A soap box derby racer is a gravity-powered racing vehicle used in soap box derby competitions by children and young adults. The style of vehicle gets its name from the materials used to build the cars during the sport's early years -- soap cartons and orange boxes. Today, the All-American Soap Box Derby organisation has officially regulated their design and construction, and all racers must build their cars according to "stock," much like modern NASCAR vehicles.
Familiarise yourself with the rules of soap box racing. The All-American Soap Box Derby rules state that a soap box derby racer cannot exceed 90.7 to 113 Kilogram in weight, including the driver, depending on which division you are competing in. You cannot officially source certain parts of any racer, such as Z-Glas wheels and AASBD-stamped axles. You must build the vehicle in accordance with the most current construction plans published by the International Soap Box Derby company. Download the official rule book from the All-American Soap Box Derby website.
Purchase official parts from the All-American Soap Box Derby website to build your soap box derby racer. These are available in stock or superstock. Purchase the appropriate parts according to the class you intend to race in. These kits contain all the parts you'll need to build a stock soap box derby racer, except the Z-Glas wheels, which you must buy separately from the same website.
Download and print out the plans for your class of soap box racer. These will explain how to build your racer so that it complies with the regulations of the official All-American Soap Box Derby organisation.
Assemble your soap box racer. Start with the floorboard, which is the vehicle's wooden chassis. Follow the instructions for drilling and installing the steering, brakes and bushing in the wooden floorboard. Then bolt on the rear and front axles and brake assembly. Do not tighten these axle bolts yet.
Attach the wheels to the front and rear axles of your soap box racer. Slide them on and secure them with metal clips. Place one of the included washers between the Z-Glas wheel and the clip securing it to the axle.
Elevate the floorboard of your racer on wooden support boards and lower the plastic racing shell over the chassis. It should fit snugly. Check that all braking and steering cables can move freely, then attach the plastic racing shell to the floorboard with screws and washers. Start with the screw at the nose of your car, then screw the shell through all other remaining screw holes. The All-American Soap Box Derby rules state there can be no gap between the plastic shell and the wooden floorboard of your vehicle at the front.
Test that the steering and brakes work well. Make sure the soap box racer's body does not snare or obstruct the steering and brakes. Add foam passing around the rim of the soap box racer body for the comfort of the driver, and add any official or sponsorship decals to the exterior of your racer.
Align the front and rear axles with a long, straight wooden ruler to ensure your soap box derby racer travels in a straight line. Adjust the front and rear wheels so they're balanced and level, then tighten the axle screws you left loose earlier to secure the alignment against shifting.
Go through the checklist supplied in the soap box derby racer plans to ensure you have installed each component correctly. Make final adjustments to optimise performance -- including adding weights, airfoils and spindling axles. Double-check all modifications against official All-American Soap Box Derby plans and rules to ensure your racer is not considered "modified" and disqualified.
All-American Soap Box Derby rules state that the driver must build his own competing soap box derby racer and must be under the age of 18 when competing. The rules allow adult supervision, but there should be minimal assistance in constructing your soap box racer to avoid disqualification.