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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

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Page 1: Off-Road, Downhill Skateboard - Andy Ruinaruina.tam.cornell.edu/hplab/downloads/StudentProjects/Meacham... · Off-Road, Downhill Skateboard 3 ABSTRACT The goal to design a new off-road,

Off-Road, DownhillSkateboard

Senior Design Project for The Sibley School of Mechanical andAerospace Engineering

Course: MAE 491

Prepared for: Professor Andy RuinaEngineering SchoolCornell Universityemail: [email protected]

Prepared by: Michael MeachamGraduating Mechanical EngineerCornell Universityemail: [email protected]

A mountain board designed from the ground up.

Draft 1: May 16, 2004Draft 2: May 20, 2004

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CONTENTS

Abstract......................................................................................................... 3

Introduction................................................................................................... 4

Design........................................................................................................... 5I. Overall Design Goals...................................................................... 5II. Overall Design Elements................................................................6III. Detailed Design - Pre Parts-Purchasing........................................9

a. Deck.................................................................................... 9b. Frame.................................................................................. 10c. A-arms................................................................................. 11d. Suspension..........................................................................12e. Steering Pivot Arm...............................................................13f . Swing Arm........................................................................... 15g. Wheel and Wheel Hub.........................................................16

IV. Post Part-Purchasing / Fabrication / Design Changes..................16a. Ball and Socket Joints......................................................... 17b. Suspension..........................................................................17c. Deck.....................................................................................18d. Steering Pivot Arm...............................................................18e. Deck Stiffness......................................................................20f. Board Steering and Stability................................................ 21g. Brakes................................................................................. 23h. Wheels.................................................................................23

Discussion / Conclusion................................................................................ 24

Acknowledgments......................................................................................... 25

Appendices................................................................................................... 26I. Appendix A - Purchase List............................................................. 26II. Appendix B - MATLAB Code For Shock Geometry........................27III. Appendix C - Dimensions..............................................................28

a. Frame (front)........................................................................28b. Frame (side)........................................................................ 29c. Frame (top).......................................................................... 30d. Swing Arm (front, top)..........................................................31e. A-arm (top).......................................................................... 32f. Steering Pivot Arm (top, front)............................................. 33g. Steering Points.................................................................... 34

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ABSTRACT

The goal to design a new off-road, downhill skateboard is firstaccomplished by studying current mountain boards for sale. These mountainboards 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 fullyindependent suspension, steer-by-lean action, 10" inflatable tires, a wide wheeltrack, and disc brakes. It features a steel frame and A-arms, with a deck thatpivots above. The pivoting action of the deck controls the steering of all fourwheels.

During manufacturing and testing, certain design elements are changedand added. Four-wheel steering can be converted to two-wheel steering quicklyfor more stable high-speed runs. Skateboard stiffeners are added to the deck,which gives this skateboard a natural skateboard feel. Disc brakes are notattached due to funding and time constraints, but testing shows that the board isperfectly functional and fun to ride.

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INTRODUCTION

Skateboarding, snowboarding and surfing are all successful industries.Every recreational sport involving a board, where the rider can carve turns byleaning the board in the direction of motion, has always drawn lots of attention toitself. People have a natural liking for maneuvering through an environmentsimply 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 ofboards, known as “mountain boards,” is that it is far more complicated to designa device that will not trip over large bumps and can take the abuse of a mountaintrail, while still giving the rider a comfortable ride with steer-by-lean action.Current mountain boards are just modified skateboards. They use the sameprinciples for the steering mechanism, offer little suspension, and not much moreground clearance. They are really designed for smooth, dirt roads, not mountaintrails. In order to truthfully tap into this environment, a mountain board must becompletely redesigned from the ground up. A rocky, bumpy path has littlesimilarity to a smooth, dirt or paved path.

An off-road, downhill skateboard should be compared to downhillmountain biking more than skateboarding. It is necessary to have a heavy,stable frame with fully independent suspension. This will allow the rider tocontrol the board, even at high speeds with many bumps and objects on thepath.

This project will take mountain boarding where it was meant to be. It willhelp to gain the attention that it deserves. With ten-inch tires, over six inches ofground clearance, and almost five inches of travel in each wheel, this mountainboard is nothing like current mountain boards. It will be able to go down steeptrails, but still offer safety to the rider with hand-controlled disc brakes on all fourwheels. It will be able to go over much larger rocks and bumps, but the user willstill feel a smooth, controllable ride.

