragstrip.
Describing the flywheel as a disc bolted to the back of the crank doesn’t do it justice. No, this round piece of metal is multi-talented: When the clutch engages and the disc clamps onto it, it’s part of the friction package. It also helps dampen engine harmonics.
Its material determines its weight, which in turn determines its personality: Heavy iron or steel flywheels do a great job of storing engine power, then releasing it to help get a heavy vehicle moving.
Lightweight aluminum flywheels don’t store up much engine power, but they’ll let your engine rev quicker. Aluminum flywheels work best in light vehicles with high numerical gears and/or a good amount of power and torque.
Again, SFI-approved flywheels are the best choice for both performance and safety.
There are three different types of linkages that connect the clutch pedal and the rest of the clutch system: mechanical, hydraulic, and cable-release. They all use clutch-pedal movement to engage and disengage the clutch, but they do it in different ways.
A mechanical linkage uses solid connections—like a pivoting bellcrank and adjustment rod—to push/pull the clutch fork and engage/disengage the clutch. This type of linkage was used in Corvettes through 1981.
A hydraulic linkage uses fluid pressure, a high-pressure hose, and a master cylinder and slave cylinder to engage/disengage the clutch. Because the system uses hydraulics, it doesn’t have a clutch fork. This linkage type is more popular in late-model vehicles, and it was added to Corvettes for the ’84-up model years.
A cable-release linkage uses a simple cable and quadrant setup to push/pull the clutch fork and engage/disengage the clutch.
When a clutch pedal is depressed, a throwout bearing moves the pressure plate away from the clutch disc. When the pedal is let out, the throwout bearing moves the pressure plate toward the clutch disc. The pilot bushing sits in a hole in the rear of the crank, and the front of the trans’ input shaft slides straight into it, keeping it aligned and preventing bearing wear.
Single-Disc vs. Multi-Disc System
Single-disc clutch systems, as the name implies, use one disc. This type of clutch reigned for decades, and it had to hold up to some of the most powerful engines ever made. So when clutch builders needed more holding power to live behind those big engines, they simply added more brawn. That meant a larger disc with more friction, and a larger, stiffer pressure plate. It held the power, but it also resulted in much heavier clutch effort—and less-streetable manners.
That all changed when multi-disc systems became available for street cars. Multi-discs are exactly as they sound: Instead of a single disc, these systems use two or even three. There are other differences too: The discs are separated by a thin, O-shaped metal ring called a floater plate. Street applications use a strapped floater plate—the straps connect to the flywheel and “drive” the floater, which reduces rattling when the clutch is pushed in. A stand-driven floater plate secures the floater more positively using bolts and nuts; it is stronger and better for racing, but also more prone to rattle.
There are several advantages to multi-disc clutch systems: When everything else is equal, torque capacity is doubled in a twin-disc kit, and tripled in a triple-disc kit. Pedal effort is substantially lighter—in many applications, a multi-disc clutch pedal is easier to work than the factory single-disc system it replaced. And speaking of light, multiple- disc systems use smaller-diameter discs, which can not only save weight over larger single discs, but improve the clutch’s responsiveness as well.
While they are usually more expensive than a single-disc kit, the multi-disc systems’ high power-holding characteristics and low pedal effort make them a perfect match for today’s high-horse street vehicles. In fact, the fastest, most powerful production Corvette ever—the 638-horse ZR1—runs a dual-disc clutch straight from the factory, and the new C7 Stingray will use a similar configuration when it goes on sale this year.
C5/C6 Clutch Tips from the Experts
For some insight into ’97-up Corvette clutches, we picked the brains of two experts in the Corvette clutch market: Bob Scheid of McLeod and David Norton of SPEC. Here’s what they had to say:
VETTE magazine: How much torque can a stock Corvette clutch handle?
SPEC’s David Norton: The last new stock C5 clutch we had tested at 436 foot-pounds. That is right at breakaway, so the wear rate would be very high.
McLeod’s Bob Scheid: They were designed to hold the factory power during street use. If you exceed that power level or use it at the track, then you run the risk of clutch failure.
Most clutches will give you indicators that you are overtaxing them: Getting that dreaded clutch smell is a sure sign; so is a change in the pedal position when the clutch engages. An overheated stock disc glazes, and a worn disc thins out. When either one happens, the clutch will no longer hold the power it used to, and it may start to slip.
C5s and C6s are different from most other cars in one aspect: Their pressure plates are diaphragm-style units like most modern cars, but when pushed to high rpm, that diaphragm can get sucked in and cause shifting problems.
VM: What are the C5/C6 clutch’s weak points?
Norton: The ratcheting mechanism is the C5/C6 clutch’s weakness. There are too many parts [that can] fail, and they hinder high-rpm actuation. The units are fairly low in load, but not terribly low. The C5 units didn’t have enough feedback to assist the hydraulic system in returning the pedal.
