Continuous Variable Transmission (CVT) Working Principle


 We already know that the job of the transmission is to change the speed ratio between the engine and the wheels of an automobile. The transmission use two gears with different diameters. So that the rotation from the crankshaft, can be change to make it has more torque with lower speed.

But how about automatic transmission ? did this thing using several gears or not ?

In this article, we will learn about CVT transmission. Which is this transmission, widely use in automatic motorcycles.

Basic of Automatic Transmission

Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts.

To change the speed ratio, CVT use variable diameter pulley. This pulley has ability to change the diameter, according to the engine speed.

To make it easier, I will dissemble every part of continuous variable transmission on motorcycles.

You can see, this component called drive pulley. While the other one, is driven pulley. Drive pulley is installed on the crankshaft section. So that, this pulley will rotate when the engine run, and the speed is influenced by crankshaft rotational speed.

While the driven pulley, located In the back. Attached with the wheel, and the speed is same as wheel rotation.

Both of the pulley, connected by a V belt.

As I tell before, that this transmission using variable diameter pulley. It refer to drive pulley, and driven pulley. Both of them has ability to change their diameter, but in opposite way.

It mean, when drive pulley increase the diameter, the diameter of driven pulley will shrink.

The basic of CVT to change the speed ratio

The basic is very simple. In low RPM, the drive pulley always on the smallest diameter. While the driven pulley, in maximum diameter. In this position, the drive pulley has smaller diameter than the driven pulley.

So, it will decrease the output speed. And the torque, is increase.

When we pull the throttle, engine RPM raise. The faster the engine RPM, the bigger the drive pulley’s diameter. So, this RPM increase will rise the diameter of drive pulley. When the drive pulley’s diameter is raising, the driven pulley will decrease it’s diameter automatically.

It make, both of the pulley have same diameter. So the engine speed is continued to the wheel in same speed.

In high RPM, the drive pulley’s diameter keep increase. It make the drive pulley has bigger diameter than the driven pulley. So the wheel rotation, will be faster than the engine RPM.

But how the drive pulley change the diameter ?

The drive pulley, use centrifugal force to change the diameter. To be more understand, let’s dissemble the drive pulley.

The drive pulley consist of three main part. The frame. This is a static component, that directly connected to the engine crankshaft. The slide plate. This is a moving component, that has function to make the diameter bigger, or shrink.

While this cylinder components, called roller. It’s like a ballast to control the movement of slider part. The roller are installed around the shaft. When it rotate, the centrifugal force will rise. Centrifugal force is an outflow force from the rotational movement.

In simple way, the centrifugal force will make the roller stay away from the axis. But, due to roller have crooked path, the roller movement will push the slider plate.

Each plate is made of two 20 degree cones facing each other. A belt rides in the groove between the two cones. When the two cones of the plate are far apart, the belt rides lower in the groove. and the radius of the belt loop going around the pulley, gets smaller. When the cones are close together, the belt rides higher in the groove. and the radius of the belt loop going around the pulley gets larger.

To keep the belt tight, the rear pulley will adapt the front pulley diameter. This pulley, has a spring that always push the plate in close position.

Another main component on automatic transmission, is centrifugal clutch.

This clutch will make the wheel stay quite, while the engine run on idle RPM. This clutch located in the driven pulley, at the wheel axis.

It use a series of component like a brake shoe. This shoe, connected to the driven pulley. In idle RPM, we can see the gap between the shoe and the drum where the drum connected to the wheel. But if the speed increase, the shoes will move outway. It will lose the gap, so the wheel will rotate according to the pulley rotational speed.

That’s how CVT works. It is very simple right ? what do you think ? type in comment below.