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DESIGN

I. Overall Design Goals

To create a list of design goals, current mountain boards were first studiedand problems assessed. An “all-terrain” mountain board that one can buy instores uses trucks to steer the board. A skateboard truck is a simple mechanismthat attaches the axle of the wheels to the board at a specific angle. When theboard leans, the axles are forced to rotate around this angled pivot point. Themechanism works well for riding on streets or smooth, dirt roads. The problem isthe lack of fully independent suspension. As can be seen by the picture, thesuspension is designed to take the shock of small bumps, not large objects. Thesuspension it contains is the flex of the board, the trucks, which utilize smallsprings as the return force, and "egg shocks" below the rider's feet, which absorb

around 3 cm of travel. The steeringand suspension are not independentof one another at all. When onewheel travels up, the other musttravel down. This feature cannotwork properly while turning, as it willchange the turning radiusconsiderably. To account for this, thetravel in the wheels is kept at aminimum, and the ride is unsmooth.

Trucks also limit the distancebetween the left and right wheels.Since the entire axle turns, a longaxle will result in large longitudinalmotion in the wheels. This will resultin bump steer, the undesired steeringwhen a wheel travels up or down. Ifthe wheels were to rotate about their

own independent axis, then the wheel track could be much larger. For off-roadsituations, a large wheel track is preferable for stability. Current mountainboards, much like skateboards, are too easily tipped over while riding because ofhow narrow they are.

The radius of the wheels can range from about 5 to 8.5 inches in currentmountain boards. The designs for other boards are very similar to this one, inthat they utilize trucks and have no independent suspension in the wheels.

Because of these concerns with current mountain boards, the designgoals of this downhill board are the following:

Figure 1Mongoose UniCamb All Terrain BoardCourtesy of mountainboardshop.com

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

- fully independent suspension – The suspension must beindependent of steering so that while in a turn, the rider can stilltravel over objects.

- large wheel track for stability

- larger tires for getting over bumps – A wheel will steer on anindependent axis. One wheel’s steering will not necessarilyaffect another’s, except through the mechanical connection tothe deck.

- four-wheel steering – In the spirit of a skateboard, there will beno front or back. A rider can get on the deck either way he/shechooses and it will work the same way. Furthermore, thisfeature allows for many more tricks where the board changesdirections.

- hand controlled disc brakes – Because of the dangers of off-road skateboarding, and the predicted weight of the board, discbrakes will be placed on all four wheels. They will be handcontrolled, with one lever controlling the “front” brakes, andother lever controlling the “back” brakes. The independentcontrol of the front and back brakes will allow the rider to controlthe skidding of the tires and add to the functionality of the board.

- maintain the general feel of skateboarding – If a rider is skilledat skateboarding or mountain boarding prior to using thisproduct, 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 framethat is independent of the deck and of the wheels. This allows the deck to pivotabove the frame and control the steering through the use of steering links, ratherthan a solid truck. When the deck is leaned by the rider, a pivot arm that hangsbelow the deck will move around the circumference of a circle. Tie-rods will beconnected at the end of this pivot arm and also connected to the steering arms atthe wheels. A-arms will be connected to the frame and allow for the verticaltravel of the wheels.

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When the deck is leaned in a direction, two pivot arms will move, and allfour wheels will be turned in the proper direction.

Figure 3Features the leaned deck and turned wheels. The steering arms allpoint toward the middle of the board causing the wheels to turn in

the appropriate direction when the deck pivoted.

Figure 2Front view. Features pivoting deck with pivot arm.

Deck

Frame Pivoting Arm

steeringarms

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The suspension will be connect to the bottom A-arm, travel through thetop 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 theground. These shocks will be about one foot in length. Shorter shocks arepreferable to avoid such tall shock towers. The rear shocks on mountain bikesare high performance, fully adjustable shocks that are typically between 7 and 8inches in length. However, these cost a minimum of $250.00 each. Becausethis design needs 4 shocks, the funding constraints made this unfeasible, and thelonger, cheaper shocks must be used.

Figure 4Features shock connected to lower A-arm, traveling through upper

A-arm, and connecting to shock tower.

