Summary: A strong thread, attached to the ball weight - the attached thread goes around the top pulley and back down to the rear axle. The thread is wound around the gear on the rear axle. Notice that this car has a rear axle, connecting the rear wheels together and keeping them perpendicular to track. Axles are generally illegal for cub scout pine wood derbys, but not
Image Dimensions: 901 x 421
Image originally found here.