Traditional snowmobiles are large enough to carry two passengers, travel on dual skis up front and a single track in back, and weigh 400 to 600 lb. They are powered by two or four-stroke engines cranking out 80 to 160 hp and cost $8,000 to $13,000 or more. Several companies make snowmobiles for trails and off-road “mountain” trekking, as well as multiperson touring versions, but they all have the same basic layout and design.
A small California-based company, Sierra Snow in Granite Bay, has designed and built a new kind of powered snow bike they call the PSV Snow Runner (Personal Snow Vehicle). Driving one is said to be a cross between riding an off-road motorbike, jet ski, and snowboard. They are more maneuverable, run cleaner, and cost less than traditional snowmobiles.
The Snow Runners started as clean-sheet designs, but they borrow technology from motorcycles, racing karts, mountain bikes, and snowboards. This is likely due to the fact that the developer Jim Wade, has extensive experience designing and racing motorcycles and karts, and enjoys biking, snowboarding, and snowmobiling.
Wade started with a list of requirements for his new take on powered travel over snow. His PSVs would be:
- Lightweight, high power, and made using offthe- shelf technology.
- Small enough to fit inside a midsized SUV, minivan, or pickup-truck bed.
- Rigid yet simple, with a chassis along the lines of racing karts and motorcycles.
- Easy, stable, and safe to ride for all age groups and abilities.
- Environmentally friendly: low emissions and minimal noise.
- A range of sizes — small to large.
- And less expensive than full-sized snowmobiles.
The first Snow Runner designed was the SR-85. Its 18Âhp comes from a two-stroke 85-cc engine fueled by a gas-oil mix. The second Snow Runner, the SR-125, has a larger 125-cc engine that burns the same fuel but cranks out 30Âhp. Wade plans on upgrading these engines soon to cleanerburning four-stroke engines. This should make the PSVs environmentally friendlier and let the vehicles avoid current and future restrictions against two-stroke vehicles in places like Yellowstone National Park.
Snow Runners are expected to drive through powdered snow over 3-ft deep, so the design had to ensure the radiators for the liquid-cooled engines had access to airflow. Riders have been known to block the radiators with snow. Company engineers are working on a fix for that problem.
The engines put their power through five or six-speed manual transmissions, just like motorcycles. The transmission then turns the 10-in.-wide drive track, a feature lifted from snowmobiles. It is studded with paddles that push Snow Runners through the snow. The SR-125, however, has a longer drive track with larger paddles to take full advantage of its 30 hp.
As you can imagine, braking on snow is tough. The PSVs are all equipped with hydraulic brakes based on those used on mountain bikes.
“Snow is so variable, it’s always a little bit of a crapshoot how the stopping aspect of the design is going to work,” says Wade in explaining his choice to install hydraulic disc brakes on his Snow Runners. “Often on snow, the best you can do is lock up the drive track and use the drag of the paddles to stop. The small hydraulic disc brakes used on mountain bikes have the power to do just that.”
For both the SR-85 and 125, the engines and drive tracks mount in a common high-strength, lightweight frame made of TIG-welded (tungsten-inert-gas) 4130 tubular steel, a design feature used on go-karts. Snowmobiles use a heavier steel and aluminum chassis.
Wade and his engineering team redesigned the seat for the SR-85 and SR-125 to save weight, which freed up space underneath for a high-performance exhaust system.
Gas tanks on both PSVs hold enough fuel for about 1.5 hr of riding time. Wade is confident he can find space for a larger fuel tank that will double the fuel capacity and give Snow Runners a 3-hr range.
One of the more-difficult engineering challenges was building a front end that supported the Snow Runner, provided a comfortable ride for the driver, and gave it crisp steering in both deep snow and on groomed winter trails. The initial design used a single snowmobile ski attached to a fork and shocks similar to those on mountain bikes. The design went through several different combinations of ski sizes and steering geometries. While performance improved, Wade was not satisfied.
Fortunately, Wade had one of those inspirational “Aha” moments, and the solution came to him: use a ski shaped more like a snowboard, make it as wide as the rear drive track, and add fins on the bottom for control and stability. So Wade built and patented a 10-in.-wide ski, the same width as the drive track, and added fins to the bottom. After some trial and error in sizing the ski, they got the desired stability and handling.
The Electric Snow Runner
When the engine supplier for the SR-125 backed out on an agreement, Wade and his company ended up with time on their hands. Wade used this time to develop a battery-powered Snow Runner, the SR Electric, with roughly the same power as the SR-125.