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III. Detailed Design - Pre Parts-Purchasing

This section will describe in detail, the designed parts prior to thepurchasing of components. The designs are based on the information providedby vendors for various parts. Many design elements are changed after the partsare purchased and more information is gained on them. Those changes will beoutlined 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 about135cm in length. This length will allow for a comfortable distance between theriders feet. Notches are to be cut out from the edges to allow for the suspensionto pass through. The ends will not be for standing, but instead will be left foraesthetic reasons. This will let a person who has never seen the product beforeknow that it is a board that he/she can stand on. Since the deck will be attachedto the frame in only two places, it needs to very rigid if the rider is to stand in themiddle. A snowboard deck may not be as rigid as needed and braces may needto be formed.

Figure 5The Deck. Features a top view (left), showing the holes for the connection to the

frame and an isometric view (right)

holes for pivotconnection

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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 framelength 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 thefront and back. These angled sections allow the frame to hit a large bump andbe forced over it, rather than hitting it and coming to a sudden stop. Whentraveling over a bump, the front wheels generally go up and over it, and thenclearance is required in the middle of the skateboard to ensure that the framedoes not grind along the bump.

Figure 6Side view of frame.

The cross section of the frame is a rectangle for the length where thesuspension, A-arms, and deck attach. This rectangle should be as small aspossible, 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 theirdistance apart is completely controlled by the height of the cross section. Theother constraint is the steering. As the deck pivots about its axis, the pivot armswings about that same axis. If the cross section is too small, then the tie rodswill hit the frame tubing on a sharp turn.

Figure 7Features cross section of frame, and the various

constraints that affect its size.

angledsection

raised middle

shock towerdeck pivot point

tie rod

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The reason to make the rectangle as small as possible is so the A-armscan 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 whiletraveling vertically. As the suspension takes the load, the wheels move up withrespect to the frame. But they also travel around a circle with the pivot point atthe frame's edge. The longer the radius, the less horizontal motion per verticalmotion, which will reduce tire scrub when the suspension retracts. Making surethat everything fit using SolidWorks, the inner cross section came out to be 3.5inches wide and only 2 inches tall.

4130 Steel is used as the material for the frame because it needs to bedurable, 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". Itswall thickness is 0.060". These dimensions yield a cross sectional moment ofinertia 0.013 in.4. With a yield strength of about 66 ksi, this steel can take amoment of 2,288 in.•lbs. The frame is likely to be the weakest in bending whenthe middle of the frame is resting on a bump, and the upward load on the wheelsis removed. The weight of the rider (200 lbs) will put the frame in 3-point bendingwith 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 thethree end-points and the ball and socket joints that allow motion, but keepvibrations at a minimum. In order for the steering and the suspension to work,

the wheel side of the A-arms mustallow for two degrees of freedom.The wheel must be able to turn, andit must be able to travel vertically.The frame side of the A-arms onlyneeds one degree of freedom toallow for the travel of the wheel.However, for simplicity, a ball andsocket joint will be used for all threeconnections. The top A-arm doesnot feel a load from the suspension;it is only there to keep the tireperpendicular to the ground. Again,for simplicity in manufacturing, theupper and lower A-arms are to beidentical.

6.4"

9.3"

Frame Side

Figure 8A-arm

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The A-arms will be made from the same 4130 steel as the frame sincethey too will be susceptible to impacts. A 0.5" round stock with 0.049" wallthickness has a cross sectional moment of inertia of 0.0018 in.4. It can take amoment of 475 in.•lbs. As it is shown in the next section, the shock will belocated at the very corner of the arm. If a wheel takes a maximum upward loadof 100 lbs while riding, then the maximum moment taken will be 400 in.•lbs. Thebottom 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 thesuspension is capable of producing. Therefore, the metal will hold, assuming thewelds penetrate properly. If it fails, it will most likely be because of a rock orbump hitting the A-arm, instead of hitting the wheel or the frame. This is why theA-arms are kept wide at 6.4" - to make them as strong as possible in thatsituation.

d. Suspension

Many factors contribute to the stiffness of the suspension. The spring rateof the shocks, the length of the shocks, and where they are mounted all effecthow smooth the board will feel when riding it. A MATLAB code is written with thehelp of Jacob Timm to determine the ideal geometries of the shocks. For costreasons, the only shock that is available is one foot in length and has a maxtravel of 2.5 inches at 500 lbs. The following graph is produced by the MATLABcode that is found in Appendix B.

