Continuously Variable Transmission

What a CVT actually is

Every conventional gearbox — manual, dual-clutch, or planetary automatic — works by selecting one of a small number of fixed gear pairs. Each pair is a discrete ratio; between them there is nothing. A continuously variable transmission throws that idea out. Instead of a set of gears it has two variable pulleys, each made from a pair of cone-shaped halves called sheaves, and a single belt running between them. By sliding the sheaves of each pulley together or apart, the belt is forced to ride at a larger or smaller radius, and the gear ratio changes continuously through an unbroken range. There are no steps, no fixed ratios, and — in normal operation — no shift shock at all.

The core insight is geometric. The belt has a fixed length, so it can only wrap a fixed total amount of pulley circumference. If you squeeze the driving pulley's halves together, its cones push the belt outward to a larger effective radius. To keep the belt length constant, the belt must simultaneously sink to a smaller radius on the driven pulley, whose halves open up to let it drop in. The two radii are mechanically coupled: one goes up exactly as the other comes down. The ratio at any instant is just the ratio of those two radii.

The mechanism: sheaves, belt, and clamping pressure

A modern automotive CVT has three working parts plus a launch device:

• Primary (driving) pulley. Bolted to the engine side through a torque converter or wet clutch. One sheave is fixed; the other slides axially under hydraulic pressure. Moving it inward raises the belt's radius here.
• Secondary (driven) pulley. Connected to the final drive and wheels. Again one fixed sheave, one movable. It carries a spring and hydraulic pressure that maintain clamping force on the belt so it cannot slip.
• The belt. Not a rubber V-belt but a steel push-belt: several hundred trapezoidal steel elements threaded onto two stacked bands of thin steel rings. Uniquely, it transmits power by compression — the elements push each other around the arc — rather than by tension like an ordinary belt. This is the Van Doorne push-belt, the enabling invention that made high-torque CVTs possible.
• Launch device. Because the belt cannot transmit torque from a standstill at infinite ratio, a torque converter (Jatco, Subaru) or a wet multi-plate clutch (Honda) couples the engine to the primary pulley once moving.

The whole behaviour hinges on clamping pressure. The sheaves must squeeze the belt hard enough that it never slides on the cone faces under the transmitted torque, yet not so hard that friction losses balloon. Getting that pressure exactly right — high enough to prevent slip, low enough to stay efficient — is the central control problem of every CVT, and the dominant cause of failure when it goes wrong.

The governing equation, with real numbers

The instantaneous ratio is simply the radii at which the belt rides:

i = R_driven / R_driving

Because the wrapped belt length L is constant, the two radii are not independent. Treating the centre distance C between the shafts as fixed, the wrapped-length constraint couples them: as one radius grows the other must shrink so that L stays the same. The pulley half-angle β (the angle of each cone face, typically 11°) sets the leverage between axial sheave motion and radial belt motion:

ΔR ≈ Δx / (2·tan β)      with β = 11°  →  ΔR ≈ 2.57 · Δx

So a small axial sheave displacement Δx produces a radius change ΔR about 2.6 times larger — the shallow cone is a mechanical amplifier. Take a representative passenger-car unit with each pulley's effective radius ranging from 30 mm to 70 mm:

Low ratio (launch):    R_driving = 30 mm, R_driven = 70 mm
                       i = 70 / 30 = 2.33 : 1   (engine spins 2.33× the wheels)

High ratio (overdrive):R_driving = 70 mm, R_driven = 30 mm
                       i = 30 / 70 = 0.43 : 1   (overdrive — wheels outpace engine)

Ratio spread:          2.33 / 0.43 = 5.4 : 1   total coverage
Add final drive ~5.7:1 → overall launch ≈ 13:1, overall cruise ≈ 2.4:1

That ~5.4:1 belt spread is competitive with a six-speed automatic, but it is delivered as a continuum: the controller can sit on any ratio in between, not just six discrete ones. A typical primary clamping pressure runs 1 to 6 MPa, generating axial forces of 20 to 60 kN on the sheave — which is why CVTs need a dedicated high-pressure hydraulic pump and tightly controlled fluid.

Efficiency — the CVT's Achilles heel

The CVT's great virtue is keeping the engine efficient; its great vice is that the transmission itself is less efficient than a good geared box. Two losses dominate. First, the hydraulic pump must continuously supply the high clamping pressure that holds the belt against slip; that parasitic pumping power is always present, even at steady cruise. Second, the steel belt has unavoidable losses from elements sliding minutely against the rings and against the pulley faces. The net result: a belt CVT runs about 86–93 % efficient mechanically, against 92–97 % for a modern lock-up planetary automatic or a manual.

The reason CVTs still win on fuel economy is that the engine-side gain outweighs the transmission-side loss. An engine is only at its best brake-specific fuel consumption in a narrow island of its map; a stepped box can only park there in a couple of gears, while a CVT can hold it there continuously. The 5–10 % engine-efficiency gain comfortably beats the 3–5 % transmission-efficiency penalty in the compact, low-torque cars where CVTs are used. In high-torque or performance applications the maths flips, which is one reason you almost never see a CVT behind a large engine.

