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The evolution of clutching and the CVT

RELATED TOPICS: CLUTCHES | PARTS AND ACCESSORIES
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Clutching is an art that has evolved for many years. The CVT transmission is the mechanics behind why snowmobilers feel the instant exhilaration of power when we hit the gas.
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Arctic Cat 10.75-inch roller cam secondary.
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the manufacturers tackle clutching a little bit differently. Cat uses an RPM sensing primary and roller secondary, semi- similar to Polaris’ P85 primary and P2 secondary. Ski-Doo has a more vertical setup with the TRAIII primary and QRS secondary. Yamaha has a button primary.
For those outside of snowmobiling “CVT” might sound like a calculus term, or a new fifth grade acronym, LOL. However, for us sledders CVT, stands for Continuously Variable Transmission and it’s one of, if not the most vital system on a snowmobile.

The main objective of the CVT is simple, to make the engine perform at the proper engine RPM. Unfortunately, this is difficult to achieve without the correct components and tuning. Operating at the proper rpm produces peak performance for the machine at all times. The main benefit of a CVT is there is no need to shift, just push the throttle and the CVT does the rest of the work. It will produce a smooth up-shift with never having to hold a clutch lever/pedal and shift a gear, thus eliminating the need for the clutch and gear box.

In order to keep your sleds clutches working correctly, or to make performance changes with your clutches, you must have a solid base of knowledge on how they work. That is what this history lesson is for!

How clutches work

CVTs of today look very similar to those of 30+ years ago, but they have made drastic improvements in efficiency, belt life, durability, strength, and performance. The original primary snowmobile clutch (also known as the Drive clutch) used the same basic principles of the primary clutch today. The focus is that rpm creates force. The more rpm produced more force is generated. The force is the “squeeze” of the clutch sheave on the snowmobile belt that begins to drive the sled.

I’m often asked, “how does the primary clutch work?” Many believe that the snowmobile clutch, in its basic form, should be simple to use and tune. BUT, explaining the concept and then using and tuning with this concept are two completely different scenarios. The easiest way to explain the ‘snowmobile clutch at a cocktail party is to stand up straight and let your arms hang loose at your sides, now spin around as fast as you can. You will notice your arms will begin to pull away from your body. This is what the primary clutch is doing. As the RPM of the engine goes up, the arms inside the clutch are pushing the sheaves closer together and creating the belt squeeze to drive the snowmobile.

Now that you have everyone looking at you, add 5-pounds to each hand and do it again, the extra weight requires more rpm to raise the same amount. This relates to HP. The more HP, the more weight we can spin at the same rpm, compared to no weight.

Primary and secondary clutch movements have basically stayed the same over many years. The primary clutch uses arms and a spring inside it while the secondary uses a helix and spring. These are the 4 “basic” tuning parts of the snowmobile CVT (clutch arms, primary spring, secondary spring and helix). The imperfection that all tuners have strived to change in the CVT has been the percentage of slip that is created during the initial start of the shift and during the shifting out of the clutches. Slip is the main inefficiency factor in CVT clutches and belts. This inefficiency also creates heat which is transferred to the belt and clutches. This heat diminishes belt life, and performance. Naturally, the largest changes in clutching technology have focused on correcting these problems throughout the evolution of the CVT. New spring tensions, spring tolerances, adjustable clutch weights and arms, multi angle helixes, improved weight and ramp arch angles, and multiple armed clutches are some of the more notable changes of the last 30 years.
A history built on racing

Past history has shown racing and technology advancements in snowmobile clutching go hand in hand. Through my 20 years of racing I have been fortunate enough to have some great conversations with some “seasoned veteran tuners” regarding the early years of tuning. In the late 70’s aftermarket performance was almost non-existent. Racers were the tuners, and thus the secrets of tuning were kept extremely close and not shared with anyone other than their own team members. Then, some of these racers noticed that there was a potential income stream for selling their secrets. Soon performance parts started hitting the main stream and mass production “aftermarket performance parts” for sleds took off.

However, advancements in primary and secondary clutch tuning components happened separately. The primary tuning and performance parts were years ahead of the secondary. Peak engine rpm operation was the determining factor. Like we said, clutching to run the proper RPM is most important and keeps the machine in its power band at all times.

One of the first items tuned were the springs. The OEMs typically only had 2-3 primary springs and 1 secondary for their entire model line-up, but the racers were looking for higher engagements, improved acceleration, better throttle response, and just plain more tuning options. Snowmobile spring manufacturing in the early 80’s was quite crude by today’s standards and consistency of spring rates was poor. Spring rates are the most important part of a spring, so the inconsistency of the primary spring rate (up to 20% from spring to spring) made tuning tough. Changing the spring when your set-up required the same spring rate, and inconsistency required racers to go through multiple springs in hopes of finding one with similar enough rate to perform close to identically.
A common compression spring rate today for a Polaris or Arctic Cat would be 150-320, which means 150lbs of compression when starting the compression of the spring and 320lbs of pressure when at a fully compressed shift. A 20% variance would put this spring anywhere from 135-165 starting compression rate to a 288-352 ending rate. That’s a lot of change! Trying to repeat a performance run with these large percentage changes of variation would be nearly impossible.

