Abstract: An annular bladder providing both circumferential expansion and rolling movement normal to its circumference in response to inflation thereof, said bladder having a cylindrical outer wall extending between a forward edge and a rearward edge and being turned radially and axially inward at the respective edges, the bladder comprising a first and a second circumferential zone and in said outer wall a transition zone, said first zone extending in a forward direction from said transition zone, said second zone extending in a rearward direction from said transition zone, said first zone having greater resistance to circumferential elongation relative to the second zone and said second zone having lesser resistance to circumferential elongation relative to the first zone, said transition zone being located a predetermined distance forward of the rearward edge of said bladder when the same is uninflated.