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Final Project: BMX Bike
EML 4024C: Engineering Design Practice
Anthony Frisco
Steven Hellmann
Patrick Olski
Peter Rowan
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Table of ContentsIntroduction...............................................................................................................................................3
Mechanical Design.....................................................................................................................................3
Drivetrain Design...................................................................................................................................3
Chain..................................................................................................................................................3
Inner Chain Link...............................................................................................................................4
Outer Chain Link..............................................................................................................................6
40 Tooth Drive Sprocket...................................................................................................................7
10 Tooth Rear Sprocket..................................................................................................................12
Crank................................................................................................................................................15
Pedal.................................................................................................................................................18
Main Frame..........................................................................................................................................21
The Fork...........................................................................................................................................32
Handle Bars......................................................................................................................................40
Wheels/Tires.........................................................................................................................................45
Tires:.................................................................................................................................................45
Wheels/Pegs:....................................................................................................................................48
Assembly..................................................................................................................................................56
Frame/Handlebars and Fork..............................................................................................................56
DriveTrain............................................................................................................................................58
Wheels/Tires.........................................................................................................................................62
Mechanism Model Analysis....................................................................................................................64
Mechanism Kinematics/Dynamics.........................................................................................................66
Introduction.........................................................................................................................................66
Motor on Crank...................................................................................................................................66
Analysis....................................................................................................................................................68
Stress/Strain Analysis..........................................................................................................................68
Flow Analysis.......................................................................................................................................71
Conclusion................................................................................................................................................75
References................................................................................................................................................76
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IntroductionThe design and analysis of a BMX bike demonstrated the process of design, 3-D
modeling, motion study analysis and simulation on the selected entity. SolidWorks was used to create a 3-D model of each individual parts comprising of the main frame, sprocket and chain, handle bars and seat, front fork and the wheels, tires and pegs. Each component was design to scale having referencing a full scale bike. Each component was assembled and constrained together, enabling full realistic motion. A motion study was performed on the complete model, as well as two simulations. The first being stress/strain analysis on the frame of the bike and the second being a flow analysis, where a constant velocity was applied to the front of the model and the pressure on the model and the changing velocity across the model was analyzed.
Mechanical Design
Drivetrain DesignThe first step in modeling the drivetrain was to research what type of sprockets and
chains were used for common bicycles and BMX bikes. We found that most chains had a .5” pitch, with varying roller sizes, but the most used was a 3/8” diameter roller, or outer pin. Next we had to decide the gear ratio of the drive sprocket, which is connected to the pedals, and the rear sprocket, which is connected to the back wheel and propels the bike forward. This ratio was decided to be 4:1, with 40 teeth on the drive sprocket, and 10 on the rear sprocket.
ChainFigure 1
A chain is comprised of two separate links, interconnected in a pattern that allows a pivot about two cylinders on either side.
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Inner Chain LinkThe Inner link seen in figure __ has a roller cylinder with a diameter of 3/8”, and a hole
with a diameter of .312” to fit the Outer pin cylinder. These circles were drawn on the front plane, and extruded to half of the roller width.
Figure 2 Figure 3
The sidewalls do not require a specific geometry, except to not interfere with the outer chain link, and were designed using two circles and arcs tangent to the circles, and then trimming the circle’s unneeded side arc. In addition, the sidewall also had a hole the same size as the outer pin hole of the roller, so a circle was made to hollow out this part of the extrude. The side wall was extruded offset to the end of the roller cylinder and merged to form a single body. The inner circles for both the sidewall and roller were set to be concentric and the equal radius, with a predetermined distance between them equal to the pitch of .5”Figure 4 Figure 5
The sidewall and roller cylinder were both mirrored across the front plane to complete the inner chain link.
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Figure 6 Figure 7
Figure 8 Figure 9
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Outer Chain LinkSimilar to the Inner chain link, the outer link had the same sidewall dimensions, so the
sketch was copied and pasted into a new part to keep the dimensions consistent. Figure 10 Figure 11
However this had a separate extruded cylinder for the outer pin. This was done in two separate extrudes, the first to extrude the sidewall, a set distance offset less than the inner chain link sidewall, but greater than the width of the sprockets. Next the outer pin circle was extruded from the front plane, and past the hole in the sidewalls of both links as seen in figure __ Figure 12 Figure 13
Both of these features were then mirrored across the front plane to produce the complete Inner Chain LinkFigure 14 Figure 15
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Figure 16
40 Tooth Drive Sprocket
The drive sprocket was known to be just below 7” diameter, with a 2” diameter hole for the crank. The basic geometry was sketched and extruded the width of the sprocket .625”Figure 17 Figure 18
Next, the profile of two teeth were drawn, according to calculations based on how many teeth, the pitch diameter, and the roller cylinder of each link. This profile was cut extruded from the main sprocket.
