CVT gearboxes

Two weeks ago, I wrote that I have driven a motor vehicle with 12 gears, and several readers challenged me to reveal what make it was.

They felt that this number was too large for an ordinary vehicle. Well, I have a surprise for them: these days there are many cars on the road with an infinite number of gears.

These are the vehicles with the so-called continuously variable transmission (CVT) gear boxes.

The principle is simple: instead of shifting gears in steps from gear 1 to 2 to 3 and so on, why not devise a system that can change the gearing ratio seamlessly?

Currently, there are three widely used designs, but one of them is the most common, and it borrows an idea from bicycles.

Geared bicycles have two sets of sprockets; one on the back wheel and the other on the pedal axle. The rear set usually has five or six sprockets of different sizes while the front one has two. This combination produces a total of ten or twelve gears.

When changing the gears on a bicycle, a lever pushes the chain from one sprocket to the next. Now even though the change from one gear ratio to the next feels quite smooth, it occurs in steps because of the diameters.

Now, what if we replaced the rear set of sprockets with a cone-shaped piece of metal? Wouldn’t that produce a continuously variable range of gearing ratios? Yes it would, but with a big challenge: there would be no teeth for the chain to grip on.

To overcome this problem of losing grip, CVT gearboxes employ a special kind of pulley, which has two cones whose smaller ends face each other. These are mounted on a common axle in a manner that allows the distance between them to be variable, meaning that the effective diameter can be changed.

Two sets of such variable pulleys are used: one at the input side of the gearbox, the other at the output. And instead of a chain, the two pulleys are linked with a tightly fitting metallic “V” belt.

When the car is starting to move, the cones at the input side are at their widest separation while those at the output side are at their closest.

Thus, the input pulley has a smaller diameter than the output one. This produces a high gear ratio; equivalent to gear one on an ordinary gearbox.

As the car gains momentum, a hydraulic mechanism gradually pushes the cones of the input pulley closer together, thereby increasing the diameter over which the belt is running.

At the same time, the cones of the output pulley are pushed apart to reduce the effective diameter.

The result of these actions is a reduction of the gear ratio. This reversal of diameters continues as the speed of the car increases. Eventually, the gearbox goes into overdrive where the output rotates faster than the input.

CVTs produce a very smooth ride and greater acceleration. In addition, the car’s engine runs steadily without the need for high revs during pick-up. This gives the vehicle excellent fuel economy. A CVT vehicle will outrun a similar automatic, and do that on less fuel.