Continuously Variable Transmission

Story by Costa Mouzouris// Photos by Costa Mouzouris
July 2 2024

An efficient drive system, but it lacks rider interaction with the machine.

Unless you ride a scooter, your bike doesn’t have a CVT. A CVT, or continuously variable transmission, changes ratios automatically, seamlessly, and infinitely. But before diving into the CVT, we must first understand why an engine needs a transmission in the first place.

There is a rev range between idle speed and redline, in which all engines produce an optimum amount of torque. Not enough revs and torque is too low; the engine will struggle to pick up revs and risks stalling. Rev too high and torque drops off again; the engine will spin needlessly while providing little in terms of acceleration. The different ratios in a transmission allow the engine to operate within this optimum range for any given road speed.

Low gear — high crankshaft revs compared to rear-wheel rotation — for example places the engine within its optimum operating range at low speed, and each gear ratio gets progressively lower (crank speed drops versus wheel speed), thus keeping the engine within that range as speed increases. The more ratios there are in a transmission, the more efficient it will be at matching engine speed to road speed. This is why many cars have gone from three-speed to 10-speed automatics. When it comes to efficiency, however, nothing beats a CVT.

A CVT has no gears, but rather, a drive and a driven pulley, each with moveable faces; between the two pulleys rides a V-belt. The drive pulley is connected to the engine, and its faces move toward each other as speed increases. When the drive pulley’s faces are spread apart, the V-belt rides near the centre of the pulley.

In contrast, the driven pulley’s faces spread apart with speed; when they are close together, the V-belt rides far from the pulley’s centre. This combination produces a high ratio, allowing the crankshaft to spin at high revs for a comparatively low road speed. As the speed increases, the drive pulley’s faces begin to come together, pushing the drive belt toward the outer edge of the pulley, while at the same time the driven pulley’s faces are spread apart by the tightening belt, which subsequently gets closer to that pulley’s centre. This changes the ratio between the drive and driven pulleys.

There is an infinite number of ratios between the lowest and highest ratios, which is why a CVT is so effective at keeping an engine within its optimum operating range at all speeds.

A simple mechanical system brings the drive pulley’s faces together. On the outside of one of the pulley faces there is a series of steel balls riding inside their respective ramps. The ramps are oriented radially from the centre toward the outer edge of the pulley face. As speed increases, centrifugal force (we’ll call it that; physics nerds please refrain from contesting its existence) draws the balls away from the centre and up their ramps, thus pushing the pulley face toward the belt.

Alternately, the driven pulley’s moveable face is spring-loaded, and it is pushed outward by the drive belt. This system is fully automatic; if you’re running at top speed with the pulleys in their lowest ratio (highest gear) and begin climbing a hill, you will begin to lose speed. This reduction in speed will lower the centrifugal effect on the drive pulley’s steel balls, causing them to move toward the centre, and spreading the pulley’s faces apart.

At the same time, the spring in the driven pulley will push its faces together and force the belt outward. Pick up speed when the road flattens out and the process reverses. The gear ratio automatically varies, thus keeping the engine within its operating range. The two pulleys work together, and a failure of either one (like a broken spring in the driven pulley) will cause the CVT to stop working.

There’s one thing missing: The clutch. All bikes have a clutch between the engine and the gearbox: it disconnects the engine from the transmission, and it is needed to get the bike going from a stop and to facilitate gear changes. A CVT also has a clutch, but it, too, is automatic. Its centre is connected to the engine, either directly or through the belt, and it contains a series of spring-loaded friction shoes. These ride inside an outer drum that connects to the drive pulley.

When the engine is idling, spring pressure retracts the shoes and they do not contact with the outer drum. Applying some throttle spins up the crank; centrifugal force acts on the friction shoes, causing them to overcome the spring pressure and expand outward to contact the drum. This gradually connects the engine to the CVT, and you have forward motion. The shoes retract if the speed gets low enough.

While a CVT is efficient and inexpensive compared to other transmissions, it’s definitely not the most engaging to ride.

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