The Technical Side of the Continuously Variable Transmission(CVT)

Belt type CVT transmission have really been around for ever.  In fact, my 1981 Arctic Cat, Snow Mobile had a continuously variable transmission that operated on the very same principle as the one in our new 2013 Subaru, XV Crosstrek.

Shown is a snow mobile, rubber belt driven CVT

However, since we just bought a vehicle equipped with a CVT to use to tow our tear drop camping trailer, suddenly I was interested in finding out every detail I could about how reliable the CVT is, what can happen to it, and if anyone has had any problems towing with one.

This new interest was brought about because of one time when I was pulling the trailer, gave it too much gas when it was cold, and felt what I would later find out was a "shudder"

It's easy to understand how the belt and variators( variable belt pulleys) work to give you an infinite number of gear ratios allowing you to always get the most torque out of the engine, no matter the speed.

Shown at the link below is the belt driven CVT.  This model is driven by a fiber reinforced rubber belt.

Below is a white board tutorial I did to try and explain the concept of the cvt

However, once you add a STEEL CVT belt instead of the normal rubber one, things become much more interesting...

The advantage to the steel belt is that it is stronger, and since it is bathed in oil, it never needs changed.  Similar to a timing chain in an engine.

Suddenly you have to start to think about the fluid dynamics of the CVT oil with its additive pack.

Wikipedia has a misquote on how the belt driven CVT works.  I have not only read the Wikipedia article, but the original masters thesis that the Wiki author plagiarizes to write the page.

In this wiki, it is stated that the "CVT fluid is there to ensure that the steel belt "never" touches the variators or pulleys"

In fact, the CVT fluid is there to ensure that the belt never touches the variators WHEN THE GEAR RATIO IS CHANGING.  Meaning when the CVT belt is traveling up and down the variators vertically to find the right gear ratio.

After the transmission finds the appropriate gear ratio, the oil film of the cvt has to get out of the way, and allow the steel belt to lock up directly with the surface of the variator to propel the vehicle.

Companies put additives in the CVT fluid to protect the variators and belts when this happens. One additive they can use is zinc-chemical, which on the molecular level, forms little pads on the cvt variators so that when the belt gets pressed into the surface of the pulleys, there is not metal to metal damage that occurs, scoring the variator surfaces.

So, the first question is, how reliable is it? 

Well there have been some reported problems with these transmission for sure.  There have been many slipping belts(causing sure failure), there have been instances of the quick lube oil change shops stupidly putting in the wrong Automatic Transmission Fluid after a "transmission flush", and many instance of shuddering. 

However, there has also been wide use with the CVT transmissions and especially in Europe.  That means there are literally millions of these transmissions on the road, and only a small fraction of them that are problematic or fail.

So what can cause a slipping CVT belt?
Many things, but a few sure ways would be gunning a cold transmission from a stop, not allowing the CVT to engage before switching form reverse to drive before giving it throttle to drive off, and exceeding the listed towing capacity in general, and under certain situation like high heat, prolonged hill climbing and such.

A: Gunning a car with a cold CVT transmission:  Before the CVT fully warms up, the transmission fluid can be thick, which means it doesn't pump as well.  This in turn means that it is possible that the transmission might not be able to apply the needed horizontal clamping pressure on each side of the variator onto the steel belt to ensure it locks up tight between the surface of the belt, and variator, producing a damaging metal on metal slide. 

B. The CVT takes more time to situate itself when switching from Drive to Reverse, or vice versa.  While it is performing this task, the transmission drops horizontal pressure to the variators.  If one tries to switch out of reverse and quickly gun it, it is possible that the transmission might not be able to apply the needed horizontal pressure in time to lock the belt to the pulleys and once again, you get a metal to metal slide.

In this type of slide, it is possible, or even likely that the steel belt(think chain) can rub on the variators causing it to chew up the surface of the pulleys.

C. When you exceed the listed towing capacity, you are putting a lot more strain on the belt and pulley(variators) in the direction perpendicular to the horizontal clamping force of the variators.  You can overwhelm the friction caused by the horizontal force on the belt, and have shearing. Meaning there is too much strain and the belt can tear loose of the pulley, and slip along the variator, causing damage.  This is more likely on prolonged up hill climbs, and in hot weather.

