Where to place weights on pinewood derby car

Understanding the physics of weight placement

The key to a fast pinewood derby car lies in understanding and manipulating physics, specifically inertia and potential energy. Weight placement dramatically affects how these forces impact your car's performance. The goal is to maximize the potential energy at the starting line and minimize the energy lost to friction and air resistance as the car races down the track.

A car with a higher potential energy (achieved through strategic weight placement) will translate that potential into kinetic energy more efficiently. This kinetic energy is what drives the car down the track. If that energy is dissipated (through friction, wobble, poor aerodynamics) the car will slow down.

The ideal center of gravity (cg)

A lower and further back center of gravity is generally preferred for pinewood derby cars. Imagine a pendulum; the lower the weight is on the pendulum, the faster it swings. The pinewood derby car acts in a similar way, though on an inclined plane. Aim for a center of gravity roughly 1 inch in front of the rear axle.

Why rearward? This helps to keep the front wheels raised slightly as the car goes down the track. Reducing the contact of the front wheels against the track reduces friction and increases speed. A more forward CG will keep all four wheels firmly planted, increasing drag and slowing the car.

For example, let's say you have a 5 oz car. Placing 4 oz of weight near the rear axle and 1 oz distributed elsewhere will help shift the CG further back.

Selecting the right weight materials

Density is crucial when selecting weight materials. Denser materials allow you to concentrate more weight in a smaller area, further contributing to a lower and more strategically positioned center of gravity. Popular choices include:

• Tungsten: The densest readily available material, offering the best weight-to-volume ratio.
• Lead: Historically popular but now often prohibited due to environmental and safety concerns. Check your local derby rules!
• Steel: A good, readily available option, though not as dense as tungsten.

Consider using a combination of materials. For instance, you could use tungsten for the bulk of the rear weight and smaller pieces of steel for fine-tuning the weight distribution.

Weight placement techniques

There are several methods for securing your weights to the car body. Some common techniques include:

• Drilling and Inserting: Drill holes into the car body and insert weights (e.g., tungsten cylinders) secured with glue or epoxy. This allows for precise placement and hides the weights.
• Adhering with Epoxy: Use epoxy to glue weights to the underside or rear of the car. This is a simpler method but may not be as aesthetically pleasing or secure.
• Weight Pockets: Carve out pockets in the wood to hold weights. This method keeps the weights flush with the car's surface.

Regardless of the technique you choose, ensure the weights are securely fastened. A loose weight shifting during the race can drastically affect your car's performance and stability.

Fine-tuning and testing

After placing the weights, it's essential to test and fine-tune your car's performance. Set up a test track (even a small inclined plane will do) and observe how the car behaves. Make adjustments to the weight placement as needed. Pay attention to the car's tracking, wobble, and speed.

Slight adjustments can make a significant difference. For example, even moving the center of gravity a fraction of an inch can noticeably affect the car's speed. Keep track of your changes and their impact on performance to optimize your weight placement.