This article first appeared in Race & Rally Magazine, Oct. 1995
Adjustable clutch weights? They are here! The Thunder Shift Kit will change the way clutches are tuned from this day forward. Even you can be a clutching “guru”.
The new Thunder Shift Kit from Thunder Products could change the way that you tune your Polaris, Arctic Cat, and Comet clutches from this point on. (Now also available for Yamaha and Ski-Doo.) The concept is simple, yet so effective that it is patented. With this kit you can create weights all the way from 39 to 65 grams. The shift pattern of the drive clutch can now be custom tailored to the power curve of the engine. No longer will you have to grind a set of weights to experiment with “what if” ideas, or settle for the “off the shelf” options. One set of weights will take care of shift RPM, dead spots, over or under loading the engine virtually anywhere in the shift pattern. Never before has a clutch kit offered such a level of tunability. Whether you are a clutching expert or just want to go faster, all tuners will recognize the precision tuning potential that this kit offers.
How would you like a 50 gram weight that allows you to vary the clutch engagement by about 400 RPM just by moving the location of the mass around? The shift pattern can be varied by 800 RPM with even more combinations that weigh exactly the same. It is just a matter of WHERE the weight is. Most tuners were limited in ability to simply change the total amount of mass in each weight (say from 50 to 52 grams). Now you can not only change your total mass of the weight, but have the ability to move that mass around. This gives you much greater control of the engine shift pattern.
The Thunder Shift Kit is a set of weights that have three carefully placed threaded holes in the arms which allow you to add or subtract weight to the arm at each of these holes in 0.3 gram increments. An assortment of about fifty pieces is included that consist of four types of small hex head bolts (with nylon locking patches) and two types of shims, allowing you to place an exact amount of weight at each hole location. The bolts and washers are made of three different types of materials which provide for almost endless combinations. The arm is offset from the pivot to provide room for the bolts and washers to be attached. Each arm by itself weighs from about 39 grams without and fasteners, and fully loaded can weigh up to 65 grams. This should provide adjustability for any engine from 400 to 900 cc.
The curvature of the arms is very similar to that of the Polaris #10 weights. The non-bushed (and now bushed) hole for the pin is located slightly lower, and there is more mass located closer and lower in relationship to the pin location. In comparison to typical Arctic Cat weights, they do not have the engagement notch that seems to cause the all too familiar clutch rattle. Engagement can be varied from about 4600 to 5200 RPM just by varying the amount of weight in the hole closest to the pin.
One of the biggest problems with primary clutching has been the limited tunability with regular weights. Off the shelf weights are usually selected to achieve the desired shift RPM, and you are pretty much stuck with the bottom end and midrange that those weights provide (unless you are into grinding custom weights). This is where the multi angle helixes have become so popular in their ability to compensate for our inability to “properly tune” the shift pattern of the drive clutch to the power curve of the engine.
A machine that upshifts quicker with a steeper angle helix would respond just al well to using a weight with more mass closer to the pivot. The steeper helix loads the engine down low and kills the over rev, allowing it to upshift faster. The Thunder Shift Kit will allow you to load the engine as quickly or as slowly as you would like, in three distinct tuning areas. Just watch your tachometer. If you’re over revving, add weight until you come down to the desired RPM. Likewise, if the RPMs are too low in a particular area, remove some weight. The important thing to remember is: the pivot hole controls the bottom end, the middle hole controls the midrange, and the tip hole controls the top end. Anything in between can be tuned with a combination of two holes.
The fact that piston ported engines and reed valve engines both come with the same basic type of OEM weights demonstrates a level of compromise. The greater torque of the reed engine allows you to load the motor quicker (more weight in holes 1 & 2) in comparison to a piston ported which has to be allowed to upshift faster (less weight in holes 1 & 2) to get into it’s power band.
This compromise continues whether it is rider weight or snow conditions. A heavier rider or wet, sticky snow will cause the engine to be loaded down more. Rather than replace components (or just live with it) you could now make adjustments.
Back at the end of April we made an early morning run to meet with Jeff Meidinger of Dakota Performance in Wishek, ND. Jeff had been performing much of the development and prototype testing of the Thunder Shift Kit through the winter and had set up a drag strip for us to do some tuning. The machines we had for testing were a Polaris Super Sport and XCR 600, ad Arctic Cat ZRT 800, and Jeff’s 732cc Polaris (bored and stroked 600 cc motor) with 46 mm carbs, and SLP triple pipes that dyno at 153 hp. We were able to control the engine RPM and tailor the shift pattern to each engine using the same set of weights, by using different combinations of fasteners. The Thunder Shift Kit improved the 732 by four sled lengths in 900 feet in comparison to the previous “best set up”. The stock clutching in the ZRT 800 was somewhat flat from 0 to 60 mph, but from 60 to over 100 it would just fly. After installing the Thunder Shift Kit we were able to tune the flat spot right out of the ZRT and gained eight sled lengths in 900 feet.
This could easily be the last clutch weights you’ll ever buy. Jeff Meidinger of Dakota Performance says, “I am very excited about the simplicity and tunability of this kit. From the stock and after market set ups out there I can take the Thunder Shift Kit and apply it to any sled and improve it considerably. Anybody with basic clutch knowledge can tune his or her sled to flat out arm straightening response, and still have excellent backshift at cruising speeds or hard trail riding. Big gains on the drag strip, radar runs and other applications are also test proven. The Thunder Shift Kit has all the clutching answers for any sled.”
Nine time winner at Eagle River World Championships, Steve Houle said, “After trying the Thunder Shift Kit for the first time, we knew it was going to be a great product. These weights give you the capability of changing the holeshot, midrange, or top-end shift RPM independently. One set of weights gives literally thousands of combinations to work with. The only thing better than its performance is its simplicity. Anyone with basic clutch knowledge can tune it!”