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Transmission & Clutch

The Clutch

How a friction clutch connects and disconnects the engine and gearbox: a disc clamped between the flywheel and a spring-loaded pressure plate, released by the pedal. Covers the bite point, controlled slip and why slipping makes heat.

Beginner12 minute readInteractive 3DFull lesson

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After this lessonThe Manual Gearbox

The idea

Beginner explanation — switch to Technical for the engineering detail.

An engine has to keep turning even when the car is standing still, and it cannot start from zero under load. The clutch solves both problems: it is a switch that connects the spinning engine to the gearbox, or disconnects it, and it can also connect it gradually.

It works by friction, a bit like pressing your hand on a spinning record to make it turn. Bolted to the end of the engine's crankshaft is the flywheel, a heavy steel disc with a flat face. In front of it sits the clutch disc, a thin plate covered on both sides with tough friction material, which is splined onto the gearbox input shaft. Behind the disc is the pressure plate, held in a cover that is bolted to the flywheel. A powerful, dish-shaped diaphragm spring pushes the pressure plate forward, squeezing the clutch disc tightly between the plate and the flywheel.

With your foot off the pedal, that clamp is so strong that the disc turns exactly with the engine, and the engine drives the gearbox. When you push the pedal down, a release bearing presses on the middle of the diaphragm spring. The spring flexes like the base of an oil can, its outer edge pulls the pressure plate back, and the disc is freed. The engine keeps turning but the gearbox no longer has to.

As you lift the pedal again, there is a point where the plate just starts to grip the disc: the bite point. Between the bite point and fully up, the clutch slips. The disc is dragged round, but more slowly than the flywheel. That gentle slip is exactly what lets a car pull away smoothly from rest. Slip also rubs the friction surfaces, which turns energy into heat and slowly wears the disc, so it should last only as long as it needs to.

What you will learn

  • Name the parts of a single-plate dry clutch and say which turn with the engine and which with the gearbox
  • Explain how the diaphragm spring clamps the disc and how pressing the pedal releases it
  • Describe what happens at the bite point and while the clutch is slipping
  • Explain why slipping the clutch makes heat and wears the friction facings
  • Relate a clutch's torque capacity to clamp force, friction and radius

Try it yourself

Select gears and see how ratio trades rotational speed for torque between input and output shafts. Open the full gearbox lab

1st gear
Ratio
3.70 : 1
Input
2,000 rpm
Output
540 rpm
Torque
× 3.55
Road
≈ 15 km/h

illustrative · 100× slower

1st gear, ratio 3.70 : 1, clutch engaged (locked). Engine 2,000 rpm, output 540 rpm, torque × 3.55, road speed ≈ 15 km/h.

Gear
Engaged
2,000 rpm

Step by step

Use “Show in 3D” to jump the animation to a step; the step currently playing is highlighted.

  1. Step 1: Engaged: clamped by the diaphragm spring

    Now playing in the 3D scene

    Foot off the pedal. The diaphragm spring pushes the pressure plate forward, squeezing the clutch disc against the flywheel. The disc turns exactly with the engine, so all the engine's torque goes into the gearbox.

  2. Step 2: Pedal down: the clamp released

    Push the pedal to the floor. The release bearing presses the middle of the diaphragm spring, the spring's outer edge pulls the pressure plate back, and the disc floats free between the faces. The engine keeps spinning, but the gearbox is disconnected, so you can change gear or stop without stalling.

  3. Step 3: The bite point

    Lifting the pedal slowly, the plate starts to press the disc again. At the bite point the disc begins to be dragged round and the car starts to strain against its brakes. In the lab only about a seventh of the engine's torque is getting through here.

  4. Step 4: Controlled slip: pulling away

    Further up, the clutch grips harder but is still slipping. The disc turns slower than the flywheel, and the car accelerates smoothly as the gap closes. Once the speeds match, the clutch locks and slip stops.

Key components

Select a part to highlight it in the 3D scene.

How it flows

The Clutch: flow diagramClutch pedalRelease bearingDiaphragm springPressure plateFlywheelClutch discGearbox input shaft
Read the diagram as text
  • Clutch pedal → Release bearing (cable or hydraulics)
  • Release bearing → Diaphragm spring (presses)
  • Diaphragm spring → Pressure plate (lifts or clamps)
  • Pressure plate → Clutch disc (friction)
  • Flywheel → Clutch disc (friction)
  • Clutch disc → Gearbox input shaft (torque)
  • Clutch pedal: Driver's foot
  • Release bearing: Pushes the diaphragm fingers
  • Diaphragm spring: Pivots on its fulcrum
  • Pressure plate: Clamps or releases
  • Flywheel: Turns at engine speed
  • Clutch disc: Friction faces both sides
  • Gearbox input shaft: Splined to the disc

Go deeper

Optional. Open a panel for the engineering detail — or switch to Technical above.

The engineering explanation4

The common passenger-car clutch is a single-plate, dry, diaphragm-spring clutch. The flywheel and the pressure-plate cover are bolted together and always turn at engine speed; drive straps let the plate move axially by a millimetre or two while still turning with the cover. The clutch disc sits between them and is splined to the gearbox input shaft, whose nose runs in a pilot bearing in the crankshaft.

