Our closed tunnel starts with a heavy-duty, blue plastic barrel, which is completely surrounded with a welded steel frame and then two parts are firmly attached together. The inner diameter of the barrel is slightly over 22", and the barrel length is 24". The back edge of the barrel has velcro around it, allowing easy attachment or removal of our nylon sleeve. Inside the barrel, we add traction strips to reduce slipping. For long-lasting service, the welded steel frame is finished with dark blue powder-coating. We offer either red or blue nylon sleeves, allowing you to choose an all blue obstacle or blue with a contrasting, red sleeve.