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Polar ATAC Clutch
By Stephen Payne

Principle of Operation:This clutch uses a fixed ramp comprising of an engagement angle and a shifting angle.The moveable shieve is positioned by a roller, linked to the spider. The roller moves up and down the ramp based on spring pressure, amount of weight attached to the roller and the RPM of the engine.The engagement area of the ramp is where the roller contacts initially.The angle of the engagement area, the amount of weight on the arm and the spring pressure will determine the actual engagement RPM.The angle of the engagement area and spring pressure will have the most affect of engagement RPM.Steeper angles and higher spring pressure increase engagement RPM, while shallower angles and softer springs reduce engagement RPM.

The roller is attatched to the spyder with the arm. Centrifugal force pushes the ramp toward the belt.

Different angle ramps are available for different engine and snowmobile weight combinations. The shifting part of the ramp determines the running RPM of the engine while moving. Shallow ramp angles and/or lighter weights result in more engine RPM while shifting. Steeper angles and/or heavier weights reduce engine RPM while shifting.

As engine speed is increased from idle, the roller will force the ramp to move towards the belt. The ramp is attached to the moveable shieve, so that they move together.

The roller should reach the end of the engagement area at the same time as the moveable shieve touches the belt. This will result in a smooth engagement. If the roller reaches the end of the engagement ramp and there is some clearance between the moveable shieve and the belt, this will cause a harsher engagement. The harshness will be a function of the belt clearance.

As the roller moves on the shifting part of the ramp it will cause the moveable shieve to position closer to the fixed shieve, causing the belt to move up and change ratios. The up-shift will be a function of throttle position and load for a given ramp angle and roller weight.

The spring pressure has most effect on engagement RPM and very small amount on the shifting speed of the engine. A heavy spring will affect shift RPM a small amount.

The weights are bolted one on each side of the arm where the roller is located. The weights are available in different sizes to adjust the running speed of the engine with a given ramp angle.

There is a large diameter shim between the moveable shieve and the spider to vary the roller position at engagement. Two thick shims (.06 ea.) engage at 4500 RPM with a six degree engagement angle. Engagement will go up 500 RPM for each .03 in shim thickness reduction. The smaller shim fits over the post to accommodate varying belt widths.

Different sizes of shims are available to fine tune roller position and belt clearances.

The clutch will come calibrated for a specific snowmobile. Modification to the engine, driven clutch or weight of the snowmobile will require fine tuning.

The following are examples:

  1. Engagement is fine, but shifting RPM is too high. The corrective action in this case is to increase the amount of weight on the arm (7 gram to 8 gram), if the RPM is 600 RPM or less. If the RPM is 600 greater than is required, a ramp with a steeper shifting angle is needed (40 to a 45 degree angle).
  2. Engagement too low. The corrective action in this case is to install a stronger spring (200-400 RPM). If more RPM is required, then a ramp with a steeper engagement angle is required (15-12 degree).
  3. Engagement is too harsh. The corrective action in this case is to remove belt clearance. Changing the thickness of the small shim to a thinner one will reduce belt clearance (0.020 inches will have a noticeable effect).

Contact Us:

Polar Motorsport International Inc.
171 Catherine Avenue
RR2 Carleton Place, Ontario
Canada K7C 4L5