by Ollie Brower
The model in Fig. 1 was used to conduct ground clearance tests. The results of these tests are shown in Fig. 2. As you can see, drag decreased sharply as the ground clearance increased in the 0 to 1 inch range (0. 200 - 0. 066) , moderately in the 1 to 2 inch range (0. 066 - 0. 058) , noticeably in the 2 to 3 inch range (0.058 - 0.052), slightly in the 3 to 4 inch range (0.052 - 0.048) , and almost undetectable in the 4 to 6 inch range (0.048 - 0.047).
These tests were conducted with Reynolds numbers equivalent to a sdeed of 25 mph. At 30 mph these data would be slightly higlier.
The model in Fig. 1, showed an airflow turbulence under the tail area. This turbulence increased with less ground clearance. The flow of air was wrapping around from the sides of the model at the rear to the underside causing airflow separation and more drag. The underside of your racer will not come upward as drastically as the model, but be careful not to slope the underside up more than 4 inches from the bottom of your car.
From strictly an aerodynamics point of view for the model tested, the best ground clearance to have is between 32' and 5 inches. This does not take in account the wheels, axles, and axle trees. The more ground clearance you have, the more the axles are in the airstream (see Figure 3), creating more drag (Sr. Div. only). This also does not take into account the effects a lower center of gravity racer has when it goes through a track surface transition. Racer A has a ground clearance of 3", and only 8 1/2" of axle exposed on each side. Racer B has a ground clearance of 5", and has 10 3/4" axle exposed to the airstream. This exposure can be further reduced by taking advantage of the allowable 1/8" prebow. This prebow should be done as Figure 4 illustrates. The bend should take place after the axle exits the racer.
From the data studied, I recommend a ground clearance of three inches. Th added drag from 312, to 5 inches (0.003) is more than offset by the reduct in drag by having less axle exposed, and the increase in energy by having lower center of gravity.
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