The traditional manual transmission may be declining in popularity, but for the great majority of enthusiasts attending track days, road races and autocrosses, the old-school stick-and-clutch setup is still the weapon of choice. We may see a day when the desire for true manual transmissions in new cars is fueled more by nostalgia than performance numbers, but they aren’t going …
How Does Clutch Magic Happen?
For your car to move, there needs to be a mechanical connection between the engine and the drive wheels. Sounds simple, right? But for your car to not move–which is something cars do occasionally–that connection needs to be momentarily severed. In manual transmission cars, that decoupling is handled by the clutch assembly.
In a typical single-disc clutch (we’ll deal with multi-plate clutches in another part of this section) there are two main components: The pressure plate assembly is fixed to the flywheel of the engine, and the friction disc is attached to the input shaft of the transmission.
The friction disc rides on a splined input shaft and is allowed to “float” freely along that shaft. Its travel is constrained by the flywheel on one side and the pressure plate on the other side. When the engine is turning, the flywheel and the pressure plate assembly spin together. When the clutch is fully engaged, the pressure plate presses the friction disc against the flywheel, transferring torque from both the flywheel and the attached pressure plate assembly to the clutch friction disc. This disc is attached to the transmission input shaft, so torque can travel into the transmission and down the line.
Whether the throw-out bearing is actuated by a cable or hydraulic line, it does the same basic job: It actuates the diaphragm spring, which lifts the pressure plate from the floating friction disc and disengages the clutch. This facilitates both starting the car from a standstill and shifting gears.
When the clutch pedal is depressed to disengage the clutch, the pressure plate lifts off the friction disc, and because the friction disc is allowed to float along the input shaft, neither the flywheel nor the pressure plate assembly can adequately transfer any torque. The pressure plate is lifted from the disc via a diaphragm spring attached to the pressure plate.
A diaphragm spring is a disc-shaped spring that lifts its outer circumference when the inner part is depressed. That oversimplifies the importance of the diaphragm spring–lots of clutch operation and feel is dependent on the characteristics of this spring–but that’s the basic mode of operation, regardless of what kind of clutch you’re talking about. But there are lots of variables contained within that basic system. First is friction.
When a clutch is fully engaged, most of the torque is transferred via the clamping force of the mechanical components pressing against each other. But friction between the clutch disc and the flywheel and pressure plate is still a key factor–especially during the engagement and disengagement phases of operation.
Too much friction and the clutch can be “grabby,” making smooth starts difficult. Too little friction and the clutch may not be able to transfer all of the engine’s torque to the transmission–even at full clamp load.
Since we mentioned friction, you know that friction’s old pal heat is right around the corner. Heat alters the friction characteristics of any system. Anyone who’s ever hit the brakes for Turn 10 at Road Atlanta and gotten, “Doo, doo, dooooooo. We’re sorry, your braking could not be completed as requested. Please hang up and try your braking again later if you don’t die,” knows that heat affects friction. We’ll talk about those ramifications in a minute.
The next variable is driveline lash. When starting from a dead stop, the clutch must be engaged progressively and allowed to slip to get the car moving until the speed of the input shaft matches the speed of the engine. The engine can only slow down so much without shutting off, so that speed imbalance must be taken up through either clutch slip or, if you’re trying to impress your posse, tire slip.
The drawbacks of constantly spinning the tires to get the car moving are pretty clear to anyone older than 17 or so, so that clutch needs to slip and progressively transfer that torque until speeds match and it can be fully engaged.
Here’s the actual hardware depicted in the above diagram. The friction disc, located between the flywheel and pressure plate, forms the link between the engine’s torque and thrust at the wheels.
However, friction slip is not really a linear process. Typically, force builds between two surfaces to a point where the force overcomes friction in a fairly sudden fashion. Even the most sophisticated materials will experience this slip-grab-slip-grab cycle, which manifests as what we commonly know as clutch chatter.
This is where those springs on the clutch disc come in. Most clutch discs are actually multi-part affairs, with a friction disc and a separate hub connected by springs. These springs compress before fully transferring torque through the disc, providing a bit of damping for any surface friction irregularities during the engagement phase. Additionally, friction materials or rubber or other damping materials can be placed between the hub and the disc to further alter the effects of variable friction on the engagement phases.
So, Why an Upgrade?
Now let’s look at why you may want to upgrade that system and where to focus those upgrades.
Heat Management: You may have noticed the word “friction” popping up in the previous descriptions a lot. Whenever a clutch is disengaged and then reengaged, friction is produced as the clutch components take up those mismatched speeds. Obviously, those friction events will be more intense and more frequent during a track lap that requires 10 high-rpm shifts than during a gentle cruise down the highway.
So, a high-performance clutch is designed with heat management and resistance as prime characteristics. Usually this manifests as different friction materials on the disc–metallic and ceramic compounds instead of organic materials common in OEM clutches–but more thermal-resistant materials can even be used on the pressure plate assembly as well.
The tradeoff is typically a more aggressive friction characteristic during engagement. Metallic and ceramic compounds have a higher coefficient of friction than organic materials.
More Friction: But that previously mentioned tradeoff with metallic and ceramic compounds can be an upgrade of its own. More friction means more and quicker torque transfer at lighter clamping forces relative to organic friction materials. More friction can also mean less drivability, but modern clutch designers have done amazing work to make high-performance materials and designs extremely livable.
More Clamping Force: Clamping force is important to the total amount of torque a clutch can transfer, so as engine output rises, clamping force may need to rise as well. The clamping force of a clutch is dictated by the strength of the aforementioned diaphragm spring, and although some trickery can be done with arm lengths and leverages, more clamping force typically means that more force is going to be required to actuate that spring.
That translates to higher required pedal pressures. Again, there are ways to mitigate these effects: through tuning of finger lengths and fulcrum points on the diaphragm spring; through the hydraulic actuation system of the clutch; or even through the rod or cable systems that actuate the clutch on some cars.
Bottom line: An increase in clamping force does not always mean a fully corresponding increase in pedal effort. In fact, a dramatic increase in clamping force is usually available with only a small increase in pedal effort.
Several individual components make up the clutch disc: The hub and friction plate are joined by springs damped by pliable material that masks small imperfections in engagement.
Less Weight and Polar Moment: You may have noticed that the heaviest part of the clutch–the pressure plate assembly–is attached to the engine. That means whenever you want the engine to accelerate, it has to spin up the mass of that clutch assembly as well.
Reducing the mass of the clutch assembly reduces the mass that the engine has to accelerate, along with the overall mass of the car. Reducing the overall diameter of the clutch reduces the leverage that its mass has, reducing the force the engine needs to use to accelerate it.
High-performance clutches typically reduce weight through the use of lighter materials, like aluminum or even titanium. Properly using these materials–which can be costly themselves–may require more expensive manufacturing techniques, pushing up the price as weight decreases.
Puck Clutches: What about puck-style clutches? All of the functional concepts that we’ve discussed about traditional full-disc clutches also apply to puck-style clutches, which replace the full circle of material with multiple pucks of friction material attached to a star-shaped disc.
Advantages include, obviously, less rotating mass, but the reduced friction area means clamping loads must increase, so friction coefficients must be higher. Because of this, puck-style clutches are typically only for competition applications where shifts happen quickly and drivability is not a concern.
