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

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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 offroad 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.

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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.

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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 motion in the wheels. This will result Mongoose UniCamb All Terrain Board 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:

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Off-Road, Downhill Skateboard

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steer-by-lean ability – Like any other boarding sport, the rider must be able to lean and have the steering respond quickly and smoothly.

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fully independent suspension – The suspension must be independent of steering so that while in a turn, the rider can still travel over objects.

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large wheel track for stability

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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.

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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.

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hand controlled disc brakes – Because of the dangers of offroad 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.

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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.

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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.

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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.

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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)

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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. deck pivot point

shock tower

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.

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Off-Road, Downhill Skateboard The reason to make the rectangle as small as possible is so the A-arms can be longer while still achieving the same wheel track. If the frame is wide, then the A-arms will be shorter, causing the wheels to move horizontally while traveling vertically. As the suspension takes the load, the wheels move up with respect to the frame. But they also travel around a circle with the pivot point at the frame's edge. The longer the radius, the less horizontal motion per vertical motion, which will reduce tire scrub when the suspension retracts. Making sure that everything fit using SolidWorks, the inner cross section came out to be 3.5 inches wide and only 2 inches tall. 4130 Steel is used as the material for the frame because it needs to be durable, easy to weld and cut, and good with impacts against rocks and bumps. The square tubular steel chosen has an outer cross section of 0.75" x 0.75". Its wall thickness is 0.060". These dimensions yield a cross sectional moment of inertia 0.013 in.4. With a yield strength of about 66 ksi, this steel can take a moment of 2,288 in.•lbs. The frame is likely to be the weakest in bending when the middle of the frame is resting on a bump, and the upward load on the wheels is removed. The weight of the rider (200 lbs) will put the frame in 3-point bending with a moment of about 1,450 in.•lbs. With two frame members at this location, the frame will not yield.

c. A-arms The major design aspects of the A-arms are the relative distances of the three end-points and the ball and socket joints that allow motion, but keep vibrations at a minimum. In order for the steering and the suspension to work, the wheel side of the A-arms must allow for two degrees of freedom. Frame Side The wheel must be able to turn, and 6.4" it must be able to travel vertically. The frame side of the A-arms only needs one degree of freedom to allow for the travel of the wheel. However, for simplicity, a ball and 9.3" socket joint will be used for all three connections. The top A-arm does not feel a load from the suspension; it is only there to keep the tire perpendicular to the ground. Again, Figure 8 for simplicity in manufacturing, the A-arm upper and lower A-arms are to be identical.

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Off-Road, Downhill Skateboard The A-arms will be made from the same 4130 steel as the frame since they too will be susceptible to impacts. A 0.5" round stock with 0.049" wall thickness has a cross sectional moment of inertia of 0.0018 in.4. It can take a moment of 475 in.•lbs. As it is shown in the next section, the shock will be located at the very corner of the arm. If a wheel takes a maximum upward load of 100 lbs while riding, then the maximum moment taken will be 400 in.•lbs. The bottom A-arm is also in tension because of the angle of the shock. In tension, the steel tubes can take a load of around 4,600 lbs., far more than the suspension is capable of producing. Therefore, the metal will hold, assuming the welds penetrate properly. If it fails, it will most likely be because of a rock or bump hitting the A-arm, instead of hitting the wheel or the frame. This is why the A-arms are kept wide at 6.4" - to make them as strong as possible in that situation.

d. Suspension Many factors contribute to the stiffness of the suspension. The spring rate of the shocks, the length of the shocks, and where they are mounted all effect how smooth the board will feel when riding it. A MATLAB code is written with the help of Jacob Timm to determine the ideal geometries of the shocks. For cost reasons, the only shock that is available is one foot in length and has a max travel of 2.5 inches at 500 lbs. The following graph is produced by the MATLAB code that is found in Appendix B.

