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How Clutches Work - In The Clutch

Aaron Bonk
Feb 19, 2014
How clutches work clutch assembly 01 Photo 1/7   |  
The clutch assembly is made up of four major components: the disc (lower right), pressure plate (upper right), flywheel (upper left), and throwout bearing (lower left). Puck-style discs like these are best suited for high-performance applications where a higher coefficient of friction is required, often at the expense of factory-smooth engagement.

Nobody's accusing you of not knowing how to pounce on the clutch pedal, pick a gear, let off and push on the gas to get things moving, but, chances are, how exactly the clutch assembly gets all of this done isn't as sorted out for you. Don't feel bad. Your car's clutch will never be as captivating as some carbon-fiber conversation piece nor safeguard your masculinity like an oversized turbocharger will, which means you're not the only one who doesn't really care how all of it goes down.

But you should. It's the clutch that makes sure engine torque makes its way to the transmission where it's ultimately distributed to the wheels. A worn or inferior clutch can reduce the amount of power that's transmitted or, worse, can keep you from shifting gears altogether. That's because the clutch doesn't just do its part in transmitting torque from the crankshaft to the transmission's input shaft; it also interrupts everything to keep your transmission's insides from blowing to smithereens when changing gears.

How it all takes place isn't terribly complex: start the engine, engage the clutch by releasing the pedal and torque transfers from the crankshaft to the transmission. Disengage it by stepping on the pedal and the engine continues to spin but without transferring torque to the gearbox. Wait, what? That's right; your first mistake is assuming that the clutch engages when you stab the pedal. As it turns out, the opposite is true.

The Flywheel

It all begins with the flywheel, which is more important than you might think. A large steel, chromoly or aluminum disc, the flywheel doesn't just play intermediary between the clutch assembly and the crankshaft, allowing the two to interface with one another, it also assists with rotational balancing, dampens vibrations and engages with the starter for initial fire-up. Without one, all sorts of meaningful things won't happen.

The flywheel bolts directly to the crankshaft and features threaded holes around its circumference for fastening the clutch to. Because it's bolted to the crankshaft, the flywheel rotates at a 1:1 ratio. The heavier it is, the harder it is for the engine to rev up quickly, which makes a lightweight flywheel a popular entry-level mod. But lightening a flywheel too much is an easy mistake to make. The engine's rotating assembly relies on flywheel mass to store potential energy. The lighter it is, the less help it'll be at spinning everything around. Imagine your engine spinning in slow motion. In between cylinders firing, the crankshaft naturally wants to slow down because of internal friction. The flywheel's stored energy, or inertia, keeps that from happening. The more it weighs, the more effective it is at doing that.

How clutches work pressure plate 02 Photo 2/7   |  
The pressure plate bolts directly to the flywheel, which bolts directly to the crankshaft. The clutch disc is compressed in between the pressure plate’s and flywheel’s face’s until the clutch pedal is depressed, at which time the disc is released.

The Disc

The clutch assembly is made up of two major components: a disc and a pressure plate. The disc is fairly simple. Here, a friction-based material surrounds another steel disc on both sides, which is sandwiched in between the flywheel and the pressure plate. At the center of the disc is a splined opening that allows it to slip over the transmission's input shaft. The friction surface can be made up of all sorts of materials depending on the disc's intended use. OEM applications that provide smooth engagement often use a mixture of paper, cotton and pieces of copper or brass wire melded into a resin mixture that surrounds the disc. High-performance discs that care more about grip and friction are typically made of ceramic and metallic materials. Such materials are more durable and allow the clutch assembly to hold more power than what the factory intended but don't feature the same sort of smooth engagement as an OEM disc. High-performance materials like these typically surround the disc in segments, commonly known as buttons or pucks, instead of the entire disc. Finally, a series of springs can be found surrounding the center of most discs that are designed for street use that help dampen engine vibrations and the initial shock of engagement. Often times these are eliminated from high-performance applications because of the likelihood of their failure.