The quick lube stores putting in the wrong ATF fluid, what can happen?
Standard ATF fluid for cars with planetary(regular) type gears is much thinner than CVT oil.  The thinness of the oil will not prevent the steel chain from touching, rubbing on the variators and chewing up the surfaces when gear ratios are changing.  In essence, you are letting the metal CVT belt file down the surface of the variators, which will absolutely lead to a transmission pan full of metal shavings, and catastrophic failure.

DON'T LET THE QUICK LUBE SHOPS TOUCH YOUR CVT TRANMISSION.  TELL THEM TO STAY OFF OF IT!

In the end you are likely not saving yourself money, you will cost yourself thousands of dollars in repair bills.  If you want someone to do something to your CVT, seriously as much as I hate to admit it, take it to the dealer!

CVT shuddering, what can cause it? 

In short, microslips.. These aren't like the a slip that can necessarily cause major damage to the transmission.

A. Not letting the transmission warm up(thick CVT fluid not allowing enough horizontal pressure to be placed on the variators.

B. Worn out CVT fluid that additive pack suspended in the oil is past its useful lifetime, thereby  preventing a high belt surface to variator surface high friction coefficient.




Tutorial:
Press your palms together as hard as you can, then try to slip them back and forth.  Now wet them, and try the same thing... This is like the belt slipping on the pulleys(variators).  Except unlike your hands, the belt slipping on the variators(pulleys) will damage them...

A. The phenomenon that makes it so your hands don't slip easily is a higher friction coefficient.

B. When the engine is cold, think of not being able to press your palms together as hard. They will slip easily.

C. When you are towing a trailer that is too heavy, think of someone else grabbing your hands and pulling them apart at a right angle to the direction you are pushing them together.

Of course, the transmission puts A LOT more pressure on the belt than you are capable of on your hands.  Also, instead of a nice soft rough surface like your palms, inside the transmission, the surfaces are extremely hard, ultra smooth steel that had a film of oil in between the surfaces, right before it was pressed out to lock up the surfaces.

Are you gaining an appreciation for the forces involved to propel a roughly 3500 pound car towing a trailer with a smooth steel belt, and pulleys?


Summary:

I. Warming this car up before you drive it hard is way more important than on a manual transmission, or even a standard automatic transmission car in order to allow the CVT fluid to flow correctly applying the proper clamping force to the steel belt.

II. Under hard use like towing, constant extreme acceleration, driving in extreme heat, and up and down large mountainous hills, you will want to have your CVT fluid changed, BY THE DEALER earlier than is recommended.  You don't want the oil to thin out loosing viscosity thereby allowing the steel belt to rub the variators while the car is hunting for the proper "gear". Plus, once the additive pack wears out, the anti wear materials are rendered useless.

III. The steel belt driven CVT really is a thing of beauty. The car will almost always be in the right "gear".    However, it will not stand up to the abuse that other transmissions can take.  There are no gears with teeth that lock the drive gears and engine torque together like in other transmissions.

IV.  High heat on this transmission plays a more critical role because for one, the CVT fluid can thin out, which once again will allow the belt to rub off material on the pulleys(variators).

V. The torque converter(what allows your car to idle while it is in gear) locks up much quicker in these transmission over a standard auto trans.  This prevents heat build up, and makes the car more fuel efficient.  The torque converter is able to do so because of the infinite, and very high gear ratios possible in this type of transmission.  The transmission is capable of a really "low granny gear" as well as an overdrive capability all in one.  It is like having a 20 gear transmission!

I have a robust all wheel drive Subaru which has a CVT, but Nissan is putting CVT transmission's in their 245 horse power, Murano SUVs, which should indicate how strong the CVT really can be.

The CVT really does drive BETTER than a standard Automatic with gears, however for reliability, I would still choose a standard Automatic over one.  Having said that, our only choice in our new XV Crostrek was either a CVT(which they had lots of), or a manual transmission, which they only had one of.

We chose the CVT for my wife.



technical ariticles

first article

Second article

Yale course on fluid dynamics I found helpful