performs the best in ASME competitions. The following .... The uni-weave carbon offered the best specific mechanical .... maintain 120 rpm in the highest gear.
R. Kozak, M. Slichko and B. O’Toole, “Design and fabrication of a composite human powered land vehicle”, Proceedings of the 41st International SAMPE Symposium and Exhibition, March 1996.
DESIGN AND FABRICATION OF A COMPOSITE HUMAN POWERED LAND VEHICLE Raymond Kozak, Michelle Slichko, and Brendan O’Toole University of Nevada Las Vegas, Department of Mechanical Engineering Las Vegas, Nevada 89154-4027
ABSTRACT Composite prototype development can be accomplished in a short amount of time with limited resources and experience. This is important for small organizations wanting to develop new ideas with composite materials. The structural components for a two-wheeled human powered vehicle (semi-recumbent bicycle) have been designed and built primarily from composite materials. Most of the design and fabrication work has been completed by people with less than one year of experience working with composites. An overview of the design process is presented which includes: selecting the structural configuration and materials, selecting a frame cross-sectional geometry, going from two-dimensional sketches to three-dimensional drawings, designing and making foam molds, material selection based on performance, price and availability, stress analysis, material characterization, experimental verification of stress analysis, component testing, planned full scale vehicle testing, and unplanned full scale impact testing. The vehicle produced during this process won first place in the 1995 ASME Region IX Human Powered Vehicle design competition.
KEY WORDS: Design, Composites, Bicycles, Prototypes
1. INTRODUCTION 1.1 Design Objectives The Human Powered Vehicle (HPV) competition is an event sponsored by the American Society of Mechanical Engineers in the collegiate Southwest Region IX. Each year three separate events (a design competition, sprint race, and relay road race) are held on three consecutive days at one of the universities in the region. A twowheeled semi-recumbent HPV concept was chosen as the basic configuration. This
vehicle style holds most of the current international land speed records and generally performs the best in ASME competitions. The following design goals were determined about one year prior to the 1995 competition: • • • •
Total weight (fully faired) less than 178 N (40 lb) Minimize overall frontal area Optimize rider positioning for optimum power Recruit an excellent design team and manage them well
The ultimate goal was to have a vehicle that could reach a speed of over 80 km/h (50 mph) on level ground with about 800 meters (0.5 miles) of acceleration and deceleration room. 1.2 Team Building The design team included about 20 students and 5 faculty members from the Mechanical Engineering and Kinesiology Departments. The team took an open, interdisciplinary approach to the entire process. This was a hands-on project from day one for anyone interested in participating. Freshmen were teamed with seniors to get used to the design experience and enthusiasm was high all year as steady progress was made in each of five major committees.
2. OVERALL DIMENSIONS 2.1 Optimum Rider Configuration Rider geometry is defined by the hip-to-crank line, mean hip angle, and the back angle as shown in Figure 1. Results of a cycling performance study (1) show that the maximum anaerobic power output can be developed when the riders trunk is in a vertical position and the back angle is between 75° and 80°. The mean hip angle can be determined by analyzing the motion of the upper leg as the pedal travels through its circular path. The mean hip angle is defined as one half the angle made by the rider’s thigh as it travels through its motion. The relative position of the crank center and hip joint can be determined by analyzing the motion of individual riders.
Back angle
Hip joint
Mean hip angle
Crank center
Hip-crank line
Pedal
Figure 1: Rider Geometry Various body dimensions were measured on fifteen rider candidates and a range of motion analysis was conducted for each one. Figure 2 shows the toe, heel and knee paths for all of the riders as well as their hip, shoulder and head positions relative to the crank center. The
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frame must be able to accommodate at least three different riders during the relay road race. Unfortunately, two of the most active students and potential riders differed in height by about 30 cm (12 in). To minimize the length of the vehicle, the front wheel was placed below the rider’s lower leg and the cranks were located above and forward of the front wheel. A small diameter front wheel was necessary to minimize the angle of the hip to crank line. This positioned the largest rider in a semi-recumbent position and maintained a low profile for the vehicle. The smaller riders are positioned higher in the vehicle by means of a replaceable seat which also moves their trunk angles closer to the upright position. The head position results in a 13.5° field of vision below the horizontal which is better than that of a large car. This rider position determined the general shape of the frame as illustrated in Figure 3. Rider Position
35.00 top of head
25.00
knee travel
15.00 shoulder joint toe circle
5.00
hip joint
crank circle
-50.00
-40.00
-30.00
-20.00
-10.00
0.00
10.00
-5.00
heal circle Note, All data points are referenced from crank center and the horizontal axis is the hip/crank line -15.00
Figure 2: Critical Body Positions for Fifteen Riders 2.2 Frame Geometry The frame geometry shown in Figure 3 was developed by connecting the required hardware with a line that did not interfere with the rider’s motion. The centerline of the front of the frame is 56 cm (22 in) above the ground providing enough clearance for the cranks. At the fork tube, the height of the frame centerline is 48 cm (19 in). The frame drops to a centerline height of 15 cm (6 in) to maintain an optimum hip position. The frame then slopes upward to a centerline height of 41 cm (16 in) at the location of the rear axle. In the back, it rises steeply to 86 cm (34 in) making a push bar and fairing support. A steel prototype was fabricated to test stability and control of this configuration and it proved very useful later for optimizing the steering geometry and as a practice vehicle for the riders.
