The present invention relates to sports racquets, for example tennis, squash, badminton, and racquetball racquets. Such racquets have a head portion containing an interwoven string bed, a handle, and a shaft portion connecting the head portion to the handle.
High performance sports racquets normally have a frame made with a composite material such as carbon fibers embedded in an epoxy resin (known as a “graphite” frame). Other materials, such as tungsten or titanium, may also be included in the frame, for example at select locations.
Advances in materials technology permit racquet frames to be made lighter and stiffer, and also allow the racquet to be designed with characteristics geared towards certain types of players. For example, it is well known that the addition of weights at various portions of the racquet head can affect the playing characteristics of the racquet. Adding weight at locations spaced from the longitudinal axis will increase the polar moment of inertia, making the racquet more stable in the case of off-center hits. Adding weights to the head portion also will increase the mass moment of inertia (swing weight), making the racquet more head heavy. Depending on where the weights are located, such weights may advance the sweet spot (center of percussion) and increase the moment of inertia about the center of gravity.
It is also well known that the frame design can be varied to affect the stiffness of the racquet. For example, increasing the cross-sectional frame height tends to make the racquet stiffer in bending. Making the frame more box-like increases torsional stiffness. The bending as well as torsional stiffness of the frame are also affected by the orientation of the carbon fibers in the composite material. Thus, if greater bending stiffness is desired, the layers of composite material forming the frame can be oriented so that more of the carbon fibers extend axially. If greater torsional stiffness is desired, the layers of composite material that make up the frame are oriented so that more of the carbon fibers are oriented at an angle relative to the longitudinal axis.
While it is known that the properties of a sports racquet can be changed through these various techniques, the design of a sports racquet is complex due to the way that forces are transmitted through the racquet. For example, when a ball strikes the string bed, the strings partially transmit the energy of the ball impact to the head portion of the frame through torsion. Such force is transmitted to the handle through a combination of torsion and bending, depending upon the particular the location on the frame.
Racquet design under the current state of technology is based on diverse optimization criteria with the goal of principally obtaining frames which are both rigid and lightweight. Such features are important in that they provide greater control of the sports racquet and a reduction of the force that the player's arm must apply.
The known methods for reducing the weight of the racquet are predominantly the use of improved materials and varying the frame shape.