snowmobile design and snowmobile sound basics

Track / Suspension - The track of the snowmobile, which like the CVT implementation, is specific to a snowmobile generates a large percentage of the noise heard on a modern snowmobile. The track generates noise due to its design and intended purpose which is to effectively provide traction on the snow. To provide this traction the track must have "lugs" which bite into the snow and provide resistance to the track slipping across the top of the snow. These lugs also act as a large fan when they are not embedded in the snow and hence generate airborne noise. The track is driven by a pair of drive cogs which interface to the track either by placing a tooth through a hole in the track or through pushing on a set of rubber bumps or knobs on the inside of the track. For the drive cogs to be able to push on the track without deforming it the track must act stiff at the cog/track interface locations in the longitudinal direction, along the direction they are pushing. The track is designed to handle this load by placing rods, typically a type of fiberglass rod, equally spaced in the track. These rods cause the track to be stiff in the vertical direction at their respective locations since they displace the softer rubber that otherwise would have been there. The presence of these rods then generates a spike of energy into the rear suspension every time they pass under or over an idler wheel. The idler wheels are a required part of the suspension for low snow conditions. There has
been considerable work to lower the noise generated by the track and rear suspensionthrough drive cog redesign, idler and support wheel placement, suspension geometry, and track design itself. In many cases, the track/suspension system may actually be the dominant noise source on a modern snowmobile if the snowmobile has a well designed factory supplied intake an exhaust system installed on it.

Radiated chassis noise - The last major noise source on a snowmobile is the chassis itself. The chassis radiates or re-radiates noise generated by the noise or vibration input into the chassis from the engine, drivetrain components, and the suspensions. Reduction of this noise has been and is a continuing concern and plays a pivotal part in the design of each new chassis which is developed. Over the last 5-6 years the snowmobile industry has adopted many of the automotive design and analysis principles to reduce the noise including finite element modeling of prototype or concept chassis, transfer path analysis of prototype chassis, materials selection, and an understanding of how to design and optimize engine and powertrain mounting systems to reduce radiated noise and chassis vibration. All of the manufacturers now use these technologies very effectively and have made a large investment in time and money to become proficient in their use. Prior to the last 5-6 years none of these technologies was used on a widespread basis to optimize and understand potential designs. The continued reduction in radiated chassis noise will happen as the snowmobile manufacturers gain an even better understanding of the dynamic behavior of their chassis, however in many cases it will come with the cost of
added weight and potentially much higher costs as exotic materials may be required to keep the weight of the snowmobiles at an acceptable level.