pedicab report

An Analysis of Material Strengths
For Dirtnail Pedicab #001
By Luke Iseman
luke@lukeiseman.com
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Table of Contents
Building Style............................................................................................................................................4
..............................................................................................................................................................8
Figure 1: Sol-Train constructed with grid beam construction method..................................................8
Figure 2: Loft bunkbed / living area constructed with grid beam construction method.......................9
Figure 3: Wood-hauling solar tractor constructed with grid beam construction method....................11
Figure 4: Solar car constructed with grid beam construction method.................................................12
Figure 5: Solar car, without shell........................................................................................................13
Structural Steel.........................................................................................................................................13
Interconnecting Fasteners.........................................................................................................................14
Bolts.....................................................................................................................................................14
Figure 6: Bolt identification and tensile strength information............................................................16
Figure 7: Tensile stress area table........................................................................................................17
Figure 8: box of 5-inch bolts used.......................................................................................................18
Nuts.....................................................................................................................................................19
Figure 9: Nylon Locknuts Utilized In Build.......................................................................................19
Washers................................................................................................................................................19
Figure 10: Washers Utilized in Build..................................................................................................20
Wheels......................................................................................................................................................20
Hubs, Spokes, Rims.............................................................................................................................20
Tires, Tubes.........................................................................................................................................20
Figure 11: Tires Utilized......................................................................................................................21
Figure 12: Tubes Utilized....................................................................................................................22
Figure 13: Comparison of Tires...........................................................................................................22
Ground Surface........................................................................................................................................22
Plywood...............................................................................................................................................22
Tread....................................................................................................................................................22
Figure 14: Non-Slip Tape Utilized......................................................................................................23
Seating Surface........................................................................................................................................23
Plywood...............................................................................................................................................23
Stuffing................................................................................................................................................23
Fabric...................................................................................................................................................23
Side supports............................................................................................................................................23
Appendix 1: Telespar Structural Information...........................................................................................24
I. Building Style
This pedicab is built utilizing a style of construction referred to as “grid beam building.” The
main advantages of this style are:
● Encourages reuse of materials between projects
● Allows rapid creation of complex structural shapes
● Prevents unnecessary expenditure of materials, money, and time on welding
Grid beam building has been employed to create a variety of robust structures, including but not limited
to trains, tractors, and electric cars. See figures 1-5 for illustrations of some of these structures.
Figure 1: Sol-Train constructed with grid beam construction method
Figure 2: Loft bunkbed / living area constructed with grid beam construction method
Figure 3: Wood-hauling solar tractor constructed with grid beam construction method
Figure 4: Solar car constructed with grid beam construction method
Figure 5: Solar car, without shell
For more information about the grid beam building method, please review:
● How To Build With Grid Beam: A Cheap, Easy, and Affordable System for Constructing Almost
Anything. Available at Bookpeople, http://www.amazon.com/How-Build-Grid-BeamConstructing/dp/0865716137
● Grid Beam Building System: http://gridbeamers.com/
II. Structural Steel
The steel employed throughout the construction process is galvanized, perforated, rectangular
steel tubing, of 12 gauge thickness and 2 inch size. Commonly referred to as Telespar, this steel
possesses a minimum yield strength of 60,000 psi.
Taking proof strength as 95% of yield strength:
proof strength = 60,000 psi * .95 = 57,000 psi minimum proof strength
Our area of contact between joints is equal to .590 square inches, giving us a minimum proof strength
per joint of 57,000 pounds per square inch * .590 square inches = 33,630 pounds minimum structural
steel strength per joint.
Additionally, this metal is manufacturer-listed as possessing an allowable movement of 15,405 pounds.
See http://www.alliedtube.com/pdf/Uni_Telespar.pdf , included with this document, for source of yield
strength, area of contact between joints, and allowable movement. You may verify that the Telespar is
2 inches across by measuring with a ruler, and the Telespar brand is visible at different points on the
material.
III.Interconnecting Fasteners`
1. Bolts
All bolts used structurally are of grade 5 steel and 3/8-inch diameter. These are a medium
carbon steel, quenched and tempered, and possess a minimum tensile strength of 120 ksi.
See figure 6 for my source of this data, available online at http://www.unifiedeng.com/scitech/bolt/boltmarks.html and other locations.
Given a tensile strength of 120 ksi, we can find the shear strength as follows:
shear strength = 60% of tensile strength = .6 * 120 = 72 ksi
This procedure is explained at
http://www.mechanicsupport.com/articleBoltTensileStrength.html and various other
locations.
From http://www.efunda.com/designstandards/screws/screwunc.cfm , included as figure 7,
tensile stress area for a 3/8-inch bolt is .0775 square inches. This allows us to calculate
shear stress per bolt:
72 ksi = 72,000 psi 72,000 pounds of force / square inch
72,000 lbs of force / square inch * .0775 square inches = 5,580 lbs
Thus, each bolt is capable of 5,580 lbs shear stress.
See figure 8 for a photo of the box of 5-inch bolts used. Note the head markings indicating
grade 5.
Figure 6: Bolt identification and tensile strength information
Figure 7: Tensile stress area table
Figure 8: box of 5-inch bolts used
2. Nuts
All nuts used structurally throughout this build are nylon locknuts. See figure 9 for a
picture of the nuts used.
Figure 9: Nylon Locknuts Utilized In Build
3. Washers
All washers utilized in this build are of the flat style. See Figure 10 for a picture of the
washers used.