Ground Clearance

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Off-Road, Downhill Skateboard This graph shows what load the wheel will feel with two different constraints, given that the shock is the foot-long one specified previously. "Distance Along A-arm" is the distance that the shock is connected from the frame. At that distance, one can look up the ground clearance and determine the load on the wheel at that clearance. For example, if the shock is connected 8 inches away from the frame along the A-arm (almost at the A-arm's corner), and you have a ground clearance of zero, meaning that you have bottomed out, then the load on a wheel will be a little over 250 lbs. This allows you to adjust what load will bottom the board out by changing the geometry of the shock. There is clearly a maximum for this graph. This occurs at max travel in the wheel and when the shock is hooked up almost at the ball and socket joint. Of course, the shock cannot be connected at that location, so the peak is actually close to the corner where the two steel tubes meet. Although information about possible load situations is missing, the suspension geometry is chosen to be at the stiffest. This is to prevent the possibility of bottoming out. Testing will surely have to be done to determine if this choice is good for riding down an off-road path. Furthermore, the vendor does not specify the preload on the shocks, so at the time of creating this graph, the preload is considered to be zero. This is most likely a bad estimate, and adjustments will have to be made. e. Steering Pivot Arm The length of the steering pivot arm effects both the steering sensitivity and the height of the deck. Both of these factors have to be balanced. From setting up different platforms, one can see that 12 inches is the absolute maximum height for a deck while still feeling comfortable standing on it. The advantage of having a swing arm hang down and follow a circular path is that Ackermann steering can be achieved. Ackermann steering is when all four wheels of a vehicle turn about the same point to reduce tire scrub. Figure 10 Deck Pivot Point

Front view - See Figure 2 for reference. The Deck Pivot Point marks the axis that the deck pivots about. This axis is shared by the steering pivot arm. As the arm swings about the deck pivot point, the steering points follow a circular path. If it rotates clockwise, then the left point has more vertical motion than horizontal motion, and the right point has more horizontal motion than vertical motion, and vice versa. This causes one wheel to turn more than another to achieve Ackermann steering.

Steering Points

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Off-Road, Downhill Skateboard If the steering points are moved farther away from each other, than the Ackermann effect becomes more sensitive. If the steering points are brought together and meet, then the wheels will turn the same amount. Using SolidWorks, all of the geometries are adjusted to achieve Ackermann steering at sharp turns. It is at these sharp turns where tire scrub will be the most noticeable.

Figure 11 Shows each wheel turning about the same point at sharp steering angle.

When determining how sensitive the steering should be, one must analyze the nature of a skateboard. Firstly, suppose you are riding a street luge. In this case, the sensitivity of the steering is extremely important. At a particular lean of the deck you will circle around a point with radius r. The street luge will mv 2 accelerate you toward this point with a force of , m being your mass, and v r being the velocity. The deck must be leaning by the correct amount such that you are not thrown off the side. Furthermore, it must be calibrated to the desired speed because this optimal lean changes with v. The reason this is so important € to shift your body weight. Once you with a street luge, is that it is very difficult lean the deck a certain amount, you have no control over how far your body weight has shifted.

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Off-Road, Downhill Skateboard On a skateboard however, the two components are independent of each other. The rider can lean the deck a certain amount using his/her feet, and then shift his/her body weight in either direction he/she chooses. Because of this, the geometric sensitivity of the steering is not nearly as important as the stiffness of the steering. If there is a way to stiffen or loosen the deck such that it springs you back to centerline harder or softer, then one could adjust the effective sensitivity of the board for faster or slower runs. If you are pushing harder to achieve a turn radius, then your body weight is easily shifted to account for the centripetal motion. I will not design for this until the skateboard has completed construction, and I can test for the natural stiffness, if any, that the steering has.

