Sports racquet frames include a head, which supports interwoven main and cross strings for hitting a ball, and a handle which is gripped by the player. The conventional method of string placement and support is to drill holes through the racquet frame, and secure the strings in the string holes so that the main strings extend parallel to the longitudinal axis, and the cross strings extend perpendicular to the racquet axis.
One of the problems of conventional sports racquets is that the power, or coefficient of restitution, varies at different locations on the string bed. For example, due to the fact that the typical racquet head has a generally elliptical shape, the laterally offset, outlying main strings are much shorter than main strings which are located closer to the center of the racquet head. The result is that the racquet has greater power near the longitudinal axis than at laterally offset locations.
This is exactly the opposite, however, of what is desirable. When the ball hits the string bed laterally off-center, the racquet tends to twist in the player's hand, resulting in a loss of power for the return stroke. Yet, the string bed, rather than providing greater power in these regions which might offset some of the twisting movement, also provides less power due to the shorter main string lengths.
The power of the racquet also tends to be non-uniform along the length of the racquet, decreasing toward the outer tip of the racquet. This is due to two reasons. First, the further out on the string bed the ball hits, the greater the bending moment about the handle. Second, when the ball hits the racquet at a location away from the center of gravity, a rotation is imparted to the racquet, which absorbs energy and reduces power, i.e., the amount of energy returned to the ball. The further out on the racquet the ball lands, the greater the rotation imparted to the racquet.
It is difficult in racquet sports to ensure that the ball hits the racquet strings at exactly the same spot each time. Thus, for two identical swings, the ball will rebound at different speeds depending upon where it contacts the strings. This means that the distance the ball will travel before hitting the court varies, and the player must therefore allow for a greater margin of error to account for inaccuracies in the return shot.
Commonly owned Head U.S. Pat. No. 3,999,756 discloses a string pattern that compensates for the non-uniform power distribution across the string bed by varying the spacing between strings. Head discloses that, due to their shorter lengths, the strings nearer to the racquet frame are stiffer than the strings in the middle of the string bed, e.g., the center main strings. Head discloses that, by increasing the string density near the center of the racquet, the string bed will have a more uniform feel.
It has also previously been proposed to provide racquetball racquets with a "sunburst", or fan-shape string pattern, in which the distance between adjacent main strings increases toward the outer end of the racquet. This has the effect of lengthening the outlying main strings slightly, and therefore acts to reduce somewhat the variation in power across the strung surface. The distance by which the lower ends of the strings can converge, however, is limited by the fact that only so many holes can be drilled into the frame, in a confined area, without weakening the frame to the point that the racquet fails during play. Thus, known sunburst patterns have relatively little effect in compensating for unequal main string lengths.
There has also been at least one proposal to compensate for the variation of power along the axis of the racquet. Commonly owned Davis U.S. Pat. No. 5,037,098 discloses a sports racquet frame in which the cross-sectional height of the frame, in a direction perpendicular to the string bed, is maximum at the tip and decreases continuously toward the handle. The result is a frame which becomes increasingly stiff toward the tip, helping to compensate for the increased bending moment.
It would be desirable to provide a string bed pattern which at the same time produces more uniform power characteristics both across the string bed and along the racquet axis.