ME 493: Final Report Spring Packable Downhill Ski Bike

Transcription

1 ME 493: Final Report Spring 2016 Packable Downhill Ski Bike Sponsor: Zdenek Zumr Group members: Brian McShane Ekendra Goravani Jesse Harris Nha Nguyen Nick Pauly Sheik Ameen Academic advisor: Dr. Faryar Etesami

2 Executive Summary The intent of this document is to communicate the results of the final design and build of the Packable Downhill ski Bike project. The overall goal of this project was to design a device that an individual will use to descend a ski slope. This design is primarily intended for a person with compromised knee joints, so as to ease the impact on one s self during the descent. The design was also required to meet several other secondary objectives. One being the portability of the ski device when ascending the slope. It was required to be compact and of reasonable weight. Another objective was for the device to have an integrated braking system, that will be intuitive to operate. The original concept of the original Downhill ski Bike was developed by a previous Capstone group. While the design and build was completed by this group, the customer required further refinement of the base design. The design improvements that were incorporated and are discussed in this report not only improve the device when in use, but also make for a robust design that will be user friendly. The previous design was tested and found to be a very good geometry for the rider. It was primarily constructed of chrome moly tubing with welded joints. This design did not incorporate a means of braking. Additionally, the design did not allow for easy transportation with an overall weight that was more than ideal. 1

3 Table of contents Introduction Mission Statement...4 Design Requirements....4 External Search...5 Internal Search.6 Product Design Testing Final Component Designs....9 Conclusion.19 Appendix A: Full Product Design Specifications..20 Appendix B: Bill of Materials...21 Appendix C: Major Component Selection.22 Appendix D: Test Apparatus..23 Appendix E: Assembly Details..24 Appendix F: Production Drawings

4 Introduction The design of the Ski Bike in its use, one would ascend a ski slope carrying the device, in a collapsed form. When at the summit, it would be assembled and used to descend the slope. The Ski Bike is a single rider, non motorized, tandem ski device, where the rider has control of the forward ski for changing direction via a handle. Motion is dependent on the gradient of the slope. 3

5 Given the extent of the changes, all that will be reused from the previous design are the skis which are provided by the customer and the main geometry of the rider position. The bike was designed with the specific goal of assisting an individual with compromised knee joints descend a downhill ski slope without impacting the joints by way of a shock absorption device. Mission Statement A fully functional downhill ski bike was to be designed, tested and delivered to the customer by the first week of June. It was to be easy to transport in terms of its compactness, and weight, which was not exceed 12.6 lbs. It was to have a hand lever operated braking system that would provide a means of stopping in both snow and packed ice conditions. The intended customer is an individual of approximately 170 lbs with medium strength and build. The aft mounted shock absorption device was to be built into the ski bike to reduce impact on the rider on uneven terrain. The final design and build will be kept under $ total. Main Design Requirements A detailed PDS report was developed for the purpose of meeting the main design requirements of the project as included in Appendix A. It showed the design criteria to be met as well as the level of importance of each item. In brief the requirements were as follows: Easy to transport while ascending the slope Have a robust braking system to operate in both snow and ice. Relatively light in weight for portability A fail safe shock absorption system that operates in cold weather.. Allow for easy assembly of the bike from collapsed to ridable form. Aesthetically pleasing design. The overall cost of the ski bike to be under a $1,000. 4

6 External Search The external searches were made on two fronts as follows: Taking a look at existing downhill ski bike designs While the concept of a ski bike is by no means a new one, many of the offerings in the market seemed to be based out of Europe. Additionally, the prevailing market for foldable road bicycles were also looked at for conceptual ideas. Potential off the shelf components that can be used for the design The ski bike shares many similarities with a traditional bicycle and therefore will be compatible with numerous off the shelf parts such as handlebars, suspension and headsets. The ski bike shown in Figure 1 is typical of the models currently on the market. This design is of a fixed geometry including the skis. Figure 1 : The Launch ski bike by Lenz Sport. This design incorporates both a front and rear suspension system as well as a seat. The skis themselves are hard mounted with no option for a packable state. The majority of the ski bike industry is built around extreme downhill sport use so the design focus is vastly different than design criteria of this project. Packability is the number one design consideration but is a feature that is not found on most ski bikes because they are generally carried up the ski slope on a chair lift. 5