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Figure 19 Figure 20
The cut extrude was then patterned around the entire sprocket 40 times with an angular offset of 9 degrees, to complete the circle.
Figure 21 Figure 22
The rest of the changes to the sprocket, were purely aesthetic, and based off of sprockets seen on other BMX bikes. The first was to create a raised section to strengthen the inside of the sprocket due to future cuts removing some of the material. A couple circles were made on the face of the sprocket and extruded near the teeth that would not interfere with the chain sidewall.
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Figure 23 Figure 24
This new feature was then filleted, and the teeth were chamfered. The chamfer on the teeth was not only aesthetic, but also provides a smoother transition for the links to attach with the sprocket, because if there is any error or offset as the bike is moving, the chamfered edges will guide the chain to center about middle of the sprocket.
Figure 25 Figure 26
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Figure 27 Figure 28
To add finishing touches to the sprocket, a design was made with multiple splines, arcs, and constraints, to be cut extruded, and give the sprocket a bit of character, while still being structurally sound due to the added thickness of the wall.Figure 29
The last step was to create a circular pattern of this cut extrude 5 times around the sprocket.
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Figure 30
Figure 31 Figure 32
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10 Tooth Rear SprocketThe smaller rear sprocket was created with a bit more simple of a design, due to learning from the first one, how to make the profile quicker and more simply. The pitch was known to also be .5”, with ten teeth, which set the angular distance between each tooth to 36 degrees. The profile of one tooth was made in a 36 degree cross section, using arcs tangent to each other, and perpendicular to the outside of the angular cross section. This would create a uniform tooth to be patterned 10 times. The many constraints used included coincident, symmetric, perpendicular, parallel and tangent. This one tooth was extruded with midplane, to the same thickness as the 40 tooth sprocket.
Figure 33 Figure 34
The edge of the tooth was then chamfered, to help guide the chain to the middle of the sprocket, and reduce the chance of misalignment and slip. Next the circular pattern was applied to this section, to create all 10 teeth.Figure 35 Figure 36
To create the hole for the rear axel to pass through, a circle was drawn and extrude cut with the diameter of .43”, the same as the rear axle.
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Figure 37 Figure 38
The rear sprockets also typically have a cylinder section that protrudes from the face of the sprocket to provide structural support, and also helps keep the sprocket perpendicular to the axle. The final steps were to draw these two circles, and extrude the outer cylinder from the plane to one side.
Figure 39 Figure 40
Figure 41
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Figure 42
Figure 43
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CrankFigure 44 Figure 45
The first step for the crank was to create the connecting shaft or cylinder which each crank lever is connected to. This shaft had a diameter of 1.45”, and was extruded midplane until it went past the frame of the wheel well, on both sides.Figure 46 Figure 47
Next, the crank lever was created, with an arbitrary shapeusing parallel, equal and tangent lines, as well as a circle at the end to be extruded to connect the pedal to.
Figure 48 Figure 49
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The pedal connecting shaft had the same diameter hole as the one on the pedal, to ensure a good fit, and was extruded the width of the pedal.Figure 50 Figure 51
This same process was done to the other side of the crank shaft, which could be done with a mirror feature, or copying and pasting the sketch, and extruding it again. Figure 52
Lastly, some reference geometry was created to help mate the crank to the frame, with a correct orientation, and location relative to the sprockets, and to give them a surface to mate to. This was done with a couple circles, and extruding them around the crank shaft.
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Figure 53 Figure 54
Figure 55 Figure 56
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PedalFigure 57
The pedal was created to resemble other BMX pedals found online, but with my own design. To create the basic shape, an altered parallelogram was created, and extruded but with a draft of 5 degrees inward. Figure 58 Figure 59
Then another sketch was made inside of this one, in order to create the hollow shape along the length o the pedal. This also had a drafted extrude of 5 degrees inward.Figure 60 Figure 61
Next, the width needed to be hollowed out, and a shape was drawn, symmetric on both sides of where the axle would be, and cut extruded through the entire pedal.
