Introduction: How to Create a Downhill Ski-Bike
Living in a frigid northern climate enables its residents to enjoy snow for many months during the winter. For avid mountain bikers, that can hinder the amount of riding time on fast, packed dirt trails. Not wanting to be slowed by snow in the winter months, I set out to combine my passion for mountain biking with the speed of downhill skiing. This mash-up of sports yields an incredibly fun, challenging and extreme sport of its own: Ski-biking.
This instructable documents my journey using the engineering process as I designed, built and rode my own version of a ski-bike. Several ski-bike instructables already exist for quick and easy ski-biking fun. However, if you want to build a serious snow shredding machine, you will want to follow this Instructable as I describe the crucial aspects of ski-bike design and the implications of various design choices.
Below are the steps of the engineering process that I employed.
• Define the problem or need
• Research existing intellectual property and history
• Conceptualize & brainstorm
• Establish design requirements
• Product design
• Production planning and tool design
• Test & Analyze
• Repeat (coming soon!)
Step 1: Research
There was no advantage for me to "reinvent the wheel" so instead of trying to develop everything on my own, I first conducted research on the internet to find out 1) what was already developed, 2) what designs or characteristics I wanted to utilize, and 3) designs or characteristics I wanted to avoid. Another useful research trick is to understand how the sport has evolved over the years. That helps to confirm a potential design isn't outdated or purposefully avoided by the industry.
Despite their lack of mainstream use (and possibly even awareness), Ski-bikes have been around in various forms since the 1960's! And just as your standard bicycle or set of skis, these designs have significantly improved over the years. In the following steps, I’ll describe how I used some of the information I gathered to influence the design of my own ski-bike.
Step 2: Design Requirements
I established the essential design requirements by assessing the riding environment of the ski-bike and the factors required to make the project successful. My design requirements were to:
• Replace the wheels on a functioning bicycle with one or more skis
• Support a rider up to 200 pounds without breaking
• Be able to turn left and right
• Ride down a gradient, with a riding stance similar to a standard bicycle
This may seem like very few requirements, but a design fulfilling these requirements will produce a functioning ski-bike. My design fulfills each of these requirements and adds additional design characteristics that enhances the performance. The additional design characteristics are covered at length in the following step.
Step 3: Design
The following categories each have a direct effect on the performance and handling of the ski-bike. I used my engineering background and knowledge of bicycles to optimize my design.
Skis: I decided to use two skis on my ski-bike which would result in a front ski for turning and a rear ski for weight support.
Ski Length: Long skis are typically better for deep powder snow. For the small, mostly snow packed and icy Midwestern hills I would be riding, a long ski length is not necessary. Many of the ski-bike manufacturers or conversion kits I looked at did not post the lengths of the skis for reference. From the few stats I did find, the skis usually ranged from about 90 cm to 100 cm. I decided to make my skis slightly shorter because I was hoping to quicken the handling.
Ski Width: There isn't much discussion about ski width for ski-bikes. I believe there is a range of optimal width for skis on a ski-bike and a large majority of downhill skis fit in this range. However, if the ski is too wide it begins to resemble a snowboard which has great stability, but has poor turning capabilities. Too narrow and it begins to resemble the blade on a hockey skate which has little stability, but excellent turning capabilities. There isn't a way to control the width of your skis unless you consider the width before deciding which skis to use. The skis I had available were typical downhill skis from the mid 1990’s and seemed to have a good balance between being too wide or too narrow so I did not pursue other options.
Mounting Brackets: To ensure my ski-bike could support a rider of 200-lbs, I had to devise a strong way to attach two skis to a bicycle frame. I chose the front and rear dropouts as the locations to connect to the bicycle because they are designed to bear the weight of a rider, are square to the frame and offer simple mounting options. The flat top of each ski was a natural location to mate to. I designed two (a front and a rear) mounting brackets that would be secured to separate skis and attach to the front and rear dropouts of the bicycle with axles.