The torque the clutch can carry before it slips is T_c = μ · F · r_m · n, where μ is the friction coefficient of the facings (roughly 0.3–0.4), F the clamp load from the diaphragm spring, r_m the mean facing radius and n = 2 the number of friction faces. Designers give it a margin over the engine's peak torque (the lab uses 1.3×) so it does not slip in normal driving, while keeping pedal effort reasonable. A diaphragm spring helps here: its load–deflection curve falls after a peak, so the effort at the pedal drops as the clutch releases.

Pedal travel acts through a cable, or a master and slave cylinder, on the release bearing. The first part of the travel is free play, taking up the clearance to the diaphragm fingers. The fingers then lever the spring about its fulcrum ring in the cover, and the outer rim lifts the plate away. In the lab's illustrative characteristic the clamp load falls smoothly over the take-up, and because of the capacity margin the clutch stays locked until the pedal is about 30% down.

When the clutch slips, torque still passes (limited by its momentary capacity) but the disc turns slower than the flywheel. The power entering at engine speed is T·ω_engine, the power leaving at disc speed is T·ω_disc, and the difference, P_heat = T · (ω_engine − ω_disc), becomes heat in the facings, flywheel and plate. A hill start or launch with high slip speed can put several kilowatts into the clutch for a few seconds, which is why prolonged slipping glazes facings and why worn clutches start slipping under full load. Cushion springs under the facings smooth the take-up and torsional damper springs in the hub absorb engine vibration. All lab numbers are illustrative.

Key concepts5
Torque capacity
The most torque the clutch can carry before it slips: friction coefficient times clamp load times mean facing radius times the number of friction faces.
T_c = μ · F · r_m · n (n = 2)
Bite point
The pedal position where the plate first grips the disc and torque starts to flow to the gearbox.
Slip speed
How much faster the flywheel turns than the clutch disc while the clutch is slipping.
Δn = n_engine − n_disc
Slip heat
Power lost as heat while slipping: the torque being passed on times the slip speed.
P_heat = T · (ω_engine − ω_disc)
Free play
The first part of the pedal travel, which only closes the gap between the release bearing and the diaphragm fingers.
Common misconceptions4

  • MythA clutch is either fully on or fully off.

    RealityBetween the bite point and fully engaged it slips by a controlled amount. That partial engagement is exactly how a car pulls away smoothly.

  • MythHolding the car on the bite point on a hill does no harm.

    RealityThe clutch is slipping the whole time, turning engine power into heat and wearing the facings. Use the handbrake or footbrake to hold the car.

  • MythA slipping clutch is always a worn clutch.

    RealityEvery normal start involves slip. A worn clutch is one that slips when it should be locked, for example engine revs rising without the car speeding up in a high gear.

  • MythResting a foot on the clutch pedal is harmless.

    RealityEven light pressure can load the release bearing and reduce clamp load, wearing the bearing and letting the clutch slip under heavy load.

In the real world4
  • If the engine revs rise but the car barely speeds up when accelerating hard in a high gear, the clutch is probably slipping and the facings may be worn.
  • A clutch that only bites right at the top of the pedal travel often has worn facings or needs its free play adjusting.
  • On a hill start, hold the car on the handbrake, find the bite point, add a little engine speed, and release the brake as the car starts to pull, so the slip lasts only a second or two.
  • In the lab, try the Slipping and Bite point pedal positions in 1st gear: the live numbers show how much power is being turned into clutch heat.

Common questions

Short answers to the questions people most often ask about this topic.

What does a clutch do in a car?

It connects and disconnects the engine from the gearbox. With the pedal up, springs clamp a friction disc between the flywheel and a pressure plate so they turn together; pressing the pedal releases the clamp so you can change gear or stop without stalling.

What is the clutch biting point?

It is the pedal position where the friction disc starts to grip and transmit torque. Around this point the clutch slips deliberately, letting the gearbox side match the engine's speed smoothly as you pull away.

What are the signs of a slipping clutch?

Engine speed rises but the car does not accelerate to match, especially in high gears or uphill, sometimes with a burning smell. It usually means the friction lining is worn or contaminated, so the springs can no longer clamp it firmly enough.

Check your understanding

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0 of 5 answered

Question 1 of 5

With your foot off the pedal, what holds the clutch disc against the flywheel?

The story behind it

Who worked this out, and what they were up against. Optional.

1891

Émile Levassor

Car builder, Panhard et Levassor

Moved the engine to the front and put a clutch pedal and gear stick in the driver's reach.

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  • Go deeper (optional) — Optional: open a deeper panel or switch to Technical.
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After this lessonThe Manual GearboxBeginner: how a car moves · 0 of 9 done

Earlier in this route you skipped How a Car Works: The Big Picture, The Four-Stroke Cycle, Torque and Power.

Sources & attribution

Licence
AutoVerse original content

This lesson is original AutoVerse writing. The links above are suggestions for further reading, not the source of this text. Numbers shown in the 3D simulation are illustrative, not measured engineering data.