To build a soapbox racer, cut out a rectangular base frame from plywood. Purchase two pairs of large rubber wheels, and connect the pairs with wooden or metal axles. Mount the axles under the front and back of the base. Add a seat and footrests to the top of the racer. Bolt a long piece of wood to the side of the base; the wood should rotate down to touch the ground and act as a brake.
Handmade soapboax racers can use a variety of wheels. When in doubt, choose 20-inch wheels.
When building the brake, measure from the soapbox racer base to the ground, and multiply the result by 2.5 to get the length of the plank. Mount the plank to the base at the center to create ample braking leverage.
For a more sophisticated soapbox racer, create a steering system. Attach the front axle to the center of the base with a bolt so the axle rotates. Cut two long pieces of rope, and fasten one to each end of the front axle. To steer the car, pull on the rope in the direction of the turn.
Improve the aerodynamics of the racer by building rounded sides and a top out of plywood. For recreational purposes, any shape is functional.