7 3/8 x 9 1/4 T echnical / Build Your Own Electric Vehicle / Leitman / 373-2 / Chapter 5
Chapter 5: Chassis and Design C rw at average wind= 7.5 mph 1.2 1.4 a vg sedan 1.6
C w factor at V= 5 mph 3.180 3.810
C w factor at V= 10 mph 0.929 1.133
C w factor at V= 20 mph 0.299 0.374
C w factor at V= 30 mph 0.163 0.206
C w factor at V= 45 mph 0.159 0.185
C w factor at V= 60 mph 0.063 0.082
Cw factor at V= 75 mph 0.047 0.062
4.440
1.338
0.449
0.250
0.212
0.101
0.076
Table 5-5 Relative Wind F actor C w at Different V ehicle Speeds for Three C
rw
Values
Aerodynamic Drag Force Data You Can Use Table 5-6 puts the Cd and A values for actual vehicles together and calculates their drag force for seven different vehicle speeds. Notice that drag force is lowest on a small car and greatest on the small pickup, but the small car might not have the room to mount the batteries to deliver the performance that you need. Notice also that an open cockpit roadster, even though it has a small frontal area A, has drag force identical to the pickup truck. To use Table 5-5 and Table 5-6 with your EV, pick out your vehicle type in Table 5-6, then multiply its drag force number by the relative wind factor at the identical vehicle speed using the appropriate Crw row for your vehicle type. For example, the 3,800-lb. Ford Ranger pickup truck of Chapter 10 has a drag force of 24.86 lbs. at 30 mph using Table 5-6. Multiplying by the relative wind factor of 0.250 from the bottom row (Crw 5 1.6) of Table 5-5 gives you 6.22 pounds. Your total aerodynamic drag forced is then 24.86 1 6.22 or 31.08 pounds.
Shape Rear Airflow If you’ve seen a movie of a wind tunnel test with smoke added to make the air currents visible, you’ve noticed a vortex or turbulence area at the rear of most vehicles tested. Those without access to wind tunnels notice the same effect when a semitrailer truck blows past you on the highway. As with the falling raindrop shape, a boat tail or rocket ship nose shape is the ideal. While this is difficult to achieve, and no production chassis designs are available to help, you can benefit from rounding your vehicle’s rear comers and eliminating all
Vehicle C d A Small car 0.3 18 Larger car 0.32 22 Van 0.34 26 Small pickup 0.45 24 Roadster 0.6 18
V= V= V= 5 mph 10 mph 20 mph 0.35 1.38 5.52 0.45 1.80 7.20 0.57 2.26 9.04 0.69 2.76 11.05 0.69 2.76 11.05
V= 30 mph 12.43 16.20 20.35 24.86 24.86
V= 45 mph 27.97 36.46 45.78 55.93 55.93
V= 60 mph 49.72 64.82 81.39 99.44 99.44
V= 75 mph 77.69 101.28 127.17 155.37 155.37
Table 5-6 Aerodynamic Drag F orce F d at Different V ehicle Speeds for T ypical Vehicle C d and A V alues
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