“There were several unknowns about the power requirements and what kind of battery works best for a small, electric, snow vehicle,” says Wade. “Several consultants offered to run power simulations, but they wanted to charge more than it would cost to just make an educated guess and build one. So we decided to skip the simulations.”
The team selected a 30-hp pancake motor based on its power and torque ratings being close to those of the 125-cc engine in the gas-powered PSV. The 48-V battery pod they designed uses 16 LiFePO4 cells, which have good power but slightly less energy density than the LiCoO2 batteries typically found in consumer electronics. But LiFePO4 batteries offer longer lifetimes and faster discharge rates. And they are less susceptible to heating problems due to overcharging. Fortunately, the battery pod could be sized to fit under the seat where the exhaust system was mounted on other Snow Runners. The next version of the SR Electric will likely have a battery pod with a higher voltage, giving the SR Electric V 2.0 longer run times.
The current pod will survive over 5,000 charging cycles, which Sierra Snow estimates should last over 5 years (assuming it is used every day over four winter months). The pod weighs 70 lb and costs about $1,000. “But battery costs keep coming down, so it’s hard to put a fixed price on the pod right now,” says Wade.
Motor speed is ultimately determined by the driver working a motorcycle-type twisting electric throttle, which sends a signal to an off-the-shelf motor controller. The motor turns a chain drive with gear ratios chosen to give the PSV a speed range appropriate for trail riding. Wade plans on upgrading the next-generation of SR Electrics to use a quieter and lighter timing-belt drive instead of a chain.
The added weight of the battery, which had the PSV pushing 200 lb, had practically no effect on handling or ride after Wade adjusted the spring rates and shocks on the front and rear suspensions, The flat torque curve of the motor gives the SR Electric plenty of off-the-line acceleration and the power to pull itself and a rider through handlebar-deep snow.
Wade came up with an innovative solution to the twin problems of recharging batteries and getting consistently good performance from them despite frigid temperatures. The battery cells are packaged in a pod that users can remove from the frame in less than 3Âmin. The pod and its transfer cart slide into a charging/conditioning station designed at Sierra Snow. Less than 35 min later, the pod is ready to be slid back into its mount on the PSV with enough power to provide 70 to 80Âmin of thrills and chills to the next rider. The station also monitors and adjusts the charge while keeping battery temperatures above 50°F and conditioning them for maximum output. So ideally, riders would always remove the pod from the charger and install it in a Snow Runner right before going out for a ride. Afterwards, they would return it to the station until the next ride.
Battery-powered PSVs should be fine for traveling around known areas and trails. And its quiet and clean operation makes it suitable for the most pristine of parks. The silent operation is also drawing interest from the military, which is looking for “stealthy” vehicles.
Wade has almost everything he needs to take his Snow Runners to market including the completed plans and engineering details, sourcing for parts, business plans and financials, and a long line of interested customers. The only thing lacking is investors who can help him make it happen.
The Original Sno-Runner
Although Jim Wade designed the Sierra Snow Runner starting from scratch, he did have a bit of inspiration: the Sno-Runner. It was a small, personal snowmobile designed, manufactured, and sold by Chrysler through its Outboard Marine Div. from 1979-81. They originally built the vehicle for the U.ÂS. Marine Corp, but it didn’t perform well in powder snow, so the Marines passed. The Marines did, however, pay Chrysler for the design and tooling.
Sno-Runners were powered by a 134-cc Power Bee engine that generated 7 hp. The vehicle was small, simple, and light enough to be broken down and packed in the trunk of a car.
Chrysler was nearing bankruptcy in the late 70s and it decided to market the Sno-Runner to the public in the hopes of generating some much-needed proÂfits.
When prototypes were introduced at a snowmobile exposition in Alaska, dealerships and the press fell in love with it. Unfortunately, Chrysler hurried the Âfinal design and cut some corners, resulting in a 30% loss of power. To make matters worse, engineers hadn’t accounted for the fact many Sno-Runner owners would use their “snow bikes” in mountains where thin air sucked away another 30% of the power. Still, they sold about 28,000 of them before demand cratered. Chrysler liquidated its stock, selling it to C.O.M.B. Liquidators. C.O.M.B. rebranded the vehicle as the Sno-Rabbit and sold them offÂ.
There are still Sno-Runner enthusiasts out there and several sites on the Web sell parts and upgrades.