MID TERM REPORT Long Distance Dogsled Senior Design Project 2006-2007 The University of Maine Team Members Lucretia Yentes Oai Ha Luis Estevez Jesse Remillard
Table of Contents Page Introduction 1 Design 4 Runners. 4 Breaks 6 Seat 7 Composite joints 7 Materials 8 Conclusion.. 8 Works cited page 9 Appendix Aerodynamics/Carbon fiber shell. Directionally dependant flex joint Rear steering mechanism (rudder) Custom sled bag I I II III III i
Introduction The goal of this project is to design and build a dogsled using our knowledge of mechanical engineering to aid us in improving current models. We are collaborating with Terry Atkins, an experienced musher and veterinarian. Terry will race the sled in this year s Iditarod, provided the sled is completed in a timely fashion with suitable durability. This is a clean slate project, so before even beginning preliminary design work we first had to research the project thoroughly. Our research includes the Iditarod race in order to understand the rigors our sled will have to withstand, as well as the basic buildup of a sled including the technical jargon needed to understand the sport. During this background work we made contacts with several people from the dog sledding community who have been and continue to be incredibly helpful, both with acquiring necessary materials and understanding the trade of building dogsleds. As mentioned above, we began our research with the Iditarod race itself. The Iditarod Trail has been in existence much longer than the competition. It began as a supply route from the coast to inland mining camps. The first Iditarod race was created in 1973 by Dorothy G. Page and Joe Redington Sr. in commemoration of the 1925 serum run when diphtheria struck Nome. The race was also meant to help keep the culture of dog sledding alive after snowmobiles took over as the primary means of transportation during the winter months of Alaska. The race is approximately 1,150 miles long and begins on March 8 th. There is a ceremonial start from anchorage in which the winner of a yearly auction gets to ride along. The real start begins from Wasilla and then proceeds on to its famous finish in Nome. In addition to each musher s individual preferences are the race requirements for sled eligibility. There are various checkpoints throughout the race that the musher can store equipment and supplies in advance but the sled still needs to be able to hold a generous amount of provisions. The official requirements for the sled are as follows: No sails or wheels on the sled Must be able to carry 200lbs 300lbs including the following required items: o A sick or injured dog o Cold weather sleeping bag weighing a minimum 5lbs o An Ax at least 22 inches long weighing a minimum of 1 ¾ lbs o A pair of show shoes with area of 252 inches each and bindings o 8 booties for each dog o One stove and pot capable of boiling 3 gallons of water at a time o A veterinarian notebook o Enough fuel to boil 3 gallons of water o Cable gang line or cable tie out to secure the dog team o Food for the dogs and musher The aforementioned items dictate the size requirements of the sled. Since most mushers do their best to minimize weight, a minimal bed size of about six feet is used on almost every sled. Sleds are traditionally made from wood, metal and various ties and little has changed on today s sleds, with the exception of the plastics now used. Part of this is due to the extreme temperatures (-120 F to -40 F) the sleds face but it is also very hard to beat those traditional materials as far as their durability and performance. Figure 1 shows a typical sled so that the reader may better understand both the information above and the design section of the report. 1
Figure 1 Parts of a Dogsled. 1 2
There are many different types of dogsleds used throughout the world but the type used, almost exclusively, in the Iditarod is the raised toboggan sled. It has the needed storage capacity and the ability to glide over deep snow. The sled depicted in figure 1 (pg 2) is a toboggan sled, a typical Iditarod sled would have a bed raised anywhere from 4-8 inches off the runners. Key parts of the sled and their purpose are as follows: Runners The base is traditionally made from wood but is now often made of aluminum or other metals. All sleds used in racing today utilize some sort of plastic runner system on the bottom. This is because of the superior qualities of plastic when it comes to friction and durability. Plastic is also relatively cheap to machine and can be replaced easily. The most popular type of plastic used in dog sledding is ultra high molecular weight (UHMW). Cargo bed The cargo bed is the resting area for the sled bag and must support the weight of all the supplies as well as aid the sled in gliding on deep snow. It is often made of UHMW plastic for the friction properties when the sled is riding in deep snow but is sometimes made of other materials. Brushbow Leading the way on all dogsleds is the brushbow used to fend off otherwise harmful stumps, trees or rocks that threaten to damage the more vulnerable parts of the sled. Again UHMW is the material of choice for most sleds due to its durability and machining ease. Handlebars The obvious purpose is to give the musher something to hang on to. Less apparent is its use to twist the sled over to one side or another in order to steer the sled to some degree. Brakes There are at least two brakes which can be used while the sled is in motion: the drag brake and the regular brake. The drag brake simply slows the sled down and could be used to control the sled on down hill sections of trail. The regular brake is used to stop, slow or even help guide the sled on portions of the trail. Also, a snow hook is used to keep the sled in place when the dogs are harnessed but the musher is not ready yet. The need for a snow hook may sound surprising but sled dogs are incredibly high energy and do not have the patience to sit still while the musher prepares the sled. 3