7 Internal Search The internal search in many aspects was understood to be the more complex of the two searches. This was due in part to the fact that having done the external search, no current designs on the market met the design requirements set forth by the customer. Which in this case meant an almost ground up design. This led to many potential choices for the following major components: Main frame Material and structure design Rear swingarm Material and structure design Collapsed state Multi part vs folding design Multi part (if) Tooling and ease of assembly Aesthetics Overall ergonomics and visual appeal The design decisions were made using a points assignment system as shown in Appendix C. Each major component was assigned a total of five points and were rated on seven design selection criteria. Three distinct frame designs, two swing arm designs, two head tube diameters, and two handlebars made the initial short list of considerations. Design Testing The design was mainly tested for weld strength durability to determine critical structure integrity. The testing was done to establish and verify three points. To ensure design chosen would withstand the loading it is intended for To test the chosen design to failure To evaluate if a dimensionally smaller design could withstand the expected loading induced by a rider The weldments were subjected to both tension and compression at different points of the weld bead. The strain that the joint was subjected to was measured using strain gauges and with the 6

8 aid of LabView software. The force that was required to propagate a crack was calculated on both the welded structures to look at the feasibility of each, as the final design. Figure 2 shows the proposed design and the weld joint that is tested. The loading simulates the weight of a rider. Figure 2: Shows the design of the ski bike with headtube geometry that is subjected to the strain test along with the load direction which will simulate the weight of a rider. The fixture is shown in figure 3, along with the placement of the bottle jack, strain gauges, and the primary weld locations. The weld on 1 x 3 frame was able to withstand the load of up to 630 lbs transferred through the steel tube. Although this might be enough for normal circumstances where the only concern is the weight of the rider, but when things such as fatigue and impact loading from collisions are taken into account, maximum load of 630 lbs seemed rather fragile. Then, for the safety of the rider, 2 x 6 frame was the choice where the weld was able to sustain the load of 6715 lbs and beyond applied to the steel bar, to the point where the steel bar plastically deformed. At this stage, the main concern is not the weld failure, but the strength and material of the steerer tube of the bike. Another concern with the use of Aluminum is that unlike steel, it has a finite fatigue life. As such, the longevity of a design would need to take into account the loading as well as the cycles. Given that in the case of the Downhill Ski Bike the exact loading and cycles are unknown, the 2 x 6 design would give the extra assurance of safety. 7

9 Figure 3: Experiment setup for the small aluminum head tube (top) and for the large aluminum head tube (bottom), with locations of strain gauges and the bottle jack. Given the testing results, the final design was modified to only have two welds along the vertical walls common to the head tube and the square tube. This was decided to be ample to meet the design requirements. 8

10 Final Component Designs A summary of each main manufactured component is looked at below for a top level view of the main features of each item. The off the shelf items are listed on the BOM in Appendix B. Main Frame Figure 4: After milling of the main frame body. Large holes for weight reduction and small hole for thru axle. The group decided on 2 x 6 x.125 Aluminum rectangular tube for the main frame material as part of the original design that was selected. The 6 dimension provides superior resistance to deflection in the vertical orientation for the main frame even with the hole cut outs. Our design is like an I Beam with 2 walls where the load is carried at the outermost regions where the majority of material should exist to resist bending. Conversely, for side loading of the main frame, the moment of inertia distance is 2 with a great deal of material at the outermost region. Side loading of this part is not seeing a lot of force unless one was to jump on the side of the bike with the intention to break it. A head tube weld test was conducted to see if our design geometry and welds could withstand forces reasonably beyond what we expect the bike to encounter that yielded extremely favorable results. In fact our test apparatus broke after transmitting over 6000 pounds of force to the head tube and the weld or areas nearby never failed. 9

