Clutching Is Key

Sport ATV, Utility ATV — By ATV Mag on June 22, 2006 at 12:00 pm

Superior to manual-shift models, centrifugal clutch systems automatically find the best shift ratio, providing better overall performance and versatility. They allow the rider to concentrate on driving by not having to worry about finding the right gear. Furthermore, clutch systems have an infinite number of ratios they can tap into, thereby maximizing the available power. Unlike manual-shift models, centrifugal clutch systems can be tuned and adjusted to assure top performance, no matter what riding conditions you face or what modifications you’ve made to your machine. Understanding the “continuously variable transmission” is a requirement, if you hope to realize its potential.

Knowing How Things Work
The clutch system is comprised of a drive clutch and a driven clutch, which is linked together with a drive belt.
Action begins with the drive or primary clutch, which is attached to the engine. Inside the clutch is a collection of “weights,” in the form of rollers or arms, and a spring. As engine rpm increases, centrifugal force builds, causing the weights to overcome the tension of the spring, thereby closing the clutch and forcing the belt to climb. This is the upshift portion. The driven clutch or converter on the other hand, is torque and load sensing, receiving its signal to act from the engine and from the wheels. The converter is regulated by a cam or helix along with a spring. Both the front and back clutches work together to control the upshift until the load from the wheels is greater than the torque output of the engine. The converter then becomes dominant and downshifts, returning the belt to a more powerful ratio. This is the backshifting portion.
The capacity of the clutches to “talk” to each other and work as a team, are dependent upon the tuning components of each clutch. The spring rates, cam angle and amount of front clutch weight, have all control over upshift, backshift and rpm, which are the driving forces that moves an ATV.
Where Did The Power Go?
The manufacturers calibrate each of their models for “all-around” use. They know you will be using your ATV for a variety of riding and working conditions and try to strike a happy medium. But what happens when you spend more time playing hard or subject your machine to more severe work duties? And what happens when you upgrade to better performing tires or other components? Under all of these circumstances, the factory calibrations could be holding you back, and probably are. It is also not uncommon for a factory calibration to be somewhat lacking, even if you leave your ATV completely stock and do just average type riding.
Subjecting your ATV to more demanding use creates more load for the engine to overcome. The engine struggles to make rpm, thereby compromising horsepower output. Once out of the “power-band,” performance is fading and your ATV is not as capable as it could be.
Also consider this: Recreational ATVs do not produce high horsepower numbers. When you install tires that hook up better, or attach plows to push snow, all of a sudden you are left with even less available horsepower. With such relatively low power at your disposal, even under normal loads, clutching becomes vitally crucial.

Reclaiming Lost Horsepower
Recalibrating the clutches can restore power, and capitalize on the power you have, if performed correctly. Changing to the right combination of weight, cam-angle and spring rate is the trick. All clutch tuning components, front and back, affect shifting and rpm to various degrees. Clutching for optimum rpm will allow you to reach maximum top speed, but just as important is preserving sufficient rpm through the upshift and backshift to maintain maximum power.
Peak rpm and upshift is achieved primarily through the amount of drive clutch weight and the curvature it pushes on, but is also influenced by the clutch spring, converter tension, and the converter cam angle. More weight translates into less peak rpm and a faster upshift, while less weight means more rpm and a slower up-shift. Less spring rate in either clutch will allow for a faster upshift but will reduce rpm. More spring rate in either clutch will have a slower and more powerful upshift because rpm is increased.
Backshifting takes its cue from the converter’s cam or helix angle. A cam with less angle will backshift quicker, which can get you back into the power faster because it keeps the rpm higher. The smaller cam angle will also provide a more powerful up-shift for the same reason. More cam angle will backshift slower but will allow for a quicker upshift. Keep in mind that a larger cam angle will reduce rpm overall. Some manufacturers equip certain models with fixed cam angles only, making working with the spring your only option. In those applications, a lighter spring has the same effect as a larger-angle cam, while a heavier spring acts like a smaller-angle cam.
You can see that, by changing any one component, there is always a tradeoff. The tuning components in both clutches must all be taken into account to successfully arrive at a final setup. All the various manufacturers have somewhat different style clutches and tuning components, but serve the same purpose and can be approached in the same manner.
A correct clutching setup will keep your ATV in the powerband at full throttle, and in the power or close to the power when shifting up and down.

High Altitude Operation
Many ATV riders are operating in the mountains where the air gets thinner the higher you go. Less air means yet more loss of horsepower. A 50-hp engine at sea level will be reduced to just over 35 hp at 10,000 feet of elevation. To compensate for the shrinking horsepower as you climb closer to the clouds, a lighter weight calibration together with heavier springs and less cam angle is needed to keep rpm levels high enough to capture whatever horsepower remains. It is very easy to completely run out of power at high altitude without recalibrating the clutches. Remember, what works at 5,000 or 6,000 feet won’t work above that. Know the altitude parameter you will be operating within, and adjust accordingly.

Where To Find Help
Clutch tuning can be a daunting experience, even if you are versed and experienced in the subject. Getting the right setup for you and your ATV takes time and practice. If you need assistance or guidance, help is right around the corner with companies like Erlandson Performance Inc., Team Industries, Dalton Industries or CU Tech Distribution. All companies have engineers with loads of experience concerning clutch systems. They produce custom clutch kits for almost every model and every type of use.
The single most important item in terms of performance for your ATV, is to maintain your clutching and run the correct calibration. Once you get the right setup, you’ll have power on tap when you need it most.