Although most clutches are made up of a single disc, it isn't uncommon for high-performance clutches to include two or even three of them. Here, multiple smaller discs allow for increased surface area when compared to one single disc. Multiple-disc setups feature center plates between each disc that serve as extensions of the flywheel. Such configurations are popular since they're able to increase friction and therefore grip without increasing pressure, which can be detrimental to crankshaft bearings and thrust washers as well as hard on leg muscles.

How clutches work organic faced disc 03 Photo 3/7   |  
Organic-faced discs like these along with sprung hubs allow for smooth engagement without the chatter or shuddering that’s often associated with puck-style clutches. The plastic alignment tool shown, which is removed after installation, simulates the transmission’s input shaft for proper alignment during assembly.

The Pressure Plate

The pressure plate, which compresses the disc in between itself and the flywheel, is really just a spinning, spring-loaded clamp. With the clutch pedal at rest, the pressure plate pushes the disc against the flywheel, allowing the assembly to spin as a unit, thus transmitting torque from the crankshaft to the transmission's input shaft. Once the clutch pedal is depressed, the pressure plate releases, allowing itself and the flywheel to spin independent of the disc, which prevents torque from being transmitted from the engine to the gearbox. Generally made of cast iron or steel, the pressure plate uses a diaphragm-type spring to apply force against the disc once engaged, which is fastened to the assembly by a series of straps. High-performance pressure plates typically feature stiffer diaphragms that can withstand additional torque and stronger straps that are capable of higher pressures.

The Throwout Bearing

At the heart of the clutch assembly is something you care very little about: the throwout bearing. You really should, though, because without it, not a whole lot can happen. Connected to the clutch fork, which interfaces with the clutch pedal by means of a cable or hydraulic actuator, the throwout bearing sits against the pressure plate's diaphragm, applying force against it when engaged. Once force is applied, the diaphragm releases tension, allowing the disc to spin independent of the assembly.

How clutches work throwout bearing 04 Photo 4/7   |  
The throwout bearing sits against the pressure plate in either a push-style or pull-style arrangement that compresses the diaphragm and releases the disc when the clutch pedal is depressed.

The throwout bearing is often the wrongfully accused culprit to curious drivetrain noises. For example, rattling or grinding sounds at idle that are likely coming from the transmission are almost always misappropriated to the throwout bearing. Understand how the throwout bearing works, and it's easy to see how it's almost never to blame. To be sure, the throwout bearing does very little and doesn't spin until called upon, when the clutch pedal is depressed and it's moved toward the rotating pressure plate.

How clutches work fulcrum arm 05 Photo 5/7   |  
A fulcrum arm that’s controlled via the pedal assembly by either a cable or hydraulics pushes or pulls the throwout bearing along the input shaft, toward or away from the pressure plate.

When Bad Things Happen

Diagnosing a slipping clutch isn't hard. Start by rapidly accelerating in first gear, then shift into second gear as you normally would. If engine speed rises or doesn't drop after the clutch pedal is released, then you can blame the clutch. A transmission that's difficult or impossible to select gears in can also be traced back to a faulty clutch.

How clutches work multi disc clutch 06 Photo 6/7   |  
Multi-disc clutches allow for increased clamping force by means of multiple, smaller discs instead of a single large one. The result is more surface area, which means better performance.

Screeching, grinding and chirping noises are a whole other set of problems, almost all of which are often wrongfully blamed on the clutch. Start by diagnosing any drivetrain sounds with the engine running, the transmission in neutral, and the clutch pedal released. Most grinding or growling noises here can be traced back to the transmission's input shaft bearing. For chirping sounds, slowly step on the clutch pedal. If the noise stops, blame the clutch fork for being improperly lubricated. If it becomes worse, then look to the throwout bearing for damage. Finally, most squealing sounds that are present at all times but change pitch whether or not the clutch is engaged or disengaged can be blamed on the flywheel's internal pilot bearing.

How clutches work exedy clutches 07 Photo 7/7   |  
Choosing the appropriate clutch can be confusing. At the very least, you’ve got to know how much torque your engine’s capable of producing before choosing anything.
By Aaron Bonk
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