f. Swing Arm The swing arm has four main parts to it. The axle holds the bearings for the wheel. The steering arm is pulled by the tie rod, which causes the wheel to pivot about the axis created by the two ball and socket joints at the ends of the Aarms. A metal plate is attached to hold the brake calipers. This plate will move with the wheel in every degree of freedom except for spin, which will allow for accurate braking. The last component is the axle in which the ball and socket joint are connected to the A-arms.

metal plate and caliper assembly

axle between ball and socket joints

steering arm

wheel axle

Figure 12 Swing Arm

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Off-Road, Downhill Skateboard Two swing arms will be constructed like the one shown here, and two mirror swing arms will be constructed for symmetry.

g. Wheel and Wheel Hub The tires are 10" x 3" tires. They have a four-bolt pattern in the rim, and therefore a new wheel hub will have to be machined to hold the brake rotor in place. This new hub will have a four-bolt pattern on one side, and a six-bolt pattern on the other for the rotor. It will also hold the bearings to ensure that the rotor has no wobble as it spins.

IV. Post Parts-Purchasing / Fabrication / Design Changes

Figure 13 Complete Skateboard

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Off-Road, Downhill Skateboard a. Ball and Socket Joints 1/4"-28 rod ends are used for all ball and socket joints. There are 32 rod ends on the skateboard: three for each A-arm and two for each tie rod. 1/4"-28 nuts are welded to the ends of the half-inch tubes to allow for the rod ends to be screwed in.

b. Suspension The shocks were preloaded to about 90 lbs. They were also easily taken apart so that washers could be added to the spring, causing it be preloaded even further. Because of this, the shock geometry is changed. They are still connected to the corners of the A-arms, but now the other ends are connected to the same point. This lowers the angle of the shocks for aesthetics and strength in the shock towers. The lower angle also reduces the amount of roll the frame may experience on strong turns. It achieves this because of the added force on the A-arms from the frame in the horizontal direction.

Figure 14 Features the shocks mounted at same point and nylon washers for added 0.375 inches of preload.

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Off-Road, Downhill Skateboard c. Deck The deck has gone through a major revision. Because of the newly design shock mounts, the deck length had to be shortened, and the aesthetic curved edges forgotten. To ensure rigidity, which was not guaranteed with a snowboard, the deck has been made out of half-inch plywood and reinforced with carbon fiber. There are four layers of carbon fiber on each side. The carbon fiber and epoxy used has a strength of about 20 ksi, and each layer is 0.02" thick. Analyzing the carbon fiber alone without the plywood, the moment of inertia of the cross section is 0.176 in.4 and it can take a moment of 13,333 in.•lbs. This is more than enough to hold up the 1,450 in.•lbs. that a rider may produce. However, the reason for the added carbon fiber is not to take the bending necessarily. In my experience, carbon fiber can be crushed easily with local damage. I have many bolts going through the deck, and high torque is experienced when turning. The added carbon fiber is to ensure that no local damage occurs. Also, metal plates are used anytime there are bolts to spread the load to a wider area over the carbon fiber. The deck may be over designed, but little weight is added with carbon fiber, and there was no added cost. Skateboard grip tape is wrapped around the deck for added friction under the rider's feet. The usage of bindings is determined to be hazardous after testing. Often the rider must jump off when traveling too fast or if he/she loses control of the skateboard. Testing shows that the friction due to the grip tape is enough to keep the rider safely on the board.

d. Steering Pivot Arm The steering pivot arm is redesigned to have a threaded rod be the connection between the deck pivot point and the steering points that push and pull the tie rods. The reason for this change was to make the height of the pivot arm adjustable through the use of nuts around the threaded rod. This did work for the purpose of finding a good height, but the threaded rod, being 1/4"-20 was not strong enough to take the torque that the steering applied to it. An analysis on the tire scrub will show why.

Figure 15 This rod was changed to a threaded rod, but this idea did not work because of unforeseen torque.

Point of failure.

