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Pinewood Derby Stories and Photos from Maximum Velocity
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Designing a Car From Scratch
I have been asked several times, "How do you come up with your
designs?" The obvious answer is 'imagination', but really I am not the
most imaginative person around. A better answer is that I think about
car designs on a regular basis. I am also a 'doodler'. When I have a
piece of paper and a pencil I tend to sketch out car profiles. I have
found if I do this enough, I eventually come up with something useful.
Of course there is more to designing a car than just sketching out a
profile. The car must also meet various criteria, some imposed by the
rules, and others that are needed for good performance. In this
article I will share with you some of the things I have learned about
car design, which I trust you will find useful when it comes time to
design your own car.
DESIGN CONSIDERATIONS
When the derby race date is approaching and time is at a premium, it
is tempting to draw a few lines and start cutting. But before you do
that, ask yourself a few questions:
1. Weight: Depending on how much wood you remove, you will need to
add between 2.5 to 3.5 ounces of weight. Too often car builders don't
consider the need to add weight until after the car is shaped and
painted. At that time it is difficult to add any weight, much less
add enough weight at the best location.
So, when designing a car, make sure to consider the following:
- Weight Amount: Make an estimate as to how much weight must be
added to achieve maximum weight. The amount will depend on the amount
of wood that is removed and the density of the wood. When I design a
car, I use previous models as a guideline, and I build several
prototypes until I get it right. If desired and time permits, you can
build prototypes as well. But if not, the weight uncertainty can be
dealt with (see 'Weight Placement' below).
- Weight Type: Determine the type of weight (shape and material). The
density of the weight makes a huge difference as to how much space
will be required to hold the weight (See Figure 1 which shows the
relative density of three popular weight types). Clearly much more
space must be reserved for zinc weight than for tungsten weight.
Figure 1 - Density Comparison of Popular Weight Types
- Weight Placement: For best performance on the most popular track
lengths (32 to 45 feet) the center of gravity (the front-to-back
balance point of the completed car) should be located between 1 and
1-1/4 inches in front of the rear axle. To achieve this balance point,
a 'rule of thumb' is to place 1/3 of the added weight behind the rear
axle, 1/3 on top of or just in front of the rear axle, and the final
1/3 about 1-1/2 inches in front of the rear axle.
I suggest embedding all of the weight in the car, as opposed to having
it hang below the bottom of the car. I oftentimes embed about 2/3 of
the weight in the car such that it is not visible. This is done with
holes, or with pockets that are covered with wood filler. The
remaining 1/3 of the weight is embedded into the bottom, so that it
can be adjusted at the weigh-in. If the amount of weight needed is
uncertain or if you don't have access to a scale, you can create
holes/pockets for the final 1/3 of the weight, but wait until the
weigh-in to install it.
2. Axle Location: For standard wheelbase cars (such as BSA kits),
best performance is generally achieved by using the axle slot closest
to the end of the block as the rear axle. This is just the opposite
of the car pictured on the kit box.
3. Wheel Base: If allowed by the local rules, extend the wheel base
to the maximum length. Long wheelbase cars go straighter, are more
stable, and allow the center of gravity to be located further towards
the rear of the car. For the opposite reasons do not shorten the
wheelbase.
4. Aerodynamics: The fastest cars tend to be low-slung and
aerodynamic. While the amount of effect from wind drag on a derby car
is a secondary speed factor, it is prudent to recognize the presence
of wind drag and avoid design features that would increase this
effect. Thus, use a basically aerodynamic design, and avoid the use of
accessories that could catch air such as sails, streamers, etc. Also,
rounding off the leading and trailing edges of the car serves to
reduce aerodynamic drag.
5. Symmetry: An important design factor that affects the speed of a
car is left-to-right symmetry. By this I mean that the left side of
the car should be a mirror image of the right side of the car (see
Figure 2 for an example of a non-symmetrical car). Why is symmetry
important? Because a car that is not symmetrical will be heavier on
one side, and will tend to 'steer' to that side. Thus, after the
wheels/axles are adjusted to make the car roll straight, there will be
an increase in friction as the car tries to turn, but the wheels/axles
prevent the turn. This extra friction will slow the car down.
Figure 2 - Non-Symmetrical Car
6. Blunt Point: Avoid designs that have a sharply pointed front end.
There are two reasons for this caution. The first is that most tracks
have round rods that hold the cars at the starting line. A car with a
sharp point will tend to slip off of the rod, which could cause the
car to start at an angle. The second reason for avoiding a sharp point
is that the point may not trigger the finish line sensor until the car
has progressed a little bit past the finish line. This could be
important in a close race. So, make sure that the nose of the car is
blunt so that it will sit properly at the starting line and properly
trip the finish line.
SKETCHING
Now that you know where the weight will go, get some graph paper.
Then in full scale, draw the side profile of the car, then the top
view, the bottom view, and the rear view of the car. Note the
location of weight pockets and holes. If you don't like what you
have drawn, just do it again. It's much cheaper and easier to throw
away ideas on paper than to throw away a partially worked block of
wood!
MULTIPLE PARTS
Once you have decided on a design, determine whether the design can be
cut directly out of one block, or if it would more easily be built
from two or more pieces of wood. For example, the faring on the back
of the car in Figure 3 is a separate piece of wood glued to the body.
In Figure 4, the wings, the side air intakes, and the faring are
separate pieces of wood.
Figure 3 - Car with Faring
Figure 4 - Car with Multiple Added Pieces
DECORATIVE ITEMS
Finally, decide if you will add on any decorative items such as
engines, exhaust pipes, etc. If so, then mark any holes/pockets
required for the item. In Figure 5, the roll cage is constructed from
aluminum rod. To mount the rods, holes were drilled in the top of the
body. In Figure 6, shallow pockets were made to accommodate the pipes
and engine. These pockets ensure that the items are firmly glued to
the car.
Figure 5 - Roll Cage
Figure 6 - Multiple Car Parts
CONCLUSION
Designing a car on paper can be fun, and will give multiple benefits:
1. Save you time by minimizing rework.
2. Achieve better performance by ensuring that weight is properly
accommodated.
3. Simplify construction by identifying holes/pockets that need to be
created, and helping you identify multiple parts.
