Golf clubs, particularly golf irons, comprise a head with a hosel and a shaft which is attached to the head by fitting the shaft into a bore in the hosel. The hosel is attached to and integral with the head. The head includes a heel, a bottom sole, a toe, a planar striking face, and a backside. The head of a conventional golf iron has a longitudinal length heel-to-toe substantially greater than the width between the striking face and the backside. On the other hand, woods generally have a relatively low length-to-width ratio. Also, the toe side of the striking face in a conventional golf iron is higher than the heel side of the striking face. This keeps the center of gravity in the center of the blade.
Golf irons typically include a set of eleven irons, numbered one (long) through nine (short), a pitching wedge, and a sand wedge. The eleven irons of a set conventionally have varying degrees of loft angle and lie angle. The loft angle of an iron is the angle between a vertical plane which includes the shaft, and the plane of the striking face of the iron. The lie angle of an iron is the angle between the shaft and the ground (horizontal plane) when the tangent to the sole directly under the center of mass is in the horizontal plane and when the shaft lies in a vertical plane.
The loft angle, as the name suggests, determines how much loft is imparted to the ball when it is struck by the tilted striking face. The lie angle of the iron assures that, when the golf club is swung properly, the sole of the iron will contact the ground evenly so that the striking face will not tend to twist inwardly or outwardly.
The toe defines a side face on the head between the striking face and the backside. The side face in an iron extends generally back and toward the heel. The face of the toe in top view thus defines a curvilinear line between the striking face and the backside. This is known as negative draft. The outermost point on the toe (compared longitudinally with respect to the center of mass) is typically at or near the edge between the toe and the striking face. The toe in known negative draft golf irons generally extends no further away longitudinally from the center of mass than a line perpendicular to the plane of the striking face and tangent to the outermost point on the edge between the toe and the striking face. For example, the edge on some irons is beveled and this results in the outermost point being on the outside edge of the bevel.
In another aspect of a negative draft golf iron, a straight line approximates the face of the toe and is defined by passing a horizontal plane through the head. Two points on the plane define the line. The first point is the intersection of the plane with the toe edge of the striking face. The second point is the intersection of the plane with the toe edge of the backside. These two points in the horizontal plane define the imaginary line across the side face. The line across the side face and the plane of the striking face together form an angle which is 90.degree. or less. In known negative draft golf irons, the outermost point on the toe is at or near the edge between the toe and the striking face.
For example, U.S. Pat. No. 1,532,545 illustrates a golf club head with negative draft in which the length-to-width ratio is substantially high. As shown in FIG. 3 of the '545 patent, the toe face tapers back from the planar portion of the striking face toward the heel of the club. Generally, the negative draft design facilitates casting of the head and removal of the head from a mold.
U.S. Pat. No. 2,447,967 describes a negative draft golf club head in which the length-to-width ratio is relatively low. The toe face tapers back and toward the heel at an angle of 90.degree. or less.
U.S. Pat. No. 1,582,836 describes a metallic golf wood head with a convex toe. The head is hollow and includes an abutment behind the striking face to provide strength and to provide offset and take up the shock from hitting a golf ball. A counterweight is cast into the rear portion of the head. This head, as with solid heads, places the mass of the club primarily behind the head. The amount of mass on the toe does not significantly impact the performance of the club.
U.S. Pat. No. 3,035,839 describes a multipurpose golf club which has a length-to-width ratio between those of conventional woods and irons. As with the wood described above, the toe is convex. The head is provided with a first sole surface on an outer toe portion and a second sole surface on a heel portion. The first sole surface intersects the striking face at a first acute angle. The second sole surface intersects the striking face at a second larger acute angle. With the second sole surface resting on the ground, the striking face has a substantial amount of lift. With the first sole surface, the pitch of the striking face is reduced. Depending on which sole surface is positioned on the ground, the club can be used for long putts and chip shots or relatively short putts. The weight of the head is concentrated toward its heel end to aid the player in making relatively long shots.
With many known golf clubs, it is desirable to hit a golf ball on the "sweet spot" or in line with the center of mass of the club. Golf balls hit with such centered shots have truer flights and travel longer distances than off-center shots. Hitting the ball slightly off-center on the striking face creates problems with the control of the direction and flight of the golf ball. The golf club tends to twist and vibrate, which absorbs the striking energy of the golf swing. The twisting and vibration impart undesirable spin to the golf ball as it leaves the striking face. Spin causes the ball to hook or to slice. The ball does not travel in the direction desired by the golfer nor does the ball travel as far as it would for a solid, on-center shot. A golf club with a larger sweet spot on the striking face helps compensate for an off-center shot by a golfer. Generally, a club with a larger sweet spot has an increased mass moment of inertia about the center of mass. The effect of an increased mass moment of inertia is a reduction of the twisting and other vibrational forms of energy loss experienced from a shot hit on the striking face at a point other than the center of mass.
There are several known methods of increasing the mass moment of inertia for a golf club. These methods include increasing the weight of the club and re-distributing the weight of the club. However, increasing the mass moment of inertia of a golf club head is restricted by a number of factors that relate to the playability requirements for a golf club. Increasing the weight of the club is not always possible nor desirable. For example, the total mass of the club head is determined by the shaft length, the grip weight, and the desired swing weight. Increasing the mass of the head may result in a heavier than desired swing weight. A golf club which is too heavy cannot be swung with a proper stroke. This results in off-center shots, loss of control, and shorter flights of the golf ball.
Another method of increasing the mass moment of inertia concentrates the majority of the club weight at the heel and the toe. Such a design reduces the golf club's tendency to twist when the golf ball is hit off-center. In a golf iron, such weight distribution is accomplished by providing a cavity in the back of the head of the golf club. The mass removed from the cavity of the club is placed around the perimeter of the backside and concentrated at the heel and the toe.
U.S. Pat. No. 4,621,813 describes a set of golf irons having such a cavity back. The material which would otherwise be in the cavity is redistributed in the heel and toe. The redistributed material improves the resistance to twisting of the club head upon off-center impact with a golf ball. Further, these golf irons have negative draft toes.
U.S. Pat. No. 3,059,926 describes a golf iron having an increased moment of inertia. A plug of material is added on the backside near the toe. This club also is a negative draft golf iron with the toe face forming an angle of 90.degree. or less with the striking face.
However, other playability requirements also affect the distribution of mass in the golf club. These include the length and shape of the sole of the club and its contact area with the ground during a golf swing or shot. For instance, a blade with a longer length has its mass positioned farther from the center of mass. But a longer length blade has an increased sole length and this results in significantly higher ground drag. High ground drag means that the club is dragging on the ground during the shot. This may result in a slower club speed during the stroke through the golf ball and in a decreased flight distance for the golf ball. Also, a longer leading edge may inhibit the taking of a proper divot.
Thus, there exists a need in the art for a golf iron having an increased moment of inertia without the loss of desirable club dimensional and playability characteristics.