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Off-Road, Downhill Skateboard When standing on the skateboard, a wheel can feel a force upwards of 150 lbs, depending on what the rider is doing. Assuming the coefficient of friction between the tire and the road is 1.0, the friction force that tire scrub could produce is 150 lbs. This force creates a torque about the axis of steering, which is 3 inches away from the friction force. This torque is equal to 450 in.•lbs. The tie rod connects to a point that is also 3 inches away from the steering axis. Therefore the tie rod can push on the steering point with a force of about 150 lbs. There are two tie rods, so the combined force is 300 lbs. The vertical distance between the steering points and the point of failure is 2.75 inches. The threaded rod was taking a max torque of around 825 in.•lbs. The unthreaded diameter of a 1/4"-20 rod is 0.2 inches. The yield strength is 60 ksi for a grade 2 threaded rod. The threaded rod, with a cross sectional moment of inertia of 7.85 x 10-5 in.4, can take a moment of only 47 in.•lbs. The large torque that this threaded rod feels was completely overlooked. This problem was corrected. The threaded rod was used to get the exact height desired, but then it was replaced with a solid, welded, half-inch steel rod. With a cross sectional moment of inertia of 0.003 in.4, it can take a moment of 798 in.•lbs. This is almost the same as the max torque that could be applied. With the additional thickness due to the welds, this pivot should not fail, and has not yet with hard testing.

Figure 16 Half-inch steel rod takes the place of weaker threaded rod.

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Off-Road, Downhill Skateboard The steering points are also moved outward. Holes were drilled at the points that were designed for, but the steering felt slightly more smooth with the points farther away from each other, near the edge of the frame.

e. Deck Stiffness The skateboard was working in testing, but as previously predicted, the steering was hard to control because there was no return force back to centerline. Different types of springs were tried such as bungee cords and thick elastic rubber cords. After many days of debating with friends (John Darvill, Jacob Timm, and Josh Christensen), the obvious solution was thought of. The stiffeners from a skateboard were removed and added to the bottom of the deck.

Figure 17 Features two stiffeners, which stiffen the deck's rotation with respect to the frame.

As the deck pivots, these hard plastic pieces compress. The bolts which hold them in a compressed state can be tightened and loosened, which will effectively change the stiffness of the steering. Using the same stiffeners as found on a skateboard gives a simple solution that creates a very similar feel to skateboarding.

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Off-Road, Downhill Skateboard f. Board Steering and Stability The four-wheel steering works well, meaning that at slow speeds, the skateboard turns at a short turning radius, and the rider is able to carve around objects with accuracy and control. However, after bringing the skateboard to the Cornell Plantations, where it was able to pick up more speed over a long distance, it is determined that at a little over 10 mph, the board becomes unstable and oscillates violently. Further testing showed that if the rider leans to the front of the board, these oscillations can be controlled more, and higher speeds can be reached. Leaning far forward is not a good solution as it is sometimes difficult to do, and if the rider forgets for even a little bit, he/she can be thrown off immediately. After a discussion with Professor Ruina, holes are drilled into the frame near the steering points to allow the tie rods on the "back" to be connected rigidly to the frame. This turns the skateboard into a front-steer only board.

Figure 18 Holes drilled into the frame allow the tie rods to be secured rigidly.

Because the tie rods can be adjusted in length, the alignment is made perfect in the rear wheels when switching between four-wheel and two-wheel steering. Although this design change does not follow the original goals stated, it is necessary at higher speeds. Four-wheel steering can still be used effectively at low inclines and at low speeds. For example, a rider can carve around bushes in a low-grade field with four-wheel steering, and cruise down a steep, rocky path with two-wheel steering. The design change is an addition to the skateboard's functionality.

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Off-Road, Downhill Skateboard The following pictures show high-speed turns at the Cornell Plantations. The two wheel steering works perfectly in keeping the board controllable and stable.

Figure 19 Frame-by-frame shot of me going down the steep part of the Cornell Plantations. The board is in twowheel steering mode and stays perfectly stable.