Design One of the first design tasks was to decide which dogsled type to use. It was evident from early on that, in order to have a versatile sled capable of handling the various terrains and conditions of the Iditarod, the raised toboggan style dogsled would be the sled of choice. From there, the components for the sleds were chosen and designed to accommodate the chosen sled type, meet design objectives and optimized for performance. Figure 2 Overall Dogsled Dimensions. Runners Our design for the runners is a parabolic shape like that of a downhill ski (fig. 3, pg 5). We are using the Rex runners built by Prairie Bilt Industries as our base. The Rex runners are extruded aluminum with an H cross section. One major difference between the mechanics of the Rex runners and that of a down hill ski is that the runners are rigid. A down hill ski counts on the fact that as the ski is carving into the side of a hill, the ski is flexing from tip to tail in a reverse camber. This reverse camber is necessary so that the whole parabolic edge of the ski is in contact with the snow, which produces the turn. Since our runner is not flexible we must account for this reverse camber in our design. The plan is make side cuts that will modify the runner so that it has a parabolic shape like that seen in figure 3 (pg 5). Also, we plan to cut an angle on the bottom side from tip to tail as seen in figure 4 (pg 5). Both are necessary in order for the ski to turn as designed. 4
Figure 3 - Basic Parabolic Ski Shape with labels 8. To calculate the radius of the side cut the following equation may be used: Side- cut Radius = ( L / 4) + d 2d 2 2 The side-cut radius is the approximate turning radius. In order to have a tight turn, there needs to be a small cut radius, which means the side-cut depth should be as big as possible. Figure 4 Reverse Camber of Parabolic Runner Design. In order to incorporate this parabolic shaped runner, we had to redesign the frame as well. We needed to design the sled s steering mechanism so that the musher could force the runners to tip over on their sides. This tipping action enables the runners to engage their curved sides into the snow properly. After a lot of discussion about how it would work, we made a test rig out of wood with simple pin and hinge connections (fig. 5, pg 6). The idea behind this was that the connection between the runner and the stanchion must be stiff in order to translate the lateral force applied at the handlebar into the tipping of the runner. The bed is held with pins so that it stays mostly flat and allows everything else to move from side to side. We also put a hinge at each corner of the handle bar so that the angles there could deform. 5
Figure 5 Rear View of Test Rig. Brakes We designed a breaking system (fig. 6) that performs much like the current brakes (fig. 7) being used today. In addition, we added the ability to engage the breaks on either the right or the left side independently. Because these brakes essentially use two flat plates with sharp tips at the bottom to stop the sled, having only one side engaged will act as a pivot point which should aid in the steering of the sled. Figure 6 Proposed Braking System for Design Project Dogsled. Figure 7 Typical Brake Bar Style Braking System of Modern Dogsleds. 6
Seat The seat is something that is not typical of most dogsleds today. We have included a seat in our design for a couple of reasons. First, the seat is to suit the personal preference of our musher. Second, due to the grueling conditions of long distance dogsled races such as the Iditarod, any rest a musher may get while out on the trail will be beneficial. The plan is to fabricate a composite piece to connect the seat to the sled. This will reduce the weight and provide some additional shock absorption. Composite Joints Composite joints will be used to connect the handle bar to the stanchions to allow for deformation of the structure in the lateral direction, while resisting movement in the fore and aft direction. Essentially, this will function much like the hinges on the test sled (fig. 5, pg 6) but also incorporates some stiffness so the sled will not move side to side until a sufficient force is applied. Composite Joints Figure 8 Composite Joints Between Handle Bar and Stanchions. 7
