Have some basic questions. If I wanted to make the primary clutch engage at a lower RPM would I go to a spring with less tension? Would I add more weight? When do you add weight vs. change springs? Once you get close to your ideal spring do you fine tune with weight?

 

You do the opposite.Get yourself the Aaen clutch tuning manual and it will real enlighten you to how these things work.

 

weight controls the RPM max the engine can turn. This is where you find out what RPM your engine makes max HP and then add or subtract weight to dial in that RPM.

Spring rate is engagement RPM and full shift out. You also need to take into consideration the secondary clutch helix ramp angle and spring tension.

Yep, if you really want to know buy the book from Aaen on clutch tuning. The other option is to get a kit already to go for your application. The trial and error has already been done for you.

 

Good info, thanks. If I tinker with the clucth I will get the book from Aaen first. And even if I don't tinker with the clutch I would like to know more about tuning. More than likely I will buy a kit as suggested since it's proven. Although, it might be nice to slightly tweak those setting for rider preference, trail type, and altitude.

 

FYI

Clutching For Power Sled running like a pooch? Understanding how a clutch works is the first step in unleashing your sled’s hidden power. Last fall I had an interesting discussion with a friend of mine — a long-time snowmobiler who is brand loyal to his blue machines. His 700 was fast, but he was tired of getting smoked by smaller-displacement sleds.
“I think I need pipes,” he said.
“Pipes are good. They’re lighter and they definitely sound cool,” I replied.
But the more I talked to him, the more I realized he didn’t want pipes at all. He wanted his sled to be trailable. And he didn’t want to change jets every time the mercury varied a few degrees. I convinced him to try dialling-in the clutching instead.
We took his sled to an open field we use for testing and set up timing lights. We tried many combinations of weights, springs and helixes. In the end, we had 18 more grams of weight in the primary clutch and had dropped the sled’s 60-foot times by more than a full second. A 700 cc triple that was getting beaten by 600 cc twins was now running faster than an 800 cc twin at the 500-foot mark. The interesting thing is, most people — even veteran snowmobilers like my friend — would never guess how radical a change clutching adjustments can make.
How is this possible? To answer, we must look at how a snowmobile’s clutches transfer power. We’ll use a Black Magic clutch kit we received for a 2002 Arctic Cat ZR 800 to illustrate.

BACK TO BASICS
A snowmobile has two clutches: a primary, or drive clutch; and a secondary, or driven clutch. They are connected by a ribbed rubber belt. The primary clutch consists of a stationary sheave, a moveable sheave, a spider assembly, a spring, three (or four in some cases) cam arms, and a cover. The exception to this is a Ski-Doo primary, which doesn’t have cam arms like the other three brands. Instead, a Ski-Doo has TRA arms and ramps, though the two systems work basically the same way. The secondary clutch consists of a stationary sheave, a moveable sheave, a spring, a helix and a retainer plate.
The primary clutch bolts directly to the crankshaft of the motor. Whenever the motor is running, the primary clutch is spinning. The secondary clutch connects to the jackshaft of the snowmobile. It only spins when the primary clutch begins to shift out and grabs the belt.
Both clutches move two ways. They spin on their respective shafts and they slide in and out. At idle, the belt is at the bottom of the sheaves on the primary clutch. As rpm increases, the primary clutch senses torque input from the motor. At a certain rpm, it will turn fast enough that the cam arms will begin to move and overcome the pressure of the spring in the primary. When this happens, the moveable sheave slides in on the shaft of the fixed sheave, thereby pinching the belt and causing it to spin the secondary clutch. This is called engagement, and it’s critical to a snowmobile’s performance. If engagement rpm is too low, the sled will bog; too high, and the sled will jerk forward and damage the belt.
As rpm continues to increase, the moveable sheave on the primary clutch continues to slide in on the shaft of the stationary sheave. As this happens, the belt climbs up the sheaves. When the sheaves are all the way together, the clutches are fully engaged. This is commonly called “shift out.” At this point, the belt will be at or near the top of the sheaves on the primary clutch.
Like engagement, shift out is also critical to the performance of a snowmobile. Shift out should occur just before the peak torque point of a snowmobile’s engine. If it occurs before this point, the sled won’t be utilizing all its power. If it occurs after, the sled will over-rev and power will drop off on top end.
As rpm increases and the belt moves from the bottom to the top of the sheaves on the primary clutch, just the opposite is happening with the secondary clutch. At idle, the belt sits at the top of the sheaves on the secondary clutch. As rpm increases and the belt begins to climb the sheaves on the primary, the sheaves of the secondary are being forced open by the pressure of the belt. When the secondary clutch is fully shifted out, the belt is at the bottom of the sheaves. At this point, the snowmobile is in (or very close to) a 1:1 drive ratio.
In essence, the secondary simply reacts to input from the belt, and that input is determined by how the primary is reacting to power output from the engine. READ PART 2
Clutching For Power, continued A REAL LIFE EXAMPLE
So how do all these parts work to make a snowmobile go? A good way to explain this is to look at how a clutch kit modifies the performance of a sled's clutches. Our Black Magic kit covered all the tuning variables: it included a softer-compound belt, a new helix, new primary and secondary clutch springs, two sets of cam arms, and helix shims. Let’s look at what we are trying to accomplish with this kit and why.
A snowmobile should be clutched to run where the engine makes the most power. A fan-cooled 440 will require a different clutch setup than an 800 cc twin simply because the two motors make different amounts of power at different rpm. In a nutshell, a fan-cooled 440 won’t need as heavy a clutch setup as an 800 cc twin.
One set of cam arms in the kit was two grams heavier than stock and the other was four grams heavier. The heavier weights are designed to load the motor more and use more of its power. However, if we just put the heavier cam arms in, the clutch would load the motor more but our engagement would drop because the spring would quickly get overloaded by the weights. In other words, our sled would bog off the line. (Alternately, if the cam arms were too light, the clutch would engage at too high an rpm because the cam arms would not be able to overcome the spring.) That’s why Black Magic sends a heavier primary spring with its kit — the cam arms and the primary spring work together to determine how the clutch reacts to engine rpm.
Now that we have our heavier cam arms and primary spring installed, it’s time to look at the secondary and its parts. Once the primary engages at the desired rpm and begins to pull the belt, the secondary comes into play.
The helix and the spring determine the rate at which the secondary sheaves open. A helix is basically a ramp that the secondary retainer plate rides on as the secondary’s sheaves open and close. The angle of the helix ramp determines in part how the secondary performs.
Helixes come in two basic configurations: single and dual angle. A single-angle helix has a ramp that allows the secondary to open at a constant rate. A dual-angle helix allows the secondary to begin opening at one rate then changes that rate, usually shortly after engagement.
The helix in our kit had a dual angle. A steep angle at engagement allows the secondary sheaves to shift out quickly. As the rollers travel along the ramp of the helix, they encounter the second angle, slowing down the movement.
For trail riding, this works great. We get a nice jump off the line because the belt drops quickly into the sheaves, but we also get a nice back shift when we let off the throttle because the rest of the helix ramp is at a shallower angle. This allows the secondary sheaves to come together or “back shift” quicker. If we had a steep ramp across the entire helix, we’d get good acceleration but poor back shift. In other words, when we let off the throttle (in a turn perhaps) the clutches would stay engaged to some extent. When we got back on the throttle, the sled would bog because the clutches would still be shifted out.
The secondary spring, unlike the primary spring, works two ways. It compresses and releases like the primary spring, but it also works torsionally — it twists inside the secondary as the sheaves open and close. The stiffer secondary spring in the kit will make the secondary sheaves resist opening more than the stock spring did, so the sled will grip the belt more in the secondary. The belt will have more force put into it as a result, and it will also back shift with more force.

THE KICKER
We’ve broken down the different clutch components and what they do. We’ve also modified our clutches to make them work differently. So why isn’t this a guarantee that our sled will run better? Because there is no universal clutch setup that will work for all riders.
Snowmobiles are set up by each factory for an average rider weight, yet a heavier rider will require a different clutch setup than a lighter rider. This is because a heavier rider in a sense robs horsepower from the engine.
Seven pounds of weight equals about one horsepower. That means our Cat is making 8.5 hp less when a 220-pound rider is aboard, versus a 160 pounder. That’s the difference between a 500 cc and a 600 cc twin!
If we clutched an 800 cc Cat to optimum levels for a light rider, and then had a heavyweight get on it, it would be the same thing as if we put cam arms that were too heavy into the primary clutch. A tuner has to compensate for rider weight with weight in the cam arms.

THE CLUTCH COMPROMISE
While there is no universal clutch setup for a snowmobile, a clutch kit is a good compromise. Most make a sled run better, come with clear instructions, and are easy to install. They are a good alternative for those who don’t have a lot of time and money to spend dialling-in a sled’s clutches.
However, a complete kit isn’t always necessary; individual parts are also available. Many racers use adjustable cam arms that add and subtract weight by use of screws and washers. The advantage to this is you don’t need a box full of weights to find the right setup. Also, each sled manufacturer offers a number of spring, cam arm and helix options.
Want to make your sled perform better? Look at the clutches first.

CAM ARMS
Ever wonder how cam arms work? It’s all about weight, and where it’s placed.
• Weight near a cam arm’s pin controls engagement and backshift.
• Weight in the middle of the arm controls midrange.
• Weight near the tip controls top end.

To illustrate, let’s look at two extreme examples:
Snowcross sleds rarely go over 50 km/h, but they accelerate as hard as any drag sled. A drag race setup would not work for a snowcross sled because a snowcross sled needs a quick back shift. Cam arms in a snowcross sled’s primary clutch have most of the weight at the pin and in the middle of the arm. This makes the primary clutch engage hard and run hard from engagement through midrange. However, the sled struggles on top end. The clutch also back shifts quickly when the rider lets off the gas in a corner or off a jump and the power is “right there” when the rider gets back on the gas.
In drag racing, the objective is to get from point A to point B in the shortest amount of time, without worry about whether the sled back shifts or is “rideable.” For this reason, the clutches need to be tuned so they load the motor as much as possible for the entire length of the run. A drag engine has as stiff a spring as possible in the primary with as heavy a cam arm as the motor can handle. The secondary most likely has a steep-angle helix to get the belt low in the secondary as quickly as possible. Those who have been to a drag race may have noticed that the sleds coast quite well at the finish of the race because they don’t back shift well.

 

more FYI

More Performance For Your Polaris-May’99

Editors note: Olav Aaen is the owner of Aaen Performance, which specializes in high-performance mods for Polaris ATVs and snowmobiles. He graciously agreed to provide Dirt Wheels readers with his years of expertise in the fine tuning of the Polaris automatic transmission.
When Polaris first introduced their ATV line, the transmission was new to the ATV market. To Polaris it was old hat, a direct transfer of technology from their successful snowmobile line.
The automatic V-belt transmission is easy to use; you can basically forget about clutching and shifting gears, it is all taken care of for you. Being the new kid on the block with a strange concept to those who were used to riding with gearboxes, the Polaris Variable Transmission (PVT) took a lot of ribbing in the beginning, but also gained a lot of converts who liked the easy operation.
AUTOMATIC vs. MANUAL SHIFTERS
Multi-speed, manual shift gearboxs give you very definite changes in acceleration rates as you shift gears, and the smooth transition from low to high range with the PVT transmission is often deceiving, because you don’t feel any distinct shiftpoint. Lining up the PVT next to a gearbox machine can often be an eye-opener.
In my experience, the automatic transmission is usually the winner in a drag race, if the engine output and the weight of the machines are the same. This is because once the PVT transmission starts shifting, it holds the engine on the power peak, while the gearbox-equipped quad has to run up and down the power curve as the rider shifts gears. As a result, the PVT rider gets more power to the ground, and while the gearbox rider is busy shifting gears, the PVT pulls away. Another advantage of the PVT is that it gives the rider an uninterrupted flow of power which helps in heavy load conditions such as mud, sand, and long uphills.
STOCK VS. MODIFIED
The PVT is calibrated from Polaris to run well in all conditions, and to have a smooth takeoff that can handle larger loads. Tuning on the transmission is necessary if you want a higher stall speed for better acceleration, or if you added aftermarket speed parts such as pipes, cylinder porting, bigger carbs, big bore kits, etc., If you add speed parts that are designed to produce the power at a different engine speed than your stocker, you have to retune the transmission so it can run at the new power peak, or you will more than likely lose power.
In spite of its intimidating appearance, the Polaris PVT is actually easy to work on. There is a large selection of tuning parts from Polaris and from aftermarket speed shops. Once you dive into this transmission, terms such as flyweights, helix angles, spring rates and preloads will be a new part of your language as you communicate with your dealer and the various aftermarket speed shops.
HOW A PVT WORKS
To be able to tune, you first have to understand how an automatic transmission works. The Polaris transmission uses two sets of sheaves connected by a V-belt. The driving sheaves are mounted to the crankshaft, and the driven sheaves are mounted on the gearbox input shaft. The driving sheaves on the engine are often referred to as the primary clutch, and the driven sheaves on the gearbox are often called the secondary clutch. All you have to do to get moving with a PVT is to hit the throttle and let the engine speed build up from idle.
A set of flyweights in the primary clutch works against a pressure spring, and when the centrifugal force from the engine RPM is strong enough to overcome the preload of the pressure spring, the outside sheave starts to move toward the belt. At the engagement point, the belt is gripped by the sheaves and the machine starts moving forward. This is referred to as the engagement speed.
The rpm at which the belt starts shifting into higher ratios is called the shift speed. The trick for the tuner is to come up with a combination of springs, flyweights, and helix angles that puts the shift speed at the power peak of the engine.
GET THE TOOLS FOR THE JOB
To accurately tune your PVT clutch you will need to invest in a good engine tachometer. You have to know what rpm the engine is running at, in order to know how how each change affected your engagement speed and shift speed. There are a number of tachs which work on the Polaris ATVs. We prefer the large VDO playback tach, where you can replay your test run in 1/3 slow motion and see exactly how the RPM changes.
Another good investment is the Avenger
This tach is digital, and it also doubles as an exhaust temperature gauge, so you can keep track of your carburetor tuning at the same time. Both these tachs are expensive, but are safe to operate because they both have playback functions so you can keep your eyes on the road when you are testing. If you only want a quick reference, the digital "Tiny Tach" sells for only $58 and is acceptable as a reference tool.
TESTING, 1, 2, 3,
To test the transmission you need a 1/4-mile stretch where you can accelerate from a standstill to top-end, and record the changes in engine speed. Ideally, the engine speed should be controlled by changing to heavier or lighter flyweights, but changes can also be affected by spring rates and helix angles.
Increasing the engagement speed is usually accomplished by installing a spring with higher pre-tension in the primary clutch. The flyweights need a higher centrifugal force to overcome the higher pre-tension, and as a result the engine speed will increase before engagement with the belt.
The pressure spring is easy to change. After you remove the transmission cover, you remove the belt from the transmission. To change the spring you first need to unscrew the center mounting bolt. You then unscrew the six bolts holding the cover in place, and you are ready to change the spring. Getting the cover back on with a stiffer spring can be a hassle. A simple trick is to get a large washer that fits over the center mounting bolt and screw the cover in to where you can install the 6 cover bolts.
MORE CHANGES
If you want larger changes in engine speed, you will have to change the flyweights. Lighter flyweights will increase the shift speed, while heavier weights will reduce the shift rpm. Flyweights are designated in grams, and if you wanted a higher shift speed, you might go down from the stock 55 gram weight.
There are 3 flyweights in each primary clutch. They are mounted with short bolts in the movable sheave, and they push against rollers in the center spider. To remove the flyweights they need to be away from the spider. If the belt and cover is off this is easy, just push the movable sheave all the way closed. If the cover is on, use a large screwdriver as a lever behind the spider and force the moving sheave over. Put an aluminum block between the spider and the sheave to hold the flyweights away from the rollers. You must remember to remove the block again before you start the engine, or it will be flying out of the clutch and cause damage to whatever it hits. It is always a good idea to install the belt and cover before you start up the engine after having worked on the transmission.
TUNING SECONDARY CLUTCH
When tuning the secondary clutch on the PVT you are going to have to remove it in order to modify its performance. To remove it, take out the center screw and slide the clutch off the splines on the transmission shaft. No pullers are necessary unless it is rusted to the shaft. On the backside of the clutch the helix is held in place by a snapring. Be careful when you remove the snapring, because the preload on the torsion spring behind the spider will force it off.
There are a number of reasons why you would want to remove the helix. You may want to tighten or loosen the torsion spring. Tightening the pre-tension by moving to the next mounting hole will increase the shift speed slightly, and improve the downshift in heavy load conditions. You may also want to change the spring altogether. Springs with higher rates increase shift speeds. Larger helix angles increase acceleration, but may reduce the shift speed too much. To correct this there are compound angle helixes which reduce the angle at higher speeds to keep the shiftspeed up. The 44-36 degree angle helix is a good example. This helix starts out at a 44 degree angle for good acceleration, and ends up with a 36 degree angle for good top-end speed.
BUTTONS AND ROLLERS
The helix angle surfaces slide against a plastic button on the stock PVT’s. There is up to 10 percent friction between these surfaces. A teflon- coated helix reduces the friction, while a rollerized clutch, where the cam rides against ball bearings, eliminates friction altogether. Reducing friction results in a secondary clutch that reacts more quickly to load changes and also accelerates harder out of corners.
Whatever setup you choose, there are a few things to watch for when you mount the helix back on the shaft. First, you should notice that there is a tooth missing in the helix spline. This missing tooth has to line up with a spline that has not been cut all the way on the shaft. The helix only goes back on again in this position. The spring goes in first in the preload holes you have selected.
The next step is where 80 percent of first-timers make a mistake! If you push the helix straight down, you will have no torsional preload, and the machine will bog down on you for the first hundred feet of travel. To preload the torsion spring you need to twist the movable sheave up to a 1/3 turn in the counter-clockwise direction and then push the helix down while making sure it engages with the sliding buttons. It sounds tricky, but it really isn’t when you get the hang of it. Have a friend help you the first couple of times. With the helix pushed down, install the snapring again.
Changing the tuning component is easy and quick once you get used to it, and it is a lot of fun to find trick combinations that give you the edge you need to blast your buddy’s stock PVT away. Practice makes perfect, but once you start tuning your Polaris clutch you will be able to dial it in for that little bit of an edge over your buddy’s machine.
If you need more help, I sell a clutch tuning handbook that details the PVT systems for both ATVs and snowmobiles. Contact Aaen Performance at (414) 552-8981.

