Clutch Basics

The Fusible Friction Link Between Your Engine and the Road

By Jeff Smith   –   Photography by the Author and Courtesy of the Manufacturers

Let the new century prognosticators spew all they want about the demise of the traditional manual transmission. For those not quite ready for dual-clutch transmissions or care even less about 10-speed automatics, there is still much to celebrate for left pedal fans when it comes to the current crop of manual transmissions for hot street cars. What we will cover here is the friction connection between the engine and the input shaft of that manual transmission.

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While dual-disc clutches might be cool and charismatic, it’s the basics about how clutches work that should be first on your list of crucial information. It’s important to understand the fundamentals of how clutches operate, and the variables involved with all the fundamental factors. Items like pressure plate design, clutch diameter, clamp load, friction material, operating temperature, and a host of other variables. Once you’ve cornered this area of expertise you will have a much better idea of which clutch will be the best one for your next project. All this makes a big difference when it comes to how you intend to use this clutch. We’ll look closer into these ideas and tie them all together to help weed out the unnecessary hype so you can choose the clutch that will best get the job done.

001 clutch assembly consists of a flywheel clutch disc and a pressure plate
The basic clutch assembly consists of a flywheel, clutch disc, and a pressure plate. When the pressure plate is torqued to the flywheel with the clutch disc in between, the springs inside the pressure plate clamp the clutch disc to the flywheel, creating a clamp load that must withstand the forces of engine rpm, torque, and load of the vehicle.

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Let’s begin with some simple fundamentals, such as how a clutch operates. The three basic components of a clutch assembly are the flywheel, clutch, and the pressure plate. The pressure plate bolts to the flywheel and, when secured, places a spring load on the clutch plate sandwiched between the flywheel and the pressure plate. When the clutch pedal is pushed in to release the clutch, the throw-out bearing applies a force to the fingers of the pressure plate to push inward on the pressure plate fingers that releases the load on the clutch.

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The amount of pressure plate force applied to the clutch is called the clamp load and is determined by the spring pressure applied by the pressure plate. The most popular basic pressure plate designs include the Borg & Beck, diaphragm, and Long style (also called a Ford lever). The Borg & Beck (sometimes called a Chrysler style) has fallen out of favor with most clutch builders, leaving the Long style as the popular racing pressure plate with the diaphragm as the most popular for the street.

002 Summit Racing illustration shows a cross section of a typical diaphragm clutch assembly with the clutch pedal pushed in
This Summit Racing illustration shows a cross-section of a typical diaphragm clutch assembly with the clutch pedal pushed in. The release bearing has moved toward the left, which pushes the fingers of the pressure plate inward to release the clamp load on the clutch. If you look closely, you can see free space between the clutch disc and both the pressure plate and the flywheel.

The Long style uses three sets of coil springs that are controlled by a thin release arm tied to each of three sets of springs. This arm can be modified with additional weight to add rpm-based centrifugal force to the base clamp load for high-horsepower engines used in competition. The Long style is not a good choice for street operation, although it can be used. Mainly it often requires a fairly high pedal effort. As a final note, Long-style pressure plates employ a different attachment pattern than the Borg & Beck and diaphragm so these pressure plates are not interchangeable.

The diaphragm, also known as a Bellville spring, offers a significant advantage over the more traditional coil spring pressure plates. As a coil spring is compressed, load increases. However, with a diaphragm, once it reaches its center pivot point when the clutch is fully released, the load is drastically reduced. This allows the user to keep the clutch pedal in during street operation at a stoplight for example, without taxing leg strength as would be the case with a Borg & Beck or Long-style pressure plate.

Another major variable is the diameter of the clutch assembly. For mild V-8 street operation, 10½-inch clutches will easily get the job done. Larger engines generating more torque and horsepower increase this to 11- and 12-inch units. A larger diameter merely increases the clutch surface area, which increases its clamp load and therefore torque capacity.

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003 a diaphragm style pressure plate spring
This image shows a diaphragm-style pressure plate spring, which offers a major advantage of street operation.

This is a good place to mention that while clutches are generally rated by horsepower, it is really the engine’s torque that is the major factor for choosing a clutch. The more torque, the higher the twisting motion created by the engine which demands a more-aggressive clutch holding capacity. Remember that horsepower is merely torque over time expressed as work. This is also why clutch companies will want to know items like vehicle weight, gear ratio, as well as engine size and whether there is a power adder.

Increasing clamp load with higher pressure plate spring force is a simple way to hold more torque but this also increases pedal effort. Another option for clutch builders is to improve capacity with an increase in diameter or with more aggressive materials using a higher coefficient of friction. For street clutches, the traditional organic material continues to constantly evolve and is an excellent choice for mild applications as this material offers the advantage of smooth clutch engagement.

004 Long style pressure plate with three fingers
Here is a Long-style pressure plate with three fingers. The fingers control three sets of coil springs that produce the clamp load. The Long-style clutch is most often employed in drag racing applications because the load can be easily tuned for both static and high-rpm centrifugal load.

