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.
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 3/4 and 1 inch 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.
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!
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
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
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.
Okay, let the drawing begin!
From Pinewood Derby Times Volume 5, Issue 1
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