Wheel Side

Ground Clearance

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Steering Points

Deck PivotPoint

This graph shows what load the wheel will feel with two differentconstraints, given that the shock is the foot-long one specified previously."Distance Along A-arm" is the distance that the shock is connected from theframe. At that distance, one can look up the ground clearance and determine theload on the wheel at that clearance. For example, if the shock is connected 8inches away from the frame along the A-arm (almost at the A-arm's corner), andyou have a ground clearance of zero, meaning that you have bottomed out, thenthe load on a wheel will be a little over 250 lbs. This allows you to adjust whatload 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 inthe 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 actuallyclose to the corner where the two steel tubes meet.

Although information about possible load situations is missing, thesuspension geometry is chosen to be at the stiffest. This is to prevent thepossibility of bottoming out. Testing will surely have to be done to determine ifthis choice is good for riding down an off-road path.

Furthermore, the vendor does not specify the preload on the shocks, so atthe time of creating this graph, the preload is considered to be zero. This is mostlikely 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 sensitivityand the height of the deck. Both of these factors have to be balanced. Fromsetting up different platforms, one can see that 12 inches is the absolutemaximum height for a deck while still feeling comfortable standing on it.

The advantage of having a swing arm hang down and follow a circularpath is that Ackermann steering can be achieved. Ackermann steering is whenall four wheels of a vehicle turn about the same point to reduce tire scrub.

Figure 10

Front view - See Figure 2 for reference.

The Deck Pivot Point marks the axis thatthe deck pivots about. This axis isshared by the steering pivot arm. As thearm swings about the deck pivot point,the steering points follow a circular path.If it rotates clockwise, then the left pointhas more vertical motion than horizontalmotion, and the right point has morehorizontal motion than vertical motion,and vice versa. This causes one wheelto turn more than another to achieveAckermann steering.

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If the steering points are moved farther away from each other, than theAckermann effect becomes more sensitive. If the steering points are broughttogether and meet, then the wheels will turn the same amount. UsingSolidWorks, all of the geometries are adjusted to achieve Ackermann steering atsharp turns. It is at these sharp turns where tire scrub will be the mostnoticeable.

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

When determining how sensitive the steering should be, one must analyzethe nature of a skateboard. Firstly, suppose you are riding a street luge. In thiscase, the sensitivity of the steering is extremely important. At a particular lean ofthe deck you will circle around a point with radius r. The street luge will

accelerate you toward this point with a force of

mv 2

r, m being your mass, and v

being the velocity. The deck must be leaning by the correct amount such thatyou are not thrown off the side. Furthermore, it must be calibrated to the desiredspeed because this optimal lean changes with v. The reason this is so importantwith a street luge, is that it is very difficult to shift your body weight. Once youlean the deck a certain amount, you have no control over how far your bodyweight has shifted.

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On a skateboard however, the two components are independent of eachother. The rider can lean the deck a certain amount using his/her feet, and thenshift his/her body weight in either direction he/she chooses. Because of this, thegeometric sensitivity of the steering is not nearly as important as the stiffness ofthe steering. If there is a way to stiffen or loosen the deck such that it springsyou back to centerline harder or softer, then one could adjust the effectivesensitivity of the board for faster or slower runs. If you are pushing harder toachieve a turn radius, then your body weight is easily shifted to account for thecentripetal motion. I will not design for this until the skateboard has completedconstruction, 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 forthe wheel. The steering arm is pulled by the tie rod, which causes the wheel topivot about the axis created by the two ball and socket joints at the ends of the A-arms. A metal plate is attached to hold the brake calipers. This plate will movewith the wheel in every degree of freedom except for spin, which will allow foraccurate braking. The last component is the axle in which the ball and socketjoint are connected to the A-arms.

Figure 12Swing Arm

metal plateand caliperassembly

axle between balland socket joints

steering arm

wheel axle

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Two swing arms will be constructed like the one shown here, and twomirror 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, andtherefore a new wheel hub will have to be machined to hold the brake rotor inplace. This new hub will have a four-bolt pattern on one side, and a six-boltpattern on the other for the rotor. It will also hold the bearings to ensure that therotor has no wobble as it spins.

IV. Post Parts-Purchasing / Fabrication / Design Changes

Figure 13Complete Skateboard

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a. Ball and Socket Joints

1/4"-28 rod ends are used for all ball and socket joints. There are 32 rod ends onthe skateboard: three for each A-arm and two for each tie rod. 1/4"-28 nuts arewelded to the ends of the half-inch tubes to allow for the rod ends to be screwedin.

b. Suspension

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

Figure 14Features the shocks mounted at same point and nylon

washers for added 0.375 inches of preload.

