Is string suspension the "holy grail" of tennis racquet design? Considering the 100 year long history of failed attempts to achieve it in a commercially viable racquet and its potential to transform the foundations of racquet design, it would be hard to argue it isn't. Almost from the beginning, manufacturers have considered various ways to achieve string suspension on a tennis racquet. Some of the earliest patents for improvements to racquets, dating as far back as 1888, include string suspension concepts.
As tennis became popular in the late 1800's, players realized that hitting a ball hundreds of times a day was hard on the body (tennis balls of the era were especially hard so that didn't help!). It was natural that engineers were imagining ways to mitigate ball hitting impact in the racquet and they recognized early on that the most direct and impactful way to address the problem was with string suspension of some kind. Suspension in principle was being explored in the automotive industry at the time as a method of separating wheels from chassis, allowing them to move independently of each other and providing comfort to passengers, so to adapt it for use in a tennis racquet was not a stretch for a creative industrialist. In engineering terms for this discussion, true string suspension on a racquet is the separation of the strings from the frame in such a way that allows the strings to move freely and independently of the frame in response to ball impact.
It should be emphasized that string suspension is not merely separating the strings from the frame. Separation is easy and can provide some degree of vibration dampening. The typical grommet strip of a modern composite racquet is a "separator" of sorts, acting as a gasket between the strings and the frame. The leather pads that Roger Federer has located under the main strings at the throat of his racquet provide separation with a degree of vibration dampening, but to separate the strings from the frame while at the same time allowing them to move freely in response to ball impact offers an entirely new kind of performance. It's the ideal model of string suspension on a tennis racquet. It's also the most difficult to achieve because it requires a sufficiently strong independent support structure for the strings which adds weight, and it's excess weight that makes execution of true string suspension in a commercially viable high-performance racquet, nearly impossible to achieve. Adding even a small amount of extra weight to the head frame is difficult to do without making a racquet unwieldy and difficult to swing.
There are some truly amazing racquet suspension designs in the past but the majority are complicated and often not practical. It's not hard to imagine how heavy and difficult to swing some would have been! Many were simply limited by available methods and materials, and being difficult to make at scale was an ever present problem. An incredible early design that achieves string suspension is described in a patent from 1888! (below left) It's from so far back that the racquet shown is in the shape of a type used to play "court tennis", a precursor to the modern game that we know today. Amazingly enough, the Phelps patent incorporates an air tube which wraps around the outside edge of the frame and provides a way for the strings to encompass the tube when woven through the frame. In theory, when the air tube is inflated the strings become separated from the frame and the inherent flexibility of the air tube would allow the tension in the strings to drop upon ball impact. Even in the days of court tennis, racquet makers were going to great lengths to incorporate string suspension into their designs!
In 1921 when Ambrose Ryder applied for a tennis racquet patent (above right), racquets were mostly made of wood and much heavier than racquets today. In fact they were likely much too heavy for most players already so to add even more weight on the head was nearly absurd. Ryder removed portions of a racquet frame to enable an an air tube to wrap around the head frame. In fact, so much material is removed it doesn't seem like enough is left to make the frame! The strings are woven through small metal collars arranged around the perimeter of the air tube. Not only has Ryder achieved separation of the strings from the racquet frame but he also enabled the strings to move independently of the frame by using an air tube for the string support structure. It's a remarkable concept that was severely hampered by its extra weight and, obvious to us now, by the materials and methods of manufacture available at the time.
A simpler approach to suspending the string-bed is shown in US Patent 1,542,177 (pictured below). Suspension here is achieved using small metal ribbons formed into near half-circles to support the string loop on the outer rim of the head frame. The idea is for the metal ribbons to compress and deflect under increased string tension due to ball impact adding a buffer and "spring action" to the response.
Another design to use an air tube like Phelps and Ryder was a patented racket from 1929 (below right). The air tube is wrapped around the perimeter of the head frame while a section of strings in each "corner" of the string-bed are allowed to wrap around the frame and essentially rest on the air tube. It's easy to see that this was going to be heavy and unwieldy, and it's definitely problematic to have an air tube exposed to damage but it was clever and it certainly could have worked to soften the string response. There are many other brilliant string suspension designs in the history of tennis racquets and together they illustrate the tremendous time, energy and resources that have been expended in pursuit of the important technological milestone of string suspension.