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Figure 3: Frame shape and wheel positioning
3. STRUCTURAL DESIGN 3.1 Frame The frame must withstand the applied loads from a rider and all the attached hardware including a full fairing at speeds which could potentially reach 113 km/h (70 mph). A tubular design with a tapered cross-sectional area was selected to maximize the structural efficiency. Round and obround shapes were used for their torsional rigidity, ease of manufacture, and aesthetics. The initial dimensions of the frame cross-sections were chosen to accommodate typical bicycle hardware, and the wall thickness was determined based on stress analysis and testing. 3.1.1 Material Selection Practical material choices for the frame were steel, aluminum, and composites. The steel prototype was too heavy and a composite design was chosen over aluminum because there was a strong interest in learning how to work with composites. It would also have been difficult to fabricate a sleek looking curved frame from aluminum. The composite design would consist of a foam core with a fiber-epoxy hand laid-up skin. Fibers considered were woven carbon, Kevlar, and glass fabrics, a carbon/Kevlar hybrid weave, and uni-weave carbon. The uni-weave carbon offered the best specific mechanical properties and was chosen as the main reinforcement material. The carbon/Kevlar hybrid plain weave fabric was chosen as the surface layer for safety and aesthetics. The Kevlar fibers would help reduce splintering during a bad crash and the hybrid fabric looked good. Mechanical properties of these materials were needed as input for stress and failure analysis. Material characterization experiments were conducted on the unidirectional carbon and a laminate consisting of one 0/90 hybrid layer, one uni-weave layer, and one ±45 hybrid layer. Initial estimates indicated that this would be the minimum laminate necessary to meet the bending, torsion, and buckling loads. Flat panels were fabricated and used to make tensile specimens. Several tubes were also fabricated. Short sections of the tubes were tested in compression and longer sections were loaded in torsion to determine shear properties. A summary of the material property data is listed in Table 1. The tests included tension in the 1 and 2 direction, compression in the 1 direction, and a torsion test. These tests provided sufficient properties to characterize the laminate for analysis. The original uni-carbon material had a lower tensile strength than the laminate because it had a lower fiber volume fraction (0.38) and had more mis-aligned fibers. A vacuum bag was used when
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manufacturing the laminate and much better consolidation was achieved, resulting in a thinner laminate with a higher fiber volume fraction (0.48). Table 1: Material Property Data
Material UniCarbon Laminate
1T
2T
1C
12
21
MPa (Ksi) 572 (83) 586 (85)
MPa (Ksi) N/A
MPa (Ksi) N/A
E11 GPa (Msi)
E22 GPa (Msi)
G GPa (Msi)
0.49
N/A
N/A
N/A
145 (21)
83 (12)
0.44
0.13
85 (12.3) 43 (6.2)
12 (1.8)
2.6 (0.38)
3.1.2 Component Testing A three-point bend test was performed on a round and obround specimen and the strain was measured at several points as shown in Figure 4. This experimental data compared favorably to strain predictions from a finite element analysis (FEA) of the tubes. The FEA model used material property data from Table 1. The flexural modulus (EI) was determined to be 0.048 Nm2 for the round tube and 0.052 Nm2 for the obround tube. The isotropic beam deflection equation, =PL3/48(EI), was used to obtain approximate values. 3.1.3 Frame Analysis and Testing A finite element analysis of the frame was conducted to determine points of high stress concentration which would require additional material. A similar analysis of another existing HPV frame was conducted to gain confidence in the FEA results. The old frame was tested statically by placing known loads in the seat area and measuring strain data at ten different locations that seemed to be areas of high stress concentration. Figure 5 shows the locations of the strain gages on the old frame. A dynamic test was also performed by riding the HPV and running an umbilical cord from the gages, through the fairing, and extending 10 meters to a chase vehicle with a portable strain indicator. The dynamic loads did not add any appreciable strain to the vehicle. The FEA strain predictions were consistently about 15% higher than the measured values. This was close enough to have some confidence in the analysis. An FEA model was then created for the new frame using the material properties calculated for the three layer hybrid laminate. Results showed that this laminate had a safety factor greater than 20 in most locations and a few areas of relatively high stress levels near highly curved sections. Based on these results, more material was added to the laminate at these locations.