Materials Materials for the other components are as follows: Table 1 List of Materials for Other Components. Stanchions & Struts ¾ x ¾ aluminum square tubing Cargo Bed 1/8 thick UHMW plastic Brush Bow UHMW plastic Stanchion Brackets Anodized aluminum Conclusion This project has been very successful. Since this is a new senior design project and there is not a lot of technical information regarding sleds in the dog sledding community, the field was wide open. We started learning about dogsleds, what was currently out there for designs, and what areas could use improvement. After taking in all the information we could gather, we had to figure out what direction we wanted to take our sled in. We had many good brainstorming sessions where we talked about the different areas we could tackle: the aerodynamic of the sled, how to make the sled turn, musher comfort, and so on. Because we didn t have all the time in the world to build our dream sled, we pinpointed the steering aspect as the place we would focus on. After many ideas on how to make the sled turn, as talked about in the appendix, we have decided to try and incorporate a parabolic runner design. We feel that since our design is a big step in a new direction, this may have a large impact on future dogsled designs. With the research and design phase mostly completed, we have entered the build phase of the project. Time is of the essence, as the Iditarod is just around the corner and we want our sled in the race. 8
Works Cited Page 1. The Anatomy of a Dog Sled. Ultimate Iditarod n.d. September 18, 2006 <http://www.ultimateiditarod.com/dogsled.htm> 2. Iditarod Rules. The Official Site of the Iditarod August 07, 2006. September 18, 2006 <http://www.iditarod.com/2-2.html> 3. Iditarod History. The Official Site of the Iditarod. n.d. December 21, 2006. < http://www.iditarod.com/learn/history.html > 4. Newell, Gregory and Newell, Denise. The Basic Dog Sled. Expedition Samoyeds February 02, 2004. September 18, 2006. < http://www.expeditionsamoyeds.org/sledbasics.html> 5. "Dog Sled." Wikipedia, the free encyclopedia. 18 Sep. 2006. <Reference.com http://www.reference.com/browse/wiki/dog_sled> 6. Little, Jon. Articulating on innovation Jeff King's tinkering takes distance mushing in new directions. Cabela's 2006 Pre-Race Coverage. n.d. September 18, 2006. < http://www.cabelasiditarod.com/coverage_2006/cov06_july08_01.html> 7. Frequently Asked Questions. Ultimate Iditarod n.d. September 18, 2006 <http://www.ultimateiditarod.com/faq.htm> 8. Physics of Skiing. n.d. December 22, 2006 < http://www.math.utah.edu/~eyre/rsbfaq/physics.html> 9
Appendix Aerodynamics/Carbon fiber shell It was clear from a look of the current dogsled designs, that aerodynamic considerations were not regarded in many of the designs. Doing an Initial analysis of drag required knowing some of the parameters involved in dogsled racing in order to find the form drag 2 that a dogsled will encounter. The formula for form drag is: F = 1 V 2 Cd ρ A Where F is the force due to form drag, Cd is the shape dependant drag coefficient, ρ is the density of the air at the typical conditions found in the Iditarod, A is the frontal area of the sled, and V is the velocity of the dogsled through the fluid (air). Thus we needed the average speed that a dogsled traversing the ice encounters as well the maximum speed that would be encountered for shorter period of times since the velocity has the most influence on the resultant force. Research showed that the average speed that a dogsled travels to be roughly 10 mph, with bursts of high speed reaching approximately 20 mph. Estimating the shape of the dogsled as a triangle (with the point going into the flow) necessitated a Cd value of 1.6 to be used in the equation for drag. The force due to form drag was calculated to be 2.266 lbf and 9.064 lbf for 10 and 20 miles per hour respectively. Thus, we concluded that aerodynamics did merit consideration. Our idea to counter the drag force was a carbon fiber shell that could be molded into an aerodynamics shape with a low Cd number. The initial analysis yielded: Positives Less force impeding the forward motion of the dogsled will result in a lower times The shell will provide a stiff structure that can afford better protection for injured dogs that must be carried There has already been substantial engineering analysis in the aerodynamics of bobsleds which can be applied to the carbon fiber shell Negatives The weight addition to the sled Impedance to gear accessibility Design and build time will be lengthy Added complexity of attaching the shell to the sled The final analysis of the carbon fiber shell was that the overall benefits to cost and time ratio would be to low to consider for this project. Further consultation with Dr. Kiran Bhaganagar added credence to our assessment. Dr. Bhaganagar s initial rough analysis was that the time to design, optimize and build a carbon fiber shell would not allow it to be built with our project s time constraints. She went on further to say that minor aerodynamic implementations would have substantial benefits when compared to the time costs associated with them. She suggested concentrating on the front and rear of the dogsled as these are higher order components than the rest of the sled. Furthermore, Dr. Bhaganagar suggested not having any corners on parts of the sled that traverse the air. She explained that rounding the corners, which lead to separation, is an easy way to gain some aerodynamic benefits with out a tremendous time penalty. I
