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TECHNICAL INFORMATION FOR POLAR CLUTCHES

How the Polar Junior clutch works,
by the guy who created it

By Stephen Payne

In the 10 years the Polar Junior clutch system has been in use, it has produced excellent results in reliability, consistency, and performance. While the stock setup works well for most cars, the optional tuning parts can really improve its performance.

Here's how it works: The drive clutch uses centrifugal* force to move a roller down a ramp and shift the clutch. (See Fig.1).The driven clutch uses a combination of springs and cam angles to control how quickly it will shift, which depends on the load needed to turn the wheels. This type of driven clutch is called "load sensitive."

THE DRIVE CLUTCH
The drive clutch has three pivoting arms bolted to the sheave(the part of the clutch that moves), with rollers on the ends of these arms. Once the engine starts, centrifugal force pushes the rollers along the ramp. As the amount of weight attached to the rollers is increased, so is the shifting force available to load the engine. The opposite is also true: when there is less weight, less force is available to move the rollers down the ramps.

The standard ramp has a shift angle of 28 degrees, changing to 22 degrees, but additional ramps angled up to 32 degrees are available. If the ramp angle is increased by any degree, the weighted roller will move faster down the ramp, it's really just like riding a bicycle: pedaling down a hill is easier then pedaling on flat ground. This will cause the clutch to shift faster and load down the engine (reduce RPM).

The drive clutch is designed to use the amount of weight on the arms to raise or lower the rpm of the engine when it is shifting on the way down the track, not for adjusting stall speed. Available for each clutch are weights ranging from 3 to 9 grams. If you find that you are using the heaviest on your car, you should change the ramp angle. The object of the game is to use the middle-range weights so, as the weather changes, you can compensate for the loss or gain of engine power.

The roller first touches a small section on the wide end of the ramp. The angle of this section determines stall speed with a given spring. Flatter angles and stiffer springs increase stall speed; steeper angles and softer springs decrease stall speed. If you want a high stall speed, it is better to change to stiffer springs before changing ramp angles.

The stock clutch stalls at about 4800 rpm with an 18 degree stall angle and 7 gram weights. This can be reduced to about 4400 rpm using a 22 degree stall angle on the ramp. The green spring will increase stall speed up to about 5200, and the heavy black and yellow spring will increase it to about 5600 rpm.

The Polar Junior drive clutch is very tunable, but remembers this: more weight or steeper ramp angles slow down the engine. Less weight or shallower angles speed up the engine. No clutch is perfect for every car. Tunability will allow you to make these adjustments for the consistency you want.

THE DRIVEN CLUTCH
The driven clutch takes the power from the drive clutch and turns the wheels on the car by way of the chain. The best system for mounting the driven clutch is on a jackshaft. This allows the best alignment and reduces flex, which helps the clutch belt last longest. The driven clutch uses a spring that is wound up by the cam to control how quickly it opens. The amount of load it takes to move the car also determines how quickly the driven clutch will shift. Tightening or loosening the spring has a small effect on how quickly the driver will shift. If the engine is mostly stock, the spring can be placed in the fourth whole in the cam. If the engine is modified (or you're not sure), use the fifth hole.

Steeper cam angles shift more quickly and increase efficiency. Shallower cam angles make shifting slower. Steeper angles are better for lighter cars that require less power to move, while shallower cams help heavier cars move better. The stock cam is 34 degrees, which will work in any car, but if your car is light, a 38 or 42 degree cam might produce a quicker e.t. The 46 degree cam is for light cars. Steeper cams can be used on heavier cars, but will need more spring tension.

What's the advantage of changing the cam? If the track is slippery, a shallow cam will not spin as much as a steeper cam. As a rule, a good-hooking track will produce lower ET's with a slightly steeper one. Increasing the spring pressure to the 6th hole will help the car be more consistent but at a slight sacrifice in ET.

Alignment dots on the Polar Junior clutch ensure that the cover is placed properly in relation to the movable sheave. The cover should be removed by using two bolts, each turned a half turn at a time, in the threaded holes near the retaining bolt.
The Polar Junior clutch uses bushing material, which should last a long time. Due to manufacturing tolerances, the clutch may not engage and shift smoothly when hot. Use a feeler gauge to check the clearance on all bushings (square or round). It should be .007 to .010 inch. If the clearance is tighter, increase it by gently scraping the inside of the old bushing with a sharp knife. With the springs out, all parts should slide on their own weight easily, and the proper clearance will help the clutch provide even better consistency.

