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CVT Basics – Constant Variable Transmissions

February 18th, 2009 · No Comments

One thing people regularly wonder about is how a CVT (Constant Variable Transmission) works. CVTs are used mostly in scooters, like the 50cc Aprilia SR50 DiTech this document describes, but some larger scooters also use CVTs, and they are even being used on some Audis now! If you’ve ever been confused about that strange little transmission in your scooter, this should help clear things up a bit.

CVTs are quite simple in design, but since every part must work in concert with all the others, it can be difficult to visualize their operation, and understand how each part fits into the whole. The drawings are not to scale, but are meant to just give you a better idea about what is going on.

This document was originally written by Scott (AKA Scootnfast) on the ApriliaForum website. I have done a bit of editing so that things make sense in a non-forum context, but all the photos and info are his, and have been reproduced with his permission.

For other articles available check out the list or read on to continue this article. People ask a lot about different roller weights, clutch springs, contra springs variators and belts. This is an attempt to put the effect these have on your CVT in simple terms.

The truth of the matter is, a new variator, springs, or weights WILL NOT increase your vehicles over all power! The power your vehicle produces depends on the engine, not the transmission. What it does do is adjust your rate of acceleration, and at what RPM your motor runs at while it is accelerating, and at what speed the motor is reving when you reach the highest gear ratio available.

Now having said both of those things, the key is to have your motor running at the RPM where it creates the most horsepower consistently while accelerating. This value will be different for different engines. For the Aprilia SR50 DiTech used in this explanation, The max power is created between 8000 – 8500 RPMs. If you are accelerating at 5000 RPM’s or 9500 RPM’s, this will decrease your acceleration because your horsepower is not at it’s peak. This goal is achieved through trial and error. Changing first your contra spring, and then trying different roller weights is how one might go about doing this. In order for you to understand which way you need to go with weights and springs, you must first understand how the entire drive train works

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