Constantly Variable Transmissions

How does Audi’s constantly variable transmission work? Like, the details. I searched the archives but couldn’t find anything. Bonus points if someone can also explain how an infinitely variable transmission works.

I don’t know about Audi’s CVT in particular, but the standard setup for things like snowmobiles and Polaris 4-wheelers is a pair of variable pulleys connected by a belt.

The drive pulley typically has a set of flyweights, springs and wedges, so that the faster the pulley is turned, the more the flyweights “clamp” the two halves of the pulley together, forcing the belt to ride further out (tapered V-belt, of course) which increases the gearing.

The driven pulley is the same- two movable halves- but this time simply spring-loaded to compensate for the diameter variations.

From what I’ve read over the years, car manufacturers were trying to do about the same thing, but with steel-link belts to better handle the torque and load (plus longevity.)

Very Simplified answer:

A CVT uses a rubber or metal belt riding between two opposed cones (opposed pointy end to pointy end). Something like this

>=<

One cone is connected to the crankshaft one to the driveshaft.

The relative distance the belt rides between the cones is changed so that the belt may ride through a large effective gear range.

One cone is connected to the crankshaft one to the driveshaft.

Oops. that should have been “pair of cones”

Ironically enough, it’s one of the oldest car transmission designs out there. Car makers quit using it because the belt technology wasn’t very good. It’s only been since the 80s (when Subaru began putting them in Justys) that they’ve started showing up in automobiles again. Thanks to better belts and computer controls the transmissions are being used again in cars. (They have been used in industrial machinery for ages, however.)

According to the Car Talk guys , the problem has always been controlling bigger engines with this sort of transmission. Some manufacturers seem to think they have the problem licked, though.

-lv