Figure 10: Washers Utilized in Build
IV. Wheels
All wheel components were sourced from Eastside Pedal Pushers. You may reach Lee, the head
of the shop, at (512) 826-3414. All components are identical in grade to or better than the
components utilized by other trailer-style pedicab companies throughout Austin.
1. Hubs, Spokes, Rims
The hub, spoke, and rims are of standard type utilized on 20-inch-wheeled pedicabs,
purchased new From Eastside Pedal Pushers. The spokes are high-strength, and spoke
count per wheel is 48. At the time of my purchase, 7 sets of the same hubs, spokes, and
rims were being sold to Heart of Texas pedicab. Having spoken with owners of several
pedicab companies, these are among the strongest wheels in use.
2. Tires, Tubes
The tires are high-pressure-rated, wire-rim treads equivalent to or higher quality than the
tires in use on both commercially available and locally made pedicabs. As is industry
standard, the pedicab utilizes thorn-resistant, extra-thick tubes. Both tires and tubes were
also purchased from Eastside Pedal Pushers.
See figure 12 for a photo of the tires, figure 13 for a photo of the tubes, and figure 14 for a
comparison of my tires to 2 other commonly-utilized pedicab tires. Specifically, the
majority of Heart of Texas and Metrocycle pedicabs utilize these treads.
Figure 11: Tires Utilized
Figure 12: Tubes Utilized
Brand
Model
Size
TPI
Bead
Max PSI
IRC
Hardies
20 x 1.95
66
Wire
125
Maxxis
Ringworm
20 x 1.95
60
Wire
110
Wire
110
Maxxis
Hookworm
20 x 1.95
60
My cab utilizes the IRC Hardies. Sources:
http://www.irctire.com/tires/hardies.html
http://www.maxxis.com/Bicycle/BMX/Ringworm.aspx
http://www.maxxis.com/Bicycle/BMX/Hookworm.aspx
Figure 13: Comparison of Tires
V. Ground Surface
1. Plywood
All plywood used on the ground surfaces is 3/4” plywood.
2. Tread
3M brand non-slip tape covers all areas of the ground surface. See figure 15 for a picture of
the tape used.
Figure 14: Non-Slip Tape Utilized
VI. Seating Surface
1. Plywood
All wood used under seating surfaces is 3/4-inch plywood.
2. Stuffing
Stuffing is all mildew-resistant outdoor-grade 2-inch filling, sourced from Hancock fabrics.
3. Fabric
Upholstery is all outdoor-grade fabric, sourced from Hancock fabrics.
VII.Side supports
Side supports are constructed out of 3/4” plywood.
VIII. Lighting system
1. Front Light
The front light is mounted to the front of the bicycle, and is a high-visibility, high-efficiency
Petzl Tikka Plus light. More information is available at
http://en.petzl.com/petzl/LampesProduits?Produit=463 .
2. Rear Lights
The rear lights are documented half-mile visibility red blinking lights. 2 are mounted on the
rear of the cab; see figure 15 for a picture of the light and specifications. Additionally, the
required 'slow-moving triangle' sign is mounted on the rear.
Figure 15: Rear Light Properties
IX. Vehicle Coloring
Pedicabs vary widely in color and style. I have chosen to utilize orange and other bright colors
to provide superior visibility for my pedicab, as being seen is in my experience one of the most
important aspects of avoiding accidents. The vehicle interior utilizes vertical stripes of distinctive
colors, further enhancing the uniqueness and attractiveness of this pedicab.
X. Vehicle Braking Capabilities
This pedicab meets or exceeds all industry- and city-recognized braking requirements via pad
brakes, as is industry standard for tow-behind cabs. Braking capabilities will be demonstrated,
both on a sloped surface and level, during the inspection.
XI. Comparison To Strength of Other Pedicabs
1. Structural Material Strength
Having contacted most major pedicab manufacturers, I was able to receive information from
Main Street Pedicabs, one of the industry's 'golden standard' companies:
Construction material: 4130 steel alloy, tubular
Material thickness: maximum wall thickness of .063”, outside diameter from 1.25” to 1.5”
See figure 16 for an image of the McMaster-Carr listing for the thickest steel possibly deployed
by Main Street Pedicabs.
Figure 16: Thickest Steel Possibly Used By Main Street Pedicabs
2. Connecting Weld Strength
Main Street pedicabs are TIG welded, which, especially with 4130 steel, makes it difficult to
evaluate the exact strength of a weld. However, assuming the welds are performed correctly:
“Chromoly has several advantages, though, IF it is welded following all of the rules. In the
normalized condition, commercial 4130 is a relatively high strength steel. Its 85,000 psi yield
strength is far higher than that of about 51,000 psi for 1020 mild steel. ”
(source: http://fourspeed.net/front-end.html )
In comparison, as per the 'bolts' section above, my bolts are 120 ksi (120,000 psi) tensile
strength.
The grid beam construction style I've deployed allows much more contact area between joints
than the typical weld employed on pedicabs, so the strength per joint is actually much greater
with my design.
So, for a typical tube-to-tube connection on a Main Street bike, assuming both tubes are .063”
wall thickness and outside diameter of 1.5', we can calculate strength as follows:
psi rating: 75,000
square inches of contact = area of circle occupied by steel = pi * .75^2 – pi *(.75-.0315)^2 ~ .15 square
inches
75,000 pounds per square inch * .15 square inches ~ 10,894 pounds of force rating for the best
intersections.
As per the “structural steel” section above, the greater contact area of my joints in comparison
to those deployed by Main Street and other manufacturers gives a greater tolerance for weight (in
excess of 30,000 lb.s for my design)
Appendix 1: Telespar Structural Information