Using Dr. Bhaganagar s analysis, we also considered making a frontal hood made of carbon fiber to place over the non-aerodynamic brush bow. It was finally decided that our time constraints required us to just round the edges of the brush bow rather than implement an aerodynamic hood. In our analysis of the aerodynamics of dogsleds, we found that though it was an area to be addressed, our current time budget only allowed us to make the minor adjustments. Future work in this area could be accomplished by an upcoming design team. If an overall aerodynamic analysis of the dogsled was done on the project, a group could make significant progress in an area that is relatively unexplored. Directionally dependant flex joint In our research our group discovered that the dogs do most of the steering in a dogsled. The dogs immediately in front of the dogsled (called the wheel dogs) basically make sure the sled stays on track. We believed we could improve on this passive method of steering. We believed the use of a joint that was flexible in one direction while stiff in the others could be implemented on the front of the runners. The front part of the runners could (through the use of a connecting flex joint) move laterally easier than up and down or forward and back. We theorized that this would permit the front of the runners to turn in the direction of the wheel dogs allowing the sled to turn easier in that direction. Carbon fiber would be an available and light material that would be ideal for use in this flex joint. Positives Adds to the ability of the wheel dogs to steer the sled Using carbon fiber for the flex joint makes getting the exact directionally dependant stiffness easy to achieve Materials and equipment needed are readily available Negatives Determining how to make the plastic runner insert move laterally along with the bend of the flex joint will have to be surmounted The weight is mostly in the rear, meaning the front runners may turn while the sled continues to track forward regardless The runners are quite expensive and cutting into them incorrectly would finish the team budget Though the idea of a directionally dependant flex joint seemed promising the negatives are too high and the ability of the flex joint to add to the steering is untested. Testing the validity of a flex joint s ability to turn the sled was not done because the time and expense associated could not be overcome for this project. If the runners were made in house, then they could be made with the flex joint already in the runners if testing deemed they could help steer the sled. II
Rear steering mechanism (rudder) The idea of a rear based steering mechanism was proposed after we examined the current steering methodology involved in a dogsled. We noticed that in order to cause the front of the sled to steer, the musher must apply a force somewhere (usually the handlebar) on the rear of the sled. Thus a means to translate the rear input to the front output requires the sled chassis to be the medium for transfer. We decided that a stern based steering mechanism where a part of the rear runners could translate laterally would eliminate using the middle of the sled to steer the dogsled along with its associated complex analysis. We speculated that a flex joint could be added to the back runners to alloy the musher to use the rear part of the runners as dual rudders. Positives The associated benefits of using a flex joint shown above Easier to analyze a steering system near the musher than one far from the musher Allowing the musher to actively steer the sled eliminates the dependence on the wheel dogs, which in turn, fatigues the dogs less Negatives The associated negatives of using a flex joint as noted above Rear rudders function by slowing one lateral side of the sled not unlike our braking system Turning via braking slows the sled down Musher must be quick with his/her feet placement Overall, we decided that the rear based steering system would leave much to be desired. The fact that our braking system works the same way makes the rear rudders redundant pieces that must be designed and built. The major flaw we found with this design was that this steering mechanism would slow the sled down too much. Custom sled bag In our research, we found 1000 denier Cordura to be the fabric of choice used in most sled bags. Further investigation of possible fabrics revealed Sunbrella to be a viable choice to replace Cordura. Designing and fabricating our own sled bag Positives Sunbrella is lighter by 1.75 ounces per square yard than currently used Cordura The Sunbrella fabric is less expensive than Cordura which lowers the cost of constructing our own sled bag Sunbrella s claimed improved strength over Cordura Sunbrella is designated for outdoor use denoting its toughness Allows us to design the sled bag as per our requirements, thus it can connect to our custom sled perfectly Sunbrella is more breathable than Cordura which can benefit the hurt dog(s) that have to ride in the sled bag Negatives III
Need to scientifically test the Sunbrella rigorously for toughness and resistance to cold extremes in comparison to the Cordura Fabricating a sled bag will be costly either time-wise or money-wise A custom sled bag with a new material could reveal weaknesses that were not anticipated or tested during the harsh conditions of the Iditarod We are still not absolutely decided against a custom sled bag. If fabrication cannot be done in house due to time constraints, an outside source may be utilized. Furthermore, if examination of the Sunbrella proves its superiority over Cordura, its use will also lower the costs involved with the manufacture of the sled bag. IV