If you’ve ever gotten into a car and wondered what that third pedal all the way to the left was for, chances are you shouldn’t have gotten into that car in the first place. It’s a clutch pedal, and it’s encouraged for use in shifting gears. You don't strictly need a clutch pedal for shifting a manual transmission, but if you don't use one you'll need to rev-match and shift only when the engine has reached a certain rotational speed. To do this successfully, without destroying your gears, you need to really know what you're doing, so we don't encourage this.
Before we understand what a clutch does, we must accept the simple nature of a car, in that all of its technology is aimed at the singular goal of getting the engine’s power to the ground. Converting heat into kinetic energy.
In a normal road going car, power is put through a transmission, to a differential which then takes the power and distributes it to the wheels. The transmission must receive this power somehow, and that is through a clutch. The clutch itself is a disc with springs and pads for creating friction, and is operated by a clutch pedal. In a rear-wheel drive platform, the power is transferred from the transmission to the differential via a drive-shaft.
When you look at a transmission from the outside, it looks like a cone that’s lying down. The larger end is flush against the side of the engine that holds the flywheel which is a large spinning disc that spins at the same speed as the engine, which is denoted by the tachometer, or the gauge reading “RPM” next to your speedometer. The clutch disc is inside of the larger end of the cone and parallel with the flywheel, and operated by the clutch pedal.
When you depress the clutch pedal, the clutch disc is distanced from the flywheel, thus allowing you to choose a gear, or go into neutral. When you release the clutch pedal while in gear, what’s called a throwout bearing pushes on a diaphragm, thereby moving the clutch towards the flywheel and making contact, creating friction thus taking the rotational inertia from the flywheel.
The clutch, since it is attached to the input shaft, transfers the power to the selected gear, which sends power through an output shaft, to a differential. This describes the clutch activity in the case of a manual transmission as equipped with a drive shaft. This setup is used in a front-engine mounted, RWD car like a Ford Mustang. In a FWD manual car, like a Honda Civic, the principle is the same, however the differential and transmission are combined into one unit called a transaxle.
That clutch needs to hold a lot of power and for a very long period of time, and the clutch that’s in your Toyota Corolla is only rated to handle a couple hundred horsepower at the most. If you tried to put that same clutch into a Corvette, the power would be so immense that the clutch disc and its ability to hold friction would be annihilated. For this reason, special performance clutches are made, where instead of a single disc clutch it’s actually multiple discs, or in some cases a stronger single disc.
Typical manual transmission cars come with single plate clutches, which is what was described earlier. You can buy aftermarket performance-oriented single plate clutches, which can run you a few thousands of dollars depending on how much power it’s rated for.
They basically work the same, except have beefier diaphragms and ceramic friction material as opposed to the more common organics found in stock clutches. These clutches come with a break-in period of approximately 500 miles before they can be driven aggressively. Until then the friction material isn’t bedded in properly and therefore won’t create as much friction.
As a decidedly more complex machine, a multi-plate clutch comes in a case, and has multiple clutch discs separated by friction plates, and all spin on a spline that runs through the transmission, or input shaft.
When the clutch is engaged, the clutch discs and friction plates all squeeze together, where the first clutch disc goes against the flywheel, and behind it a friction plate allows the second clutch disc to rotate with the first, and so on.
The reason for multi-plate clutches is it can hold a lot of torque. A clutch disc can hold a certain amount of torque, and if you increase the amount of clutches, you can increase the amount of torque that goes through the transmission.
You’d use a multi-plate clutch over a single plate performance clutch for racing applications, because you can decrease the size of the clutch discs, and therefore decrease the diameter of the clutch pack, allowing for a smaller transmission. Smaller transmissions are lighter and are more easily fitted thus allowing a more flexible weight distribution.
Single and multi-plated clutches are most common in the auto industry with manual-shifting transmissions. There are manual-shifting transmissions operated by paddle shifters, where it’s common that the clutch is operated by a magnetic field which engages or disengages the clutch. This eliminates the need for a clutch pedal, but a car with an h-pattern gearshift uses a single or multi-plated clutch.
Cars are equipped with a lot of technology for the purpose of getting power to the ground, and there are several points of contact that make this possible. The clutch is the gateway for the power to get through the transmission to the rear wheels, in a car with a transmission.
Every gas powered car with a transmission has a clutch of some kind, even automatics. If you’re thinking about upping the power on your manual transmission car, consider upgrading your clutch to one that can handle the increase.
Avid Formula 1 fan and motorcyclist, I enjoy chocolate chip cookie dough ice cream and long rides to the beach.