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Off-Road, Downhill Skateboard In two-wheel steering mode, the turning radius is twice as long per lean of the deck. This decrease in geometric steering sensitivity is also helpful in highspeed situations. The deck stiffeners are effective in changing the steering sensitivity, and therefore no geometric changes were required.

g. Brakes Because of time and funding constraints, the brakes have been put on hold. I am currently deciding whether to ever add brakes because while testing, they are rarely needed. The hills that I tend to ride on are not very long, so the need to slow down is minimal. If taken to a larger hill or mountain, brakes would most surely be a necessity.

h. Wheels The wheels used were purchased from ebay.com. The original place of purchase and the brand name are both unknown. I believe they were originally designed for a hand truck used to move boxes. The only other wheels that come close to the desired size are go-kart wheels. These are much more expensive, and sometimes much heavier. The wheels from ebay were the correct dimensions, cheap, and only weighed 3 lbs each. The bearings were replaced because the stock bearings were not sealed and were low quality.

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Off-Road, Downhill Skateboard

DISCUSSION / CONCLUSION Almost all original goals were met in this project. The major goal that was not met is the disc brakes. However, these can be added at a later time with more time and money. I predict that it would take one week to install disc brakes on all four wheels. Since I am the only person who has done extensive testing on the skateboard, I will give my honest opinion on its usability and functionality. By far, the worst aspect of the skateboard is its weight, which is about 55 lbs. Towing it up the hill is not a simple task. At first, I called this "added exercise," but now I just wish it was lighter. However, I am not sure if making the board lighter would create an unstable ride. Most likely, a small decrease in weight would go unnoticed. With the addition of the two-wheel-steering mode, the ride is extremely nice in all conditions tested. The wide track gives the rider the feeling that it is almost impossible to fall off, and this seems to be true. Any bump I hit, the board finds a way over it, and I feel almost no jerking vibration in my feet. Even though the hike is always painful, the ride down makes it well worth it. As far as the dangers of this sport are concerned, it depends entirely on what the rider is using it for. For the Cornell Plantations, I would feel completely safe with a helmet and some light padding. For a more extreme path, which has not been tested, major padding is recommended, much like the armor that downhill mountain bikers wear. The board itself can be your enemy at times. There are many metallic components on it and an abrasive grip tape on the deck. When falling or if the rider decides to bail, he/she should jump away from the board as far as possible. If you simply step off the board while traveling at high speeds, the A-arms will most likely sweep you off your feet and cut your legs badly. This skateboard may or may not be durable and reliable. I would imagine months of testing being required to make a claim about this. In early testing, some things did break or yield, but all of these parts have been updated, and I ride it with confidence, never worrying that something may snap. Concerning possible consumer interest - When I take the board anywhere, people constantly ask me questions and comment. It is safe to say that they haven't seen anything like it before, and it interests them. People either want to try riding it, or they want to see someone else try it. The only negative comments I ever receive are, "you're crazy" or, "that's insane." I suppose these two ideas were unstated goals from the beginning, and they convince me that the project is a complete success.

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Off-Road, Downhill Skateboard

ACKNOWLEDGEMENTS I. Funding Most of the funding for this project came from myself. I would also like to thank Joan Galer (mother) and Stephen Meacham (father), for their unquestioning financial support. I could not have completed the project without these two great financial sources. II. Design / Construction Special thanks to Jacob Timm for discussing almost every design aspect with me and writing the shock-geometry MATLAB code. He also spent long hours with me in the auto lab helping me finish this project on time. Emily Smith, Josh Christensen, Jon Darvill, Paul McCord, and Luke Delaney all contributed greatly with ideas during this project. The CUHEV team was always patient with me using their welder and tools in the auto lab, and they allowed me to have access to carbon fiber.

