How a CVT Works

A six-step lesson on a 3D model. See how two pulleys and a steel belt change the ratio smoothly, what the belt is made of, why the oil needs such pressure and what wears out in a CVT.

6 steps
about 5 minutes
rotate the model

Full screen

Step 1

The CVT is a gearbox without steps

An ordinary gearbox has five to eight gears and an automatic shifts them. A CVT (continuously variable transmission) has no steps: the ratio changes smoothly from the lowest to the highest, so the engine can be kept at an efficient speed. It is surprisingly simple: two pulleys and a steel belt between them. The first pulley receives rotation from the engine, the second passes it to the wheels.

Step 2

The cones slide, the belt changes radius

Each pulley is two cones: one fixed and one that slides. When the halves move closer, the belt is pushed outward onto a large radius; when they move apart, it sinks deeper onto a small one. The belt is a closed loop and does not stretch, so when the radius grows on one pulley it shrinks on the other. The ratio of the radii is the gear ratio.

A CVT transmits less of the engine’s power than a manual: a belt CVT is about 88% efficient against roughly 97% for a manual gearbox, because the belt rubs on the pulleys.

Step 3

The belt: hundreds of steel plates on bands

A CVT belt is not rubber. It is hundreds of steel plates (from about 280 to 470 in different CVTs) threaded onto two stacks of thin steel bands, 9 to 12 bands in each. The plates work in compression and push each other from the drive pulley to the driven one. The bands work in tension and hold the plates in a ring. The side faces of the plates are tapered to the cone angle, so the belt is clamped in the groove.

Step 4

Oil pressure clamps the belt

Pressure too lowthe belt slips on the cones and overheats

Pressure too highthe belt and pulleys wear faster and fuel use rises

The moving halves are pushed by pressurised oil: each pulley has its own hydraulic cylinder with a piston. Pressure at the driven pulley clamps the belt, and pressure at the drive pulley changes the ratio. That is why a CVT has a powerful pump and electronics that constantly adjust the pressure.

Step 5

How a CVT pulls away and reverses

The belt always links the pulleys, so a CVT needs a unit that connects the engine to the wheels smoothly: in some CVTs a torque converter, in others a multi-plate clutch. The belt turns in one direction only, so for reversing a small planetary gearset with two clutches sits between the engine and the pulleys: one engages for driving forward, the other for reversing. The units differ from model to model.

Step 6

What wears out and how to make it last

OilIt both lubricates and transmits pressure. Specialists advise changing it more often than the schedule; for the Chery Tiggo 4 Pro the figure quoted is at least every 50,000 km (about 31,000 miles). That is an estimate, not a factory guarantee.
Shaft bushings and bearingsIn Chery CVT25 units, specialists estimate that wear more often starts after 100,000 km (about 62,000 miles).
Belt and pulleysThey dislike slipping and overheating: if you are stuck, do not keep the throttle down for minutes.

Nobody has exact CVT failure statistics by brand. The honest way to put it: a CVT is sensitive to oil, overheating and slipping.

What to read next

Repair guides for your carsections on the transmission How an automatic works3D breakdown How a manual transmission works3D breakdown How all-wheel drive works3D breakdown

We built the 3D model after a typical belt CVT. Sources: patent US 6,468,174 (Van Doorne’s Transmissie), Wikipedia “Continuously variable transmission”, Autozine “CVT”.