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c. Deck

The deck has gone through a major revision. Because of the newlydesign shock mounts, the deck length had to be shortened, and the aestheticcurved edges forgotten. To ensure rigidity, which was not guaranteed with asnowboard, the deck has been made out of half-inch plywood and reinforced withcarbon fiber. There are four layers of carbon fiber on each side. The carbonfiber 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 ofthe cross section is 0.176 in.4 and it can take a moment of 13,333 in.•lbs. This ismore 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 bendingnecessarily. In my experience, carbon fiber can be crushed easily with localdamage. I have many bolts going through the deck, and high torque isexperienced when turning. The added carbon fiber is to ensure that no localdamage occurs. Also, metal plates are used anytime there are bolts to spreadthe 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 underthe rider's feet. The usage of bindings is determined to be hazardous aftertesting. Often the rider must jump off when traveling too fast or if he/she losescontrol of the skateboard. Testing shows that the friction due to the grip tape isenough to keep the rider safely on the board.

d. Steering Pivot Arm

The steering pivot arm is redesigned to have a threaded rod be theconnection between the deck pivot point and the steering points that push andpull the tie rods. The reason for this change was to make the height of the pivotarm adjustable through the use of nuts around the threaded rod. This did workfor the purpose of finding a good height, but the threaded rod, being 1/4"-20 wasnot strong enough to take the torque that the steering applied to it. An analysison the tire scrub will show why.

Figure 15This rod was changedto a threaded rod, butthis idea did not work

because of unforeseentorque.

Point offailure.

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When standing on the skateboard, a wheel can feel a force upwards of150 lbs, depending on what the rider is doing. Assuming the coefficient of frictionbetween the tire and the road is 1.0, the friction force that tire scrub couldproduce is 150 lbs. This force creates a torque about the axis of steering, whichis 3 inches away from the friction force. This torque is equal to 450 in.•lbs. Thetie 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 distancebetween the steering points and the point of failure is 2.75 inches. The threadedrod was taking a max torque of around 825 in.•lbs. The unthreaded diameter of a1/4"-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, cantake a moment of only 47 in.•lbs. The large torque that this threaded rod feelswas completely overlooked.

This problem was corrected. The threaded rod was used to get the exactheight 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 of798 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 hasnot yet with hard testing.

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

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

e. Deck Stiffness

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

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

As the deck pivots, these hard plastic pieces compress. The bolts whichhold them in a compressed state can be tightened and loosened, which willeffectively change the stiffness of the steering. Using the same stiffeners asfound on a skateboard gives a simple solution that creates a very similar feel toskateboarding.

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f. Board Steering and Stability

The four-wheel steering works well, meaning that at slow speeds, theskateboard turns at a short turning radius, and the rider is able to carve aroundobjects with accuracy and control. However, after bringing the skateboard to theCornell Plantations, where it was able to pick up more speed over a longdistance, it is determined that at a little over 10 mph, the board becomesunstable and oscillates violently. Further testing showed that if the rider leans tothe front of the board, these oscillations can be controlled more, and higherspeeds can be reached. Leaning far forward is not a good solution as it issometimes difficult to do, and if the rider forgets for even a little bit, he/she can bethrown off immediately.

After a discussion with Professor Ruina, holes are drilled into the framenear the steering points to allow the tie rods on the "back" to be connected rigidlyto the frame. This turns the skateboard into a front-steer only board.

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

Because the tie rods can be adjusted in length, the alignment is madeperfect in the rear wheels when switching between four-wheel and two-wheelsteering.

Although this design change does not follow the original goals stated, it isnecessary at higher speeds. Four-wheel steering can still be used effectively atlow inclines and at low speeds. For example, a rider can carve around bushes ina low-grade field with four-wheel steering, and cruise down a steep, rocky pathwith two-wheel steering. The design change is an addition to the skateboard'sfunctionality.

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The following pictures show high-speed turns at the Cornell Plantations.The two wheel steering works perfectly in keeping the board controllable andstable.

Figure 19Frame-by-frameshot of me goingdown the steep

part of the CornellPlantations. Theboard is in two-wheel steering

mode and staysperfectly stable.