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Figure 62 Figure 63
Also, the top surface needed to be hollowed out, and was done in the same manner, but required two separate sketches, of the same geometry, but shifted to account for the parallelogram shape in the pedal. This extrude was drafted at a 30 degree angle inward.Figure 64 Figure 65
Below is the bottom surface extruded cuts, done in the same manner as the top surface
Figure 66 Figure 67
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Next, a cylinder was added that would house the connecting rod to the crank. This was done with two circles and lines to match the profile of the pedal, extruded past the edge of the pedal.
Figure 68 Figure 69
Because the end of the pedal had a diagonal shape, the circular face was cut to match this surface, and was drafted at a 1 degree angle outward from midplane to truncate the pedal to crank cyinder
Figure 70 Figure 71
The finishing touches were to add fillets to the needed surfaces with 1 and 2 mm radii.
Figure 72 Figure 73
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Main Frame
Figure 74
-To create the frame a circle with diameter of 1.75” was extruded 3”
Figure 75
-a construction line was sketched and a plane was created normal to the construction line
- a circle of diameter 1.25” was extruded 7”
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Figure 76
Figure 77
-various construction lines were sketched
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Figure 78
-various planes were defined
Figure 79
-A swept feature was made using a profile and guideline sketched on the defined planes to create the top back tire rung
24
Figure 80
-an extruded cut was used to create a hollow section for the gears
Figure 81
-Two circular extrudes were made using a sketch on the defined plane to create the handlebar hub
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Figure 82
-an extrude was made using a sketch on the defined plane to create a support tab
Figure 83
-an extrude and extruded cut was made to create the seat adjuster bracket
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Figure 84
Figure 85
-an extruded cut was made of a sketch to create unique detailing in the support tab
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Figure 86
-a swept command using a profile and path was utilized in the creation of half the bottom tire rung
Figure 87
-the other half of the tire rung was created using the mirror feature
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Figure 88
-the chain tensioner was created using an offset extrude of a sketch
Figure 89
-a chamfer was made to give the sketch more realism and the chamfer and chain tensioner were both mirrored
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Figure 90
-Circular sketches of diameter 1” were thinly extruded to create the top and bottom bar of the frame
Figure 91
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Figure 92
-the edges of the handlebar hub were chamfered to creat more relism
Figure 93
-a 7” extruded cut was made to create the bike seat sleeve
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Figure 94
-a through-all extruded cut was made to finish the handlebar hub
Figure 95
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The ForkFigure 96
-a sketch of a circle with diameter 1” was extruded 7”
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Figure 97
-Rectangle was sketched and extruded 1”
Figure 98
-another rectangle was sketched and extruded 1”
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Figure 99
-a cut extrude was made using a sketch of a circle with a 1” diameter
Figure 100
-a new plane was defined
35
Figure 101
-a swept feature was made using a 1” diameter circle sketch and a path
Figure 102
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-an extrude was made using a sketch of a rectangle
Figure 103
-the extrude was filleted and chamfered to create more realism
Figure 104
-a cut extrude was made using a sketch of a circle
37
Figure 105
-the features were mirrored
Figure 106
-a new plane was created
38
Figure 107
- Sketches of two circles were created and extruded
Figure 108
Figure 109
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-an extruded cut was made using a sketch of rectangle to create a keyed groove
Figure 110
-various fillets and chamfers were made to create more realism
Figure 111
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Handle BarsFigure 112
-a swept feature was created using a .75” diameter circle for the profile and a guide curve
Figure 113
-a mirror feature was used
41
Figure 114
-an extrude was made using a sketch of a circle with a diameter or .5”
Figure 115
-a mirror feature was used
Figure 116
-a chamfer was used to create more realism
42
Figure 117
-a swept cut was made using a profile and guideline to hollow out the handle bars
Figure 118
-the swept cut feature was mirrored
Figure 119
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-an extrude was made using a sketch of a circle
Figure 120
-an extrude was made using the sketch of a circle to make the handlebar grip
Figure 121
-the feature was mirrored
44
Figure 122
-an extrude was made using a sketch of a rectangle to create the key
Figure 123
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Wheels/Tires-Note: This design is for both front and rear tires/wheels
Tires:-To create the front and rear tires, a circle was sketched on the front plane with a 20” diameter.
