I haven’t heard much discussion for years on bulge and roll of a club head, but the subject has come up rather often lately.  It is becoming a point of discussion because of various OEM commercials.  In the past when clubs were made of persimmon wood it was somewhat common to hear discussion of bulge and roll.  This was because it could easily be changed by sanding down the head.  With today’s steel and titanium heads changing the bulge and roll is impossible.

Before we go further we need to explain what bulge and roll are or aren’t, and how they effect the shot.  First, only woods have bulge and roll.  Irons do not.  All wood heads curve side to side, and up and down.  Bulge is the curvature of the face of a wood head from its heel to its toe, or the horizontal plane of the face.  Roll is the curvature of the face up and down, or the vertical plane of the face.

Bulge is the built in correction or compensation factor which under normal circumstances at impact starts the ball farther to the right on toe shots and farther to the left on heel shots in order to compensate for ball sidespin caused by such off center shots.  Bulge is one of the more misunderstood of wood design specifications.  Part of the reason is that there are two long time myths associated with it.  First, the proper bulge either adds or reduces spin to the shot.  Well, it doesn’t do either.  It starts the ball farther to the right on toe shots and farther to the left on heel shots.  Bulge is a correction or compensation factor for the clubhead’s center of gravity that causes hooking or slicing sidespin on off center shots.  Second, it has been thought that when a shot was hit off center toward the toe, the clubhead would turn open by turning around the shaft.  The clubhead does turn on toe shots, but it rotates around the head’s center of gravity not the center of the shaft.

On off center shots the clubhead rotates around its center of gravity causing a sideways movement of the point of contact.  This places sidespin on the ball, which normally causes an undesirable hooking or slicing to take place.  Bulge compensates for this hooking and slicing sidespin by intentionally starting the ball farther out to the right on toe shots and left on heel shots.  This allows the hooking or slicing spin to bring the ball back toward the target.

The curvature of the face is measured in inches by the radius of the circle it is cut to.  An example would be a bulge of 12 inches or in other words a face shaped to the radius of a 12 inch circle.  It is well understood more bulge is required the further the center of gravity is away from the face.  So, if we have a head with a center of gravity near the face with a bulge of 12 inches, if the center of gravity is located to the back of the head, bulge may need to be 9 inches.  This is why metal wood heads appear flatter, because they are flatter.  With a center of gravity much closer to the face metal woods require less bulge.  Determining the correct amount of bulge is difficult.  The ideal amount of bulge is primarily determined by the location of the center of gravity, but is based on other factors as well.

If the ball is struck in the middle of the head bulge does not come into play, the bulge is irrelevant to the shot.  It only has effect on the shot when the ball is struck off center. 

Roll is the vertical radius of the face measured from the top of the head to the bottom and is the same from heel to toe.  Roll affects loft in the vertical plane on the clubface.  A ball hit high on the face will fly higher and a ball hit low on the face will fly lower.  Back when clubmakers were experimenting and found that shots were improved by giving the face bulge, not understanding why, they thought they should give the face curvature in the other direction.  Frankly, roll only makes a ball hit high on the face go higher and a ball hit low on the face go lower.  In my opinion, if the face had no roll it would be better.  But, part of the problem in eliminating roll is the face will appear to be up to 2 degrees lower lofted than it's actual loft.  The other part of the problem is unfortunately, metal woods roll cannot be changed.

The commercials mentioned above speak as if their club is the only club with bulge and roll.  In all the years I have been building and repairing clubs, I have never seen a wood head without bulge.

If you would like the bulge and roll of your club checked contact us here at Pat Ryan Golf