When shopping for a snowmobile, some important factors you might consider are aesthetics and wind protection. Making sure a machine has low aerodynamic drag probably isn’t a priority for most sledders, but have you ever thought about how factors such as the vehicle’s weight or the hood’s length affect how well a sled cuts through the air or across the snow?
Sources Of Friction
Once a snowmobile is put in motion, there are two major sources of friction: aerodynamic drag and what I’ll call “trail resistance.”
Aerodynamic drag is made up of two components: form drag and friction drag. Form drag relates to the aerodynamic shape of the snowmobile and rider sitting on it. The less a shape disturbs the air outside of the shape’s vicinity, the more streamlined the shape is considered to be. Friction drag is the resistance to motion caused by the layers of air near the shape’s surface. A streamlined design may have less form drag but more friction drag simply because there is more surface area for the air to pass over.
Trail resistance is the drag placed against the snowmobile by the track and ski contact with the surface the vehicle is traveling over. This resistance is a factor based on the weight of the machine and rider, the track surface design and the conditions of the surface. The power required to overcome trail resistance increases according to the square of the velocity of the snowmobile.
The surfaces a snowmobile travels over vary tremendously and the size of this drag factor varies accordingly. In deep, fresh snow the trail resistance is very high whereas a dusting of snow on ice offers the least trail resistance. It’s a no-brainer to determine which condition will allow the fastest peak speeds.
Because surface conditions and the resultant trail resistance greatly affect the top speed of the snowmobile, savvy snowmobile racers will read the track surfaces and select their line based not only on the positions of a turn, but also on the lowest resistance surface. A successful racer must have the ability to “read the snow.”
Drag Affects Performance
It’s hard to think of snowmobiles as vehicles that should be, or even could be aerodynamic. With that huge footprint on the ground, a big flap hanging on the back, suspension system components hanging out all over and the driver’s knees and elbows sticking out, it’s no wonder a snowmobile’s drag coefficient is so high. Tucking arms, legs and the head behind the windshield and hood will increase top-end speed — ask anyone who competes at radar runs — but it’s impossible to tuck behind the tiny windscreens on some of today’s sleds.
Speaking in terms of how airflow affects performance, there is little chance of gaining any “free” horsepower from a snowmobile ram air induction system, other than in some racing situations. Trying to force air into an airbox for a supercharger effect isn’t a practical concept. Keeping snow out of the intake requires many stages of filtration, which reduces any possible benefit ram air might provide and, unless a rider travels at least 100 mph, a usable gain doesn’t occur.
Aerodynamics of a sled shouldn’t be a big concern for snowmobilers, except for racers who regularly run at speeds faster than 90 mph. Even a well-streamlined hood and bellypan is chock full of openings to let in air to cool the exhaust system, brakes, clutches and engine. The rider becomes part of the frontal area of the machine and slows down the sled when he sits above the windshield or when shoulders, arms or legs protrude outside the body. Small flaps at the top edge of the windshield and deflectors on the hood or side panels direct air away from the rider, but they increase aerodynamic drag.
Snowmobile design is in constant flux, but the shape of sleds has really changed since the Ski-Doo REV came out in 2003. Rider positions on the machines have changed, the frontal areas of the sleds have decreased and, generally speaking, the amount of rider protection has decreased.
Tiny windshields might look good, but they don’t necessarily make a machine faster; in many cases, they will actually slow down the vehicle. There is a concern by designers to “smooth” the airflow around the machine and rider. A well-designed, tall windshield will usually provide a more aerodynamic flow than simply letting the rider’s upper body sit up in the wind. The small, low windshields simply meet aesthetic requirements.
Aesthetic design is critically important on the showroom floor. A tiny windshield and a tight, compact hood and chassis design can make a sled look light, sleek and fast, but a customer has to be comfortable while riding the machine. Fortunately for snowmobilers, the materials, design and quality of snowmobile outerwear and helmets have done amazing things to protect riders when the machine itself doesn’t.
The mammoth alternators on today’s sleds also allow for electrically heated surfaces like handlebars for the driver and a passenger, throttle lever and seat. Electrical outlets on snowmobiles today allow for the use of heated socks, gloves, face shields, vests and suits. A rider can be equipped to ride at any temperature and at any speed without much protection provided by the sled itself.
While there are alternative ways to stay warm, some riders still want the machine to offer better protection, especially in regions where sub-zero conditions are common. Small wind deflectors that mount to hoods and handlebars have become more common and some of them work very well. Windshields are also again growing on production sleds, or at least available as an accessory.
It’s up to you to decide which sled you end up buying, but with each mile I put on, the more I yearn for better wind protection!
How Drag Is Measured
Generally measured in a wind tunnel, aerodynamic efficiencies of shapes are compared by determining the drag coefficient of each design. Mathematically, drag, measured in pounds of force, is equal to Cd•A•v2/391 where:
Cd= drag coefficient
A= frontal area in square feet
v= airspeed in miles per hour.
The horsepower required to overcome aerodynamic drag is equal to Cd•A•v3/146,600. The power required to overcome aerodynamic drag increases with the cube of the speed. Because of this cube relationship to speed, aerodynamic drag horsepower quickly passes trail resistance power losses to become the major cause of power loss at speeds more than 90 mph.