TAPP Snowmobile Clutch Tuning - Part 2 – The Shift Arms
The TAPP primary snowmobile clutch is closer to the SkiDoo TRA in its design concept than it is to a more common swing arm style like that of the Polaris, Yamaha, Arctic Cat, Comet, and others. However, a large part of the appeal of the TAPP clutch is that it is a 4-arm clutch which greatly increases the adjustability. This increase is because tuning changes can be made to pairs of arms (always across from each other to keep the clutch in balance) rather than having to make changes to all the arms in the clutch. This is a significant advantage when compared to a 3-arm clutch.
Back to the focus of this post, the shift arm. The shift arm swings or pivots on a pin at its base. At the tip, a bolt holds a roller in the center between the two sides of the arm. As RPM increases, the weight of the shift arm forces it out against the ramp. When the RPM is high enough, the arm climbs the engagement “hump” of the ramp and engages the belt.
The weight of the arm affects RPM and can be adjusted in three ways. First, different length roller bolts can be used in combination with washers under the head and nut of the bolt on the outside of the arms. The combinations of bolts, washers and heavy “race weight” washers yield a lot of adjustment in the arms. More weight on the arms makes them pull harder as they have more centrifugal force and lowers RPM, less weight shifts less aggressively and raises RPM. In effect, it’s like going to heavier or lighter weights in the more common style of clutches. The cover of the TAPP clutch must be removed to make roller bolt and washer adjustments to the shift arms.
The second way the weight of the arms can be adjusted is with different rollers. Different size rollers are available which weigh different amounts. There are the base rollers which weight 9.9 grams and plus size rollers as follows:
- Size +1 ( 16.5 mm ) – 12.5 G
- Size +2 ( 17.5 mm ) – 15.3 G
There are other considerations with respect to roller size, but we’ll save those for an advanced tuning tips post. Like the roller bolts, the cover of the clutch must be removed to make roller changes.
The third way the weight of the arms can be changed is through Allen screws inserted into the threaded holes in the ends of the arms. Inserting screws into the front side of the arms is easy and can be done without removing the cover from the clutch. As always, if screws are added always add them equally in opposing arms. Additionally, if only one screw is added per arm, say in the right side of the arm, be sure to add the screw to the opposing arm in the same location to keep the clutch in balance. Screws added to the front side of the arm will affect RPM lower in the shift curve. Screws added to the back side of the arm will affect RPM less at engagement and more equally throughout the shift curve.
So there is a lot of adjustability in the weight of the arms. How much does it make a difference? Each total gram of weight added to the arms makes about a 40 running rpm difference. I say total weight because you must always add or subtract weight in pairs of arms. For example, adding a washer (which weighs about 1.2 grams) behind the head of one bolt on a pair of arms adds 2.4 total grams of weight to the clutch. This total weight would make about a 100 rpm (2.4 x 40) difference (give or take). When adjusting engagement, heavier arms will have a lower engagement RPM than lighter weight configured arms.
In part 3 of this series, we’ll discuss the TAPP clutch ramps.
