Tuning a Polar Clutch

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Tuning a Polar Clutch

Postby Honey » Sat Jan 10, 2009 9:58 pm

Tuning of Polar Clutch

Information from an article "Cold, Hard Facts" Part 2: On the Track. Written by Stephen Payne

In "Cold, Hard Facts, part 1" (JDR January/February '96), we learned that the Polar Junior clutch is a centrifugal system: The drive clutch shifts by using centrifugal force to move a set of rollers down a set of ramps. The weights on the ends of the rollers can be adjusted for tuning (the heavier the weight, the greater the load on the engine). The driven clutch uses a combination of springs and cam angles to control how quickly the clutch will shift.

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Theory is one thing. Using the Polar Junior on the track is another. Following is a typical tuning situation to help you know what to expect the first time out with the 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 bough 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 Performance or a piece of flat bar and a tape measure. Measuring center-to-center distance The center-to-center distance be about 8 15/16 inches. If it's right, the belt will deflect only about 1/2 to 3/4 inch in the middle. To set the belt tension hold the driven clutch in your right hand and squeeze the sheaves together. With your left hand, push the belt backward around the driven clutch. If the belt backward around the driven clutch. If the belt 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.

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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 center-to-center 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 on the cam of the driven clutch so that you can slide the belt right onto the driven. (Tip: Removing the belt when towing with also help it last longer.) Getting ready The clutch comes with a 12 degree stall and a 20 degree shift ramp with four gram weights on the arm. Weights ranging from three to eight grams are remommending for tuning a Jr. Dragster because no two race cars are the same. Polar Performance does not recommend using weights heavier than eight grams. A tachometer that the driver can see will help when dialing in the clutch. Note that the clutch stalls at about 4,500 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 7,500 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. Polar claims that its clutch will cause a car to lower its e.t. by two- to three-thenths of a second. What happened?

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A view of the Polar Junior from above shows the drive cluch should be offset from the driven clutch by 5/16 inch***** The tach showed that the engine was running at 8,500 rpm most of the way downtrack. This is past the powerband (remember? 7,500 rpm). You know that adding weight to the clutch arms will reduce engine speed, so for the next pass, you replaced the four-gram weighs with six-gram weights. With the heavier weights, the e.t. was much better: 10.04. The engine was in its powerband at 7,600 rpm, and the 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. Polar Performance believes that gear ratio should be changed only to help a car reach the speed it is capable of, not to increase load. And 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:

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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 9.96 at 60.8 mph. With engine rpm at 7,650. Lets say the last problem left to solve is that the engine over-revs to 8,000 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 combination of 12/24/20: 12-degree stall (same as stock), 24-degree first shift angle (loads the motor more), and 20 degrees stock). On the next pass, rpm remain constant at 7,600 rpm. The e.t. is 9.93 at 61.1 mph. The next several time slips show consistent runs. 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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