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Designing the Car Body for Performance

I am continually amazed at the creativity of pinewood derby builders. My
talent (and the talent of my kids) tends toward straight lines and basic
geometric shapes. But there is clearly a lot of artistic talent among car
builders. Cars take all shapes from minimalist sleek designs to artistic
wood carvings. One of the fun aspects of the derby is the wide variety of
creative cars that are entered into the race.

The intent of this article is not to stifle this creativity. Instead, the
intent is to raise awareness that the fastest cars tend to have certain
design features in common. Although cars of all shapes and sizes have done
well in derby races, my observation is that overall, winning cars tend to
have several basic characteristics. Whether or not you design a car with
these characteristics is entirely up to you.

Aerodynamics - The fastest cars tend to be low-slung and basically
aerodynamic. While the amount of effect from wind drag on a derby car is
debatable, it is prudent to recognize the presence of wind drag and avoid
design features that would increase this effect. So, while I don't suggest
being overly concerned with wind drag, I do suggest using a basically
aerodynamic design, and avoiding the use of accessories that could catch air
such as sails, streamers, etc.

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. (For an example of a
non-symmetrical car, click here:

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.

Blunt Point - Avoid designs that have a sharp 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.

Minimal Wood - (This section applies only to cars that will run on a track
with a beginning slope, followed by flat section. This does not apply to
cars that will run on a track that consist of one continuous slope). The
center of gravity (COG) of the car should be positioned about 1" in front of
the rear axle. This gives the car the greatest opportunity to accelerate
down the slope while ensuring enough weight on the front wheels to keep the
car on the track (The science background for this principle will be
discussed in a future article). In order to locate the COG at 1" in front of
the rear axle a lot of the wood must be removed, and weight must be added
around the rear axle. Thus, make sure your design leaves room around the
rear axle for the added weight. Also, it is generally best to keep the car
at the maximum allowable length (typically 7"). With a shorter car, it is
difficult to add weight (unless you use a very dense material such as
tungsten) such that the COG is properly located.

Weight Amount - Design the car so that it will achieve the maximum weight
(typically 5 ounces). Thus, decide ahead of time what type of material you
intend to use for added weight and then ensure that the design can
accommodate sufficient added weight to bring the car to the maximum weight.

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 COG to be located further towards the rear of the car (faster on
tracks with a beginning slope, followed by a flat section). For the
opposite reasons do not shorten the wheelbase.


The basic design characteristics listed above are very common in high
performing cars. By implementing these characteristics, you will be on the
path of creating a competitive pinewood derby car.

From Pinewood Derby Times Volume 1, Issue 5

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