CVT versus a planetary automatic

Property	Belt CVT	Planetary automatic (8-speed)	Dual-clutch (DCT)
Number of ratios	Infinite (stepless)	8 discrete	6–8 discrete
Ratio spread	5.5:1 to 7:1	7:1 to 9:1	7:1 to 8:1
Mechanical efficiency	86–93 %	92–97 %	94–97 %
Shift feel	Stepless (can drone)	Smooth, distinct shifts	Crisp, fast shifts
Torque capacity	~250–400 Nm	up to ~1,000 Nm	up to ~600 Nm
Engine at peak efficiency	Always possible	Only in a few gears	Only in a few gears
Typical fuel economy	Best in class (compact)	Very good	Good
Cost & mass	Low, light	High, heavy	Medium
Typical use	Compacts, hybrids, scooters	Mid/large cars, trucks	Sport & performance cars

The summary: a CVT trades a few points of transmission efficiency and a torque ceiling for the ability to keep the engine permanently on its best operating line, at lower cost and weight. That trade pays off below about 400 Nm and loses badly above it.

Variants — not just belt-and-pulley

• Steel push-belt CVT (Van Doorne / Bosch). The dominant automotive type. Hundreds of trapezoidal steel elements on two ring stacks transmit torque by pushing. Used by Jatco (Nissan), Subaru Lineartronic, and most compact-car CVTs. Torque-limited to ~250–400 Nm.
• Chain CVT (LuK / Audi multitronic). A steel link chain replaces the push-belt. The chain contacts the pulleys on the ends of its pins, giving line contact and higher torque capacity (~400+ Nm), at the cost of more noise. Audi used it on longitudinal-engine cars.
• Toroidal (traction-drive) CVT. Two doughnut discs with tilting rollers between them; tilting the rollers changes the contact radii. Nissan's Extroid (2000) was the only mass-production unit; handled ~370 Nm via elastohydrodynamic traction fluid, but heavy and costly.
• Hydrostatic CVT. A variable-displacement hydraulic pump drives a hydraulic motor; varying pump displacement varies the ratio steplessly. Standard on lawn tractors, combine harvesters, and skid-steer loaders, where infinite low-end ratio and reversibility matter more than efficiency.
• Power-split / e-CVT. The Toyota Prius drivetrain — no belt or pulley at all. A planetary gearset blends engine and two motor-generators so that varying motor speed varies the effective ratio continuously. Mechanically robust and high-torque, but technically an epicyclic device, not a friction CVT.
• Bicycle / scooter CVT. Scooters use a centrifugal-weight primary pulley and a torque-sensing secondary; the NuVinci/Enviolo hub uses tilting balls for a stepless ratio inside the rear wheel.

Where CVTs actually show up

• Compact and economy cars. Nissan (Jatco JF015E/JF016E), Subaru Lineartronic, Honda, and Toyota economy lines are overwhelmingly CVT. The fuel-economy advantage is the whole reason.
• Hybrids. The Toyota/Lexus e-CVT power-split is the most-produced "CVT" on earth, in tens of millions of Prius, Corolla, and RAV4 hybrids.
• Scooters and small motorcycles. Nearly every twist-and-go scooter uses a centrifugal rubber-belt CVT — the simplest mass-market CVT there is.
• Snowmobiles and ATVs. Polaris, Ski-Doo, and side-by-sides use rubber-belt CVTs with centrifugal primary clutches for instant, clutchless launch.
• Agricultural and construction machinery. Hydrostatic CVTs in tractors and combines give infinitely variable ground speed, including creep and reverse, independent of engine rpm.
• Wind-turbine and industrial speed control. Traction-drive CVTs are used experimentally to hold a generator at synchronous speed despite varying rotor speed.

Failure modes — where CVTs actually break

• Belt slip from lost clamping pressure. The number-one killer. A worn pump, sticking valve body, or degraded fluid lets clamp pressure fall; the belt micro-slips, glazes and scores the pulley faces, and within minutes generates enough heat and steel debris to destroy itself. Once the cone faces are scored, the whole unit is scrap.
• Wrong or worn fluid. CVT fluid is friction-tuned for the specific clamping system; using generic ATF drops the friction coefficient and causes slip even when the hardware is healthy. Fluid also degrades thermally — overheated fluid loses its anti-slip additives.
• Pulley-face pitting (contact fatigue). The belt elements ride on a thin, highly stressed contact patch on each cone. Over hundreds of thousands of cycles the surface pits, raising vibration and eventually shedding metal.
• Push-belt element cracking. Each of the several hundred steel elements is loaded in compression every revolution; a fatigue crack in one element can cascade.
• Overheating under sustained load. Towing, long hill climbs, or stop-start traffic on a hot day overheats the fluid and bleeds clamping pressure. Most CVTs are torque- and temperature-limited and many add a dedicated fluid cooler; manufacturers warn against flat-towing CVT cars because the pump is not running to keep the belt clamped.
• Launch-device wear. The torque converter or wet clutch that gets the car moving sees the highest slip and heat; its failure mimics belt failure but is cheaper to fix.

Common pitfalls when specifying or living with a CVT

• Exceeding the torque ceiling. A steel push-belt CVT behind a 500 Nm engine will slip and fail. Match the unit's rated torque to the engine, with margin for transient overshoot.
• Treating it like an automatic for maintenance. CVT fluid and its change interval are not interchangeable with ATF. Skipping fluid service is the fastest way to kill an otherwise-healthy unit.
• Ignoring thermal limits. If the application tows or climbs, add a cooler and respect the temperature-limp logic; sustained high-clamp, high-slip operation is exactly the regime that destroys belts.
• Misreading the drone as a fault. The constant-rpm "rubber-band" feel under full throttle is the CVT doing its job — holding the power peak. It is not a slipping transmission, though real slip feels similar, so diagnose with fluid pressure and debris checks, not feel alone.
• Forgetting the launch device. A CVT cannot start a vehicle from rest on its own; the torque converter or clutch is integral, and its wear is often misattributed to the belt.