11 Frame Bushing Assembly Figure 5: Complete Frame Bushing Assembly Assembled. Large Diameter bushings were selected for their large polar moment of inertia which is desireable for single pivot points where side loading occurs as a larger polar moment of inertia resists in plane bending. The 2 main frame width is the other moment of inertia resisting side load bending between the main frame and the swing arm. Wider gives more moment of inertia which is one of the criteria in mind from the design we chose. The bushings are igus iglide J series bushings pn: JFI They are of newer technology for bushings that self lubricate, are impervious to dirt and other contaminants, as well as they don t expand when wet. They operate up to 194 F and can handle 5,076 psi. These bushings are also machinable and in doing so they do not lose their lubricity. Figure 6 : JFI Main Frame 2 ID Bushings Donated by igus. 10

12 Frame Bushing Outer Aluminum Race Figure 7: Outer Aluminum Bushing Race Pressed Into Main Frame Prior to Welding. This component of the bushing assembly connects the bushing to the main frame T6 2.5 diameter x.25 wall aluminum tube was turned to match it s inside diameter to the outside diameter of the bushings. Bushing Through Axle The Bushing through axle design left an approximately 1.6 diameter hole through the bushing assembly that will house the foot peg. This piece was machined to outside diameter to match the inside diameter of the igus bushings which were and Swing Arm Clamps Figure 8: Swing Arm Attachment Mechanism Swing Arm E Clamps Pre Welding. These parts are a solution to the question of how to connect the swing arm to the bushing of the main frame. They were CNC milled from 1 x T6 Aluminum flat bar. The rear end is machined to fit into the swing arm tubing which is Aluminum rectangular tubing of 1 x 2 x 11

13 This was done to make welding easier by ensuring parts were lined up and as much of the 6 degrees of freedom per part were constrained. The contact points for these clamps are 1 wide and initially required a pry bar of sorts to open them to position them on the bushing through axle. For dynamic loading while riding this Ski Transport Device, the 2 bolt design, along with the wide width of 1 per clamp and 2 clamps more than 2.25 apart gives an extremely rigid attachment for the swing arm to join to the through axle of the bushing. The surface of the Aluminum on Aluminum large area is highly frictional. This coupled with the clamping by 4 stainless steel bolts makes for a connection that will never be displaced as long as the bolts are in place. The Swing Arm Clamps were also designed to have the material for the bolts that are tangent to the circle be plentiful enough in their tangent length that excessive bolt force would not distort that material as can often happen for this type of connection when there is not enough material to accommodate the normal force of the bolts. The Clamps were fully welded to the swing arm tubes. Swing Arm Shock Mounts Figure 9: Rear Shock Mounts Post Welding with Pre Machined Titanium Rod for Shock Pins. The piece was designed to have multiple holes to give us some room to select a frame angle between the swing arm and the main frame. It s position would give the rider the angle choices that were either obtuse or acute in plan. This coupled with the selection of 3 holes in the main frame give multiple geometric and forcing options for the frame position and suspension. 12

14 Swing Arm Stabilization Cross Member Figure 10: Swing Arm Stabilization Cross Member Post Machining pre welding. This piece is designed to resist torsion in the swing arm assembly and to keep both sides of the swing arm in structural unisen. The center cutouts that include diagonals were cut out by the CNC mill. It was milled from ½ x T6 Aluminum flat bar. Swing Arm Drop Outs Figure 11 : Swing Arm Rear Drop Outs Post Welding. These parts were hand milled from 2 x T6 Billet stock flat bar. They are designed to take the point load of the rear ski assembly (including brakes) and transmit that load evenly to the 2 x 1 x.0625 Aluminum swing arm tubes evenly rather than drilling a continuous hole through the 4 walls at the back of the swing arm. These were fully welded to the swing arm tubes. 13

