Ideal Clutch Tuning

"Dear Ralph" September 27, 1995 0
Dear Ralph: Much has been made of the multi-angle helix in the driven clutch. I’ve heard and read that it can substantially improve acceleration...

Dear Ralph:

Much has been made of the multi-angle helix in the driven clutch. I’ve heard and read that it can substantially improve acceleration without hurting backshifting in most models. Snowmobile shows are rife with machinists that have “their own” version of this little item. They all claim that their version is best. What do you think ?

Howard McCollister
Crosby, MN

When it comes to clutching, you will find plenty of people who claim that they understand how it works. Many will tell you that all the others (their competition) who think they understand clutching really don’t, but they do. It is actually a very complex subject that could cover volumes of information. Here’s my two cents worth.

You have two sides of clutching, the theory side (how it is supposed to work) and the practical application (what really does work). You will find disagreements on the theory of how clutching is supposed to work, but nobody can argue with what does work. And, it is a proven fact that multi-angle do helixes work.

However, I think the (very effective) multi-angle helixes are nothing more than a “band-aid” for the inability of clutch tuners to control over-rev conditions. A steeper initial angle loads the motor quicker, resulting in improved acceleration (if the motor isn’t loaded to the point that it bogs).

True CVT clutching theory indicates that the main purpose of the secondary (driven) clutch is to provide just enough belt tension to prevent slippage, and good backshifting characteristics (the torque sensing aspect). Spring rates and helix angles should be selected to satisfy these criteria first. All RPM control (ideally) should be done by the primary (drive) clutch.

Therefore, the main purpose of the primary clutch is to control engine RPM and upshift characteristics. You want to apply the load to the engine as fast as possible without bogging it. Due to the inability (and sometimes lack of understanding) of tuners to do this (control the engine rpms with the drive clutch), they have used the “back-door” method (multi-angle helixes) to help perform the engine loading function. Still, it is a very effective way to get better performance out of a machine.

A steeper initial angle (at the top) on the helix upshifts the clutches faster, placing more load on the engine quicker – but on the flip side it slows backshift response. A lower angle gives a more responsive backshift, but a slower upshift. Typical multi-angle helixes use a combination of angles with “shift points” rather arbitrarily selected to help load the engine based somewhat on it’s power curve. If a sled has a 36 degree angle as stock, the tuner may select, say, a 42-36 degree helix for a stronger initial upshift, then come back to the standard angle for backshifting characteristics. The problem comes in selecting the initial angle, the point (on the helix) at which the angle decreases, and the final angle. Was the stock angle selected as a compromise, or was it picked as the best selection for backshifting ?

In my opinion, the IDEAL way to approach clutching is to start with a straight angle helix, with the angle selected for backshifting characteristics, and the secondary spring tension as loose as possible while still preventing belt slippage. Then all engine RPM tuning should be done with the spring and weights in the primary. The stock spring is usually pretty good, the problem is with the weights. Usually there is not enough mass up by the pivot to control the over rev (especially on the higher horsepower machines) – and this is where the multi-angle helix comes in.

Typical RPM tuning has been done by installing heavier or lighter weights, usually with the same curvature and weight distribution ratio from pivot to tip. The mass of the weight is selected more for peak shift RPM, with the low end and mid-range shift characteristics pretty much being “settled for” as is.

Grinding some of the material off of a heavier weight has been one way to change the effect of “off-the-shelf” weights. But, up until recently the art of manipulating the distribution of the mass (from pivot to tip) in a weight has been a “secret”, or confined to those who grind sets for their own custom application.

The introduction of new “adjustable” weights (Thunder Shift Kit) open up the potential to tune the drive clutch the way it should be done (to a finer degree). The adjustable feature of these weights allow the tuner to vary engine RPM in distinct phases of the upshift. Adding more mass to the pivot area of the weight has the same effect as the steeper initial angle on a multi-angle helix. Instead of being stuck with choosing a weight and it’s total mass suited for the top-end shift rpm, you can manipulate the low end and mid range and still end up with your desired shift rpm.

Granted, there will be some gains to be made with additional grinding and further tweaking with helix angles and spring rates, but for the most part we now have a greater ability to keep the RPM control up front where it belongs.

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