Spitfire Formula 4 Radial Wheels. 53.5mm x 33.7mm, 99A. Formula 4: unbeatable lasting performance Spitfire urethane. Fast.Long-lasting, abrasion resistance, smooth anti-stick slide with a controllable grip. Radial Shape: A round edge, medium width riding surface shape. Offers the versatility of the Classic shape and the width of a conical.