Other compounds, such as ceramics, Kevlar, and sintered iron offer a much higher coefficient of friction, which increases holding capacity. But these materials can make clutch engagement more aggressive and more likely to produce clutch chatter. Lately, several companies, such as Centerforce, McLeod, Ram, Spec, and others, now combine these more forceful compounds on one side of the clutch disc with more forgiving organic compounds on the opposite side. This accomplishes the goal of increasing the holding power while ensuring easy engagement.

You may have noticed that these combination clutch discs use a smaller area of ceramic or sintered iron face material. This is intentional since this concentrates a higher load (in terms of pounds per square inch) on the material to increase its operating temperature to ensure optimal performance. When combined with an organic clutch facing on the opposite side of the disc, the clutch can supply greater holding capacity without negatively affecting engagement or creating undue wear.

005 right is a typical organic facing material used in street and mild performance situations The ceramic puck material on the left is often used on more high performance situations
On the right is a typical organic facing material used in street and mild performance situations. The ceramic puck material on the left is often used on more high-performance situations. Many clutch companies will combine these two materials on a single disc to offer more holding capacity while still offering excellent everyday street engagement performance.

Within the clutch disc itself, there are multiple design functions at work. Nearly all street clutches employ what is called a sprung hub. The typical approach is six (and sometimes eight) coil springs placed between the input shaft of the clutch disc and the clutch facing. When the disc is loaded during engagement, these springs compress slightly and reduce or eliminate clutch chatter or shudder. An additional technique employs what is called a Marcel spring. This is a thin, wavy spring placed between the clutch face material and the clutch disc itself. This spring also improves engagement performance, making this process smoother and less intrusive.

006 diaphragm spring pivots on a raised machined portion of the actual pressure plate ring
A diaphragm spring pivots on a raised machined portion of the actual pressure plate ring. Certain Centerforce pressure plates substitute high-strength, steel ball bearings as the pivot, which reduces friction and pedal effort while improving engagement performance.

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One aspect that most generic clutch stories fail to address is how heat generated by the clutch affects holding power. Heat is a significant factor that directly affects overall clutch performance. Organic materials are used to enhance low temperature clutch engagement but are limited by a somewhat conservative temperature limit. This is why dual-friction–style clutches with an organic facing on one side and a ceramic material on the other have become so popular. Under high load, the clutch temperature increases, which bumps up the ceramic or sintered iron coefficient of friction, which improves the clutch assembly’s overall torque capacity. That’s why you will see the higher rated clutch assemblies using these more aggressive materials.

007 pressure plate bolts with a short guidebody that centers in a countersunk portion of the threaded mounting hole in the flywheel
Muscle car–era clutch assemblies use pressure plate bolts with a short guidebody that centers in a countersunk portion of the threaded mounting hole in the flywheel. Newer metric flywheel and pressure plate assemblies use three steel pins to center the pressure plate. The pressure plate bolt patterns are often the same, but you should never substitute an older pressure plate on a metric flywheel by eliminating the guide pins as this will not center the pressure plate on the flywheel and a vibration can be the result.

The trick is to choose the correct clutch assembly for your particular application without going overboard by “over clutching” the application. This is where a heavier duty clutch is misapplied when it’s not necessary. This can lead to engagement problems, clutch chatter, and a less-than-positive experience compounded by spending more money on a component that was misapplied, like trying to use a race clutch on the street.

Of course, choosing a clutch with less torque capacity than required can be equally as frustrating, often leading to slippage and clutch failure. This is why it’s important to discuss the application with your clutch company and to be completely honest with the vehicle specs and how it will be used. A clutch intended mainly for road course use with only occasional street time is a very different application from a clutch where it is mainly a boulevard cruiser with only occasional trips to the track.

008 clutch disc must match the input shaft spline count
Car guys are famous for mixing and matching components and this includes transmission input shaft spine counts. An old-school Muncie four-speed uses a coarse 10-spline input but the more modern TREMEC five- and six-speeds and many other performance boxes use a 26-spline input shaft. The clutch disc must match the input shaft spline count and it’s best you ensure the clutch fits properly before you install the clutch in the car.

Flywheel weight is another important consideration. Several poorly conceived media stories have appeared over the years touting the acceleration benefit of lightweight aluminum flywheels. What these stories rarely address is the loss of inertia momentum created by a lightweight flywheel. This is instantly recognized in normal driving when typical off-idle engine speed engagement results in an engine that easily stalls. With a lightweight flywheel, a much higher engine rpm is needed to build sufficient inertia to easily accelerate from a dead stop. This is why OE flywheels are usually heavier. This additional mass stores energy in the flywheel, allowing the engine to allow clutch engagement at just above idle speed.

009 Ram 900 Series clutch uses eight springs covered in polyurethane
This Ram 900 Series clutch uses eight springs covered in polyurethane around the center hub to help dampen clutch engagement for street use. Other clutch discs use five larger springs.

As an example, a typical small-block Chevy 11-inch flywheel usually weighs 30 pounds with a complete clutch and pressure plate assembly scaling in roughly 45 to 50 pounds. By reducing the weight of the steel flywheel to 25 pounds, this can improve acceleration while still retaining sufficient mass to allow for normal street operation. Of course, this can also be achieved with a lighter pressure plate.

