Snowmobile Acceleration and Braking Performance

Understanding the acceleration and braking performance of a vehicle is crucial when reconstructing an accident. As experts, we are often asked how fast a vehicle was travelling or whether a driver could have braked to avoid a collision? Answers to these questions can help establish liability for the collision and its consequences.

Decades of research have built a vast pool of data for car and motorcycle dynamics. Snowmobiles, on the other hand, have received much less attention, despite the fact that there are more snowmobiles registered in Canada than motorcycles and mopeds combined. Each year, nearly 200 fatalities and 14,000 injuries occur with snowmobile use in North America. Despite these numbers, only limited research data exist for the performance characteristics of modern snowmobiles. To help fill this knowledge gap, the acceleration and braking characteristics of a variety of modern snowmobiles were tested and measured. The goal was to generate the data needed to accurately calculate a snowmobile’s speed if its braking or acceleration distance is known.

Based on motorcycle behaviour, it was hypothesized that a snowmobile could accelerate more rapidly than it could brake. Like a motorcycle, the weight of the sled and rider shifts off the front skis and onto the rear track during acceleration. This additional weight improves traction between the track lugs (small protrusions on the track belt) and the snow. During braking however, the weight of the sled and rider shifts off the rear track and onto the front skis. The reduced weight on the rear track decreases traction between the track and snow, and produces less traction during braking than during acceleration. This is identical to how a motorcycle with no front brakes would behave.

About 500 straight-line acceleration and braking performance tests were completed in February 2010 in rural Ontario. The test sleds included both two and four-stroke engine styles, with engine power ranging from 80 to 135 horsepower. This covered the majority of snowmobiles found on Canadian and US trails. Each of the snowmobiles was instrumented to measure its motion throughout the acceleration and braking phases. Also measured was track speed, brake pressure, and throttle level. Track patterns in the snow were also documented to identify differences in marks left by different levels of acceleration and braking.

Tests were performed on a groomed/packed snow surface to simulate a snowmobile trail. According to the International Snowmobile Manufacturers Association (ISMA), approximately 80 per cent of riders use their snowmobiles for trail riding on groomed, marked trails. To keep the snow conditions as consistent as possible, the test surface was groomed regularly. Throughout our testing, monitored weather conditions were monitored and snow conditions were monitored using the International Classification for Seasonal Snow on the Ground (IACS) protocol. To limit fluctuations in human performance and to maintain a consistent rider weight, a professional rider performed all tests.

For the acceleration tests, mechanical stops were placed on the thumb throttle to limit the maximum throttle applied to either quarter, half or full throttle. The rider accelerated up to a specific speed, between 20 to 60 km/h depending on the test, and then braked to a stop. The deceleration tests involved the rider either braking fully (locked track), or allowing the snowmobile to “roll down” to a stop. Roll down is the lay term for when the throttle is released on a moving snowmobile and “engine drag” slows the track. Half of the roll down tests were completed with the engine power on, and half with engine power off. This allowed the ability to document the effect engine power had on roll down deceleration.

In terms of snowmobile acceleration, there was a significant increase in acceleration with increasing throttle position. There was also an increasing trend in acceleration for higher power sleds; however, the two-stroke 800cc engine (rated at 135 hp) did not perform better than the four-stroke 1000cc engine (rated at 120 hp). The lowest acceleration observed at quarter throttle was 0.27g, while the highest acceleration observed at full throttle was 0.70g.

From the braking data, the full braking deceleration rate across all sleds was relatively consistent. No one sled performed better than another, and all fell within the range of 0.32 to 0.42g. In the roll down tests, there was no difference between the power-on and power-off conditions, but the deceleration rate decreased as speed decreased. This means that a snowmobile in roll down will gradually slow down more gradually as its speed decreases.

Outside of these statistical results, there were a few additional observations made. First, all four snowmobile speedometers consistently over-reported the actual sled speed, often by more than 10 km/h. Second, a subtle side-to-side weight shift by the rider was often necessary to maintain directional control of the sled during the full acceleration and braking tests, particularly with the high powered snowmobiles. This alludes to the potential importance of rider experience and warrants further investigation. Third, snowmobiles slow at a fairly constant rate under full braking. For years, a theory has existed that the locked track of a snowmobile acts like a plow, thereby slowing the sled at an increasing rate as snow accumulates ahead of the locked track. The tests proved this theory wrong and instead showed that snowmobiles decelerate at a relatively constant rate. A video camera placed under the sleds to monitor snow build-up ahead of the locked track confirmed this phenomenon did not occur.

A comparison to road vehicles helps to put a snowmobile’s performance into perspective. A common four-cylinder sedan (e.g., Hyundai Elantra) is able to accelerate on dry roads at about 0.3g. A snowmobile – even a relatively low powered one – can easily accelerate at 0.5g or more. This is similar to the maximum acceleration capability of a high-end sports coupe (e.g., Porsche 911 Turbo) or a typical motorcycle. For emergency braking however, most cars can brake at between 0.7 and 0.9g on dry roads. The same is true for motorcycles when both front and rear brakes are applied. A snowmobile’s braking capability, however, is barely half that. This means it’s easy for a snowmobile to get up to speed, but not so easy to stop quickly in the case of an emergency.

So how does this snowmobile research help? With this new research data, experts can now more reliably calculate snowmobile speed if the brake or acceleration distance is known. In addition, more accurate avoidance analyses are possible since the dynamic capabilities of modern snowmobiles are now better understood. Using these new data, experts can better answer the questions needed to establish liability in snowmobile crashes.

Ken Iliadis is a senior scientist and Pamela D’Addario is part of the Toronto office transportation group at MEA Forensic Engineers & Scientists.