Off-Road, Downhill Skateboard - Andy Ruina

Transcription

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: ruina@cornell.edu
Prepared by:
Michael Meacham
Graduating Mechanical Engineer
Cornell University
email: mbm27@cornell.edu
A mountain board designed from the ground up.
Draft 1: May 16, 2004
Draft 2: May 20, 2004
Off-Road, Downhill Skateboard
CONTENTS
Abstract......................................................................................................... 3
Introduction................................................................................................... 4
Design........................................................................................................... 5
I. Overall Design Goals...................................................................... 5
II. Overall Design Elements................................................................6
III. Detailed Design - Pre Parts-Purchasing........................................9
a. Deck.................................................................................... 9
b. Frame.................................................................................. 10
c. A-arms................................................................................. 11
d. Suspension..........................................................................12
e. Steering Pivot Arm...............................................................13
f . Swing Arm........................................................................... 15
g. Wheel and Wheel Hub.........................................................16
IV. Post Part-Purchasing / Fabrication / Design Changes..................16
a. Ball and Socket Joints......................................................... 17
b. Suspension..........................................................................17
c. Deck.....................................................................................18
d. Steering Pivot Arm...............................................................18
e. Deck Stiffness......................................................................20
f. Board Steering and Stability................................................ 21
g. Brakes................................................................................. 23
h. Wheels.................................................................................23
Discussion / Conclusion................................................................................ 24
Acknowledgments......................................................................................... 25
Appendices................................................................................................... 26
I. Appendix A - Purchase List............................................................. 26
II. Appendix B - MATLAB Code For Shock Geometry........................27
III. Appendix C - Dimensions..............................................................28
a. Frame (front)........................................................................28
b. Frame (side)........................................................................ 29
c. Frame (top).......................................................................... 30
d. Swing Arm (front, top)..........................................................31
e. A-arm (top).......................................................................... 32
f. Steering Pivot Arm (top, front)............................................. 33
g. Steering Points.................................................................... 34
2
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.
3
INTRODUCTION
Skateboarding, snowboarding and surfing are all successful industries.
Every recreational sport involving a board, where the rider can carve turns by
leaning the board in the direction of motion, has always drawn lots of attention to
itself. People have a natural liking for maneuvering through an environment
simply by standing and leaning on a platform.
The next environment for this phenomenon to enter into is off-road,
mountain trails. The reason people have only just begun to design these types of
boards, known as “mountain boards,” is that it is far more complicated to design
a device that will not trip over large bumps and can take the abuse of a mountain
trail, while still giving the rider a comfortable ride with steer-by-lean action.
Current mountain boards are just modified skateboards. They use the same
principles for the steering mechanism, offer little suspension, and not much more
ground clearance. They are really designed for smooth, dirt roads, not mountain
trails. In order to truthfully tap into this environment, a mountain board must be
completely redesigned from the ground up. A rocky, bumpy path has little
similarity to a smooth, dirt or paved path.
An off-road, downhill skateboard should be compared to downhill
mountain biking more than skateboarding. It is necessary to have a heavy,
stable frame with fully independent suspension. This will allow the rider to
control the board, even at high speeds with many bumps and objects on the
path.
This project will take mountain boarding where it was meant to be. It will
help to gain the attention that it deserves. With ten-inch tires, over six inches of
ground clearance, and almost five inches of travel in each wheel, this mountain
board is nothing like current mountain boards. It will be able to go down steep
trails, but still offer safety to the rider with hand-controlled disc brakes on all four
wheels. It will be able to go over much larger rocks and bumps, but the user will
still feel a smooth, controllable ride.
4
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:
5
-
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 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.
-
maintain the general feel of skateboarding – If a rider is skilled
at skateboarding or mountain boarding prior to using this
product, then the transition time will be kept at a minimum.
II. Overall Design Elements
To achieve the previously stated goals, the skateboard will have a frame
that is independent of the deck and of the wheels. This allows the deck to pivot
above the frame and control the steering through the use of steering links, rather
than a solid truck. When the deck is leaned by the rider, a pivot arm that hangs
below the deck will move around the circumference of a circle. Tie-rods will be
connected at the end of this pivot arm and also connected to the steering arms at
the wheels. A-arms will be connected to the frame and allow for the vertical
travel of the wheels.
6
Deck
Frame
Pivoting Arm
Figure 2
Front view. Features pivoting deck with pivot arm.
When the deck is leaned in a direction, two pivot arms will move, and all
four wheels will be turned in the proper direction.
steering
arms
Figure 3
Features the leaned deck and turned wheels. The steering arms all
point toward the middle of the board causing the wheels to turn in
the appropriate direction when the deck pivoted.
7
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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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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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.
10
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.
11
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
12
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
13
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.
14
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
15
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
16
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.
17
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.
18
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.
19
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.
20
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.
21
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.
22
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.
23
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.
24
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.
25
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
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:
26
Subtotal
$69.98
$143.80
$0.00
$47.00
$0.00
$849.23
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)
27
III. Appendix C - Dimensions - All Dimensions In Inches
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

Off-Road, Downhill Skateboard - Andy Ruina

Transcription

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