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Round Specimen layout Gages read the 90 direction strain
load 5 .38
Gages
2 4
3 1 Foam Core
2.25 core
.75
5
1.0
6
1.0
Gages Support
Support
Gages 1,2,5,6 read the 0 direction strain
All dimensions in inches
Figure 4: Flexural Component Testing Of Sample Frame Member
Y
Support cross section 1.00 x 1.50
X
Frame cross section 1.75 x 3.75
Load Point
13.0
22.0
1
3
20.0 5,6T 10
12.0 4 9
2 5.0
7,8T 37.0
22.5
Support Point B
Support Point A
Dimensions are given in decimal form Strain gage numbers are in shadow
0,0 Reference point
Figure 5: Location Of Strain Gages On 1994 Frame 3.1.4 Frame Fabrication The fabrication of the frame occurred in three main steps. The foam core was prepared, the templates for the composite fabric were made (and the fabric cut), and the actual lay-up was done. The foam core was hot-wire cut to the approximate shape using templates, then sanded to the final shape as shown in Figure 6. Aluminum drop outs were added in the foam to support the rear wheel. All of the brake, gear, and computer cables were embedded inside the foam so that they would be internal to the composite frame. The dry reinforcement was cut into templates and the entire frame was laid-up in one
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operation and vacuum bagged. The cured frame and drop outs are shown in Figure 7, and the internal rear brake cable is seen exiting the seat support.
Figure 6: Rear Dropouts Being Placed Into The Foam Core
Figure 7: Cured Frame And Drop Outs 3.2 HPV Components 3.2.1 Bonded Joints The component inserts were installed after the frame was fabricated using a chopped fiber and epoxy bonding compound. The frame was drilled to accept the insert and the compound was injected into the gap shown in Figure 8. Figure 9 shows how the compound serves as a bonding agent in the interface regions and as a reinforcing pad to reduce stress concentrations at the frame-to-insert site. A cross section of a typical bond site can be seen in Figure 10. The diameter of the outer flange was determined to be 47.7 mm (1.9 in) for an insert with a diameter of 38.1 mm (1.5 in).
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Figure 8: Insert Site Preparation
Figure 9: Bond Test Specimen The chopped fiber/resin mixture was injected into the cavity around the housing insert, effectively creating a bridge between the frame walls and forming a housing bonded to the bracket and the frame walls. A mold was necessary to form the exterior flanges. The fiber length for the mixture is dependent on the fiber and resin material properties The critical fiber length for a carbon fiber/epoxy mixture with a 10% fiber volume is 3.7 mm (.15 in). Seven different mixtures were tested and the results summarized in Table 2. The 6.35 mm carbon fiber mixture had the best compressive and tensile results, as indicated by the critical fiber length calculation. Component testing of full scale bonded joints showed sufficient strength for the HPV application.
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Composite Tube with foam core
Chopped fiber Housing
foam core
Bottom Bracket Insert
foam core
Frame wall Side view
Section view
Figure 10: Insert Bond Site
Table 2: Chopped Fiber/Epoxy Test Data Material Pure Epoxy 3.18 mm Carbon 6.35 mm Carbon 6.35 mm 40C Carbon 12.7 mm Carbon Cotton Flock Glass Balloons
Tensile Modulus GPa (Ksi) 1.2 (180) 2.8 (399) 3.8 (547) 3.0 (442) 3.4 (495) 1.4 (201) 1.3 (188)
Tensile Strength MPa (Ksi) 51.2 (7.43) 50.3 (7.29) 68.6 (9.95) 38.8 (5.63) 24.8 (3.60) 34.0 (4.93) 18.6 (2.70)
Compressive Compressive Modulus Strength GPa (Ksi) MPa (Ksi) 0.67 (97.3) 42.5 (6.16) 0.41 (60.0) 43.2 (6.27) 0.40 (57.4) 59.0 (8.56) 0.26 (37.3) 31.1 (4.51) 0.83 (121.0) 19.4 (2.81) 0.38 (55.1) 29.2 (4.24) 0.59 (72.0) 13.6 (1.97)
3.2.2 Drive Train The goal of the gearing analysis was to determine a gear range to achieve a velocity of 80 km/hour (50 mph) with a maximum crank angular velocity of 120 rpm using 1118 watts of power (1.5 hp). To attain the desired velocity a two stage system was used by inserting an intermediate drive shaft in the drive train. The drive train is illustrated in Figure 11 where C* is one of the crank gears, BT and BS are the intermediate gears, and A* is one of the rear cluster gears. The vehicle speed is determined using the following formulas: Output angular velocity = (crank angular velocity) x (C*)/(A*) x (BT)/(BS) Vehicle Velocity using a 700c rim = (Output angular velocity) x (37.1/1057) Vehicle velocity vs crank cadence is provided in Figure 12. The riders found it difficult to maintain 120 rpm in the highest gear. Note the intermediate gear group will be changed after the sprint race. Specificity of performance requires that we use a different gear ratio for the road race and sprint race.