Ideally, I would have made the mounting brackets out of steel for its strength to weight ratio, but I was without access to a welder. So instead, I chose to build my mounting brackets out of wood because it is relatively strong in compression and is easy to work with. To fasten the wood together, I chose to use 2.5 inch deck screws for their strength.
For the front mounting bracket, I decided to fix a fork mount on a triangular wooden base to connect to the bicycle fork. A fork mount is a quick release mechanism that (when closed) secures the fork to whatever the mount is fastened to. A picture can be seen in the photos. I subtracted the fork mount height and used some trigonometry to design a wooden base for the fork mount to sit on top of.
For the rear mounting bracket, I wasn't able to use another fork mount because the spacing of the rear dropouts on my bike frame was 135 mm and the fork mount spacing is only 100 mm. This meant designing a complete hub and axle assembly directly into the bracket. For simplicity and because the ski/bracket assembly would only be rotating a few degrees around the axle (unlike a typical bicycle wheel that rotates around continuously), I also designed the "hub" out of wood. Drilling a hole through a 135mm long 2x4 the same diameter of the axle allowed for the small amount of rotation needed.
Axle Height: I wanted the axle height to be similar to a 26" mountain bike to mimic a standard bottom bracket height, but decided to reduce the height by a few inches to keep the center of gravity low. The final design called for the axles to be 12" above the ground, not including the thickness of the ski.
Axle Position (relative to the ski mid-line): I wasn't able to find any information about the optimal positioning of the axle relative to the mid-line of the ski. I hypothesized that positioning the axles behind the mid line would be optimal, forcing the skis to react primarily to the terrain ahead. Conversely, if the axle was ahead of the mid-line, the back of the skis could react too much to the terrain, pushing the nose of the skis down increasing the chances of catching an edge and crashing. I decided to position the axle 4 inches behind the mid-line on the front ski and 5 inches behind the mid-line on the rear ski.
Suspension: There are numerous websites showing homemade ski-bikes that are completely rigid (no suspension), but I wanted to have a full suspension bike frame to absorb bumps and jumps. I chose to use the frame from my full suspension mountain bike.
Head Tube Angle: The angle of the head tube (measured from horizontal) affects how quickly the ski-bike steers. A very steep angle can turn the ski-bike quickly, but is harder to control with speed and would be prone to catching the opposite edge, resulting in the rider being tossed over the handle bars. A very slack angle has stable but unresponsive steering, and turns the ski-bike slowly. Since the frame I used for my ski-bike was a downhill mountain bike (and had a slack head tube angle) I wanted to keep the head tube angle of the completed ski-bike the same as the original bike with wheels on it. That meant keeping the dropouts an equal distance off the ground.
Center of Gravity: The center of gravity plays a big role in the handling and feel of a ski-bike, just as it does on a standard mountain bike. The lower the center of gravity, the more stable the bike feels and roll time from one ski edge to the other is reduced resulting in quicker handling. However, that also results in lower ground clearance for the pedals or footings. Generally, the center of gravity of a bicycle is most directly affected by the height of the bottom bracket because that’s where the majority of a rider’s weight is transferred down to the bike.
Bottom Bracket Height: The bottom bracket height affects the center of gravity of the bike and at what angle the foot pegs or pedals on the ski-bike contact the snow. This height is determined by the frame geometry and the axle height. Since I did not have the capacity to modify the geometry of the bike frame I chose to alter the axle height as covered above in the Axle Height section.
Footings: This is a very crucial design characteristic because your feet hold the majority of your weight. There are two common designs utilized by other homemade ski-bikes. One is to use the bicycle crank arms and pedals and the other is to create a custom foot platform in the bottom bracket shell of the bike frame.
