The case is see-through. Start the engine and drive away to watch the pulleys swap sizes, or slide the ratio yourself. Drag to turn it, scroll or pinch to zoom, and click a part to use it.
Try itPress Drive away. The engine holds its speed while the belt climbs the drive pulley and sinks into the driven one.
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Meet the parts
Thirteen parts, one belt
Here it is taken apart, with the case off. Tap any part of the picture or pick a name below. The engine end sits at the top left, the two pulleys and the belt in the middle, and the gears to the wheels underneath.
Hover or tap the picture. Orange rings mark every part.
Parts diagram
Follow the power
Seen from above with the case off: the engine is on the right, and the drive goes left through the converter and the planetary set, round the pulleys and back out through the gears to the wheels. The numbers follow that path.
How it works, step by step
Fourteen steps, one idea
The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.
Step 1 of 14
Engine speeds and wheel speeds
Demo · the ratio
Slide the belt
Push the drive pulley’s sliding sheave in and the belt climbs to a bigger circle. The belt can’t change length, so on the other pulley it sinks deeper, all by itself.
The ratio is the driven radius divided by the drive radius: how many times the engine turns for one turn of the output. Set an engine speed, then slide from low to overdrive and see what reaches the wheels.
Low, 2.4 : 1. Small drive circle, big driven circle. The driven pulley gets 2.4 times the engine’s torque, but turns slowly.
Overdrive, 0.42 : 1. The output turns 2.4 times faster than the engine, for quiet cruising.
2.40 : 1ratio: engine turns per output turn
30 / 71 mmbelt circle, drive / driven
830output shaft, rpm
18car speed, km/h
Demo · gears against a CVT
Same car, two gearboxes
Two identical cars pull away side by side to 100 km/h. One has a five-speed automatic, the other a CVT. Watch the revs: the gearbox saws up and down, while the CVT holds the engine where it works best.
–gearbox: 0 to 100 km/h, s
–CVT: 0 to 100 km/h, s
–gearbox: fuel used, mL
–CVT: fuel used, mL
Demo · clamping force
Squeeze it or it slips
Steel on steel, with oil in between: the belt only carries the power by friction, so the sheaves must squeeze it very hard.
The needed squeeze grows with the engine’s torque and with a smaller belt circle on the drive pulley, so low ratio is the hardest case. Too little and the belt slips. Too much and the oil pump burns fuel making pressure nobody needs.
Oil pressure behind the driven sheave does most of the clamping.
The spring keeps a grip before the pump has built any pressure.
–clamp force needed, kN
–clamp force applied, kN
–safety margin
–oil pump power, W
More to know
Kinds, history and care
Six ways to make a ratio without steps
Push beltThe one on this page: steel blocks on steel rings, pushed between two variable pulleys. Used in most CVT cars.
ChainA chain of steel links replaces the belt. Pins through the links rub on the sheaves and pull the power round. Used by Audi and Subaru.
Rubber beltThe variator in scooters and snowmobiles. Spinning weights push the sheaves together as the engine speeds up, with no oil pressure or computer.
ToroidalRollers sit between two doughnut-shaped discs. Tilting the rollers moves the contact to a bigger or smaller circle on each disc.
HydrostaticAn engine-driven oil pump drives an oil motor. Changing how much oil the pump moves per turn changes the ratio. Common in tractors and diggers.
Power-split hybridA planetary gear set and two electric motors share the work. No belt at all, but the same stepless feel. Toyota’s hybrids work this way.
From a sketch to millions of cars
Leonardo da Vinci’s notebooks include a sketch of a stepless drive.
Milton Reeves builds a variable-speed pulley drive for sawmills. By 1896 he has fitted one to a car.
The DAF 600 arrives with Variomatic: rubber belts, spinning weights and engine vacuum move the pulleys.
Hub van Doorne founds Van Doorne’s Transmissie to build the steel push belt.
The Subaru Justy, Ford Fiesta and Fiat Uno go on sale with steel push belts.
The Toyota Prius brings the power-split hybrid, an electric CVT.
Nissan’s toroidal Extroid and Audi’s chain-driven multitronic arrive.
CVTs are common in small cars, hybrids, scooters and snowmobiles.
Keeping one happy
Use only the fluid the maker specifies. The belt grips through a thin film of special CVT fluid, and the wrong oil can let it slip.
Change the fluid on schedule, and sooner if you tow or drive in heavy traffic. Old fluid loses the additives that help the belt grip.
Don’t hold the car on a hill with the accelerator. Use the brake: slipping heats the fluid.
A whine that grows with speed, a shudder as you pull away, or revs that flare without more speed are early warnings. Have it checked before the belt or the sheaves wear.
Stay within the towing limit. A heavy trailer at full throttle heats both the fluid and the belt.
Come to a complete stop before you select reverse or park.
Things worth knowing
“It sounds stuck”The revs stay put while the speed climbs, which feels odd to drivers used to gears. Some makers now program fake shift points into their CVTs.
A belt that pushesAlmost every belt pulls. In this one the steel blocks push each other along, and the rings just keep them in a loop.
Thinner than paperEach pack has 12 steel rings, each about 0.18 mm thick: about as thick as two sheets of paper.
Squeezed with tonnesAt full torque in low ratio, the sheaves squeeze the belt with about 30 kN, the weight of three small cars.
Reverse racesDAFs with Variomatic could go as fast backwards as forwards, and Dutch drivers held races in reverse.
Too good for F1In 1993 Williams tested a CVT that held its engine at peak power on every straight. The rules soon demanded stepped gears.
Every Mechanism
