Thunder Shift Kit Fine Tunes Clutching
By Steve Janes

When it comes to maximizing clutch performance for optimum shift patterns, bottom end and top end performance, most of us don't have a clue how to fine tune the primary clutch. Our dealer may tell us to use certain weights with certain springs. And when we do, we may think we have solved the mystery in clutch setup. But basically, we still don't have a clue.
Throw into this question all the clutch components available today that claim to be "miracle cures" for all things. There are springs, weights, rollers, helixes...did I mention Springs and weights? And then bear in mind that every simple adjustment on one end changes the operation of the clutch at two or three other ends.

Right. Still no clue.

But suddenly, something comes along which seems to make some sense. Miraculously, all the secrets of the universe begin to unfold as two and two equals four and so forth. But it's such a simple, basic concept you have to wonder why it wasn't thought of before. That's the Thunder Shift Kit.

Last winter we spent some time with Lonn Peterson of Thunder Products, Richmond, MN, who told us we'd be impresses with his new Thunder Shift Kit. And although Lonn has had a good record with us when it comes to unique products which really work, we were somewhat apprehensive about this one. Something about putting screws in weights seemed just a little too simple.

Lonn came to our test facility with Jeff Meidinger of Dakota Performance, Wishek, ND. Since it required two, we figured this was going to get complicated.

Thunder Shift Kit (patented)
Here's the simple explanation of the clutch kit. You take a regular Comet-style clutch weight which fits Polaris and the new Arctic Cat clutches and the Yamaha YXR clutch. Each weight has three threaded holes where weighted fasteners can be added. The weight weighs about 39 grams. Washers for really fine tuning weight from .4 to 1.4 grams. The trick is to add fasteners and washers to the weight until it reaches the optimum weight. Depending on the performance you're looking for, that weight can be added close to the pin or far from the pin - holeshot vs. top end.

Now it's best to calibrate your tach since the ultimate performance will come with optimum rpm for your engine. The tach can easily be calibrated if you have the right tool...or it can be sent off for calibration. Once you determine the optimum rpm, you add mass to your weight until you're taching out where you want. Then it's a matter of rearranging the mass location on the weight to create that smooth, stable ride.

Right now you're probably thinking: "Wait a minute. How do I know what weights is the right on my clutch weight?"

For our tests, we were using a Polaris XCR 600 which uses 10 M Red weights, which weigh about 44 grams. So we started with 44 grams on the Thunder Shift Kit weight (using a calculator to reach the total and making the fasteners heavier near the post and decreasing in mass on the tip).

Field Tests
On the first run, we were turning too many rpms on the top end with a bounce-down in the tach. (Optimum rpm should be 8500.) With stock 10 M Reds we engaged at 4900 rpm ran up to 9000 rpm before settling all the weight down to 8200 rpm. With our first Thunder Shift Kit set-up we engaged at 4600 rpm, ran up to 8500 rpm before settling down at 8200.

We didn't mind the engagement, but we needed more rpm on the top, so we left the fastener closest to the pin alone, removed the fastener from the middle spot and added some weight on the fastener closest to the tip, making our weight 45.5 grams. The result: Engagement at 5000, peak rpm at 8600, settling at 8200. Again we needed more rpm so we took even more weight from the tip - now our weight is down to 45 grams. The result: engagement at 4900, peak at 8600, and settling down to 8400.

From the seat of our pants, this seemed to be an excellent shift pattern, much better than with the stock weights (and we were changing the weights back and forth to make certain there was a difference).

Our final adjustment was to take .4 grams off the first and third fasteners, bringing the weight down to 44.2 grams. Using a radar gun and computer, we noted a two mph change for the better with just this fine-tune adjustment. In backshifting potential, this translates to 27 feet in three seconds - a pretty good shot coming out of a corner.

We then went through the same process with an Arctic Cat Powder Special. Our results were even better. We picked up 3.5 mph and an unbelievably smooth shift pattern in just an hour's worth of tuning. Most of the improvement came in midrange and top end.

Lonn has done some serious R&D to make certain the kit stays together and performs as it should. He has tested the weights at 17,000 rpm on a chisel point, putting as much stress on them as possible. And they have passed every test.

No-Brainer Testing
Once Lonn and Jeff left town, it was time for the real testing. If this Thunder Shift Kit is as simple as they make it sound, then a real moron...such as a snowmobile journalist...should be able to make it work. Right?

Well, actually, right.

Taking another kit and working with an XCR 440, I managed to find almost 3 mph and an extremely smooth shift pattern in just a couple hours of testing. And the best part about it was that I did it myself.

The Thunder Shift Kit comes complete with three weights and billet fasteners and washers. The fasteners have a Nylock patch (commonly used in aircraft parts) which keep them in place during the tuning stage. The kit also includes Lock-Tite so that when you're done tuning, you can secure the weights without concern for them working their way out.