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Off-Road, Downhill Skateboard

APPENDICES I. Appendix A - Purchase List

Item Promax Disc Brakes 12" Hydraulic Shock Carbon Fiber 4 10" Wheels Deck Stiffeners 1/2"x1/2"x6' Square Steel Tubing (0.0625" Wall Thickness) 1/2"x6' Steel Tubing (0.049" Wall Thickness) Rod Ends Aluminum Spacers 5/8" i.d. Ball Bearings 5/8" i.d. Tie Rod w/ Rod Ends Spindle Rod End 1/4"-28 Rod End 1/4"-28 Nuts, Bolts, and Washers Nuts, Bolts, and Washers

Place Of Purchase pricepoint.com jackssmallengines.com CUHEV ebay.com Taken From Skateboard mcmaster.com mcmaster.com mcmaster.com mcmaster.com jackssmallengines.com jackssmallengines.com jackssmallengines.com jackssmallengines.com Cornell Auto Lab Bishops Hardware mcmaster.com

Price Quantity $34.99 2 $35.95 4 $0.00 1 $47.00 1 $0.00 2 $8.38

4

$33.52

$19.33 $6.00 $3.86 $2.40 $22.95 $10.85 $6.30 $0.00 $50.00 $50.00

3 20 4 8 4 4 17 12 1 1

$57.99 $120.00 $15.44 $19.20 $91.80 $43.40 $107.10 $0.00 $50.00 $50.00

TOTAL:

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Subtotal $69.98 $143.80 $0.00 $47.00 $0.00

$849.23

Off-Road, Downhill Skateboard II. Appendix B - MATLAB Code For Shock Geometry % input geometry travel = 5; wheeldiameter = 10; hubheight = 2; hubwidth = 2; wheelwidth = 3; hubclearance = (wheeldiameter - hubheight)/2; clear = linspace(travel,0,100); % array of points from 0 to the maximum ground clearance % input geometry and shock characterstics armlength = 9.26; shockfree = 12; shockmaxdef = 2.5; shockmaxload = 500; shockrate = shockmaxload/shockmaxdef; theta = asin((clear-hubclearance)/armlength); xpivot = armlength*cos(theta); ypivot = armlength*sin(theta); inc = .1 x = [0:inc:armlength]; xprime = zeros(length(x),length(theta)); yprime = xprime; shocklength = xprime; Fs = xprime; phi = xprime; Fsy = xprime; Fwy = xprime; Ms = xprime; y = zeros(length(x),1); i = 1; for xn = x, yprime(i,:) = xn*sin(theta); xprime(i,:) = xn*cos(theta); y(i,1) = sqrt(shockfree^2-xprime(i,1)^2)-yprime(i,1); shocklength(i,:) = sqrt((y(i,1) + yprime(i,:)).^2 + xprime(i,:).^2); Fs(i,:) = (shockfree - shocklength(i,:))*shockrate; phi(i,:) = atan((y(i,1) + yprime(i,:))/xprime(i,:)); Fsy(i,:) = Fs(i,:).*sin(phi(i,:)); Ms(i,:) = Fsy(i,:).*xprime(i,:); Fwy(i,:) = Ms(i,:)./(xpivot+hubwidth+wheelwidth/2); i = i+1; end contour3(clear,x,Fwy,100) xlabel('Wheel Travel'); ylabel('Distance Along A-arm'); zlabel('Load On Wheel'); title('3D Graph For Positioning Shocks'); figure; plot(x,y)

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Off-Road, Downhill Skateboard III. Appendix C - Dimensions - All Dimensions In Inches a. Frame (front)

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Off-Road, Downhill Skateboard b. Frame (side)

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Off-Road, Downhill Skateboard c. Frame (top)

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Off-Road, Downhill Skateboard d. Swing Arm (front, top)

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Off-Road, Downhill Skateboard e. A-arm (top)

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Off-Road, Downhill Skateboard f. Steering Pivot Arm (top, front)

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Off-Road, Downhill Skateboard g. Steering Points

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