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force secondary chain
2 gear crank group (C*)
(BT) (A*) (BS) 7 gear rear cluster
2 gear intermediate group
primary chain
Figure 11: Drive Train
65.0
60.0 Vehicle Velocity with Sprint Intermediate Group (36/24)
55.0
50.0
45.0
Vehicle MPH
40.0
35.0
30.0
25.0 25 23 21 19 17 15 13 12
20.0
15.0
10.0
5.0 40 rpm
60 rpm
80 rpm
100 rpm
120 rpm
Crank R PM
Figure 12: Vehicle Velocity vs. Cadence 3.2.3 Power Requirements Laboratory testing showed that our riders can produce over 1118 w (1.5 hp) for short periods of time and an average power output of 373 w (.5 hp) over longer periods. The power required at any velocity can be determined from the following equation (2): powerrequired = [ ( Whpv + Wrider) + 1/2 Cd V2 hpv Ahpv] Vhpv where: W Cd V
= = = = =
rolling friction weight total of rider and vehicle drag coefficient air density at 25C and STP desired velocity
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Value .01 250 lbm (113.4 kg = 113.4 N) .12 (faired) or .88 (unfaired) 0743 lbm/ft3 (1.190 kg/m3) 15 to 70 mph (6.71 to 31.29 m/s)
Ahpv = vertical projected area 741.6 in2 (.4785m2) Inspection of Figure 13 shows drastically different power requirement profiles for the faired and unfaired vehicles. The power required for an unfaired vehicle at 64 km/h (40 mph) is over 1490 w (2 hp), which is not humanly possible, even for very short periods of time. The sprint test conducted indicated a maximum velocity of 56 km/h (35 mph) for the unfaired vehicle which corellates to a power output of 1192 w (1.6 hp) as can be seen in Figure 13. The power required for the faired vehicle at the same velocity is only 335 w (.45 hp) which would be quite attainable for a trained rider. Looking at the upper end of the faired vehicle graph it is recognized that the power requirement at 104 km/h (65 mph) is equal to our best riders maximum horsepower. Hor sepower R equir ement 3.5
Horsepower
3 Unfaired Rider cd = .88
2.5 2
Faired Rider cd = .12
1.5 1 0.5 0 15
20
25
30
35
40
45
50
55
60
65
70
Velocity in MPH
Figure 13: Power Vs. Velocity For Faired And Unfaired Vehicle
3.2.4 Front Forks The fork crown and drop-outs were manufactured from aluminum. Two foam core blades were bonded to the crown and drop outs and three layers of 6k braided tubular carbon fiber were layed up over the entire assembly. Figure 14 shows the design and Figure 15 is the actual forks. The composite/aluminum shear strength was determined to be 3.4 MPa so the lap length was designed accordingly. A specimen was tested in compression for fatigue at 0 to -2.224 kN, at a frequency of 0.5 sec for 10,000 cycles with no loss of stiffness or strength. 3.3 Aerodynamic Fairing 3.3.1 Fairing Design Figure 16 shows a three-dimensional control volume of the frame and rider which represents the minimum volume of space the fairing must surround. The control volume was then wrapped in a series of NACA airfoils (Cd = 0.12) at 5 cm consecutive elevations (planes parallel to the ground). After fitting foils to each elevation’s control volume, a rough three-dimensional shape for the fairing was obtained. The frontal area was 0.48 m2 which would result in a drag force of 17 N at 80 Km/hour using: Drag = CDAus2/2 where CD = drag coefficient, A = cross sectional area, = fluid density, and us = free stream velocity. The overall length of the fairing was 2.9 meters. The airfoil shapes are
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Figure 14: Fork Design
Figure 15: Forks With Carbon Fiber Blades
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shown in Figure 17. The fairing was fabricated in a three step process. A foam plug was sculpted, then a female mold was made off of the plug, and the final part was laid-up in the female mold. 3.3.2 Fairing Experiments The first aerodynamic experiment involved a race between a professional cyclist on his favorite bicycle and one of our student riders on an old fully faired HPV. The pro cyclist hit a top speed of 60 km/hour and the HPV reached 72 km/hour. Normally the pro cyclist is much faster. This simple test was conducted to demonstrate the need for reduced drag. A tuft test was also performed on the old fairing. Small strings of yarn were taped to the fairing and it was filmed in motion. A single dead spot near the upper back showed signs of flow separation so the shape was modified on the new fairing.
