How does a clutch work?
An engine cannot stop spinning, but your car has to stand still. Take a clutch apart in 3D, watch a dished steel spring clamp and let go, then find the bite point and try not to stall.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A sandwich that spins. A clutch is a friction plate squeezed between the engine's flywheel and a pressure plate. The flywheel, pressure plate, spring and cover all turn with the engine. Only the plate in the middle is joined to the gearbox, on a splined shaft it can slide along.
- The spring clamps, your foot lets go. A dished steel diaphragm spring squeezes the plates with about 3.5 kN all the time. Pressing the pedal pushes the spring's fingers in. The spring tips on its fulcrum rings like a see-saw, its rim lifts away, and the plate floats free with less than a millimetre of gap each side.
- Friction sets the limit. The most twist a clutch can pass is faces × friction × squeeze × radius: 2 × 0.3 × 3,470 N × 0.0835 m, about 174 N·m here. Ask for more, or let oil or wear cut the grip, and it slips. The slip turns engine work into heat.
- The bite point. To pull away, the plates must slip for a second or two while the car catches up with the engine. Let the pedal up too fast and the standing car drags the engine below about 300 rpm: a stall. Hold a car on a hill with the clutch and every kilowatt becomes heat.
- Springs that hide the engine's jerks. A four-cylinder engine fires twice a turn, so its flywheel speeds up and slows down about 27 times a second at idle. Coil springs in the plate's hub soften that. A dual-mass flywheel splits the flywheel in two on soft springs and keeps nearly all of it out of the gearbox.
- Other ways to do it. Motorcycles stack several small plates in oil. Scooters use weighted shoes that fly out and grip by themselves. Gearboxes lock gears with toothed dog clutches. A car's air-conditioning compressor is switched by an electromagnet.
| Term | Meaning |
|---|---|
| Friction plate | The disc between flywheel and pressure plate, with a ring of friction facing on each side. It sits on the gearbox input shaft. |
| Diaphragm spring | A dished disc of spring steel with fingers. Its rim clamps the pressure plate, and its fingers are the levers that release it. |
| Clamp load | The force squeezing the friction plate, about 3.5 kN in a small car. |
| Torque capacity | The most twist a clutch can pass before slipping: rubbing faces × coefficient of friction × clamp load × mean radius. |
| Bite point | The pedal position where the plates pass just enough twist to start the car moving. |
| Slip | The engine side turning faster than the gearbox side. Heat made each second equals the twist passed times the slip speed. |
| Release bearing | A bearing that lets the still release fork push on the spinning spring fingers. |
| Judder | A shudder on pulling away, caused by glazed, oily or warped plates grabbing unevenly. |
| Dual-mass flywheel | A flywheel made in two halves joined by soft springs, so the engine's firing pulses do not reach the gearbox. |
About 160 years from sliding teeth on a mill shaft to a computer that works the clutch for you.
- 1891 · Engine, clutch, gearbox: the layout is set (Émile Levassor, Panhard et Levassor, Paris, France)
- 1905 · Hele-Shaw's many plates (Henry Selby Hele-Shaw, England)
- 1910 · Linings that can take the heat (Herbert Frood, Chapel-en-le-Frith, Derbyshire, England)
- 1910 · One flat plate (George Borg and Borg & Beck, Moline, Illinois, USA)
- 1938 · One dished spring replaces many (George Hunt and General Motors, Detroit, Michigan, USA)
- 1958 · The Super Cub frees the left hand (Soichiro Honda and Takeo Fujisawa, Japan)
- 1985 · The flywheel splits in two (LuK and BMW, Bühl, Germany)
- 1999 · Asbestos is banned (United Kingdom and European Union, Europe)
The full story, with 29 moments, charts, people and 45 sources: glassbox.how/e/clutchclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
clutchclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/clutchclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
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Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
js/physics.js |
The sums, with no 3D in them: the diaphragm spring's load curve, the pedal linkage, torque capacity, the launch simulator, torsional vibration and the other clutches |
js/clutch.js |
The 3D clutch built to real sizes, the fork, pedal, cable and hydraulics, the cut-away, chart boards and readouts |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |