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.
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 ($5.95). Contact Aaen Performance at (414) 552-8981.