-Circle was extruded to a distance of 1.75”
Figure 124
-A circle was sketched on the front face of the previous circle with a 16.5” diameter.
-An extruded cut was applied to this sketch, creating a hollow circle
46
Figure 125
- A fillet of 0.05” was applied to the edges of the circle
Figure 126
-A plane was created on the side of the tire and the pattern shown below was sketched and extruded on the surface of that plane
47
Figure 127
-A circular pattern was applied to this sketch around the face of the tire, repeating 160 times
Figure 128
-Fillets of 0.15” were applied to the inside faces of the tire
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Figure 129
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Wheels/Pegs:-A circle was sketched on the front plane with a diameter of 16.5”
-Extruded to a distance of 1.45”
Figure 130
-A circle is sketched on the front face of the circle with a diameter of 15.13”
-Extruded cut applied to new sketch
Figure 131
-A 0.20” fillet it applied to the inner edge of the circle
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Figure 132
-A new sketch is created on the front plane, a circle with diameter of 0.43”
-Extruded midplane at a distance of 3.64”
Figure 133
- Sketch a circle on the front plane with a diameter of 1.35”
-Extrude midplane a distance 1.75”
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Figure 134
-Sketch a circle on the front plane with a diameter of 2.70”
-Extrude blind 0.10”
Figure 135
-Create another 2.70” circle on the front plane
-Extrude 0.10” offset from the surface
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Figure 136
- Create a new sketch on the front plane of the design below. The distance between each line is 45 degree
-Use circular pattern
-Extrude midplane 0.30”
Figure 137
-Fillet the edges of the design 2.00” as shown below
53
Figure 138
-Sketch the inlays as shown below inside the wheel
-Use circular pattern to apply across entire wheel
-Extruded cut through all
Figure 139
-Fillet the edges of the cuts 1.3”
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Figure 140
-Sketch a circle with a diameter of 1.50” on the front face of the outermost extrusion
-Extrude 4.1” blind
Figure 141
-Sketch a circle with diameter 1.28” on the front face of the face
-Extrude 4.1” back into the wheel
55
Figure 142
-Fillet the edge of the peg 0.10”
Figure 143
-Mirror the peg across the front plane
56
Figure 144
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Assembly
Frame/Handlebars and ForkTo begin the assembly, the frame was the first piece to be inserted because it is the main
piece which every other is attached to. The fork connect to the front section of the frame through a hollow shaft, allowing the fork’s shaft to slide up from underneath and into it with a concentric mate. The fork shaft also has a cap at the top, which was used to mate it coincident with the top of the frame, to keep it from sliding out. The fork and handlebars need to rotate, and thus were not constrained to the frame any more than these necessary mates, but were constrained to the handlebars through the stem. The stem is the connector piece for the horizontal section of the handlebars to the vertical shaft of the fork. This was done with two coincident mates, and a concentric mate for the shaft
Figure 145
Figure 146
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Figure 147
Figure 148
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DriveTrainThe drivetrain was comprised of six components: the two sprockets, an inner and outer
chain link, a crank, and a pedal. To begin, the 2 equal pedals had an extruded hole that allowed the crank cylinder to concentrically mate to them.
Figure 149
The edge of the pedal, was also coincident with the face of the crank, which guaranteed it would not slip off.
Figure 150
The last constraint was to concentrically mate the crank, to the tube of the frame which it is attached, and to prevent sliding out, a coincident mate along a flange to the frame.
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Figure 151
The sprocket was concentric to the pedal shaft, and to constrain the rotation of the crank to the sprocket, the top plane of the crank was coincident with the right plane of the sprocket.
Figure 152
The rear sprocket also had the same constraints, but instead to the rear axle of the wheel. Also, the rear sprocket had a gear mate from both 40 and 10 tooth sprockets, at a 4:1 ratio.
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Figure 153
Also, the faces of the sprockets were mated together, coincidentally to center the sprockets together.
Figure 154
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Figure 155
The chain was very tedious to mate, due to each link having to be individually concentric mated to the previous links roller or pin, and then coincident mated with an inside and outside side wall to position them in line with the sprockets.
Figure 156
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Wheels/TiresThe tire and wheel frame had the front planes and top planes coincident mated together, and then the two tires were gear mated at a 1:1 ratio.
