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

34 Pages·2004·4.21 MB·English

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 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.
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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
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:
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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.
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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.
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.
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