010 engagement dampening also comes in the form of a thin wavy spring called a Marcel
Additional engagement dampening also comes in the form of a thin, wavy spring called a Marcel placed between the clutch facing and the hub.

As an example, let’s say you’re about to upgrade to a new TKX five-speed transmission with a 2.66:1 First gear ratio from the original Muncie 2.20:1 close ratio box. This deeper First gear ratio offers more leverage and the potential opportunity to reduce the flywheel weight slightly while not sacrificing driveability.

Companies like Centerforce, McLeod, Ram, Spec, and others offer several tiers of single-disc clutch assemblies that are aimed at a particular power level and vehicle application. For example, McLeod offers four levels of street single-disc clutches from Street Level to Extreme Street. Centerforce also offers similar packages with Centerforce I, II, and Dual Friction kits and Ram has three levels. Each is aimed at a particular performance level to help you match the right clutch for your situation. These kits are rated by horsepower but it’s still best to discuss your application with the clutch company for a specific recommendation.

011 mild clutch with soft engagement is more important than holding 500 lb ft of torque
For mild street cars like our ’65 El Camino with short 17-inch tires, a wide-ratio Muncie four-speed, and 3.08:1 gears, a mild clutch with soft engagement is more important than holding 500 lb-ft of torque. This small-block El Camino weighs roughly 3,600 pounds and is mainly a street cruiser.

This basics story has really just skimmed the surface of how the various details affect clutch performance, but this information should offer a solid foundation from which to begin the search for the right clutch. A basic understanding of how clutches operate makes the decision process much less confusing.

012 Spec Stage 2 clutch for an early Chevy muscle car with a 10 spline input shaft might be a good choice
For cars with a more aggressive power package and upwards of 500 hp, this Spec Stage 2 clutch for an early Chevy muscle car with a 10-spline input shaft might be a good choice.
013 single versus dual clutches
In a more in-depth Part II article we will dive into single versus dual clutches and the various crossover points where it is more likely to choose a dual- versus a single-disc clutch assembly.
014 machine shop surface it with a Blanchard grinding stone to ensure the flywheel is flat and straight
If you are planning to reuse your existing flywheel with a new clutch, have a machine shop surface it with a Blanchard grinding stone to ensure the flywheel is flat and straight. The people at Ram maintain this is a crucial step that should not be overlooked. This used flywheel certainly doesn’t look acceptable and if you’ve just invested $500 or more in a new clutch assembly, why cheap out with a used flywheel? At least have the flywheel properly machined to ensure the friction surface is flat.
015 check for bellhousing alignment for a new clutch assembly
It’s important to check for bellhousing alignment for a new clutch assembly. The spec for total indicator run-out is 0.005 inch. Also check for vertical transmission mounting alignment, which is also 0.005 inch. Checking that the bellhousing is aligned with the crankshaft centerline is critical to ensuring proper clutch performance.
016 place this raised portion closer to the pressure plate
This may seem elementary to some, but it’s easily overlooked. All street clutches come with a raised portion of the clutch hub for the integrated springs. Always place this raised portion closer to the pressure plate. If the disc is installed upside down, the hub can hit the flywheel bolts and not release properly and you will have to disassemble the entire system to flip the clutch disc and hope the clutch hub was not damaged.

Chart 01

Friction Materials
Material Application
Organic Stock applications
Kevlar Composites Street performance
Ceramic-Metallic High Performance, higher load and heat capacity
Sintered Iron Mainly used in race clutches—highest torque capacity but wear is accelerated with harsh engagement
Sintered Bronze Pure race applications


Sidebar 02

Other Parts

In addition to a clutch, pressure plate, and flywheel, there are a few other important parts you’ll need to complete a proper clutch conversion. Let’s start with the pilot bushing or pilot bearing. Make sure the bushing is not worn or elongated. If so, it’s possible the bellhousing is not aligned, which is an important step to check to ensure proper clutch performance. Many problems blamed on improper clutch operation can be traced to bellhousing alignment.

Most new clutch assemblies supply the proper pressure plate bolts. If you are mixing and matching parts, make sure to use the correct fasteners. Never use locking star washers under the flywheel attaching bolts. Always use a new release bearing with a new clutch. This is just cheap insurance. Many companies like Centerforce, McLeod, and others supply a new release bearing with the clutch along with a plastic dummy shaft to align the clutch pedal with the pilot bushing. Check with the clutch company because if the clutch does not come with one, you will certainly need one to complete the installation.

For older muscle cars using a release arm, carefully inspect the arm for wear marks or cracks around the pivot point and where the arm contacts the release bearing. It’s also a good idea to replace the ball stud that the release arm pivots on. This is a universally overlooked part that is often extremely worn. Replace both it and the release arm if there is any question. Your clutch will thank you for your attentiveness.


(928) 771-8422

Holley Performance (Hays)
(866) 464-6553

McLeod Racing
(714) 630-2764

Ram Clutches
(803) 788-6034

Spec Clutch
(205) 491-8581

Summit Racing
(800) 230-3030

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