If you drive a car with a manual transmission, you know what to do to move the car: press the clutch, select a gear, and release the clutch while pressing down on the gas. Because the engine is constantly turning and your wheels are not, the clutch also allows you to disconnect the wheels from the engine without killing it. Pretty simple, right? But, what’s happening to your car when you do that? The clutch connects two rotating shafts so that they can either be locked together to spin at the same speed, or detached to spin at different speeds. These actions provide the two basic functions of your Mustang’s clutch:
To gradually apply engine power when the car is moving, and
To interrupt power to avoid gear crunching when shifting, or stalling out when stopping.
When you’re driving, this engages the clutch and allows power to transfer from the engine to the transmission and wheels. When the clutch is disengaged (while shifting), the power transfer stops, and the engine continues to turn without force to the wheels.
To fully understand how a clutch works, however, you need to know all of the parts of the clutch system, and how they work together.
There’s more to the clutch than just the disc, pressure plate and flywheel. There are actually seven different parts to the system that you should know about.
The Engine: Pretty basic – the engine generates the power needed to move your Mustang.
The Flywheel: Bolts to the engine crankshaft, and provides a friction surface for the clutch.
The Clutch Disc: A friction surface that transfers power from the engine to the transmission.
The Pressure Plate: Provides the clamping power needed to lock the clutch to the flywheel.
The Throwout Bearing: Sometimes called the Release Bearing, this part reduces the friction between the clutch fork and the springs in the pressure plate.
The Clutch Fork: Provides a lever action to engage or detach the clutch.
The Transmission: Provides a lot of selectable gear ratios, which allows the driver to match the power the engine puts out to changing driving conditions.
So, how do these components work together? Good question! Let’s look at each of these parts a little more closely.
You already know that the engine runs the car. Without it working well, your car won’t function. The backbone of the engine is the crankshaft, and it is the hardest working part of your engine. The crankshaft drives all belt-driven accessories, such as the water pump, the alternator, the A/C and the fan. But, its primary function is to change the give-and-return motion of the piston and rod into a circling motion that is transferred to the transmission, and drive wheels. As the fuel/air mixture in the cylinder burns, it forces the pistons down. Each piston is connected to the crankshaft by a connecting rod. As the piston goes down, the connecting rod causes the crankshaft to turn. This isn’t a big deal if your car is a daily driver that you never race. For highly modified cars putting out extreme amounts of power, this task can be hard on the engine.
The flywheel does a lot of things – it acts as a balancer for the engine, reduces vibrations caused by cylinders firing, and it provides a smooth surface friction surface for the clutch. But the flywheel’s main function is to transfer engine torque (the turning effort produced by the pressure from the crankshaft on the pistons) from the engine to the transmission. The flywheel connects the clutch and the driveline to the engine. One side is bolted directly to the crankshaft, and one side is bolted to the clutch assembly.
Sandwiched between the flywheel and the pressure plate, the clutch disc is covered with friction material on both sides. The center of the disc – called the “hub” – is splined to match the splines on the input shaft of the transmission. Each clutch disc comes with a set of springs, located in the hub, called the torsion damper system. These springs are designed to cushion the engagement by absorbing a portion of the impact when the disc is squeezed between the flywheel and the pressure plate. In street applications, the central hub is a separate part connected to the clutch with marcel cushion springs. This absorbs any engagement impact.
In racing applications, there is no central hub. Because there is no torsion damper and no marcel, you feel instant power.
When your foot is off the clutch pedal, the clutch is engaged, and the springs push the pressure plate against the clutch disc, which in turn presses against the flywheel. This locks the engine’s crank shaft to the transmission input shaft, and makes them spin at the same speed. When your foot depresses pedal, the clutch and pressure plate are pulled away from the flywheel, and no power is being transferred from the engine.
The Pressure Plate is, basically, a spring-loaded clamp that is bolted to the flywheel. As you can tell by the name, the pressure plate presses the clutch disc and allows for the transfer of power to the transmission.
When the clutch pedal is pressed down, this bearing – also called the Release Bearing – moves toward the flywheel. It pushes in against the pressure plate’s release fingers and moves them against the force of the plate’s springs. This action moves the pressure plate away from the clutch disc, interrupting the flow of power and reducing friction.
This piece provides a lever action to engage or disengage the clutch. It forces the throw-out bearing into the pressure plate.
The Transmission provides several selectable gear ratios, which allows the driver to match the engine output to a variety of driving conditions. Say you’re starting from a dead stop – as you accelerate, the engine needs to spin quickly to make the necessary power to move the car. Once you’ve reached the speed limit, your car needs less power to maintain speed. So, the transmission uses a high gear ratio (lots of power, but not much speed) during initial acceleration, when climbing hills. It uses a low ratio (lots of speed, but not much power) when you’re cruising.