What is a CVT gearbox?

Just what is a CVT gearbox? Well, do not worry, because here at the TorqueSteer Workshop I will try to decipher and decode this mystery, and hopefully educate and entertain you at the same time.

Loads of vehicles comes with one of these new contraptions, the Toyota Rav4 and Mitshibishi ASX just to name a few, which means it is not something that started in super expensive, high end performance cars and filtered down to normal, everyday runabouts, and there is a reason for it, see if you can figure out why.

To start off with, there are many different types of CVT options available, all offering a different method to achieve the end goal, which is to get the car moving. It also has its own set of pros and cons, and each subcategory of these designs has its limitations.

 

What Is a CVT box?

The simplest, easiest way to try and explain this wizardry, is to simply say the following; A CVT (Continuously variable transmission) is a small, efficient gearbox always seeking to get the optimum “gear ratio” to provide for either economy, power, or speed. You could also say it is a single speed gearbox.

Source parkers.co.uk

How does it work.

This is where things get complicated, purely because there are so many different variations on this concept, but the main principle in all of them stays the same. There are two conical, or adjustable, cylinders over which a belt or chain runs. These cylinders face each other in an opposite direction

The front cylinder gets power from the engine (input shaft), and turns the pulley, which in turn turns a belt or chain. The rear cylinder (output shaft) is tasked with keeping the belt tight, and turning the wheels.

By being able to change the diameter of the cylindrical cone the belt runs on, the power delivered is effectively changed to the right gear needed for the job. What makes the CVT different from automatic or manual boxes, is the fact that there are no seat gears, meaning the engine can “always” be in the perfect “position” for power or economy.

CVT Diagram

In case you missed it in science class, here’s how the diameter part works. A Large Pulley in front and a small pulley at the back translates to speed (top gear). The small pulley (output shaft) has to turn more times to keep up with the larger pulley (input shaft) when rotating. If you would turn this around so that the small pulley is now in front (input shaft), and the large pulley (output shaft) is at the back, the larger pulley would have to move slower to stay with the small pulley, resulting in increased power, but decreased speed (first gear).

A simple example of a CVT in action is a scooter. You twist the accelerator and it goes, revving happily until you reach the maximum speed.

Pros And Cons.

Pros.

It is a cost effective way for manufacturers to get the power to the wheels. Being easy to construct, and having very little moving parts, means having to maintain it is also very low on the list.

It provides quicker acceleration than conventional gearboxes, simply because there is no fixed gear. As engine speeds differ, this system can compensate and get the most out of the engine without the engine itself working any harder.

Cons.

Though power, economy and speed is a plus, the CVT gearbox is only about 88% effective in delivering the power, which is less than conventional gearboxes.

Allowing the motor to rev at any speed, the noise coming from the front will sound a bit odd to the old school car enthusiast. It will sound like a slipping clutch.

When you put foot, an automatic will feel like it’s winding up, and then burst with power, while the CVT does this in a smooth and almost lazy fashion.

Now, I was wondering why this technology didn’t filter down from high end exotics like the DSG gearbox did, and the answer is simple, yet very disconcerting. It’s not effective in high powered vehicles.

Some History.

I also thought this was a rather new concept, but I was very wrong.

The CVT concept was first designed and explored in 1490 by none other than Leonardo Da Vinci himself. The first car to have this gearbox is the DAF (a dutch car maker in the ‘50s) but failed because of the limitations it had when having to deal with more than 100 horsepower.

The improvement of technology made these gearboxes capable of handling more power, with the spike in interest happening between the 90’ and 2000, with Subaru actually offering the CVT in the Subaru Justy, a car built for Tokyo. Even the 1987 Fiat Uno designated for the European model had a steel belted CVT.

Subaru Justy
I could be mistaken, but I think this is a Subaru Justy

That’s it for now, and although the workings are dumbed down, the principles stay the same.

If you have any questions for the workshop, send a mail to juan@torquesteer.co.za, and I will try to get some form of answer. Eventually. Maybe.

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Juan Loubser

I am an old fashioned 30 year balding, ham fisted egghead, with a love for mechanical engineering, Victorian era architecture. There was always something about cars, and up to this day I cannot put my finger on it, the easiest way to explain it would be to like it to something you just understand, you get it, you don’t know why or how, but it’s something you've understood all your life. With Torquesteer I have created a couple of Goals. One is to tour the little market town of Todmorden, England in a MG B roadster, and the Second is to do the US route 66 in true american muscle style. Driving a Tesla is also on the list.

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