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For some that are following these series of posts, the following 3 articles may appear to be oversimplied, BUT for most, they will, fairly well, describe how the CVT system functions in pretty simple language. If you are a puriest, JUST substitute the word,
UTV, whenever reference is made to the word Snowmobile. :)


Before attempting a limited overview of how CVT(continuously variable transmission) works, I would HIGHLY URGE everyone to read these articles..

How does the clutch system work? Brochure from the Gates Corporation, pretty straight forward explanations as well as some good picture examples of a clutch working.
How does the clutch system work? - Gates Snowmobile & ATV Belts


The next article was found by Cajun, who is a moderator on atvquadsquad.com, and was referenced in his thread on that site called: CVT explained/ Tuning info
U of Maine-this pdf goes a little more in depth to the theory and tuning aspects of the CVT. Just remember cams=flyweight, ramp=helix
http://www.umaine.edu/MechEng/Peterson/Classes/Design/2004_5/Groups/CSC_Model/clutchreport.pdf
U of Maine

As Cajun replied:
I think this sentence from the last articles says it all:
“Locating the power peak will be the first step to tuning the clutch. Once found, it can be determined if the clutch is holding the engine to operate at its power peak.”


The last article was printed in a writeup by George Sappanos in
www atv.off-road.com.
CVT Clutch Tuning Basics - Part 2: Understanding the Fundamentals
One of the items I found interesting was the terminology referring to both the primary & secondary clutches.

http://atv.off-road.com/atv/article/articleDetail.jsp?id=190896
Understanding the Fundamentals

I have made mention a number of times to a book by OLAV AAEN entitled
CLUTCH TUNING HANDBOOK (currently updated 2009)
For Serious Racers and Anyone Who Wants More Performance From Their
Variable Ratio Belt-Transmission. Many tuners feel that this is the HOLY GRAIL for clutch tuning.

This book should be an ABSOLUTE MUST HAVE, due to the nature of topic discussions, charts & graphs. For some, it could be a little overwhelming at your
FIRST Reading. But just take your time and at some point, the light bulb should come on
and you’ll be saying to yourself, “NOW I KNOW WHAT THEY WERE TALKING ABOUT”
I have found various prices, however all of them are pretty close to $30.00, which is about the cost of purchasing just 1 spring.
Sources for the book may be found at the following:
AMAZON.com
Aaen Performance - High Performance products for Snowmobiles and Polaris ATV's


NOW, everybody is armed and ready for Clutch changes.
N O T
If you are totally serious about trying to milk every ounce of Torque RPM and shave every second off your run time, then you need to either check or monitor, all of the following:

Spring Height, measure to make sure that the coils parallel to each other and that coil spring length is within tolerance
If a running a new belt, it’s got to be cleaned (soap&water) before any breakin AND
It’s a good idea to blow any dust off the belt and from inside the clutch cover.
Clutch surfaces should be Clean & free of any black scuff marks, etc. I normally use
Acetone on a clean cloth and wipe the surfaces of both clutches.
All Bushing in both clutches should be serviceable, meaning check for free rotation with
no dragging or sticking.

Ok now you’re ready to begin some changes & testing. BUT REMEMBER
JUST CHANGE ONE ITEM at a time and thoroughly document any & all conditions,(ie, temperature,humidity,elevation,etc) as well as the ITEM that you changed and what effect it had on your test runs.
Above all Don’t Do What I tried early on, AND THAT was trying to over compensate or relate one tested item by radically changing another one. ALL REMEMBER, the primaryj(drive)clutch and the secondary(driven) clutch will effect each other. What you’re tring to perform is a BALANCING act between the two items.

Just as Cajun referred to, you are basically trying to get the springs and weights in the primary clutch adjusted to the point that the engine RPM’s stay at a particular RPM range. Then you begin working the the spring and helix in the secondary clutch to get both the shiftout & backshifting that fits whatever your riding style is.(ie, mudding,trail riding, racing, etc) I’ve been told that the IDEAL would be a slight overrev at engagement, then holding the RPM’s at whatever the peak horsepower is from the engagement point all the way to the shiftout point, after which the RPM’s would continue to climb(not as fast) until you hit the rev limiter. SOUNDS SIMPLE doesn’t it.
Trust me, the combination of the primary and secondary clutches are ENDLESS and the point mentioned above in my opinion is impossible to get exact. What you want to do is get it as close as possible, THEN, QUIT and be happy with your success.
I’ve said previously and will continue to bring this topic backup.
JUST BECAUSE I GOT IT TO WORK FOR ME,MY MACHINE AND MY RIDING HABITS, DOES NOT AND WILL NOT MEAN THAT IT WILL WORK FOR YOU. The very BEST that anyone can do, is to get you started in the RIGHT direction, but after that the old saying applies
IF IT’S MEANT TO BE…..IT’S UP TO ME

OK, now where do we go from here. I will briefly explain how and what I do,
SO IF THIS IS NOT THE CORRECT METHOD or ANYONE has suggestions, FIRE AWAY.

I start with the Primary Clutch and trying to get my RPM’s at the level that I have chosen. Everyone has their own power range varying between 5800-6200 RPM’s for a stock engine. I have been told to try to clutch for a 1000-1500 RPMs below the power RPM level, so that you still have some wiggle room and don’t run out of Torque RPM when you need them. I will play with the flyweights, increasing and decreasing the gram weight and/or try different profiles (curvature) depending on whether I’m going after Torque or Speed. My suggest here is that if you intend to change clutching in the future or change engine mods, GET YOUR SELF A GOOD SET OF ADJUSTABLE WEIGHTS. The may appear a little pricey upfront, but when you consider a set of fixed weights will run $90+ buckets, the price really doesn’t look that bad. A good set of adjustable weights should have a GOOD base weight(this depends upon your overall engine setup) AND the ability to either add or take away weight in the following areas.
Heel=Bottom END
Middle=MidRange
Tip=Top END
Now the above being said, there is always some overlap between the area, so you’ve got to really be observe the tach RPM’s and document your ranges. My opinion is not to use the same PROFILE weight, if I’m attempting to initiate the first Clutch change and I try not to alter the Secondary to change RPM’s. However, sometimes, this is the only option left.
After I get the RPM’s where I want them, THEN, I start with the Secondary Clutch. This is where I establish my upshift and backshift patterns. And as said earlier, THIS IS a personal preference. The combo of Secondary Spring & Helix Angle is what controls these functions. I believe that the Secondary Spring relates to and/or controls the side pressure(force) on the belt. Not enough, you get belt slip, Too much, you lose efficiency. Unfornately, I’m not come up with REAL slick way to determine either one, short of using a temperature gun to get heat readings from both clutches and the belt.

Guess that’s about it for this Part. Post your responses, GOOD or BAD, I’m always open to a better setup or method for clutching.
 
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