As materials technology and methods of manufacture have advanced, string suspension designs for racquets have advanced along in unison. US Patent 3,884,467 of 1975 (below left) marks a shift in suspension evolution on three very important fronts. The first is a new kind of frame made of metal which offered new possibilities for manufacturing and certainly had the potential to be much lighter than what came before it. The second was the use of small coil springs to achieve suspension which made it far simpler than previous designs. The third is its use of plastic and in particular its use of plastic to make a sufficiently strong but relatively lightweight string support structure. Essentially all of the key qualifications necessary for the making of a commercially viable high-performance string suspension racquet were improved over previous designs in this patent. It certainly would be too heavy by today's standards and it likely was too heavy even by the standards of its day, but the design definitely moved string suspension a big step closer to becoming a commercial reality.
The design shown in US Patent 10,004,950 (below right) is very much an upgraded and modernized version of the patent from 1975. It's a commercially viable version. The frame is now made of reinforced composite plastic instead of metal, and the small metal springs are now reconfigured in molded plastic. In a way it's the culmination of generations of string suspension designs which rely on complex mechanisms like coil springs and while this improved version may be much lighter than its predecessor, its physical complexity still makes it heavy, plastic or not. As shown in the illustration, it's lightweight enough to be used on the throat of a racquet but it's still too cumbersome and heavy a mechanism to be located around the head frame without making the racquet unwieldy.
As shown here in this summary, there is no doubt that the application of string suspension to a racquet represents a major performance leap for racquets. It makes possible a paradigm shift in performance and it's the reason why countless hours, over the course of more than a century, have been spent toward perfecting its application in a commercially viable high-performance product.
A number of themes resonate in the history of string suspension evolution and knowing them can help to understand the characteristics of the ultimate embodiment. One quality that stands out from one version to the next is a push toward simplicity. Particularly in an implement like a racquet, which can take a serious beating in the course of play, any additional features or performance enhancements such as a spring must be simple and not easily damaged. Another quality of suspension evolution is a push toward efficiency. This goes hand in hand to some degree with simplicity, and both go hand in hand with a reduction in weight. Complex mechanisms tend to be inefficient and often have to be made heavier than usual to make up for deficiencies. Complexity tends to make things more difficult to manufacture and mass produce too. As suspension designs have been simplified and the use of mechanisms minimized they've become lighter and more efficient, and closer yet to a high-performance product.
It's easy to see that a final step in achieving the ultimate string suspension design on a high-performance racquet would be to eliminate spring mechanisms altogether. In both designs pictured above, it's the spring mechanism in all its complexity that's responsible for most of the excess weight on the racquets. Eliminating it would make possible the lightest weight string suspension design ever, but eliminating the spring is not so easy. Strings under tension in a racquet require a very strong structure to hold them in place and plastic on its own is not strong enough. If the spring mechanism is eliminated, it suggests that the support structure itself must be both the spring element and support structure at the same time. That's some serious multi-tasking! Is string suspension possible in this ideal form?
Yes, modern high-tech string suspension is not only possible but it has arrived at last. For the first time ever on a commercially viable high-performance tennis racquet, string suspension is here in its ultimate form. It's the culmination of a century of sometimes radical string suspension design exploration distilled into one super-simple, super-efficient, super-strong and super-lightweight element called a Zipstrip (pictured below). All of the springs, tubes, screws and other complexities of former designs are gone, replaced by a simple tube made of reinforced plastic. It's strong enough to carry the full load of the stringing while remaining flexible, and it's light enough to be located all around the head frame where it comprehensively transforms the racquet response. It's the result of more than 20 years of targeted development inspired by 100 years of suspension exploration. It marks a paradigm shift in racquet design and it's only available from BOLT.
Technical Footnote: A discussion of string suspension designs is incomplete without some mention of "pulley" type suspensions. Designs based on the pulley concept have been relatively easy to manufacture but the result is only vaguely similar performance to the compression based suspension designs featured here. They can achieve some vibration dampening due to separation of the strings from the frame but the concept is inherently flawed and results in no other apparent benefits. Some notable pulley type designs that made it briefly to market include the Mitt Rocker and the Wilson Rollers racquets. The ideal form of string suspension as featured here is compressive action suspension and because it's been nearly impossible to achieve in a practical way, pulley type designs have been irresistible to designers seeking to capture compressive suspension performance. For a more detailed analysis of the "pulley suspension concept" go to: Top 4 Myths of Racquet Technology .