Figure 157
The wheels both had 2 pegs attached to them, at their centers, with a symmetric mate, and also the front and rear pegs aligned with the frame to be symmetric to the fork and main frame respectively.
Figure 158
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Figure 159
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Mechanism Model Analysis
Shown in the table below are the results for the mass properties for the complete assembly as well as each individual parts that made up the assembly. Properties that are included below include density, mass and center of gravity, as well as various moments of inertia. The results that are given are to scale and are a direct result of the design choices. The majority of the BMX bike was AISI 4130 steel, a moderately lightweight and durable type of steel. The front and rear tires as well as the seat are made of rubber, and the wheels and pegs are made of Hexcel AS4C carbon fiber to increase the aerodynamics of the wheel and decrease the weight of the overall bike.
The overall mass of the final assembly was found to be 66.46 lbs. Comparing to the average BMX bike weight of about 25-30lbs, there is a considerable difference in the design. Causes of this stem from various pieces of the assembly such as the front fork and stem weigh an approximately 9.85 lbs. which is 14.8% of the final product. This is because these pieces are solid steel and are not hollowed out much like a normal BMX frame. Also, the front and back tires each weight 6.36 lbs., making up 9.6% of the final assembly per tire, a number much higher than desired.
A possible solution to counteract this large mass would be to hollow out every aspect of the frame including the front fork, stem and pedals. Proof of this result are shown from the main frame itself, where the support beams are hollowed out to reduce mass and in return decrease the overall mass of the bike by about 10 lbs. In addition, some pieces such as the pedal can possibly have varying materials that could better improve the overall mass of the system.
Shown below are the mass properties for the overall assembly and the individual parts that make up the assembly. The first table shows the mass properties of the final assembly, showing not only the mass of the body, but the moments of inertia with respect to different points on the final assembly such as the center of mass. The second table shows the mass properties and material type for each individual part. The density and mass distributions for each individual part are also shown below, displaying show certain parts and their materials can affect the whole system.
66Table 1- Mass Properties of Final Assembly
Table 2- Mass Properties of Assembly Parts
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Mechanism Kinematics/Dynamics
IntroductionThe BMX Bike that was modeled required only one motor on the crank. The pedals will receive a force that will turn the gear, this motions is a rotating force that will be transmitted to the smaller sprocket at a 4:1 ratio through the chain. The small sprocket will then turn the rear tire at a 1:1 ratio, which is then transmitted to the ground.
Figure 160
Motor on Crank
Figure 161
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This motor was designed to simulate the force that would be applied with little to no jerk, the motor increases in acceleration for 4 seconds. Once this was done plots of the motion at different points was generated from the analysis, this shows that the tire is moving 4 times faster than the crank, this matches what is calculated from the gear ratios.
Figure 162
Figure 163
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Analysis
Stress/Strain AnalysisFor the BMX Bike that was modeled the stress/ strain analysis was performed on the frame and front
fork, this was done with realistic forces that could be seen buy these parts. The point where the tires connect to the frame and front fork were constrained to be the “ground” and the forces of a person were placed on the points were the handle bars and crank for pedals connect.
The two forces applied to the test segment were 400N and 200N. The 400N force was place in the crank section to simulate a person standing on the pedals, while the 200N force was placed in the section that the handle bars mounts to. The Static Stress from these forces gave a maximum of approximately 1.3e^7 N/m^2, this value proved to be below the Yield Strength of the material chosen of 4.6e^8 N/m^2. The Static Strain in the frame and fork showed a maximum of approximately 3.6e^-5.
Figure 164
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Figure 165
The materials chosen for these two parts where Chromoloy Steel (4130), this material was chosen because it is a favorite among BMX Bike manufacturers and riders. It also provides a high yield strength to stand up to the abuse of riding a performing stunts. Aluminum is also a choice as well but with a lower yield strength of 2.757e^7 N/m^2 the bike could experience a load that may surpass this value.
The Stress and Strain Analysis of the handle bar can be seen in the figures below showing that the material chosen is correct to handle the force of 200N on the grip section. For this analysis the stress maximum is at the point where the bar connects the two sides together, at this point there will be a material inconsistency from the weld that holds the bar in place, this weld if done properly will not cause the material to be weakened.