PART 2: ON THE TRACK

Following is a typical tuning situation to help you know what to expect the first time out with Polar Junior. If you've already been running it for a while, keep reading. The following information may help you work out a few bugs.

First, let's presume you have installed a Polar Junior clutch on your Jr. Dragster. You made or bought a jackshaft motor mount and everything bolted up fine. The next step is to set center-to-center distance and offset. Use the special tool available from Polar or a piece of flat bar and a tape measure.

MEASURING CENTRE-TO-CENTRE DISTANCE
The centre-to-centre distance should be about 8 15/16 inches. If it's right, the belt will deflect only about ½ to ¾ inch in the middle. To set the belt tension, hold the driven clutch from moving with your right hand. With your left hand, push the belt backward around the driven clutch. If the belt is too tight, you won't be able to push it. If it is too loose, it will go very easily. When the belt tension is right, it will just go backward. At idle, the car will roll ahead slightly by dragging on the belt. A new belt might squeal a bit, but after it's broken in, it won't squeal unless it's too tight. Proper belt tension makes for easier staging.

MEASURING OFFSET
For the belt to be in perfect alignment between the drive clutch and the driven clutch, the two parts must be offset by 5/16 inch. Offset is measured from the back of the moveable sheave on the driven clutch (with the belt off) to the backside of the fixed face on the drive clutch. Proper offset makes the belt last longer and helps the clutch perform at its best.

If the centre-to-centre distance and offset are correct, putting the belt back on should be easy. With the belt already on the drive clutch, just push and turn slightly clockwise on the cam of the driven clutch so that you can slide the belt right onto the driven. (Tip: Removing the belt when towing will also help it last longer.) I prefer to set up the driven clutch with +/- 1/16" float from aligned. This helps let the belt optimize alignment during the shift.

GETTING READY
The clutch comes with a 18 degree stall and a 28 degree changing to 22 degree shift ramp, with seven gram weights on the arm. Weights ranging from three to nine grams are recommended for tuning a Jr. Dragster because no two race cars are the same. Polar Motorsport International does not recommend using weights heavier than nine grams.
A tachometer that records the run will help when dialing in the clutch. Note that the clutch stalls at about 4800 rpm. If the belt tension is correct, the driver will feel a small tug when the car moves.

REVIEWING THE DATA
Let's say the engine in your Jr. Dragster should make the most power at about 7500 rpm. The car ran an eighth mile e.t. of 10.19 with the old clutch system, but the time slip on the first pass with the Polar showed an e.t. of 10.15. The tach showed that the engine was running at 8500 rpm most of the way down the track. This is past the power band (remember? 7500 rpm). You know that adding weight to the clutch arms will reduce engine speed, so for the next pass, you replaced the four gram weights with six gram weights. With the heavier weights, the ET was much better: 9.95. The engine was in its power band at 7600 rpm, and driver felt the car pull all the way down.

MORE ADJUSTMENTS
With the old clutch, the gear ratio was changed to help load down the engine. With a 54 inch circumference tire, a 16 tooth top gear, and a 75 tooth bottom gear, that speed is 81.8 mph. Here's the formula, where TG equals Top Gear, BG equals Bottom Gear, and TC equals Tire Circumference:
MPH = [RPM x TG}
[ BG ] TC
1056 (constant no.)

Changing to the Polar clutch increased the car's speed to 58 mph. Switching to the six gram weights brought it up to 59.5 mph. What happens when you trade the stock 16 tooth top gear for a 12 tooth? The car runs a 9.93 at 59.4 mph, with the engine rpm at 7650.
Let's say the last problem left to solve is that the engine over revs to 8000 rpm up to the 60 foot mark, but runs fairly constant after that. Changing the ramp angle slightly for the first shift will solve the problem. You change to a combination of 18/30/22: 18 degree stall (same as stock), 30 degree first shift angle (loads the motor more), and 22 degrees for the last part of the ramp (same as stock). On the next pass, rpm remain constant at 7600 rpm. The ET is 9.91 at 60.0 mph. The next several time slips show consistent runs. Gearing the car for more MPH than it can go, gives a slower ET and highest MPH. The shorter gear ratio for least MPH will usually give the best ET. For consistency in bracket racing, a slightly taller gear ratio will give the best results.

That's it - a typical tuning situation with a new Polar Junior clutch. You may go through a similar process when tuning your own. Just remember that a good understanding of any component will always help you get maximum performance from it.

 
 

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