Golf club head with flexible sole

1. A hollow body golf club comprising: a body having a front end; a back end opposite the front end; a crown; a sole opposite the crown, the sole defining a sole surface; wherein a ground plane is tangent to the sole surface when the golf club head is at an address position to strike a golf ball; a heel; a toe opposite the heel, and a hosel structure having a hosel axis extending centrally through a bore in the hosel structure; a strikeface positioned at the front end and defining a geometric center, and a loft plane tangent to the geometric center; wherein the geometric center further defines a coordinate system having the geometric center; the coordinate system comprising an X axis extending through the geometric center between the heel and the toe; a Y axis extending through the geometric center and perpendicular to the X axis, between the crown and the sole; a Z axis extending through the geometric center and perpendicular to the X axis and to the Y axis, between the front end and the back end; the Y axis and the Z axis together define a YZ plane extending between the crown and the sole and between the front end and the back end; the X axis and the Z axis together define a XZ plane extending between the heel and the toe and between the front end and the back end; an indented region; wherein the indented region is defined where the sole veers inward in a direction toward the internal cavity; a sole transition point defined by an intersection of the sole and the strike face; the indented region comprises a sole curvature profile defined by an intersection of the sole surface and the YZ plane; wherein the sole curvature profile comprises a first inflection point and a second inflection point; wherein the sole curvature profile comprises a first concave section extending from the sole transition point to the first inflection point and is concave relative to the XZ plane; a convex section extending from the first inflection point to the second inflection point and is convex relative to the XZ plane; a second concave section extending from the second inflection point to the back end and is concave relative to the XZ plane; wherein the first concave section comprises a radius of curvature (R 3 ); the convex section comprises a radius of curvature (R 4 ); the second concave section comprises a radius of curvature (R 5 ); wherein the sole curvature profile further comprises a nadir and a nadir height ratio; wherein the nadir represents a point on the sole curvature profile that is closest to the XZ plane; wherein the nadir is located on the convex section; wherein the nadir height ratio is defined as the ratio of the nadir height to the radius of curvature (R 3 ) of the first concave section; and wherein the nadir height ratio is less than 0.33.

2. The golf club head of claim 1 , wherein the nadir height ratio is between 0.001 and 0.05.

3. The golf club head of claim 1 , wherein the radii of curvature R 3 and R 5 are equivalent or greater than R 4 .

4. The golf club head of claim 1 , wherein the radius of curvature R 3 is at least twice of the radius of curvature R 4 .

5. The golf club head of claim 1 , further comprises an depth and a nadir depth;

wherein the depth of the club head is measured as the furthest point from the front end to the back end, in a direction parallel to the Z axis; and

wherein the nadir depth is measured perpendicularly from the loft plane to the nadir.

6. The golf club head of claim 5 , further comprises a nadir depth ratio;

wherein the nadir depth ratio is defined as the ratio of the nadir depth to the depth of the club head.

7. The golf club head of claim 6 , wherein the nadir depth ratio is greater than 0.35.

8. The golf club head of claim 7 , wherein the nadir depth ratio is between 0.40 and 0.60.

9. The golf club head of claim 1 , wherein the radius of curvature R 5 is greater than 10 inches.

10. The golf club head of claim 6 , wherein the first inflection point comprises a depth;

wherein the first inflection point depth is measured between the loft plane and the first inflection point along a direction perpendicular to the loft plane.

11. The golf club head of claim 10 , further comprises an inflection point depth ratio;

wherein the inflection point depth ratio is defined as the ratio of the first inflection point depth to the depth of the club head.

12. The golf club head of claim 11 , wherein the inflection point depth ratio is greater than 0.25.

13. The golf club head of claim 12 , wherein the inflection point depth ratio is between 0.25 and 0.45.