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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.
Summary
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
A feature article is a regular part of the Pinewood Derby Times Newsletter. To subscribe to this free e-newsletter, please visit: www.maximum-velocity.com/subscribe.htm
