Our Air (Pneumatic) Tires and Struts are designed to fit our Medium and Large Rear Walkin’ Wheels Wheelchair Frame. The metal struts snap into the leg of the wheelchair frame to help adjust to the height needed for your pet. Struts are adjustable within a range of 3 inches and come in 4 different sizes: 6", 9", 12", & 15". Air-filled pneumatic tires are recommended for very active dogs, These tires will improve traction, and are greate for off-road use. Great for hiking on rough surfaces. These tires are like mountain bike tires and have a deep tread and a replaceable inner tube. Tire Sizes available: 12" & 16".