CVT Clutch Tuning Basics - Part 2: Understanding the Fundamentals

Jan. 01, 2004 By George Szappanos
There are two options for clutch optimization- A) a scientific method, or B) a not so scientific method (but also still effective). The way a manufacturer might do it is as follows. First, the engine alone is ran on an engine dynamometer (a means to brake, or provide resistance to the engine) which is connected directly to the crankshaft without transmission. The throttle is held wide open and the dynamometer holds the engine at a low, steady rpm. Slowly, the dyno allows the rpms to increase until the engine reaches redline, all the while measuring power (torque x rpm).

Inspection of the power curve would reveal what rpm peak power occurred. The engine is then reinstalled in the ATV, along with clutch, and fitted to a chassis dynamometer (the "braking" now occurs at the rear wheels rather than at the crankshaft). From a standing start, the throttle is again held fully open and the quad begins to accelerate to top speed.

If the clutch is doing its job optimally, then the rpms will rise, and then hold at the peak power rpm while the clutches adjust the ratio, and then finally continue to rise again once the maximum range of the CVT adjustment has been exceeded. If the shift-speed rpm is off, then an adjustment is made to the roller weights.

This acceleration profile is useful in explaining the overall function of the CVT clutch system. As the throttle is applied from rest, the engine speed quickly rises, spinning the front clutch and variator assembly.

At this stage the belt is resting against the innermost part of the front clutch, and is pushed outward on the rear clutch by the squeezing force of the large torque driver spring. As the rpms rise, the rollers in the front variator are flung outward in their slots and have the effect of squeezing the front sheaves together, thus gripping the belt and starting it to move.

As the belt moves it begins to rotate the rear clutch. The rear clutch begins to spin and accelerates as well. But the quad hasn't started moving yet. Inside of the rear clutch are three brake shoes that are held in place with extension springs. Once sufficient rpm is achieved centrifugal force starts to move the shoes in the rear clutch outward against spring tension of the three little extension springs. The shoes engage on the drive drum and the quad begins to move.

"gear" cross-reference
when front sheaves are apart- when front sheaves are together-
low gear,
short ratio,
high numerical ratio
high gear
tall ratio,
low numerical ratio
The ATV now begins to accelerate and the rpms briefly over-rev as the shoes seat and settle in to the “shift-speed”. The elegance of the CVT is in this ability to maintain the rpm independent of vehicle speed… When the rpms go above the shift-speed then the increased centrifugal force pushes the variator rollers out farther, squeezing the front sheaves together more, thus slightly lowering the ratio of CVT (higher gear), and therefore dragging the engine speed back down.

Think of what happens when you shift a manual trans quad from 1st to 2nd – the rpms drop. The converse also applies, when the load increases (like going up a hill) and the rpms drop, the clutching automatically compensates by easing the front sheaves back apart, increasing the ratio (lower gear), until the shift-speed is achieved again. In practice, this happens so quickly, and on such a minute scale, that these adjustments are completely unnoticeable.


sheave's aliases
front- rear-
Moral of the story is this: the variator roller weight is the primary means to adjust shift-speed. A higher weight will have the effect of squeezing harder at a given rpm, therefore decreasing the ratio (increasing the “gear”) and decreasing rpm. Likewise, a lower weight will increase the ratio as well as shift-speed. 

Finally, once the adjustment range of the CVT has been exceeded (when the belt is all the way OUT on the front clutch) the quad can continue to accelerate, but no longer at constant rpm. The transmission will essentially act like like a manual gearbox stuck in top gear and the engine will continue to accelerate the quad until it runs out of power, or hits the rev limiter. It's important to understand this since the power of your motor will fall off quickly after the power peak. Since the CPSC has limited youth quads to barely over 15 or 20 mph, they will reach this "rev-out" point fairly early (about 22 mph in the chart).

To reach 30 or 40mph will require modification of the final drive gearing such as new transmission gears, or drive chain sprockets. Otherwise the motor will be spinning at 12,000rpm+, possibly way past its power peak, assuming the rev limiter will even let it.

The rear torque-driver spring also has a very specialized function which is to maintain enough tension on the belt to keep it from slipping. This also is a bit of balancing act because too much belt tension translates to inefficiency. It’s kind of like over-tightening the chain on your bicycle- power is wasted by over-stretching the belt. On the other hand, too little tension and the added power of your recent engine modification will vanish as heat generated by belt slipping and won't find its way to the rear tires. The trick is to find the just the right tension without overdoing it.

A secondary influence of the torque driver spring is that it has a slight impact on shift speed since the belt tension needs to be reacted against by the front sheaves. The higher belt tension tends to push the belt deeper into the front sheaves making the ratio higher, and revving the motor higher. Therefore, for a given desired rpm you would need to compensate with a higher roller weight to bring the rpms back down. After a torque driver spring change, it’s good practice to re-optimize the roller weights.

The three shoe-springs are really the only clutch parts that enjoy the freedom of rider preference. But their role in ratio adjustment is short lived – once the machine has started to move it’s all up to the rollers and torque-driver spring. Installing a very tight, stiffly sprung set of springs will provide a higher "stall" speed, effectively raising the rpm during launch. Alternately, a softer set of springs will provide a little easier, smoother engagement. After the shoes have engaged, there’s little if any effect of the shoe-springs.

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