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Off-Road, Downhill Skateboard - Andy Ruina - Cornell University PDF

34 Pages·2004·4.21 MB·English
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Preview Off-Road, Downhill Skateboard - Andy Ruina - Cornell University

Off-Road, Downhill Skateboard Senior Design Project for The Sibley School of Mechanical and Aerospace Engineering Course: MAE 491 Prepared for: Professor Andy Ruina Engineering School Cornell University email: [email protected] Prepared by: Michael Meacham Graduating Mechanical Engineer Cornell University email: [email protected] A mountain board designed from the ground up. Draft 1: May 16, 2004 Draft 2: May 20, 2004 Off-Road, Downhill Skateboard CONTENTS Abstract.........................................................................................................3 Introduction................................................................................................... 4 Design...........................................................................................................5 I. Overall Design Goals...................................................................... 5 II. Overall Design Elements................................................................6 III. Detailed Design - Pre Parts-Purchasing........................................9 a. Deck.................................................................................... 9 b. Frame..................................................................................10 c. A-arms.................................................................................11 d. Suspension..........................................................................12 e. Steering Pivot Arm...............................................................13 f . Swing Arm...........................................................................15 g. Wheel and Wheel Hub.........................................................16 IV. Post Part-Purchasing / Fabrication / Design Changes..................16 a. Ball and Socket Joints.........................................................17 b. Suspension..........................................................................17 c. Deck.....................................................................................18 d. Steering Pivot Arm...............................................................18 e. Deck Stiffness......................................................................20 f. Board Steering and Stability................................................21 g. Brakes................................................................................. 23 h. Wheels.................................................................................23 Discussion / Conclusion................................................................................24 Acknowledgments.........................................................................................25 Appendices................................................................................................... 26 I. Appendix A - Purchase List.............................................................26 II. Appendix B - MATLAB Code For Shock Geometry........................27 III. Appendix C - Dimensions..............................................................28 a. Frame (front)........................................................................28 b. Frame (side)........................................................................29 c. Frame (top)..........................................................................30 d. Swing Arm (front, top)..........................................................31 e. A-arm (top).......................................................................... 32 f. Steering Pivot Arm (top, front).............................................33 g. Steering Points....................................................................34 2 Off-Road, Downhill Skateboard ABSTRACT The goal to design a new off-road, downhill skateboard is first accomplished by studying current mountain boards for sale. These mountain boards are limited in their design and functionality in regards to off-roading. Design goals are created to make a more functional skateboard for use on off- road trails. Using SolidWorks, a design is created, which incorporates fully independent suspension, steer-by-lean action, 10" inflatable tires, a wide wheel track, and disc brakes. It features a steel frame and A-arms, with a deck that pivots above. The pivoting action of the deck controls the steering of all four wheels. During manufacturing and testing, certain design elements are changed and added. Four-wheel steering can be converted to two-wheel steering quickly for more stable high-speed runs. Skateboard stiffeners are added to the deck, which gives this skateboard a natural skateboard feel. Disc brakes are not attached due to funding and time constraints, but testing shows that the board is perfectly functional and fun to ride. 3 Off-Road, Downhill Skateboard INTRODUCTION Skateboarding, snowboarding and surfing are all successful industries. Every recreational sport involving a board, where the rider can carve turns by leaning the board in the direction of motion, has always drawn lots of attention to itself. People have a natural liking for maneuvering through an environment simply by standing and leaning on a platform. The next environment for this phenomenon to enter into is off-road, mountain trails. The reason people have only just begun to design these types of boards, known as “mountain boards,” is that it is far more complicated to design a device that will not trip over large bumps and can take the abuse of a mountain trail, while still giving the rider a comfortable ride with steer-by-lean action. Current mountain boards are just modified skateboards. They use the same principles for the steering mechanism, offer little suspension, and not much more ground clearance. They are really designed for smooth, dirt roads, not mountain trails. In order to truthfully tap into this environment, a mountain board must be completely redesigned from the ground up. A rocky, bumpy path has little similarity to a smooth, dirt or paved path. An off-road, downhill skateboard should be compared to downhill mountain biking more than skateboarding. It is necessary to have a heavy, stable frame with fully independent suspension. This will allow the rider to control the board, even at high speeds with many bumps and objects on the path. This project will take mountain boarding where it was meant to be. It will help to gain the attention that it deserves. With ten-inch tires, over six inches of ground clearance, and almost five inches of travel in each wheel, this mountain board is nothing like current mountain boards. It will be able to go down steep trails, but still offer safety to the rider with hand-controlled disc brakes on all four wheels. It will be able to go over much larger rocks and bumps, but the user will still feel a smooth, controllable ride. 4 Off-Road, Downhill Skateboard DESIGN I. Overall Design Goals To create a list of design goals, current mountain boards were first studied and problems assessed. An “all-terrain” mountain board that one can buy in stores uses trucks to steer the board. A skateboard truck is a simple mechanism that attaches the axle of the wheels to the board at a specific angle. When the board leans, the axles are forced to rotate around this angled pivot point. The mechanism works well for riding on streets or smooth, dirt roads. The problem is the lack of fully independent suspension. As can be seen by the picture, the suspension is designed to take the shock of small bumps, not large objects. The suspension it contains is