Now anyone can be a do-it-yourself clutch wizard.

Story and photos by Jerry Bassett

     We are strong believers in keeping things simple.  After all, snowmobiling is neither rocket science nor brain surgery.  However, it does have its share of "black arts."  Snowmobile clutching long has been one of those areas.  The arrival of the Thunder Shift kit may remove the mystical aura surrounding clutching.  And if it does, it will be because the designers of the Thunder Shift hold a basic belief in Keep It Simple Stupid (K.I.S.S.) technology.
     Many seasons ago we played around with simple-to-tune clutch systems like the Securistat Power Bloc and the Duclo Clutch.  Both were simple designs, but too simple and not effective enough to supplant the Comet and Polaris designs.  These two primary drive units have been at the forefront of snowmobile drivetrains for decades.
     A cult of clutch tuners has arisen and created many ideas as well as new components.  There are multi-angle helixes, notched ramps, and friction-free gizmos.  To the educated performance tuner, all of these special clutch parts make a great deal of sense, and will continue to do so.  But for many enthusiastic trail riders, including many performance-oriented riders, there is confusion.
     Lonn Peterson at Thunder Products suggests, "The Thunder Shift can remove the mask of black artistry and make maximizing a sled's potential realistic for virtually anyone who can handle a hex wrench and follow directions."
     After spending some time with Lonn this past fall, we would agree.  In fact, we suspect that you'll start hearing much more about Thunder Shift as the patented product starts making its way to consumers from distributors like Hot To Go Racing, Hot Seat Performance, Dakota Performance, Kinetic Engineering and others.  Racers and performance tuners are discovering that the Thunder Shift can put the engine on its peak power point and keep it there from holeshot to top end.  And it's not an arbitrary procedure that fails to take into account the not-so-little things like a driver's weight.  That's a potential failure of generic clutch kits.  power to weight is real.  If you weigh 275 pounds and your spousal unit weighs 115, your sled is not going to shift the same for both of you.  It will most likely over rev for the lighter weight if it's set up for the heavier rider.
     What you want is the ability to clutch specifically for how you intend to use your sled.  And it would be nice to be able to make quick clutching changes. 
     It appears that the Thunder Shift meets these criteria.  It is designed to fit the Comet and Polaris clutches as well as both the Comet/Cat and new aluminum Cat clutches.  The kit is simple.  It consists of three ramps and four types of hex-headed bolts with various weight shims that screw into the threaded ramp holes.  The range of adjustments can be made in increments of three-tenths of a gram with the total weight range running from about 39 to 66 grams.
     The secret to the system is that you can adjust the shift pattern by adjusting  the mass of the ramp.  Instead of grinding a ramp - usually via pure guess work - you can add or subtract weight from the of three positions on the ramp.  Simply add or subtract a fastener or washer.  Starting at the hole nearest the pivot point, you can affect your sled's holeshot and engagement.  The middle hole in the ramp affects midrange, the outermost position makes changes to engine rpm at top end.
     In theory, you use this kit, a calibrated tachometer and your knowledge of where the sled's engine delivers peak power to precisely tune the drivetrain.  The Thunder Shift kit comes with basic instructions and should give you an idea of the ideal ramp weight for your specific sled.  The Thunder Shift ramp weighs about 40 grams.  If you need 52 grams worth of weight per ramp, you would simply add 12 grams of additional weight to the Thunder Shift ramp.  you could start by placing four grams in each position.  Or, you could add more weight in the first position and split the difference between the other two.  However you start, you need to make a test run, watch your sled's tach and make adjustments.  For example, if the sled over revs at launch, under revs in the midrange and under revs at top end, you can add weight in the first position by removing weight from the other positions.  It should take a run of 700 to 800 feet to get the clutch to fully shift out.  Be sure that you have adequate shut down for safe testing.
     Another tip is to indicate the red line on the tach with a piece of tape.  Twist your tach so that the red line is straight up.  This lets you know safely and at a glance whether the engine is under or over revving.
     Your goal is to hit the throttle and have the engine rev up to its red line and stay there from launch to top end.  If your sled hits full power at 8250 rpm,  you want to see that tach spin to 8250 as quickly as possible and remain steady throughout the run.  Make adjustments accordingly.
     The Thunder Shift is extremely simple in concept and tuning ability.  If you add pipes or change porting, merely re-tune the Thunder Shift weights to meet the new engine requirements.  Another benefit is that you can remove the system and re-install the original equipment clutch weights when you go to sell the sled.  That way you can save the $149.95 suggested retail price of the kit and use it on another Comer, Polaris or Arctco-clutched sled.