| abstract |
At all points Pa in a zone A extending over a distance range from equal to or greater than 1 mm to less than 5 mm from the central point of a core 4, a mathematical formula, Ha 2 −Ha 1 <5, is satisfied. At any of points Pb in a zone B extending over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core 4 , a mathematical formula, Hb 2 −Hb 1 ≧5, is satisfied. Ha 1 and Ha 2 indicate JIS-C hardnesses at points Pa 1 and Pa 2 located radially inward and outward of each point Pa at a distance of 1 mm from the point Pa. Hb 1 and Hb 2 indicate JIS-C hardnesses at points Pb 1 and Pb 2 located radially inward and outward of the point Pb at a distance of 1 mm from the point Pb. |