the flex of the board, the trucks, which utilize small springs as the return force, and "egg shocks" below the rider's feet, which absorb around 3 cm of travel. The steering and suspension are not independent of one another at all. When one wheel travels up, the other must travel down. This feature cannot work properly while turning, as it will change the turning radius considerably. To account for this, the travel in the wheels is kept at a minimum, and the ride is unsmooth. Trucks also limit the distance between the left and right wheels. Since the entire axle turns, a long axle will result in large longitudinal Figure 1 Mongoose UniCamb All Terrain Board motion in the wheels. This will result Courtesy of mountainboardshop.com in bump steer, the undesired steering when a wheel travels up or down. If the wheels were to rotate about their own independent axis, then the wheel track could be much larger. For off-road situations, a large wheel track is preferable for stability. Current mountain boards, much like skateboards, are too easily tipped over while riding because of how narrow they are. The radius of the wheels can range from about 5 to 8.5 inches in current mountain boards. The designs for other boards are very similar to this one, in that they utilize trucks and have no independent suspension in the wheels. Because of these concerns with current mountain boards, the design goals of this downhill board are the following: 5 Off-Road, Downhill Skateboard - steer-by-lean ability – Like any other boarding sport, the rider must be able to lean and have the steering respond quickly and smoothly. - fully independent suspension – The suspension must be independent of steering so that while in a turn, the rider can still travel over objects. - large wheel track for stability - larger tires for getting over bumps – A wheel will steer on an independent axis. One wheel’s steering will not necessarily affect another’s, except through the mechanical connection to the deck. - four-wheel steering – In the spirit of a skateboard, there will be no front or back. A rider can get on the deck either way he/she chooses and it will work the same way. Furthermore, this feature allows for many more tricks where the board changes directions. - hand controlled disc brakes – Because of the dangers of off- road skateboarding, and the predicted weight of the board, disc brakes will be placed on all four wheels. They will be hand controlled, with one lever controlling the “front” brakes, and other lever controlling the “back” brakes. The independent control of the front and back brakes will allow the rider to control the skidding of the tires and add to the functionality of the board. - maintain the general feel of skateboarding – If a rider is skilled at skateboarding or mountain boarding prior to using this product, then the transition time will be kept at a minimum. II. Overall Design Elements To achieve the previously stated goals, the skateboard will have a frame that is independent of the deck and of the wheels. This allows the deck to pivot above the frame and control the steering through the use of steering links, rather than a solid truck. When the deck is leaned by the rider, a pivot arm that hangs below the deck will move around the circumference of a circle. Tie-rods will be connected at the end of this pivot arm and also connected to the steering arms at the wheels. A-arms will be connected to the frame and allow for the vertical travel of the wheels. 6 Off-Road, Downhill Skateboard Deck Frame Pivoting Arm Figure 2 Front view. Features pivoting deck with pivot arm. When the deck is leaned in a direction, two pivot arms will move, and all four wheels will be turned in the proper direction. steering arms Figure 3 Features the leaned deck and turned wheels. The steering arms all point toward the middle of the board causing the wheels to turn in the appropriate direction when the deck pivoted. 7 Off-Road, Downhill Skateboard The suspension will be connect to the bottom A-arm, travel through the top A-arm, and then connected to a shock tower that is connected to the frame. The A-arms will move up and down, but keep the tire perpendicular to the ground. These shocks will be about one foot in length. Shorter shocks are preferable to avoid such tall shock towers. The rear shocks on mountain bikes are high performance, fully adjustable shocks that are typically between 7 and 8 inches in length. However, these cost a minimum of $250.00 each. Because this design needs 4 shocks, the funding constraints made this unfeasible, and the longer, cheaper shocks must be used. Figure 4 Features shock connected to lower A-arm, traveling through upper A-arm, and connecting to shock tower. 8 Off-Road, Downhill Skateboard III. Detailed Design - Pre Parts-Purchasing This section will describe in detail, the designed parts prior to the purchasing of components. The designs are based on the information provided by vendors for various parts. Many design elements are changed after the parts are purchased and more information is gained on them. Those changes will be outlined in the following section. All detailed dimensions of the parts can be found in Appendix C. a. Deck The deck is chosen to be a small snowboard. A kid's snowboard is about 135cm in length. This length will allow for a comfortable distance between the riders feet. Notches are to be cut out from the edges to allow for the suspension to pass through. The ends will not be for standing, but instead will be left for aesthetic reasons. This will let a person who has never seen the product before know that it is a board that he/she can stand on. Since the deck will be attached to the frame in only two places, it needs to very rigid if the rider is to stand in the middle. A snowboard deck may not be as rigid as needed and braces may need to be formed. holes for pivot connection Figure 5 The Deck. Features a top view (left), showing the holes for the connection to the frame and an isometric view (right) 9 Off-Road, Downhill Skateboard b. Frame The frame length is designed to be about the same length as the deck. With this design, the rider’s feet must be kept within the shock towers. The frame length must allow enough distance between the rider’s feet, the shock towers, and then the A-arms. Length is also added to allow for the angled sections at the front and back. These angled sections allow the frame to hit a large bump and be forced over it, rather than hitting it and coming to a sudden stop. When traveling over a bump, the front wheels generally go up and over it, and then clearance is required in the middle of the skateboard to ensure that the frame does not grind along the bump. shock tower deck pivot point raised middle angled section Figure 6 Side view of frame. The cross section of the frame is a rectangle for the length where the suspension, A-arms, and deck attach. This rectangle should be as small as possible, but is constrained by two main factors. The first is the A-arm geometry. If the A-arms are connected to the top and bottom of the frame, then their distance apart is completely controlled by the height of the cross section. The other constraint is the steering. As the deck pivots about its axis, the pivot arm swings about that same axis. If the cross section is too small, then the tie rods will hit the frame tubing on a sharp turn. tie rod Figure 7 Features cross section of frame, and the various constraints that affect its size. 10

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May 16, 2004 Using SolidWorks, a design is created, which incorporates fully independent Skateboarding, snowboarding and surfing are all successful industries. The usage of bindings is determined to be hazardous after testing.
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