The Right Frame of Mind

Keeping Things Together

Published in the September 2008 Issue September 2008 White Out & Wide Open—The Blog, Snow Tests Jacob White

2005 Polaris RMK tunnel showing the running boards.

Even with a 140 hp fuel-injected, clean-burning, two-stroke engine with a 162-inch deep lug powder track, a snowmobile can only be as good as the chassis that powerplant and track have been bolted into.

The chassis can be the most fundamental and integral part to the way a machine performs and handles in the deep snow. Through the last 15 or so seasons, manufacturers have improved the "mountain" sled into something that snowmobilers should be proud to own.

The early 90s brought about the first real sleds dedicated for western mountain riding. Longer tracks and narrower ski stances were adopted into your basic trail model machines and the word "mountain" was used to describe these deep snow models.

The evolution of the mountain sled has come a long way since the first '94 Ski-Doo Summit 583 model was released. As manufacturers started looking for better deep snow flotation and maneuverability, the term lightweight became the driving force behind chassis design.

The switch to aluminum bulkheads reduced weight drastically while still maintaining the strength and integrity of the more rigid steel frames of years before. More recently, manufacturers have started using even lighter and stronger materials in the building of bulkheads and main frames with use of titanium and magnesium parts. As a result, chassis weights continue to decrease year after year.


Lower Center Of Gravity

Along with the change in materials used, the chassis' layout and design has improved as well. Today's rider-forward riding position has much improved deep snow maneuverability by lowering and centering the bulk of a sled's weight. The new design allows for the easy control of the machine and an overall better balance. Laydown engine designs; taller, more upright sitting positions; and taller handlebars are all improvements that have been made to enhance a sledder's ability in the backcountry to negotiate the trees, hillclimbing and deep powder.

If you need any proof as to how these changes in the chassis have improved the ride of today's snowmobiles, spend an afternoon on the older style sleds with the low "cruiser" style sitting position. After just 10-20 miles of your knees bouncing off the handlebars and your back scrunched over reaching for leverage to throw those old chassis' around, you'll better understand just what the manufacturers have improved with chassis design.

Tunnel design has improved as well. The introduction of the dropped and rolled chaincases allowed for deeper style lugs, flatter track approach angles and better snow evacuation from the tunnel, all areas that, again, improved deep snow flotation and maneuverability.

By moving the drive shaft location in the tunnel farther down and back the track can better bite the snow and slowly work its way on top. Rather than digging through the snow, the machine can float over it.


Wide Or Narrow?

Ski-Doo and Arctic Cat utilize wider running boards, which make up an important part of the tunnel. Wider running boards offer a solid platform for standing and maneuvering around the snowmobile.

On the other side of the fence, Polaris has long been a believer in narrow running boards and external heat exchangers. Theories behind the narrower boards suggest that they won't hang up in the deep stuff, allowing the machine to cut its way through the powder with less drag. The external running board heat exchangers keep the boards warm, decreasing snow buildup on the running boards, which, in turn, offers better foot traction to the rider.

When it comes down to it, both running board designs serve a purpose. Some riders like it wide, others narrow. That is evidenced by the personal preferences of the SnoWest SnowTest staff who spend a fair amount of time arguing each side.

While on the subject of heat exchangers, through the years their design on a chassis has evolved as well. Heat exchangers serve one main purpose-keeping the engine cool. But manufacturers found that by running the heat exchanger through the tunnel to the rear of the machine, the hot coolant can not only be cooled off by snow thrown under the sled by the track. The design can also help keep the tunnel area free from snow and ice buildup, eliminating unwanted dead weight while riding.

Running board design has continued to change through the years as well, from the early models with basic dimples for traction (or lack thereof) to the more open drilled hole design. Most sleds of today now offer completely open running boards for better snow evacuation and improved boot traction.


Covering Up

Another important factor on chassis design is the hood. While many think its sole purpose is to protect the engine bay, design and function goes much farther than that. Hoods are now used to deflect snow from the rider while still offering ample cold air to the engine bay to ensure your motor and clutches continue to work properly. The evolution of the hood has come from the common fragile fiberglass design to the polycarbonate rubberized versions of today. Not only are hood designs lighter, they are stronger as well.

Hoods are much more resistant to cracking and breaking, thanks to the use of the new modern materials.

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