Friday, September 27, 2013

Scaling Up: Design a Pinewood Derby Car from a Toy Car

I generally build pinewood derby 'cars' that don't look like real cars. Oftentimes it is awkward to coerce a real car design into the pinewood derby car form factor. However, many people like to use a real car as the basis for a design, and in fact, I have done so a few times. So in this article I will explain how this conversion is accomplished, and the design decisions that will need to be made. Although I will focus on scaling up from a toy car, the same principles apply when scaling down from a full-size car.

WHAT IS THE 'SCALE' OF A PINEWOOD DERBY CAR?

When building models, the 'scale' refers to the size of the model relative to the original. For example, many people have heard of 'HO Scale', a popular scale for model railroading. The HO scale is 1:87. That is, for every 1 inch in length (also width and height) of the model, the real train is 87 inches. Thus a train car that measures 44 feet long would be a little over 6 inches long in HO scale (44 feet, times 12 inches per foot, divided by 87).

Pinewood Derby Cars do not have a specific scale factor, as they are forced to be 7 inches long by 1-3/4 inches wide (instead of varying in size based on the size of the original). For many standard automobile sizes, 1:24 is pretty close, and in fact 1:24 scale decals generally work pretty well. However, the scale would be reduced when modeling a compact car, while the scale would be increased when modeling a large vehicle. So, instead of using a specific scale the designer must calculate a scale factor for the given original.

CALCULATING THE SCALE

To determine the scale, measure the length of the original vehicle, and then divide that number into 7 (inches). For example, let's consider modeling a toy car that measures 3 inches long, 3/4 of an inch wide (excluding wheel wells), and 11/16 of an inch tall (see Figure 1). With these measurements, dividing the length (3) into 7 gives 2.33(1). Thus, for every 1 inch dimension of the toy car, the pinewood derby car will be 2.33 inches. Therefore, the pinewood derby car will be 7 inches long (2.33 times 3), 1-3/4 inches wide (2.33 times 3/4), and 1.6 inches tall (2.33 times 11/16). All other dimensions, such as the placement of the cockpit and engine, the location of the body curves, etc. are calculated using the same scale factor.


Figure 1 - Hot Wheels Sweet 16 II

At this point, a tradeoff may need to be made. In this case, the width is fine, however, 1.6 inches is quite tall for a pinewood derby car. If realism is the primary goal, then maintaining the 1.6 inches is okay. However, if speed is the primary goal, then it is best to reduce the height of the car as much as possible.

If the width is too narrow, then it would need to be arbitrarily increased to 1-3/4 inches. If the width is too wide, then two options exist:

- Arbitrarily reduce the width to 1-3/4 inches, slightly skewing the final car.
- Recalculate the scale factor based on the width instead of the length. This will result in a pinewood derby car that is shorter than 7 inches; however, the proportions of the toy car would be maintained.

WHEELBASE CONSIDERATIONS

The next design issue that must be resolved is the wheelbase. When the wheelbase of the toy car is scaled up, almost certainly it will not match the wheelbase required by your local rules. For example, on the toy car in Figure 1, using the 2.33 scale factor, the front wheels would be 0.73 inch from the front of the car, and 1.16 inches from the rear of the car. Obviously this does not match the BSA wheelbase (nor any other for that matter). At this point, the following options could be implemented (based on the rules for your race).

1. If your race rules require a wheelbase distance AND a wheelbase position, then adjust the wheelbase to meet the regulations. This will certainly skew the resulting car, but there isn't really a choice in this case.

2. If your race rules require a wheelbase distance, but does not require a relative location then adjust the wheelbase to meet the required distance, but position the resulting wheelbase as close as possible to the position calculated with the scale factor. The resulting car will be skewed, but less so than in option 1.

3. If your race rules do not specify a wheelbase, then use the wheelbase calculated with the scale factor.

WHEEL WELLS

Another design issue is that of wheel wells. Many toy cars have wheel wells, so to accurately model the car the wheel wells must be included. This will require the addition of material to the sides of the car, followed by shaping the wheel wells to accommodate the pinewood derby wheels.

But wait; now we may have another issue. Measure the diameter of the wheels on the toy car, and then multiply the diameter by the scaling factor. Does the result closely match the diameter of a pinewood derby wheel? If not, the wheel wells will need to be adjusted to fit the pinewood derby wheels, which could skew the car.


Figure 2 - Hot Wheels Silver Bullet

As an example, for the car in Figure 2 (which also has a 2.33 scale factor), the rear wheels scale up to 1.16, but the front wheels scale up to 1.02. Since the standard BSA Speed is about 1.18, the wheel wells (mainly the front wheel wells) would need to be increased in size to accommodate the pinewood derby wheels.

SELECTING A CAR TO MODEL

When choosing a car to scale, you will need to consider the following factors before doing any wood working:

1. Given the rules for the local race, when the toy car is made to fit within the required dimensions, will it maintain the 'look' of the original toy? With strict wheelbase rules, try to find a toy car that will fit well within the local wheel base restrictions.

2. If you don't want to deal with wheel wells, choose a car without them.

3. Make sure to consider car weighting. You will likely need to add 2 to 3 ounces of weight, so space must be available on the car to accommodate the weight.

4. Some toy cars are quite complex. Consider whether the car can be built with your tools, or whether the design can be simplified without sacrificing the original look.

5. Actually, if you have a young boy like mine, the most difficult problem will be finding a toy car that is still intact!

EXAMPLES

Here are three examples of modeled cars:

Humvee
This Humvee in Figure 3 was modeled from a drawing on the internet. To model the wheelbase as accurately as possible, and to meet the BSA wheelbase specification, the car was shortened to 6.2 inches (see Figure 4 - blue lines show original body location, red lines show shortened location). The result is a wheelbase that matches the BSA wheelbase length, but is moved slightly forward from the standard location.

The body is made from three parts (you can see the glue lines running front to back). Before the parts were glued together, much of the middle part was cut away to eliminate weight. Even so, the final car required little to no added weight.


Figure 3 - Humvee Body


Figure 4 - Humvee Drawing Showing Shortened Body

Hot Wheels Car
This race car was modeled by a customer (see Figure 5). As you can see, the wheelbase was lengthened, the wheel wells were enlarged, but the general look was maintained.


Figure 5 - Hotwheels Car

Stock Car

This (unfinished) Stock Car body is for sale on our web site. The wheel base is lengthened to match the BSA wheelbase, but is offset to maintain the NASCAR look. The height of the car is slightly lowered (looks slightly mashed, but not overly so). 1:24 scale decals are used, but they had to be trimmed slightly as the actual scale factor is slightly larger than 1:24.


Figure 6 - Stock Car

CONCLUSION

If you need inspiration for a pinewood derby car design, your child's toy box (or the local toy store) can provide a lot of great ideas. Then, by calculating a scale factor, creating a pinewood derby car from the toy car is not as hard as it may first appear. Just make sure to consider the overall dimensions, the wheelbase, wheel wells, and the overall complexity of the design before making any sawdust.

(1) Most 'Hot Wheels' brand cars are 3 inches long, resulting in a 2.33 scale factor.

From Pinewood Derby Times Volume 6, Issue 12

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