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.
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.)|
|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
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. 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|
4. It was found that a car has less drag with a sharp tail.
Racer used in test: C. Robertson 89
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.
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 Waldrips racer. The airflow appeared to be very smooth.|
|Jon Waldrips racer is prepared for drag measurements. (Sorry, this was a pretty dark picture as published - Ed)|
|Cody Robertsons 88 racer had the least drag of all junior cars tested.|
|Cody Robertsons two cars side by side. The 88 car in foreground has a noticeably rounder nose.|
|Krista Shuffs new senior.|
|Bridgit Fredericks "Downhill Drifter."|
|Cody Robertsons 1989 championship racer is prepared for testing.|
|Ruthie Shuffs 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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