15 Head Tube, Headset, and Fork This system is critical to the steering and control of the SnowBike. There were many design decisions that had to be made prior to purchasing any materials. The three major components of this system are the headset, the headtube, and the fork. Both the headset and headtube are aluminum while the fork is CrMo steel. The headtube had to be aluminum so that it could be welded to the main frame. The fork material was chosen due to its strength to weight characteristics. The headset was selected for many reasons. The primary decisions affecting these systems were the fork diameter and the headset bearing type. The group decided a wider fork diameter would support more forcing and so a diameter of was selected and both the fork and headtube were purchased from an online supplier. Much deliberation was had in regards to the wall thickness of the fork and force/moment calculations were performed to select the most appropriate. Without knowing the amount of forcing that the fork would experience, a wall thickness of was selected for its acceptable yield strength in comparison to its low weight. The fork thickness may need to be increased in addition to decreasing the forward angle. The headtube is 2 in diameter and had to be thick enough to machine out seats for the bearing. There were plenty of bearings available to choose from that would fit our needs. The best choice for our purpose is called an external cup. This means that the bearing is semi permanently seated into a housing. This orientation was helpful since it would limit the amount of loose parts when assembling and disassembling. In the end the only loose parts were the upper and lower bearing races which make contact with the bearing itself. A silver Chris King nothreadset was purchased and dimensioned to have an interference press fit into the headtube. The headtube was cut to length, machined on both ends to accept the headset and then milled out to reduce the overall weight. A crown race was brazed to the fork at the selected location for the rear bearing race to sit against and provide compression to the headset. The fork was cut to 14

16 length based on the geometry of the frame and a hole was drilled at the base to accept the pin connection from the front binding. Suspension The suspension is another component that was selected off the shelf and can be swapped out easily for an upgrade in the future. A RockShox Kage 9.5 x 3 spring and damper shock was selected for its tunability and robustness. The shock was originally fitted with a 300#/in spring but it was swapped out after testing for a stronger 600#/in spring to support heavier riders. It was decided by the group that a spring and damper system would be more reliable and durable in the cold and snowy conditions. Handlebars and Riser Ritchey Pro Rizer Alloy handlebars were chosen for their combination of light weight (.48 lbs) and budget friendly price. The handlebars have a rise of 35mm and a sweep of 9 degrees. Foot Peg(s) Due to a last minute design change to meet the customer s request, the foot peg design was altered. The foot peg was originally designed to attach just above the pivot point and would fold towards the frame for transport. The new design is of a straight tubular cross section of Aluminum which attaches through the hollow section of the main pivot point. The foot peg will be held in place by a latch pin and can be removed for transport. Though this adds weight to the bike over the originally designed pegs, it lowers the center of gravity of the rider and takes advantage of the hollow geometry of the pivot point. Figure 12: Final foot peg design 15

17 Bindings A separate binding was incorporated into the design in order to facilitate easy pinning of the Ski Bike structure to the skis during assembly. The previous design made this task somewhat cumbersome and time consuming due to its design. The new design uses lightweight aluminum in place of an integrated steel design. Additionally, the aft binding doubles as a mounting point for the braking mechanism. The new design makes it such that, when the binding is placed on the ski, the holes common to both the ski and the bindings self align for a much easier pinning operation. The alloy chosen for the binding was 2024 T3, due to its excellent strength characteristics, easy machinability, as well as resistance to deformation. Effort was put into making the bindings lighter for the overall intention of making the Ski Bike lighter. The Ski Bike is attached to the bindings using ¼ bolts both for the forward and aft mountings. See Appendix F for dimensions. Figure 13: The forward and aft bindings shown with weight reducing geometries added. Brakes The brakes were designed to function in both hard pack snow and icy conditions. The brake system will be operated via a standard hand lever and cable, which can be found in any bike shop. The rest of the braking system was designed to be 3mm thick, 304 SS and it was to be cut 16

18 using either a laser or water jet, and then bent into the desired shape. Both the material and water jet cutting of the parts were provided by IRC Aluminum at no cost. This was a savings of about $150. The brake assembly consists of 5 different cut out pieces, two springs, a threaded coupling and 2 bolts. The final product of the cut out pieces are shown in Figure 14 and are: a support bracket to which the brake attaches to the binding, two claws which will be used to penetrate the snow, a rear support brace and a brake cable bracket. Figure 14: The cut out pieces post bending for the braking system. The claws were designed to penetrate ice using a sharp point, and in hard pack and snowy conditions, a flange was incorporated to provide greater surface area to act as a plow. An area of concern was how the brake would return to its starting position. This was achieved by the use of two tension springs that slip over the threaded coupling. The assembly of the braking system is shown in Figure 15. Figure 15: Assembled Braking System 17

