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Gearing Set-up

"Dear Ralph" September 16, 2004 0
Dear Ralph: I have a question that may be difficult to answer but here goes. You always hear people say that dealer setup can...

Dear Ralph:
I have a question that may be difficult to answer but here goes. You always hear people say that dealer setup can have a lot to do with how good a sled goes. Assuming the sled is setup per manufacturer spec (e.g. track tension, belt alignment, deflection, etc…) how can one dealer can get increased performance out of the same machine vs. another dealer’s setup? To put it another way, is manufacturer spec just a starting point/guide and if so, does it ultimately come down to trial and error/feel?

My next question deals with gearing. As noted, I have an ‘04 Ski-Doo MXZ REV 800HO w/ RER. The sled is bone stock. Best top end I saw this past winter was 108 mph on the digital speedo. That infamous group I refer to as “most people” claim that this, like most sleds, is geared for a top end speed that is impossible to reach (i.e. not enough horse power in the stock sled). If that’s true, is there something to be gained in dropping down a tooth? If so, what is the best way to find out the optimum gearing (top & bottom). Again, the dealer isn’t likely to be of much help because this would put the machine out of spec.

Finally, if you do change the gearing, what else needs to be changed. Does a gearing change impact carb, clutching (springs, weights, ramps, etc…), etc?
Fran Sarro

We could spend several pages properly answering all of these very good questions, but we’re limited on space. “Dealer set-up” can mean two things – how well the sled is set-up to manufacturer specs, and it could mean their own, proprietary calibration that they’ve found to work best for their customers in their area. In your case, dealer set-up is being restricted to factory specs. The key you mentioned is “assuming the sled is set-up per manufacturer’s specs”. There are a number of dealers who pretty much bolt the skis on, align and tension the track, do the programming bit and get the thing out the door. Other dealers will do set-ups by the book, and even check things they’ve found to cause machines to come back for service. A good set-up would align the skis and clutches (at least check them), synchronize the carbs and oil pump and cables, even temper the brake pads and hyfax. The term “set-up” is still fairly ambiguous, so yes, there can be a difference.

The manufacturer’s specifications provide an established baseline that they’ve tested and verified the safety and operation of the vehicle. Any changes made to increase the performance that deviate from this set of specs should logically indicate the manufacturer can not be expected to “make it right” if there is a problem related to your out-of-spec situation. There is plenty of adjustability to make your sled work better within the allowable specs. Most important is carb calibration and clutching calibration.

Gearing. Yes, most sleds are intentionally geared a bit tall, to the tune of 5-10 mph than the sled will be able to normally go, but you should not think of this being an “impossible” situation. It is an infrequent, yet ideal situation that gearing selections are made. Very cold morning, low elevation, hard surface, little snow to slow you down, long run. Think lake racing. The gearing is tall to keep the sled from shifting out fully and then starting to over rev. Visualize what would happen if you left your four-speed car in third gear and put the throttle down. The RPMs would climb and climb and climb, as long as you had the horsepower to pull it up to, and past the redline. Same thing goes for your sled. If the gearing is too low, you could shift the clutches all the way and the engine RPM would start to climb and climb and eventually over rev. Bad deal.

So, if you are aware of these possibilities then you can decide if lower gearing would be to your advantage. Maybe you ride at a bit higher elevation than sea level, or at temps warmer than bitter cold, or on deeper snow, or don’t line it up on the lake and hold it down for any length. All of these would make us believe we could gear down slightly from stock and see gains. Most of the time, you will. A slightly smaller top gear or larger bottom gear increases the amount of torque delivered to the drive axle. Acceleration and response should improve all the way from the bottom to the top, but not always. We’ve seen many sleds where it just doesn’t seem to make a difference, but it is usually worth asking how each sled responds to gearing changes. Think of your clutches as your transmission, and the chaincase gearing as your rear axle ratio on a car.

How do you know if the gearing is optimum? Compare your riding conditions to the “ideal”. The closer to ideal conditions, the closer stock gearing will be to ideal. As you deviate from ideal conditions, the gearing follows. As an extreme example, a rise in elevation means less power and normally deeper snow, thus gearing down is normally called for.

One way to see where you’re at is the good old magic marker on the sheaves test. You can do this anytime to see how far your clutches are opening and closing. Take a marker and draw lines from the hub out to the edge of the clutch (you can do this just on the primary for a quick easy check). Take the sled out and ride it, then shut it off and see how far up the clutch your belt went. The marker will be wiped away by the belt as it rides up and down the sheaves. If the belt isn’t coming very far up the sheaves, you know that for the conditions you just rode in lower gearing would allow you to shift out further (and would be like adding another gear to your transmission).

Most every time you do this will NOT be anywhere close to the ideal conditions the gearing is based on, so take that into consideration. You can always pick gearing for one set of conditions, just like carb jetting, but what happens when the conditions change? You get the idea.

Finally, gearing changes do affect all of the other clutching calibrations, but a one tooth change shouldn’t require a change in most cases. You may need to add some flyweight or run a steeper helix to keep the peak operating rpm where it should be. Knowing your target RPM is very important if you plan on doing anything with the gearing.

You also want to be sure you have the right length of chain for whatever gearing combination you use. It is always better to run as big of a top gear as possible, and the less you have to crank in the chain tensioner, the more efficient the system will be. Grab a calculator and do some figuring with the different gears offered for your sled and you’ll see many combinations come up with similar ratios, so plug this into the available chain lengths and see what options you have. Also realize that more torque at the drive axle isn’t going to help if you don’t have traction. A studded track will make the most of changes like this. And ALWAYS be wary of long runs on cold mornings, keep an eye on the tach!

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