 

jimmypsp700

Thanks so much for that very detailed info. That is exactly what I was looking for. To my surprise it turns out I know more about clutch tuning than I thought. My only question goes back to your situation. When you put too much weight on the primary, the engagement RPM lowered. Then to combat that you added a heavier spring to fight the lowered RPM engagement. By increasing the weight and then increasing the spring have you not just negated the affect of the weight? In other words, did all the benefit of the heavier arms get wiped out by the stiffer spring? Or did the heavier weight still have an affect but only at high RPMs? Also, do they make progressive springs (rising rate) for the primary or the secondary.

 

The info was copied online and not my own. You may want to ask Rick Ritter, but I don't think that's the case.

On my Scrambler I went to heaver springs in the primary and secondary with lighter weights, from the 55 grams stock to 53.
This raises the engagement rpm and improved back shifting.

 

polaridoo did u ever get that sled motor up and running in the atv? do u have some pics?

 

No, not yet. But it is still on the 'things to do list'. I am pretty sure it will consist of an air cooled 440 - 550 twin cylinder snowmobile engine and go into a later model Trailblazer. I have been watching Ebay for the right deal on parts to complete the project. Hopefully soon. I see most of this project as being fairly straight forward with the exception of making the pipe work. There is a brand new 550 twin on Ebay now but it's a bit more than I prefer to pay... still not bad since it is a complete motor (CDI, carbs, pull start - but no pipe).