Lots of people get confused by CVT transmissions, so I'm writing this to help out those that need a full explanation. It can be pieced together from reading various threads, but I'll attempt to explain it, plus possible mods. Just PM me if you spot any errors or need further clarification.
The CVT is how your machine converts engine momentum into forward or reverse momentum. CVT stands for continuously variable transmission. It works on the principal of balancing forces between the engine and tire rotation resistance. It does this through the use of two sets of pulleys called sheaves, and a belt that connects them. It's superior to a convention transmission because it allows the machine to maintain peak horsepower throughout the gearing range. With a conventional transmission, the engine increases RPM to increase speed. The gearing stays the same as the engine goes through it's RPM range, and then starts lower again with the next gear and goes through the range again for each gear. With the CVT, the RPM stays fairly constant, and the gearing changes instead. This means always staying in the peak power under heavy acceleration, if it's tuned right that is.
Click the link and open up the clutch diagram on Yamaha's site for reference. YAMAHA MOTOR CANADA | Parts Catalogue
There are two sets of sheaves; the primary and the secondary sheaves. They are both comprised of two plates with tapered faces. When put together, they create a V.
The primary is the driving pulley. It is connected to your engine, engaged by an automatic wet clutch. We'll leave the wet clutch discussion till later though.
The secondary is the driven pulley. It's driven by the belt that connects the primary to the secondary.
Quick video of my CVT in action.
The CVT is designed to start in the lowest gearing, and then gear up until top speed is reached for the given engine output. At a stop, the primary sheaves are spaced at the maximum distance, causing the belt to go to the lowest point on the sheaves. The secondary sheaves are together except for the distance the belt doesn't allow them to move. This causes the belt to be towards the outer edge of the secondary sheaves. This creates the greatest mechanical advantage. It's much the same as starting off in 1st gear in a conventional transmission. Once you start moving, the transmission will automatically adjust that gearing ratio to balance the engine torque output with the resistance of the wheels.
The ratio changes because of the physics of the clutch.
The primary consists of the two sheaves I mentioned before. One is fixed, and the other moves in and out on a collar. When it moves inward, the V gets tighter between the sheaves. This causes the belt to rise in the primary. The higher in the primary it moves up, the further the belt travels per rotation, increasing the speed of the belt.
The secondary responds to the primary belt height. The secondary also has a fixed and movable sheave as well. The difference is that the secondary also has a spring in it to force the plates together. The belt doesn't stretch much, so as the belt rises in the primary, it wedges the the secondary sheaves apart and moves close to the center of the sheaves. This causes the secondary to increase rpms. This means the ATV goes faster.
When working together. The Primary squeezes increasing the belt speed, and the secondary spreads, increasing the output speed.
What causes the primary to squeeze the belt and gear up is centrifugal force. At the end of the primary movable sheave is a fix plate. It's bolted securely to the end of the shaft. The Moveable sheave slides on the fixed plate and collar, with 8 weights between the plate and sheave. The weights act like wedges. As the RPM of the primary increases, the weights are forced outward and slide inside the channels on the primary sheave. This causes the weights to wedge against the sheave, forcing it inward towards the fixed sheave. Greater RPM's = Greater squeezing force.
Now you're probably wondering why the transmission wouldn't just gear all the way up when the engine increases RPM right?
The magic happens in the secondary. Although the spring keeps tension on the movable sheave, pushing it towards the fixed sheave, there is also extra force created by shaft resistance. This resistance is transferred from the wheels through the final drive gears, and then onto the secondary. On the secondary, the movable sheave is also mounted with pins that move inside corkscrew like channels. When a load is applied to the wheels from acceleration, a hill, or mud, that load utilizes the corkscrew action to increase the pressure on the secondary movable sheave. When this pressure exceeds the force the weights create on the primary, it will hold the lower gearing until the weights overcome this resistance.
So to summarize it for you. The weights cause the primary to squeeze harder as the RPM's increase. The secondary resists this gearing up based on spring pressure and resistance created by the corkscrew affect due to resistance on the tires do to acceleration, hills, or mud. It always finds that perfect balance.
Now that you know how it works, lets mess with it to make it do what we want. Since the CVT is tuned for stock everything, it needs to be tuned when tires, weight, or engine torques is changed. If you don't, you're not getting the most from your machine, or your mods.
Lets start with the primary:
You can modify the primary by changing weights, machining sheaves, adding shims and modifying the cam plate.
The weights determine the shift up of the primary. Lighter weight hold lower gearing longer, and heavier weights cause it to gear up sooner. Your goal is to weight the machine to hold your gearing under full throttle just below peak torque on a flat road. This way, when resistance is created by a hill, extra weight, or mud, the machine will hold the gearing longer, putting you in the peak torque of the engine under full throttle. Hell ya!
Shims and machining. By adding shims or machining, you can increase the lowest gearing substantially by spacing the primary sheaves apart, allowing for the belt to drop lower in the primary, and rise higher in the secondary. Shims will lower the gearing range entirely, so top speed will be lost. Machining can be done to lower gearing to increase the gearing range without losing top end speed. This is done by only machining the inner part of the sheave, and leaving the out sheave the same. Other machining removes the from the whole face, lowering the entire gearing range like the shim mod. The benefits are that it keeps the sheave angle stock for decreased belt wear. (Mad Mike's mod #2)
Here's a guide on installing shims.
How to do the shim mod.
Fixed plate mod. If you've taken some meat off your sheave and lowered the gearing range, the fixed plate mod can restore your top speed. The tips of the fixed plate where the weights ride are bent in slightly at the ends. This allows the weights to push the movable sheave more in the top end, restoring your top speed gearing. (Mad Mike's mod #3)
Take a breather if your eyes are seeing double.
The only thing you really need to modify in the secondary is the spring. The spring assists the secondary with holding gearing, and downshifting. Since it takes pressure off the pins reducing the wheel resistance force needed to return the secondary to lower gearing, it will increase the downshift rate as the spring pressure increases. As a side affect, it does increase the primary force needed to overcome the heavier spring, so weights will need to be heavier for a heavier spring to maintain the up-shifting profile the weights create. The stronger spring also increases the pinching force on the belt reducing slippage and thus heat.
Now if you're deciding on tuning your clutch, you should follow these steps.
First, start with engine mods, cause what you want from your transmission will change with power output. Then I'd do the machining, shims, and fixed plate mod. This will determine the gearing range you're working with. Change it later, and you'll have to change everything else. Next would be your secondary spring. This will affect your weighting, but your weights won't effect your spring choice. Then finally, get your weights and tune your machine in to get the gearing set to the RPM peak that you prefer.
Remember, no gearing set up is perfect for all. Rider weight, cargo weight, pulling weight, and riding conditions all have their affects on what RPM the transmission will shift best at.
Wet clutch springs can also be changed to increase wet clutch engagement RPMs. Gets you into the power a little sooner. The wet clutch is just 5-6 shoes or blocks that are pushed outward be centrifugal force. When RPMs overcome the springs holding the shoes in, they connect with the clutch housing and engage the crank with the primary. Stiffer springs will increase this RPM engagement, and are found in 500+ and 1000+ RPM. 1000's are more for racing.
Feel free to ask for edits if something isn't clear or if I made any errors.