WIND TUNNEL TESTING IN TEXAS

Derby Tech - January/February, 1990

by Bruce Finwall

On January 2 and 3, 1990 some interesting observations were made during tests of soap box derby racers in the Texas A&M University subsonic, closed-circuit wind tunnel. Six of the top cars in the country (3 seniors and 3 juniors) were involved in a total of 48 separate tests. The following is a summary of what was found.

THE PARTICIPANTS

Senior

Jon Waldrip - 1989 NDR 2nd Place
Bridgit Fredericks - 1989 NDR 4th Place
Krista Shuff - a brand new car featuring an extremely thin tail (1989 NDR 2nd Place Junior)

Junior

Cody Robertson ‘88 - 1988 NDR 5th Place (Wood Car)
Cody Robertson ‘89 - 1989 NDR Champion (Wood Car)
Ruthie Shuff - Current All-American "Wild Card" points leader (Fiberglass Car)

THE RESULTS

Senior Division (Cars were tested with axles, axle foils, and steel wheels)

Car	Cross-Sectional Area (Square In.)	Distance from the nose to the location of the girth (inches)	Drag Nose Forward (lbs.)	Drag Tail Forward (labs.)
J. Waldrip	182	50.4	1.24	1.40
B. Fredricks	157	52.0	1.24	1.38
K. Shuff	171	39.3	1.23	1.38
K. Shuff (No Axle Foils)	171	39.3	2.18

Junior Division (Cars had no axles or wheels)

Car	Drag w/ Nose Forward (lbs.)	Drag w/ Tail Forward (lbs.)
C. Robertson ‘88 (Very Round Nose)	0.82	0.89
C. Robertson ‘89 (Less Round Nose)	0.86	0.89
R. Shuff (Jr. Jetstar Shell)	0.88	0.92

OTHER ITEMS OF INTEREST

1. A piece of tape was placed on the nose of a racer to simulate the effects of an All-American V.I.P. sticker. The result was an increase in drag.

Racer used for the test: B. Fredericks

Drag with tape on nose: 1.30
Drag w/o tape on nose: 1.24
Percent increase in drag: 4%

2. A junior car was tested to see what effects nose rake (angle of attack) had on drag. A level racer was found to have the least amount of drag. All tests were done with the rear ground clearance set at 4 inches.

Racer used for the test: C. Robertson

Ground Clearance at the Nose (inches)	Drag (lbs.)
3 5/8	0.822
4	0.810
4 3/8	0.843

3. It was found that the racers had LESS DRAG with the steering cables exposed to the airstream. This test was conducted on two racers, with the same result both times.

Car	Drag w/cables (lbs.)	Drag w/o cables (lbs.)	% Increase in Drag
J. Waldrip	1.24	1.25	0.8%
B. Fredericks	1.24	1.25	0.8%

4. It was found that a car has less drag with a sharp tail.

Racer used in test: C. Robertson ‘89

Drag with round edge on tail: 0.86 pounds (Championship configuration)
Drag with sharp edge on tail: 0.84 pounds

CONCLUSION

Of the senior division cars tested, their drag was nearly identical. This was especially interesting because of the major differences in car design. The cross-sectional measurement appeared to make little difference in reducing the drag, which was a little surprising.

It was found that the drag increase due to removing the axle foils was 0.95 pounds of drag. Since axle foils are known to be worth about one car length per run, each 0.01 pounds of drag is worth about 7/8 inches on the track.

It should be noted that all of the senior cars tested are highly competitive and have well streamlined body shapes. This should just go to show that there are many ways to approach senior car design and there is plenty of room to be creative and still be competitive.

Of the junior cars tested, it was found that the rounder the nose the lower the drag. In fact, the difference in drag between the round nosed junior car and the others was greater than the difference between the radically different senior designs! It was also found that a junior car has the least amount of drag when the floorboard is level to the road surface.

Drag was reduced when the tail had a sharp edge compared to a round edge. This technique should apply in both divisions.

Of great surprise, it was found that the steering cable helps to decrease the drag when exposed to the airstream. So, it is probably not to our advantage to go through any effort to keep the steering cables within the car body (as many racers currently attempt). The reasons for this decrease in drag, are not know, but could be due to either:

1. Steamlining the rear axle foil and/or wheel.

or

2. Decreasing turbulence from the front axle.

The effects of steering cable exposure should be similar for the junior division.

Conducting the tests was a lot of fun. Some questions concerning soap box derby were answered, but other new questions have arisen due to the test.

Special thanks to Texas A&M University and their staff for all of their support, for without, these tests could not have been conducted. NOTE: This was a ONE TIME TEST. The Texas A&M windtunnel is NOT AVAILABLE FOR PRIVATE TESTS. This test was conducted for the benefit of all and the results published for all to read.

On-line Derby Tech Editors note:

The complete 101 pages of results plus additional report body are not currently available. We hope to make them available either on-line or printed at some point in the future. We will place a notice when it is available.

	Flow visualization tests were conducted on Jon Waldrip’s racer. The airflow appeared to be very smooth.
	Jon Waldrip’s racer is prepared for drag measurements. (Sorry, this was a pretty dark picture as published - Ed)
	Cody Robertson’s ‘88 racer had the least drag of all junior cars tested.
	Cody Robertson’s two cars side by side. The ‘88 car in foreground has a noticeably rounder nose.
	Krista Shuff’s new senior.
	Bridgit Fredericks’ "Downhill Drifter."
	Cody Robertson’s 1989 championship racer is prepared for testing.
	Ruthie Shuff’s fiberglass junior racer. (and others.... Ed)

Online editors note: Additional pictures of the wind tunnel test have been posted on the Zero Error website at: http://www.zeroerror.turbo2k.com/

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