HOW TO DESIGN A SOAP BOX DERBY MASTERS (SENIOR) DIVISION RACE CAR BODY

Derby Tech - January/February, 1990

by Bruce Finwall


INTRODUCTION

You are probably reading this article because you are interested in designing a soap box derby racer. You may have noticed that the current (1990) soap box derby rule book contains a series of complex rules. Have you said to yourself, "Where do I begin in designing my car?" This article will attempt to show you how to design a state-of-the-art "lay-back" styled soap box derby racer. I will show an example step-by-step how to design a car as well as include other hints and clues about soap box design. This article is designed for the novice designer, but may also provide some new ideas for the more experienced derbier. A knowledge of basic mathematics skills will be necessary in order to follow this article, so you children will find the math above their grasp. After reading this article you should be ready to design a racer like the one (Figure 1) shown below.

NOTE: It will be to your advantage to read these instructions through carefully before beginning your design work.

MATERIALS NEEDED

1. Scale (Ruler)

2. Paper

3. Pencils

4. Eraser

5. Piece of string (cut to exactly 24 1/2 inches long)

6. Calculator

7. Scratch paper

STEP 1 - WHAT YOU NEED TO KNOW ABOUT THE DRIVER

The first thing you need to do is determine how large of a race car the driver will need. Remember the car should be big enough for the driver to be able to fit.

Here is a list of driver measurements that you should take:

___ Weight

___ Height

___ Foot length

___ Hip width

___ Shoulder width (with shoulders compressed. The driver should not be "too" uncomfortable)

By far the most critical dimension is SHOULDER WIDTH. The width requirement will determine what frontal view and what top view shapes you can use. Foot length requirements will limit how low of a nose (side view) you can use. Weight and height measurements are mostly for reference use in comparing what sized drivers have previously raced in various styled cars. If, for example you weigh 100 pounds it might be a good idea to try to design your racer to look similar to one previously raced by a 100 pounder, rather than try to copy the design of a 70 pound driver.

STEP #2 DRAW THE FRONTAL VIEW

You are now ready to design the front view of your car. Several specifications that you need to consider are:

  1. Minimum girth of 49"
  2. Maximum curvature gage (Fig. 2) (note: the maximum curvature gage also applies to the bottom of the car (not shown).
  3. Minimum width of 13"
  4. Minimum height of 14"
  5. Your shoulder width
  6. Your hip width

If you want a rounder car you will have to build a car wider than 13" (Figure 2). If you are small you can build a smaller, flatter car (Figure 3) or still choose to build a rounder car.

The front view must be at least wide enough for your shoulders and hips (Figure 4). Probably, the width of your front view should be at least 1.25 times the size you actually need in the shoulder area, so that the top view of your racer will be a more streamlined shape. Top view will be discussed more in the next step.

It may be easier for you to draw only half a section (frontal) view (left or right). Use the string to make sure that you have at least a 40" minimum girth. for half a car it should be 24 1/2" minimum (Figure 5).

STEP #3 DETERMINE THE TOP VIEW SHAPE

You should now be able to determine what style of top view that you want. Think:

1. At what distance from the nose do you want your car’s frontal view (location of girth, width, and height) to be located (Figure 6)?

2. What do you want your car’s nose shape to look like (Figure 7)?

3. What do you want your car’s tail shape to look like (Figure 8)?

Here are some things to consider when making these decisions:

STEP #4 DETERMINING THE SIDE VIEW SHAPE

You should have already figured out where your frontal view will be located and what top profile you want to use. So, you have less leeway in determining your side view profile than you did in choosing your top view. Your side view must match-up to your top view.

Some ideas (shown in Figure 9) you might consider are:

  1. The side view should be 14" high at the location of your frontal view.
  2. Leave enough room for your feet.
  3. Leave room for the axles to be mounted in the car.
  4. Most electric eye timers are 8" above the road surface.
  5. You want a good line of sight out of your racer.

STEP #5 DESIGN THE CAR BODY FORMERS

At several different locations from the nose you will want to draw formers (section views), the individual cross-sections at that location. Figure 1 shows formers A through F for example.

(Hint: It might be a good idea to use only about 6 or 7 formers unless you have precise data (such as NACA airfoil data), otherwise your car may come out "lumpy" looking.)

1. Determine the width and height of your top and side profile views (Figure 10) at the location of each former. The "height" of the car is measured relative to the nose center. (usually 8" above the road surface.)

Here’s how to draw the cross-sections:

2. Draw the frontal view (Half shown. Figure 5)

3. Draw a number of horizontal lines (variously spaced) on the frontal view. (Note: When drawing an actual plan about 30 horizontal lines work well, but I will show only 5 here for simplicity) (Figure 11)

FIGURE 10. Height and width measurements used for determining the dimensions of the individual cross-sections

FIGURE 11. Lined Frontal View

4. Measure the width (length) of each of the lines. Also, measure the vertical distance from the center of the nose. (FIGURE 12)

5. Now here’s the tricky part. Calculate the coordinates for the individual cross-sections. Each cross-section is determined by 7 data points (#1 is the top of the section #2 through #6 correlate to the horizontal lines, and #7 is the bottom of the section). Table 1 shows sample calculations. Each cross-section is "proportional" in width and height (distance to the 8" nose height reference line) to the width and height determined from Figure 10.

FIGURE 12. Measuring the Cross-section

How did I calculate those numbers you ask? Well, let me show you step-by-step how I did it. Use scratch paper and a calculator to determine the coordinates for your formers.

I will use Former ‘A’ for my first example.

- At point #2

 

  Max width of car @ 4.2" from nose
Width = width at pt. #2 Frontal View = -------------------------------------------
  Max width of car @42" from nose

so width = (5.00")(3.02" / 8.0625")= 1.88"

  Distance to nose center @4.2" from nose
Height = Height at pt. #2 frontal view = -------------------------------------------
  Distance to nose center @42" from nose

so height = (7.75") (3.24" / 9.00") = 2.79"

Another example using former ‘E’

- At point #4

Width = (7.75") (5.95" / 8.0625") = 5.72"

Height = (2.50") (9.20" / 9.00") = 2.56"

5. Plot the points out on a piece of paper (Figure 13).

FIGURE 13. Plot Points for Former ‘A’

6. Draw smooth curves through the points to created the former shapes (Figure 14).

CONCLUSION

It may take you a while to get the hang of drawing racers with the method I have just explained. Don’t be discouraged if your first attempt isn’t quite what you want. Keep trying and don’t make things overly complicated.

I hope that this article has been helpful to you. These methods described are the same methods that have been used by past world champions. Have fun designing and racing your next soap box derby.


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