This Is The Replacement Walking Belt For The Free Spirit Serial 30131. When it comes to replacement walking belts Treadmill Doctor has been a leader in the fitness industry. Our walking belts are made from quality polypropylene materials, 2 ply construction with the backing made of polyester blended material. A warranty for our walking belts is offered depending on your setting you may want to opt for the Lifetime warranty when in a residential setting and the 1-year warranty in a commercial setting.