Manufacturing of springs today includes new types of wire, process improvements, and better forming tolerances. Tolerances today are held at the 5% rate in the snowmobile aftermarket. These tolerances have also made the aftermarket clutch kit sales become a larger industry as repeatability is possible from spring to spring. The customer can almost be guaranteed to have the same rate as the test machine the kits were developed on. This advancment in spring replication made tuning changes more precise and consistent.

This simple change in clutching technology (springs) made huge waves when it became available. Springs today are made in 100’s of compression combinations with multiple types of material. These new springs ushered in a new era in tuning for the average joe and also kept racers thinking outside the box.

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Clutch weights change game
As spring options became a staple in sled tuning, the next racer secret was made public. The clutch weight.
The most commonly used tuning part on the market (clutch weights) has not changed much in concept since it was first introduced. BUT, large improvements have happened ability to add weight in multiple positions, the amount of weight along the clutch arm itself, and improved shift arcs. Even though the basics of many clutch weight theories have been around since the mid 70’s (much of it from early 70’s Yamaha racing) it never quite caught on till the late 80’s. Most of it started by racers welding different clutch weights to achieve the proper gram-weight they wanted, in the proper place.

This early welding on weights was one of the only ways to change how much and where the tuner required the weight. There were issues that arose with the welding as it was not only unsafe but it changed the temper in the weights, and it was also inconsistent and hard to replicate. You often ended up tuning one machine very precisely but could not exactly replicate it to the next. So, the adjustable weight was invented and appeared quickly in the tuning market. The advantages of the adjustable-mass-distribution weight are the ability to accelerate and de-accelerate the clutch weight throughout certain shifting points of the clutch. These have become a standard in every racer and tuners’ clutching bin. The adjustable weights are now sold as part of many standard ‘trail’ clutch kits as well to improve performance, and give the customer adjustability of tuning for rider weight, altitude and conditions.

Through the late 70’s to mid 80’s there were great improvements in primary CVT tuning but it was during the late 1980’s that the move toward tuning the secondary clutch (along with the primary) began to happen. The top racers in the field had to keep moving forward as most of the racing field was acquiring the correct primary clutch parts and gaining primary tuning knowledge. So, knowledge of the helix and secondary spring separated the top minds in clutching in the late 80’s from the beginners.

Manufacturers up until the late 1980’s sold helixes in straight angles only, this was the only production option available. Just as racers were grinding on clutch weights, the straight angle helix was being modified. Soon the multi-angle helix became a staple. Many early prototypes were done entirely by hand by grinding all three ramps. A crude, but effective way to increase or decrease shift angle. These new tuning options coupled with a few new torsional spring options made huge improvements.

Reminiscing with seasoned tuners, many say they had a hard time believing some of those early helixes even worked! But it was their hard earned testing and racing that paved the way for the performance options we have today. The addition of CNC machines into the manufacturing of snowmobile performance products produced a perfectly replicated multi-angle helix. A big change from grinding the ramps and checking it out by line of sight.
The modern helix comes in an infinite amount of possible angles and compounds. The latest advancements are fully adjustable helixes. Amazingly this was built and tested in the late 80’s but never made it to market due to its reliability, material, and cost effectiveness.

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little parts, big job - The latest clutch components are fully machined and balanced to precise tolerances to help repeat and increase performance consistently on every sled.
It’s a great day for clutching
As today’s snowmobile tuning parts technology has evolved, so have the actual snowmobile clutches. We have touched on mostly on the clutch components, but the actual clutches themselves today can be completely CNC machined, can harness up to at least 8 clutch weights (many early versions only used 3), can have different sheave angles, have improved shift angles, less resistance, etc. All helping to create improved belt squeeze.

Using these improved and redesigned clutches combined with today’s internal clutching components can increase a 20 year old sleds performance without actually producing more engine horsepower. Technology is improving and so are the clutches we use on our sleds. CVT performance is advancing daily and is a great way to transfer engine HP to the driveline.

Recently, car manufacturers have started using CVT’s in their vehicles. No more transmission, no more shifting, and peak performance at all times. It’s great to see years of hard work in snowmobile clutching now being transfered to the auto industry. The tuning components for the snowmobile CVT’s over the last couple decades may have looked similar to the untrained eye, but years of experimentation by great racers and tuners show in the results produced by today’s modern clutches and components.

The smart racers will tell you the engines horsepower is the key to a fast snowmobile, but the clutching is the brains behind it all.
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Jason Houle owns Straightline Performance in Forest Lake, MN. He test rides with, and contributes for AmSnow. He has been racing, wrenching, and tuning in the industry since the early 1980s. More info: www.straightlineperformance.com
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