Figure 16: Fairing Control Volume Based On Largest Rider
Figure 17: Final Fairing Design
4. RESULTS/DISCUSSION
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Although the target speed was not attained during the competition, most of the people involved with this project were pleased with the results. Some innovative concepts were applied to the entire composites design process which was recognized by a first place finish in the 1995 ASME Region IX HPV Design competition. One key design consideration that paid off was attention to safety. A 4-point safety harness saved one rider from serious injury when she hit a hay bale at 51 km/hour. In fact, the entire vehicle withstood the impact fairly well and was operational the next day. A summary of key design features is listed below: Frame: light weight hybrid carbon-Kevlar/epoxy strong, safe frame; factor of safety > 10 internal cabling for aesthetics and safety low profile frame to reduce frontal area and increase stability sleek curved frame is aesthetic and has no corners with high stress concentrations static and dynamic testing of last year’s frame provide confidence in the FEA modeling FEA of this year’s frame provided detailed stress analysis extensive material characterization and component testing before manufacture a fully operational prototype was completed by mid-November Seat, Forks, and Power Train snap-in seat design provides same line-of-sight for all riders snap-in seats provide ability for quick rider exchanges seats molded to individuals for lumbar support providing safety and comfort seat position optimized for high power output custom hybrid forks made from aluminum and carbon composite fork design tested under static and dynamic conditions steering geometry extensively tested to find the most stable positions Fairing aerodynamic fairing designed with low drag coefficient and low frontal area fairing has Kevlar skin to withstand damage during spills and handling smooth Kevlar interior will not splinter and harm riders in the event of a bad collision fairing has carbon fiber ribs and rollbar for rider safety and shell stiffness tuft test provided insight on flow separation Rider Training riders were tested for their maximum power output and their fatigue rate rider training schedule since the Fall including power testing and endurance training all riders wear helmet and safety belt during training sessions team effort with over 30 students and 5 faculty members trimmed 15 pounds from last year’s vehicle and many pounds from our riders
Project Management extremely well organized project with effective student leadership and delegation all new students were coupled with experienced students in an active committee bi-weekly group meetings and weekly executive meetings
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5. ACKNOWLEDGMENTS The 1995 UNLV HPV team gratefully acknowledges the tremendous support from Raytheon Services Nevada, our prime sponsor. We would also like to thank UNLV’s College of Engineering, Behavioral Healthcare Options, Inc., Southwest Gas Corporation, Prime Health, Inc., Silver State ASME Chapter, Science Applications International Corporation, Prime Health, Inc., Charter Behavioral Health Systems, and Bat Rentals. The authors would also like to acknowledge all the students and faculty who have spent a great deal of time working on this project over the last several years.
6. REFERENCES 1.
D. Too, “The Effect of Hip Position/Configuration on Anaerobic Power and Capacity in Cycling”, International Journal of Sport Biomechanics, 7, 359 (1991).
2.
A. C. Gross, “The Aerodynamics of Human Powered Land Vehicles”, Scientific America, 249, (6), 142 (1983).
7. BIOGRAPHIES B. J. O’Toole is an Assistant Professor of Mechanical Engineering at the University of Nevada, Las Vegas. He has been conducting composites related research for the past eight years. He received a Ph.D. degree from the University of Delaware in 1993. R. Kozak is currently pursuing an M.S. degree in Mechanical Engineering at the University of Nevada, Las Vegas. He has over 20 years experience working in the material handling and manufacturing industries. He received his B.S. in Mechanical Engineering from UNLV in 1995.
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