19 Stem The stem is the part that connects the handlebar and the front fork of a bike. It is usually designed so that once it is set up, the handlebar and the fork will not easily be moved or separated from each other unless multiple bolts joining them are loosened first. Last year s ski bike used this standard design, which can be seen in Figure 16. The handlebar and the fork formed a T shape, which made the transport of the bike a cumbersome task when it was not in use. This year the stem was designed so that the handlebar can come off and on the bike easily. This task was achieved by first acquiring an off the shelf part called a stem riser that is made out of aluminum. The stem riser is essentially an another pipe that wraps around and extends the fork. It has a slit cut down vertically at the bottom, which allows the riser to be clamped around the top of the fork by tightening bolts. These bolts were replaced by parts called quick release skewers, which have cam levers for easy clamping and unclamping. Another bolt that needed to be replaced by a skewer was the one that goes through the shaft of the fork vertically. This bolt compresses the stem and the headset tightly, which is important in making sure the fork rotates with the bearings inside the headset. A steel cylinder of ¾ inch length and the diameter 0.02 inch less than that of the inner diameter of the fork was cut, tapped with M5 threaded hole, and was braised inside of the fork to provide threads for the vertical skewer to screw into. Details of this steel cylinder can be seen in Appendix F. Figure 16: Handlebar and fork connection of previous year s ski bike. Disassembly is not possible without tools, and is time consuming. Figure 17: Stem, stem riser, and the handlebar set up being removed from the bike. Tightening bolts are replaced with skewers, which allows for easy assembly and disassembly. 18

20 Conclusion In looking at the final design and comparing them to the PDS requirements, it can be said that overall objectives were achieved. The following table looks at each PDS objective for a more indepth evaluation. The only objective that was not met was the weight goal of keeping the overall weight under 12.2 lbs. This was a priority level two. Given the new design add ons, it was concluded that it was a workable weight in terms of portability. 19

21 Appendix A: Full Product Design Specificatio n Below is the PDS breakdown along with its level of priority. 20

22 Appendix B: Bill of Materials 21

23 Appendix C: Major Component Selection The selection process itself proved to be quite challenging due to multiple merits of each, but in the end, based on the points system the decisions were made simpler. The key concepts that were kept in mind was structural integrity and manufacturability. Note: The handlebar selection was changed from carbon fiber to Aluminum due to cost. 22

24 Appendix D: Test Apparatus Figure D1 is the picture of the test apparatus just before the experiment was conducted. Figure D1: Shows the custom built fixture that houses the test assembly. The fixture is made of steel that is welded with extra bracing for strength. The test assembly is bolted in place during loading. The bottle jack has a 20 Ton capacity, which proved to be ample for the test. 23

25 Figure D2 and D3 show the welds of the two test tubes after the load had been applied. Figure D2: Shows the crack propagation at the point where there is a transition in geometries. Additionally, necking/elongation in the 2 dia. Aluminum tube is observed prior to cracking. Figure D3: The plastically deformed steel tube is shown with no failure of the Aluminum assembly. The subassemblies that are connected by the weld has no abrupt changes in geometry, resulting in a more robust design. 24

26 Appendix E: Assembly Details Brake Assembly The pieces were bent using a vice, vice grips, crescent wrenches, and additional metal plates. The parts were secured with the vice and using vice grips, metal plates were secured at the bend areas to create a more rigid line to allow cleaner bends. The crescent wrench was used for leverage and a lot of force was applied to make each bend. The claws, back brace and cable bracket were welded together to form the complete brake claw. To assemble the system, the support bracket must first be mounted to the rear binding. With the threaded coupling running through the springs, it is then positioned to insert the bolts and tighten. The spring should be positioned with the longer ends towards the center and top and the shorter ends to the bottom and pressed out to the side so that they slide between the gaps to reduce the initial tension and make for ease of assembly. After the brakes are assembled, using a pliers, carefully move the springs that were between the gaps to the top of the plate. 25

27 Appendix F: Production Drawings Bindings 26

28 27

29 Brake 28

30 29

31 30

32 31

33 32

34 33

35 34

36 35

37 36

38 37

39 Stem 38

40 Fork 39

41 Headtube 40