Wheel bearing apparatus for a vehicle

A vehicle wheel bearing apparatus comprising:

• an outer member formed with double row outer raceway surfaces on the outer member inner circumference;
• an inner member formed with double row inner raceway surfaces on the inner member outer circumference opposing the double row outer raceway surfaces of the outer member;
• double row balls freely rollably contained between the outer and inner raceway surfaces and the balls adapted to be applied at a predetermined contact angle on the outer and inner raceway surfaces;
• at least the inner raceway surfaces of the outer and inner raceway surfaces are formed as circular arc sections, each circular arc section having two radii of curvatures, each inner raceway surface circular arc section is formed by a first circular arc section and a second circular arc section, the first circular arc section with a first radius of curvature, the first circular arc section with the first radius of curvature extends between an inflection contact point (P0) to the raceway bottom and during steady travel of the vehicle, the ball rides on the first circular arc section at a contact point (P) which contacts the ball at an initial contact angle α, and the second circular arc section with a second radius of curvature, the second radius of curvature is smaller than the first radius of curvature, the second circular section with the second radius of curvature extends between the inflection contact point (P0) to the radially outer side, the contact point and inflection point are different, the ball contacts the inflection point at a contact angle (α1), during cornering, a large moment load is applied to the ball, the contact point of the ball moves over the inflection point (P0) onto the second circular arc section resulting in securement with a greater preload, the first and second radii of curvature are set, respectively, as 1.05 d<2×first radius of curvature ≦1.10 d and 1.01 d<2×second radius of curvature ≦1.05 d (wherein “d” is a diameter of the ball) and the difference (Δα) between α1 and α being set in the range of 2° to 8° prevents the generation of excessive pre-load during steady travel of the vehicle and a deficiency of the pre-load does not occur during curved travel of the vehicle when the inflection contact point of the first and second radii of curvature is positioned at α1=α+Δα (α=30 to 45 degrees).