Figure 166
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Figure 167
The Pedal was then analyzed with a 200N force to simulate a person standing on each pedal. This analysis shows a high point of stress at a point that was not expected and passed the yield point of the Aluminum alloy that was first chosen so the material was changed to steel and the point of Highest stress strain was still above the yield but only slightly this could be from a number of causes, the first being that the design of the pedals geometry is incorrect or that because the force is applied perpendicular to the face of the pedal which is not flat but slightly tapered on both sides.
Figure 168
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Flow AnalysisFor this BMX bike design, a flow analysis was performed across the model. In this
simulation, a velocity of 15 mph (264 inches/second) was applied in the negative X direction to simulating a 15 mph gust of wind. From this simulation, the pressure on the model from this velocity as well as the change in velocity was measured across the entire model. A point on the model was then selected and from that point, the pressure and velocity differentials were measured.
To emulate the desired condition, a velocity was 15 mph was applied in the negative X direction, giving the figure below. From this simulation, the difference in velocity can be shown across the entire model, as well as the different directions the flow takes after coming in contact with certain areas of the model such as the tires. It can be observed that around the time the flow reaches the back tire, the velocity as decreased significantly from the initial, dropping to a velocity of approximately 69.5 inches per second, or 3.95 mph.
Figure 169
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Figure 170
By doing an analysis of the pressure of the flow across the model, it can be shown that the pressure differential across the frame remains mostly constant, hovering around 14.69497 lbf/in2.
Figure 171
When looking at the effects that the flow had on the model, a few things are observed. The first observation that is made is that the handle bars of the bike are by far under the most pressure from the wind, having a pressure of approximately 14.698 lbf/in2.This result makes sense because that is the area that is coming in contact with the flow first before the flow velocity begins to drop. The front wheel of the bike is the area under the second greatest pressure with a pressure of about 14.696 lbf/in2, also due to the direct contact with the initial flow. The black
74
arrows show the different directions that the flow is traveling when coming in contact with the model, with the biggest areas of deflection being the seat, due to its obscure shape.
Figure 172
In order to analyze the pressure differential across the entire duration of the flow, a specific point was selected to measure the overall differential. For this simulation, the outside edge of the back tire was selected, yielding the following results.
75
Figure 173
Figure 174
For the velocity differential, it can be seen that the spike in velocity around -6 inches comes from the flow reaching the edge at that point in time. Beforehand the flow was between the front wheel and the pedals. The spike up to 124.13 inches/sec (7.05 mph).
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ConclusionThe selection of a BMX bike allowed for the freedom to create a different type of bike
which can potentially perform better than some of the models currently available. This also created many different possibilities for performing different simulations on the body aside from the ones shown above. As shown above, the design of the 3-D model involved splitting up the design into 3 major sections, the frame/front front/handle bars, the drivetrain and the wheels/tires. By splitting up the model into more manageable sections, more time was able to be devoted to creating realistic and unique aspects to the bike and create the best design possible.
When assembling the major components together in the final assembly, numerous constraints including coincident and concentric were used to attach each part together. The complete assembly allowed for rotation of the pedals, wheels and sprocket as well as the necessary motion for the handle bars. Following the completion of the assembly, a motion analysis was performed on the assembly, applying different motors to the bike to simulate realistic motion and analyze the results. In addition a point was selected in the assembly and the position, velocity and acceleration were determined from that given point.
In addition to the motion study, two types of simulations were performed. The first simulation was a stress/strain analysis, where different forces were applied to the frame of the bike to test the areas of most strain as well as see where the strongest areas of the frame are. The second simulation involved flow, where 15 mph wind was simulated hitting the front of the bike and understanding the direction the wind will travel following impact with the bike, the velocity of the wind as well as the pressure that is applied to the frame of the bike. In addition, a certain point was selected from the frame, and values such as the temperature differential and pressure differential were analyzed off of that given face. The main purpose of these simulations is to test our initial design and assembly and determine the strengths and weaknesses of the design. This also gives the opportunity to understand how the bike behaves under certain conditions.
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References
1. "Notes on Sprockets and Chains." Notes on Sprockets and Chains. N.p., n.d. Web.2. "Comparison of Frame Materials - BMXmuseum.com." Comparison of Frame Materials
- BMXmuseum.com. N.p., n.d. Web.3. "CG Bike Co. Vert BMX Bike 2013." Dick's Sporting Goods. N.p., n.d. Web.4. "Wheels BMX." AceBMX. N.p., n.d. Web.
