Abstract: A body member of polyurethane plastic has flat bottom, side and end walls and a convex top wall. The body member is of equal width and fully encloses a core of metal having a flat top, bottom side and end walls. The core is uniform in thickness throughout its length such that the thickness of plastic of the body member will vary between the top surface of the core and the convex top wall of the body member. The core is also closer to one side of the body member than to the other to provide varying cushioning hardness. Further, the bottom surface of the block is provided with a plurality of dimples for distributing and absorbing energy in a cold shrinkage function.