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In two-wheel steering mode, the turning radius is twice as long per lean ofthe deck. This decrease in geometric steering sensitivity is also helpful in high-speed situations. The deck stiffeners are effective in changing the steeringsensitivity, and therefore no geometric changes were required.

g. Brakes

Because of time and funding constraints, the brakes have been put onhold. 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 theneed to slow down is minimal. If taken to a larger hill or mountain, brakes wouldmost surely be a necessity.

h. Wheels

The wheels used were purchased from ebay.com. The original place ofpurchase and the brand name are both unknown. I believe they were originallydesigned for a hand truck used to move boxes. The only other wheels that comeclose to the desired size are go-kart wheels. These are much more expensive,and sometimes much heavier. The wheels from ebay were the correctdimensions, cheap, and only weighed 3 lbs each. The bearings were replacedbecause the stock bearings were not sealed and were low quality.

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DISCUSSION / CONCLUSION

Almost all original goals were met in this project. The major goal that wasnot met is the disc brakes. However, these can be added at a later time withmore time and money. I predict that it would take one week to install disc brakeson all four wheels.

Since I am the only person who has done extensive testing on theskateboard, 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 itup the hill is not a simple task. At first, I called this "added exercise," but now Ijust wish it was lighter. However, I am not sure if making the board lighter wouldcreate an unstable ride. Most likely, a small decrease in weight would gounnoticed.

With the addition of the two-wheel-steering mode, the ride is extremelynice in all conditions tested. The wide track gives the rider the feeling that it isalmost impossible to fall off, and this seems to be true. Any bump I hit, the boardfinds a way over it, and I feel almost no jerking vibration in my feet. Even thoughthe 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 onwhat the rider is using it for. For the Cornell Plantations, I would feel completelysafe with a helmet and some light padding. For a more extreme path, which hasnot been tested, major padding is recommended, much like the armor thatdownhill 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 theboard as far as possible. If you simply step off the board while traveling at highspeeds, the A-arms will most likely sweep you off your feet and cut your legsbadly.

This skateboard may or may not be durable and reliable. I would imaginemonths 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 Iride 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 theyhaven't seen anything like it before, and it interests them. People either want totry riding it, or they want to see someone else try it. The only negative commentsI ever receive are, "you're crazy" or, "that's insane." I suppose these two ideaswere unstated goals from the beginning, and they convince me that the project isa complete success.

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ACKNOWLEDGEMENTS

I. Funding

Most of the funding for this project came from myself. I would also like tothank Joan Galer (mother) and Stephen Meacham (father), for theirunquestioning financial support. I could not have completed the project withoutthese two great financial sources.

II. Design / Construction

Special thanks to Jacob Timm for discussing almost every design aspectwith me and writing the shock-geometry MATLAB code. He also spent longhours 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 contributedgreatly with ideas during this project. The CUHEV team was always patient withme using their welder and tools in the auto lab, and they allowed me to haveaccess to carbon fiber.

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APPENDICES

I. Appendix A - Purchase List

Item Place Of Purchase Price Quantity Subtotal

Promax Disc Brakes pricepoint.com $34.99 2 $69.98

12" Hydraulic Shock jackssmallengines.com $35.95 4 $143.80

Carbon Fiber CUHEV $0.00 1 $0.00

4 10" Wheels ebay.com $47.00 1 $47.00

Deck Stiffeners Taken From Skateboard $0.00 2 $0.001/2"x1/2"x6' Square Steel Tubing(0.0625" Wall Thickness) mcmaster.com $8.38 4 $33.521/2"x6' Steel Tubing(0.049" Wall Thickness) mcmaster.com $19.33 3 $57.99

Rod Ends mcmaster.com $6.00 20 $120.00

Aluminum Spacers 5/8" i.d. mcmaster.com $3.86 4 $15.44

Ball Bearings 5/8" i.d. jackssmallengines.com $2.40 8 $19.20

Tie Rod w/ Rod Ends jackssmallengines.com $22.95 4 $91.80

Spindle jackssmallengines.com $10.85 4 $43.40

Rod End 1/4"-28 jackssmallengines.com $6.30 17 $107.10

Rod End 1/4"-28 Cornell Auto Lab $0.00 12 $0.00

Nuts, Bolts, and Washers Bishops Hardware $50.00 1 $50.00

Nuts, Bolts, and Washers mcmaster.com $50.00 1 $50.00

TOTAL: $849.23

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II. Appendix B - MATLAB Code For Shock Geometry

% input geometrytravel = 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 charactersticsarmlength = 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 = .1x = [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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III. Appendix C - Dimensions - All Dimensions In Inches

a. Frame (front)

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b. Frame (side)

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c. Frame (top)

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d. Swing Arm (front, top)

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e. A-arm (top)

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f. Steering Pivot Arm (top, front)

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g. Steering Points