How a CVT works - the basics

How a CVT works - the basics

Not sure how many people are still interested in tuning, but I found this old post of mine that survived the Wayback Machine. Just basic info on how a CVT works, and the effect of changing variator weights, and doesn’t even touch on other variables, like drive face mods, clutch mods (except for a little about contras), etc., but it might help you get started.

The CVT (constant variable transmission) works by changing the ratio between the front drive pulley (variator/drive face) and the rear driven pulley, on the clutch. Both have one “fixed” pulley face, and one “Moveable” pulley face.
In “low gear”, the belt is in the lowest position on the front pulley, and the highest position on the rear pulley. As the rpms go up, the vari weights slide outward, pushing the variator and the drive face closer together, and forcing the belt higher. As the belt moves higher on the front pulley, it forces the back pulleys further apart so the belt rides lower in the clutch pulley. The contra, or torque, spring in the clutch provides resistance against the clutch pulley, so the variator weights have to provide enough force to overcome the resistance of the contra spring. (This is why a stiffer contra takes more rpm to force the clutch open). At WOT, the belt should be at it’s highest point on the variator, and it’s lowest point on the clutch. When you let off the throttle and the rpms start to drop, the variator weights will start to drop lower, the contra spring will force the clutch pulleys closer together, forcing the belt up on the clutch and down on the variator, until you end up back in low gear. The bigger the change in ratio, the better, you want to get the “highest highs and the lowest lows” you can, belt-wise.

The weight of the rollers controls how soon the variator “shifts”. Lighter weights need more rpm to upshift, so they keep it in “low gear” longer, which is good for takeoff and acceleration, but if they’re too light, it won’t open the variator fully and you’ll never hit “high gear”.
Heavier weights will upshift sooner and hit high gear quicker, but if they’re too heavy the rpms will be too low and acceleration will suck. The goal is to find the sweet spot that gives you good accel and top speed. It’s kind of a balancing act.

A tach makes the tuning process MUCH easier, because you can see exactly what effect different weights have. Lighter weights will keep the rpms higher, heavier weights will lower the rpms. The goal here is to keep the rpms in the powerband as much as possible. So if you’re flat out at WOT and only hitting, say, 7800 rpm, you need to go lighter on the weights until you’re in the powerband range. On the other hand, if you’re hitting the rev limiter (8850 rpm) on a regular basis, you need heavier weights to bring the rpms down a little. So as an example, if you’re rpms are too low with 6g weights, and too high with 5g, that would tell you to try 5.5. I you want to get really serious, a lot of us shave down weights to 1/10gram increments. Use a digital scale and remove metal from the center of the weight. I was skeptical at first, but a 1/10gram change really does make a slight difference.

A lot of us use the Trail Tech TTO tach. That’s what I have. They’re small enough to mount almost anywhere, and have a very simple, single wire hookup. They’re only about $30 from Amazon. A tach is also really handy when you’re adjusting the air/fuel mix screw for the best idle. May be the best $30 you can spend on your ruck.

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