There are two specifications (bulge and roll) you will see omitted from the 2007 Hireko catalog. Although these are important design parameters in woods and some hybrids, we felt that by listing these as standard specifications they where only confusing the clubmaker. But first of all, let’s define what bulge and roll are.

Bulge: The curvature of the face of a wood (or some hybrids) from heel to toe. Bulge aids in compensating for the draw and fade spin imparted on the ball when it is struck on the toe or heel side of the center of gravity.

Roll: The measure of face curvature from crown to sole on wood (or some hybrid) clubs.

As you can see on any wood, the face is not flat, but has a slight curvature in both the horizontal and vertical planes. Bulge and roll are measured in inches, where the majority of drivers and fairways are built with 10 inch bulge and 10 inch roll. To provide an explanation, imagine a circle 10 inches in diameter. The arc or chord of that circle for the distance of the face would be the same as the club in the heel to toe or sole to crown plane. A circle with a 12 inch radius would be larger and subsequently a reduced amount of curvature where as an 8” circle is smaller in diameter and would be more curved.

In the manufacturing process, there is also a slight tolerance for bulge and roll too. The tolerance is usually +/-1 inch, which means a club with a design specification of 10" could be anywhere in-between 9 and 11 inches and be totally acceptable without showing adverse affects in directional control.

Any club fitting book will explain the gear effect on a ball when it is hit toward the toe or heel side the center of center gravity of the face and how the proper curvature of the face will help curve the ball back to the intended target line. But how do you know what the proper bulge or roll is for you or your customer? The answer is you do not know, because manufacturers do not make clubs with multiple versions of the same head with varying amounts of bulge and roll like they do loft. If they did, clubmakers would find out that one of the club heads would be predominately chosen amongst the others because it performed better. This is due to the fact that bulge and roll should be built into the club based upon the distance the center of gravity of the head is away the striking surface of the face.

In addition, adjustable weighted heads are very popular today. The effect of changing the weights can alter the distance the center of gravity is away from the face. Yet, no one discusses the negative effect of bulge and roll characteristics to the ball flight. Why? The small center of gravity shift probably doesn’t affect inducing more or less gear, especially in light of modern 460cc heads with a high moment of inertia.

The conclusion is don’t fit by bulge and roll or get caught up in the measurements. Just because you hit one club better that had a 10" bulge than one than which had 12" may have been due to several other factors. Ultimately it is up to the clubhead designer to incorporate the proper amount into the clubhead design.

by Jeff Summitt
Hireko Technical Director
jsummitt@hirekogolf.com