I chose to use bicycle crank arms and pedals to 1) eliminate the need to design an additional system, 2) save money by eliminating the need for additional parts and 3) crank arms and pedals were designed to support the weight of a rider so safety would not be a concern. The two downsides of this design is that the cranks would spin very easily without the chain providing resistance and the pedals could contact the snow at a relatively small turning angle. The remedy for the latter is to keep the pedals horizontal or rotate the cranks so the uphill pedal is at the highest position. In fact, rotating the pedals so the uphill pedal is at the highest position becomes an advantage over other footing designs because a rider can transfer their weight to the downhill pedal which lowers their overall center of gravity and increases their stability while turning.
Step 4: Construction
Bill of Materials:
One pair of downhill skis
2.5" deck screws
Salvaged rear 3/8 inch bicycle axle
Drill (w/ bits)
All of the pre-planning during the design phase removed any guess work and made building the front and rear mounting brackets straight forward.
The fork mount was designed to sit on top of the mounting bracket and the rest of the base was constructed by using trigonometry and a miter saw to cut the precise angles. The 2x4s were wider than the skis so to eliminate any overhang of the 2x4s I measured how much overhang was on each side of the 2x4 and then used the miter saw to cut a taper on each side. The front mounting bracket required two pieces of wood to create the triangular base. I drilled pilot holes in the 2x4s to prevent the wood from cracking. Deck screws were driven into the wood to fix the base together followed by the fork mount on top of the base.
To fasten the ski to the bracket, I positioned the bracket on the ski relative to the mid-line as determined in my design and made sure the axle of the fork mount was perpendicular to the center line of the ski. Using a permanent marker, I inked lines on top of the ski where it mated with the 2x4s. With the base still sitting on the ski in the correct position, I held up a deck screw to the ski and bracket to determine where to drive it in. I wanted the screw head to be flush with the bottom of the ski to minimize the additional sliding friction, but I also didn't want the point of the screw to protrude out of the 2x4. Once I determined the correct position of the screws, I used a marker to mark the points on the side of the ski where I would drill. Then I drilled two pilot holes for each leg of the bracket all the way through the ski. To determine the exact location of the pilot holes needed in the wooden legs of the bracket, I drilled screws through the ski just enough to pinpoint where they touched the bracket leg. Once all of the pilot holes were drilled, the deck screws were sunk flush up through the bottom of the skis and into the legs of the bracket.
This process was repeated for the rear ski. The only difference was the use of a 2x4 instead of a fork mount to span the distance of the dropouts. The spanning 2x4 had a 3/8 inch diameter hole drilled through its length for the rear axle to slide through. To ease the ski and bracket rotation around the axle, I placed washers in between the 2x4 and the dropouts. This required trimming the 135mm long 2x4 down by the thickness of two washers, but prevented the wood from rubbing on the dropouts and creating friction.
Step 5: Testing & Analysis
As fun as designing and building this ski-bike was, I was more excited to test it out in the snow! I also enlisted two volunteers to help with the testing. We had two sessions of riding the ski-bike at a local sledding hill and we made sure to test when the hill was vacant for the safety of ourselves and others.
The final product did not disappoint. The ski-bike worked exactly as it was designed to and I was extremely happy with it. The ski-bike combines aspects of snowboarding, downhill skiing and (of course) cycling to create a riding experience like no other! The riding experience of the ski-bike is akin to snowboarding. With the rear ski independent of the front ski, the rider is able to push the back end of the ski-bike side to side creating a slide similar to snowboarding. The steering of the ski-bike on the other hand, is functionally more similar to traditional downhill skiing. This is because when the handlebars are turned in one direction, the head tube angle causes the same side of the front ski edge to dig into the snow and track in the intended direction. This combination of handling characteristics paired with the human-to-snow interface of a bicycle makes for an exhilarating ride!
Check out the videos of the test rides!
Even though the ski-bike performed admirably, I determined several small design changes and improvements that could make the ski-bike even better for future editions. These design changes and improvements will be covered at length in my next iteration of the ski-bike (coming soon!) so stay tuned. In the meantime, check out these other helpful resources:
After painstakingly designing, building, and testing, the Instructable for Ski-bike 2.0 is finished, including HD video footage of it in action! Click on the link below.