This is a set of two Medium/Soft HPI Molded Rear Inner Tire Foams. HPI Racing Molded Inner Foams are original equipment on the rear of the Baja 5T, as well as the front and rear on the Baja 5SC. These are molded foam donuts that help support the tire for stable performance. The "MS" compound is strong and durable, especially important for proper performance on the rear of the Baja 5T. If your stock molded inner foams are worn or damaged, these will restore grip and handling to the truck.