The flywheel is the worst part of the C5 system: The castings were bad, so there were porosity issues that caused pits and warpage—and they weren’t meant to be resurfaced. They also weren’t balanced separately of the stock clutch, so balance issues existed when end users tried to reuse them without extra balancing steps.
Scheid: The factory clutch’s pressure plate and disc material were built for stock power levels, not for the power that most modified C5s and C6s are making.
VM:What important things should customers understand when choosing a clutch?
Norton: Inertia properties are commonly misunderstood. The benefits of lighter parts normally far outweigh any downsides. The trick is gathering enough info from the end user about their build, and how the car is driven, to determine how light the unit can be to optimize performance.
Scheid: That a higher “Stage” number doesn’t necessarily mean that it’s a better clutch for you. The clutch material, pedal pressure, and other design factors are what’s really important.
For street-driven vehicles, a clutch that has a high amount of holding power, yet uses a friendly disc material and has a soft pedal effort, is ideal.
Also, clutch weight affects both performance and driveability. A lower-weight unit will get more of the engine’s horsepower to the wheels, since less power is being used to rotate the flywheel and clutch. The tradeoff is that you can sacrifice some driveability if the weight is reduced too much.
VM: What should C5/C6 owners’ main considerations be when changing the stock clutch?
Norton: The normal considerations apply, like making sure the capacity covers the modification level, [as well as] driveability preferences and budget. Everyone may prioritize those considerations differently.
And they should always change the stock flywheel. The original C5 flywheels were not meant to be reused, and the stock cast units were not balanced independently from the factory. The C6 flywheels were better, but the stepped design limits performance and reliability to the self-ratcheting stock clutch design.
Scheid: The three most common concerns are holding power, driveability, and clutch life. Quality is always priority one when designing our clutches, so we must be able to supply the customer with a high-quality clutch that meets those three needs.
Clutch life has a lot to do with the way the unit is used. Of course, a clutch that is used for racing or abused on the street will not last as long. Customers will get long life out of our clutches if they choose the proper clutch for the way they are going to be using it.
VM: What are the latest technology advancements and clutch trends?
Norton: For technology, I’d say carbon/ carbon units. They are exceptional: high torque and low weight, smooth engagement, and [they’re] easy on driveline parts.
[On trends:] First, multi-discs are popular, [thanks to their] good power holding and driveability. Second, most C5/C6 owners would like improved trans shifting. Though stout, the transaxles do not shift like butter. One way to assist is with the clutch, by reducing the Moment Of Inertia of the rotating assembly and the disc itself.
We can accomplish this with our stock-appearing, lightweight aluminum pressure-plate option, which has other benefits as well. We have [also] introduced a 10.5-inch clutch option that has torque capacities similar to the 12-inch units, but with lighter discs for faster shifting. I see those units becoming very popular this season.
Scheid: Although there have been advances in disc materials, I think the greatest advancement has been in clutch design. Even though McLeod introduced the Street Twin back in the 1970s, the newest designs are street friendly—with the strapping of the floater plate to eliminate the rattle—cost effective, easy to install, and they hold the power.
Our new Modular RXT Street Twin makes clutch installation a snap. Our Modular units come fully assembled. The installer simply unbolts the old clutch and bolts the new one to the factory flywheel.
VM: How do most of your customers order clutches these days?
Norton: Most of our LS customers order Super Twins and single-disc Stage 2+ and 3+ clutches, with billet steel and aluminum flywheels. The aluminum pressure-plate option is becoming more popular; I see our new 10.5-inch set becoming very popular this year.
Scheid: Street-friendly twins such as the RST and RXT are very hot right now. That being said, not all customers need them. For those customers who have basic bolt-ons, we still sell a lot of our Street Pro and Super Street Pro single-disc units.
VM: What is your recommended clutch for a 450- to 650-rwhp “street” C5/C6?
Norton: In our line, the Stage 3+ raises the bar of a single disc to over 900 foot-pounds, with good driveability. We would recommend both single and dual-disc units at that power level, and let the end user choose based on budget and future plans that may warrant the upgradeability of the Super Twin.
Scheid: For cars up to 500 rwhp, we would recommend our Super Street Pro single-disc. The clutch has a dual-faced disc with organic lining on one side, and Miba lining on the other. A steel flywheel is recommended for street-friendly driving.
VM: What is your recommended clutch for an 800- to 900-rwhp “extreme street/ ’strip” C5/C6?
Norton: [See above.]
Scheid: For cars that make 500-800 rwhp, we recommend our RST Twin Disc. The RST has organic linings for smooth engagement. The added discs give increased holding power without the use of aggressive materials or the need for heavy pedal effort. A steel flywheel will give the needed stored energy for friendlier street driving.
VM: Should C5/C6 owners keep stock hydraulics when buying a new clutch, or upgrade?
