Derby Tech - August/October, 1992

by Bruce Finwall

A hot topic in the last year has been aerodynamics of axle airfoils. Particularly, people have wondered how much does 7" of exposed axle handicap the All-American layback masters racers? By August, just about everyone knew that the handicap could not be overcome, as the sit-up racers dominated the All-American. Now, the new question is how much does 2.5" of exposed axle handicap the All-American layback racers? For 1993 will the sit-up or layback be fastest? This article will attempt to give you some answers. Results from several wind tunnel tests as well as track tests are presented to spread a little more liqht on the subject.

First, let's try to figure out how much 7" of exposed axle handicapped the layback racers. Using an average from race results and track testinq on a standard hill like Akron, the sit-ups had about a 0.440 second advantage in a timer-swap format. In NDR trim (both cars having full axle foils) the layback had about a 0.160 second advantage. So, the difference between having 7" of exposed axle and full axle foils is about 0.600 seconds.

In the January, 1990 wind tunnel test sponsored by DERBY TECH the aerodynamic drag was determined for a car with and without axle airfoils (Table 1). As you can see, eliminating the axle foils increased the drag by a whopping 77% (0.95 lbs.). The total amount of axle exposed was 46.5", or an average of 8.7" per corner. The area of the exposed axle was 46.5" times 3/4" (axle thickness) or 34.9 square inches. The calculated increase in coefficient of drag, CD, for the exposed axles is 1.289.


Racer with Axle Foils 1.23 lbs.

Racer with no Axle Foils 2.18 lbs. (46.5" of total axle exposed)


More recently, wind tunnel testing was conducted to specificallv study soap box derby axle foils with the 1/4" gap between foils and axle. The tests were conducted in an open circuit, subsonic tunnel (Fiqure 1 shows a test of an axle foil in the wind tunnel). A variety of leading and trailing edge shapes were made and many combinations were tested. Figure 2 shows the shapes of the different axle foil shapes tested. As the testing progressed it became obvious that some axle foil shapes were inferior, thus not all possible combinations were tested. From the results found (Table 2), the general trends of each foil shape could be seen. It was found that the axle foils could decrease the drag coefficient from 1.334 to 0.229, a difference of 1.105. This result is about 14% different from results of the other wind tunnel testing, which involved putting the whole car in the wind tunnel. The difference could be due to factors like interference of the car body or wheels.



Trailing Edge

  R5 R4 R3 R6 R1 R2 None
F4 0.351 0.337 0.315 -- -- -- --
F1 0.266 0.258 0.243 0.256 0.283 0.276 0.493
F2 0.245 0.244 0.236 0.255 -- -- 0.494
F3 -- -- 0.235 0.229 0.255 0.254 --
F5 -- -- -- 0.239 0.262 0.247 --
R3 -- 0.254 -- 0.257 -- -- --
None -- 0.857 -- -- -- -- 1.334


As you can see there are several interesting trends in axle foil design. As the leading edge length is increased drag was reduced. As the trailing edge length was increased drag was generally reduced. Increasing the thickness of the leading edge foil actually increased drag slightly. Rounding the corners of a trailing edge foil helped slightly. The leading edge shape had a much greater impact on the total drag of the axle foil than the trailing edge shape.

Now, let's look and see how the numbers ad up. Track results tell us that 7" of axle foil is worth 0.600 over bare axles, so it is about 0.086 seconds per inch of axle foil in a timer-swap race. Full scale wind tunnel testing showed that axle foils are worth about 0.95 lbs.of drag at 35 MPH, or 0.11 lbs. per inch of axle foil. The wind tunnel tests of just axle foils show that axle foils are worth about 0.97 lbs., or 0.13 lbs. per inch of axle foil. Takinq an average of 0.12 lbs/in, and estimating rolling resistance and other energy losses, the time differential comes out to be about 0.1 seconds per inch of axle foil in a timer swap race, which compares pretty close to the track results.

So what will happen with the 2.5" rule for 1993? Simple math shows 2.5'' of axle foil to be worth between 0.215 or 0.25 per heat. But since a layback has a more streamlined cockpit area the difference will be 0.055 to 0.090 per heat. This is only 0.027 to 0.045 seconds at the All-American, so it should be just close enough to keep from being a complete blowout like it was in 1992. Some layback cars should make the top 9 at Akron, but the sit-up car will most likely come out on top.

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