The maximum weight for a Pinewood Derby car is 5.0 ounces, but most finished cars will only weigh half that amount. Since heavier cars have more gravitational pull and go faster, experienced builders will add weight to the car to reach maximum weight. Where to place the extra weight is a crucial decision.
Inside or Outside?
Lead weights can be purchased that attach to the outside of car to bring it up to the maximum weight. These can be placed anywhere on the car; however, in order to keep the center of gravity as low as possible, it is best to place them on the bottom. If you do this, make sure the weight is high enough to avoid hitting the bottom of the track. This can create drag or stop the car from leaving the starting line.
Adding weights inside the car creates less wind resistance than external weights. Although only adding a slight aerodynamic advantage, internal weights can add a few tenths of a second that may make the difference between a win and a loss.
Front or Back?
Placing weights on a Pinewood Derby car toward the rear of the body is often the only practical place to add weight. The narrow nose section doesn't allow adequate space for weights, especially internally. Because the front wheels guide the car, having less weight there minimizes the amount of time the car contacts the guide strip (which can slow the car down).
Where to place the weights in the rear of the car can be a process of trial and error. Placing the weights slightly more than one inch in front of the rear axle should help keep the center of gravity as close to the middle of the car as possible.
Materials to use
Weights can be purchased at many online stores dedicated to Pinewood Derby cars or at a Boy Scout Troop supply store. Lead and zinc weights are the densest and are easy to work with. Use caution when working with lead - do not put lead in your mouth, and always wear gloves.
Steel plates and household nuts and bolts can also be used, but attaching them and making adjustments can be very difficult. Holes cut for internal weights can be sealed with automotive putty, which dries within minutes and is easy to sand.