Norton: The stock hydraulics in the Vette have been refined for better durability over the years. We don’t see any issues with the C5/C6 factory setups now. But it is good to always replace the slave [cylinder] while the bellhousing is out of the car, and keep an eye on the fluid in the master cylinder, especially on higher-mileage cars. If fresh fluid starts to get dirty, the seals may be nearing the end of their lives, and actuation issues may soon arise.
Scheid: The C5/C6 hydraulics are fine, unless the customer decides to go with a pressure plate that requires much more pedal pressure. For example, we offer a Borg & Beck-style twin. The Borg & Beck-style pressure plate is ideal for high-rpm LS applications. It doesn’t get sucked in and cause high-rpm shifting problems like a diaphragm-style can. We recommend changing the master cylinder when using a pressure plate that requires more pedal pressure.
Advanced Clutch Tech With Mantic
Melbourne, Australia–based Mantic Engineering is a new name on the Corvette-clutch scene, but parent company Clutch Industries Pty Ltd has been building clutches since 1951. CI has been an OEM supplier to Holden (GM), Ford, Nissan, and Toyota. With that kind of pedigree, it should come as no surprise that Mantic prides itself on building high-performance clutches with OEM-style engineering, R&D, and quality.
In addition to a full line of single- and multi-disc clutch systems, Mantic is known for innovative clutch features. One example is company’s the ER2 Street Series covers: They use a patented Groove Design that is CNC machined on the pressure plate’s friction surface, which Mantic says increases the torque-drive capability by more than 8 percent over a non-grooved cover.
Because Mantic does extensive testing—both in the real world and on the only clutch dyno in Australia—we wanted to show you some of the terms, formulas, and calculations used in the company’s clutch design and testing. Some of it is pretty advanced, but it provides a solid understanding of the forces at work during clutch operation.
Let’s start with some basic terminology.
Clamp Load: This is the load exerted by the diaphragm to clamp the clutch disc between the pressure plate and the flywheel.
Coefficient of Friction (μ): This is the measured resistance that occurs between two surfaces.
Mean Effective Radius: This is the effective radius of the friction surface, measured from the inside to the outside of the friction area; it’s used to calculate torque capacity.
Torque Capacity: This the amount of torque that can be safely transmitted through a clutch. It is calculated by multiplying the load exerted by the diaphragm to clamp the clutch disc between the pressure plate and the flywheel, multiplied by the coefficient of friction, multiplied by the radius of gyration, and multiplied by the number of clutch discs.
Torque capacity is affected by 4 factors:
1. By decreasing the diameter, the torque capacity is reduced.
2. By decreasing the clamp load, the torque capacity is reduced.
3. By adding a second clutch disc, the torque capacity is doubled.
4. By increasing the coefficient of friction, the torque capacity is increased.
It is therefore possible for the two smaller discs in a dual-disc clutch to provide a greater torque capacity than the one big disc in a single-disc unit.
Mass Moment of Inertia (MMOI) measures the ability of the clutch and flywheel assembly to resist changes in rotational speed about a specific axis. (The symbol “I” is used to refer to the moment of inertia when making calculations.) This term is of particular interest with clutches and flywheels because to accelerate a vehicle, it’s necessary to overcome the vehicle’s resistance to acceleration, or MMOI.
The larger the MMOI number, the smaller the angular acceleration about that axis is for a given torque. Therefore, a higher MMOI number makes the flywheel accelerate slower at that torque amount. A lower MMOI, meanwhile, allows for faster gear shifts and improved engine response. Reducing the MMOI has much the same effect as adding power to the engine, enabling it to accelerate more quickly.
Decreased system weight also makes for an incremental improvement in the vehicle’s power-to-weight ratio. A dual-disc clutch system with smaller-diameter discs will actually be lighter than a single-disc system, in spite of its intermediate plate and extra clutch. This is because the effect of the weight decreases dramatically as the diameter gets smaller: Weight is proportional to the radius squared. If the radius is halved, the weight is decreased by a factor of 4.
There are two advantages to lowering the MMOI of a clutch assembly:
1. There is less inertia—and therefore less power—required to spin the clutch assembly. The net effect is that the vehicle is able to accelerate faster.
The MMOI of a non-point object is calculated by the following formula:
I = k x R2 x M (measured in kg / m²)
M is the mass
R is the radius of the object from the center of mass
k is a dimensionless constant called the inertia constant that varies with the geometry of the object in consideration. For example, k = 1 for a thin-walled cylinder around its center, or k = ½ for a solid disc around its center.
2. The clutch disc(s) will not “spin on” for as long; this allows gear changes to happen more quickly. Again, the results are faster acceleration, and less time when there is no power being transmitted to the wheels.
It is important to note that the MMOI is proportional to the radius squared. So a small change in the radius or diameter of a clutch has a dramatic effect on the MMOI. For example, an increase in diameter of 40 percent—say, from 200mm to 280mm—approximately equates to a doubling of the MMOI, or a doubling of the resistance to changing the rotation rate. In layman’s terms, that means twice the power is required to accelerate the clutch.