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Steel/Polyester String Hybrid In a BOLT

Even steel cable is comfortable in a BOLT racquet! We dared to install 16 gauge wound steel cable, at a typical tension, in a BOLT tennis racquet for play-testing, not necessarily in pursuit of great performance, but to prove a point. When we say that any kind of string, no matter how stiff and unresponsive, will play comfortably and responsively in a BOLT, even at higher tensions, we mean it. It's no bull, it's science.

To be clear, we're not recommending this stringing for tennis players generally, whether using a BOLT or not. As far as we know steel is one of the least desirable materials for tennis strings. It's stiff, not generally flexible and lacks resilience, qualities that lead to unresponsive strings. The only obvious rationale for using steel would be for a player that doesn't want to re-string their racquet ever again or, in our case, to demonstrate the amazing performance of a Zipstrip. Whether it's the stiffest polyester or stiffer yet, wound steel cable, we want players to understand the potential here - a Zipstrip can make any string comfortable and responsive. It's huge! It opens up new possibilities for creating power and spin with the stringing.

We strung the test racquet last week with 16 gauge Steel Cable (wound) mains and 17 gauge Polyester (Kirschbaum PLE) for the cross strings at 56.5/57 pounds. One can see in the video of the stringing session that the steel cable is very inflexible. The crank on the stringing machine barely turned when the steel was pulled to #56.5, certainly less than the stiffest polyester strings we've previously pulled on the machine. After stringing, the initial read of the tension was taken with a frequency based tension calculator. It's not very precise, but it provides solid reference points for tracking changes.

After four days the tension reading was roughly four pounds lower without having touched a ball. The polyester cross strings are pre-stretched so the tension drop likely indicates poor tension maintenance in the steel cable. This makes sense because steel can lack resilience. Its capacity to return to its original state prior to being stressed is minimal. This means that every time a ball is struck, the steel moves much further away from its original state than a typically resilient string. Technically, steel is referred to as a "ductile" material. It can be flexible. It can stretch but not much, and if it stretches past a certain stress point, it can "rupture" and hit the point of no return. Before we get it out on the court, I'm concerned that the resilience of the steel has disappeared and it's become "dead".

I decided the first playtest session would be brief and I wouldn't subject anyone else to the racquet until I determined it was safe. The brutal look and feel of the steel was intimidating and I certainly had some reservations about hitting with it. Obviously it would be a big problem for me to blow out my arm so I started the playtest hitting lightly. Quickly I realized it was very comfortable. I was up to full speed in no time and compressed the ball hard for twenty minutes. When finished, it was confirmed I was still in one piece and my arm was still attached in the same place, and I wasn't sore at all. The next day my arm and wrist were still okay so I scheduled the next playtest session.

I was surprised by the feel. It was more lively and powerful than expected. The launch angle of the ball release was lower than our usual polyester string set-ups. Another surprise was the lack of spin generation. Given that maximum spin typically is generated by a stiff string-bed, one would expect the steel to achieve high marks for RPMs but that wasn't the case. I could sense the taut feel of the steel cable but the impact quality was very comfortable. How can a steel string be made lively and comfortable? The string is supported with a Zipstrip.

Typically a stiff string results in a smaller sweet spot than a softer stringing. It would make sense that the sweet spot using something as stiff as steel would be noticeable smaller, even in a BOLT, but that wasn’t the case. This steel/polyester hybrid produced a huge sweet zone when combined with the Zipstrip. That kind of blew my mind because I wasn't sure we'd discover anything more than a comfortable hit. The generous sweet zone was very noticeable, even on volleys.

Another pleasant surprise was the low pitched thump the racquet made on contact. It sounded like the sweet zone is very deep and substantial, and felt like it too. It's possible that the "bass" frequency makes the sweet zone feel bigger than it is, but that is part of the experience and somehow made it more enjoyable.

Completely unanticipated was the extra weight of the steel. Of course it makes sense but it's not something to be considered with a typical stringing. It turns out the steel is more than 50% heavier than a typical string. It's enough weight to change the balance characteristics of the frame. Most frames are designed to have 15-16 grams of stringing on the head frame and this was far past that. A player would maybe have to consider using a lighter frame because the strings are so heavy. It would be a necessary but unusual adjustment to make on account of the strings.

After two and a half hours of playtesting my arm and wrist are intact, no harm done. The tension calculator indicated 2.7 pounds lost over that time on court. Relative to a typical polyester stringing, the loss is twice as big - another reason steel isn't very appealing. I enjoyed the experience but the performance doesn't compare to my favorite polyester set-up. We won't be installing steel cable in our demos anytime soon but it was worth the examination, and we proved that super-stiff strings can be used in the BOLTs without fear of the usual physical punishment associated with them. If the steel is comfortable and responsive with a Zipstrip, then almost anything will be, including the stiffest polyesters available.


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