Tech Professor: Airflow And The Snowmobile

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 th