Tennis Industry magazine


String Playtest: Head Gravity

Head Gravity is a polyester/polyester hybrid string that uses the same Crystal Core technology Head introduced in Head Hawk a little more than a year ago. According to Head, Crystal Core technology is a unique manufacturing process that utilizes a complex, multistep heat treatment, which allows its engineers to create different material properties in the core than in the outer region of the string.

With this process, highly oriented molecules on the outside of the string generate stiffness and energy efficiency for power. In the core of the string, the more crystalline structure adds dampening and touch. The molecular variations in the cross-section create the highest possible energy efficiency with optimized control.

The result, says Head, is a string that maximizes spin potential, allowing the player to control the trajectory and direction of every shot. The triangular main strings increase the friction between the ball and the string bed, while the cylindrical cross strings allow for faster “snap back.”

Head designed Gravity for the player who likes polyester and wants maximum spin and control. Head Gravity is available in 17/18 in white mains and anthracite crosses. It is priced from $16 (with an MSRP of $32) for sets of 5.5 meters (mains) and 6.5 meters (crosses), and $220 (with an MSRP of $440) for 304 foot (mains) and 356 foot (crosses) reels. For more information or to order, contact Head at 800-289-7366, or visit

In the Lab

The coils measured 24 feet 1 inch (mains) and 19 feet 7 inches (crosses). The mains measured 1.41 mm per side (the equivalent gauge in a cylindrical string would be 1.25mm, but with 25 percent less cross-sectional area), and the crosses diameter measured 1.18 mm prior to stringing. After stringing, the mains measured 1.37 mm per side and the crosses diameter measured 1.16 mm.

We recorded a string bed stiffness of 79 RDC units immediately after stringing at 60 pounds in a Wilson Pro Staff 6.1 95 (16 x 18 pattern) on a constant-pull machine.

After 24 hours (no playing), string bed stiffness measured 72 RDC units, representing a 9 percent tension loss. Our control string, Prince Synthetic Gut Original Gold 16, measured 78 RDC units immediately after stringing and 71 RDC units after 24 hours, representing a 9 percent tension loss. In lab testing, Prince Synthetic Gut Original has a stiffness of 217 and a tension loss of 11.67 pounds, while Head Gravity has stiffnesses of 193 (mains) and 182 (crosses) and tension losses of 15.31 pounds (mains) and 17.99 pounds (crosses). Gravity added 15.9 grams to the weight of our unstrung frame.

The string was tested for five weeks by 38 USRSA playtesters, with NTRP ratings from 3.5 to 6.0. These are blind tests, with playtesters receiving unmarked strings in unmarked packages. Average number of hours playtested was 24.4.

Out of the package, the Head Gravity mains feel to be the thickest 17-gauge string ever, and you’ll want to adjust your clamps accordingly. Head recommends using a little more care during stringing. Installing the triangular mains wasn’t an issue for us — aside from getting the string through some of the tighter grommet barrels — but weaving the crosses reveals just how aggressive those three apexes are. Don’t forget to readjust your clamps between the mains and the crosses!

One playtester broke the sample during stringing, five reported problems with coil memory, nine reported problems tying knots, one reported friction burn, and nine reported other problems.

On the Court

Our playtesters were effusive in their acclaim for Head Gravity. Twenty-seven of the 37 playtesters (73 percent) had great things to say about Gravity, usually praising the amount of spin they were able to generate. Consequently, Head Gravity was ninth best of the 183 strings we’ve playtested for publication in Spin Potential. Our playtesters also rated it well above average in Durability, Resistance to Movement, Tension Retention, Control, Power, and Playability. Not surprisingly, Head Gravity rated well above average overall.

No playtester reported premature fraying or peeling, two reported buzzing, and nine reported notching. Seven playtesters broke the string during the playtest period, one each at 2, 6, 9, 10, 15, 27, and 44 hours.


Although there are now special racquets designed to maximize spin, Head Gravity can help maximize the amount of spin from existing frames. The uncompromising profile of the mains should also impress those players seeking a geometric string.

Playtester comments

“I thoroughly enjoyed hitting with this string. It gave me more power and more spin than any string I’ve hit with. My regular opponents had to stand further back to return my groundstrokes. My serves had more motion and kick. Although this string broke in six hours of hitting, I would definitely hit with it again, and it just may become my new favorite string.” 4.5 male all court player using Babolat Pure Storm Tour + strung at 55 pounds CP (Yonex Poly Tour Pro 16)

“This string loves low tensions! Super spin, super feel, super response on snap back. Can strings get any better than this?! This string has found the magic space between durability and playability! Although I wouldn’t recommend this string to intermediate or beginners, it would be perfect for 4.0 plus players especially for those who play an open string pattern racquet. This string has so much going for it, spin, feel, durability, and comfort. Now I know what powers a Star Wars light saber!” 4.5 male all court player using Head Graphene Instinct MP strung at 50 pounds CP (Solinco Tour Bite/Solinco Vanquish 18/16)

“This co-poly hybrid was a game changer with the triangular-shaped mains and smooth crosses. The spin I was able to generate was awesome. Average effort produced magnificent spin results due to the excellent bite on the ball. The power for this co-poly was above average for a poly. Loved the controlled power of this test string. I would recommend this string to any customer looking to get tons of spin and control with outstanding durability. I loved this string.” 4.0 male all court player using Dunlop iDapt Force strung at 43.6 pounds CP (Wilson Sensation/Prince Synthetic Gut 17/18)

“This was another excellent test string. It took a few hits to get accustomed to its feel, but I loved it after a while. There was no movement of strings, but with much more spin potential. The thick main strings were difficult to pass through the grommets at #4T on both sides of the Steam 105. Despite the main string’s girth, the thinner cross strings slipped through rather easily. The mains exhibited a unidirectional tinting, some appearing whiter when viewed from the head to the throat and swapping this trait when viewed from the throat toward the head. The apparent poly in both directions (mains and crosses) in this hybrid was a strange combination.” 4.5 male all court player using Wilson Steam S strung at 56 pounds CP (Tecnifibre Pro Red Code 1.20/Gamma Synthetic Gut 18/16)

“The string hit really well! The shape really bit into the ball and the thicker main should last longer than other shaped polys. Definitely worth a try!” 4.0 male baseliner with heavy spin using Babolat Pure Drive Roddick strung at 60 pounds LO (Solinco Tour Bite 16L)

“Good, soft feel with good power and satisfactory spin. A top-tier string. I would use it, recommend it, and sell it.” 4.0 male baseliner with moderate spin using Wilson Steam S strung at 60 pounds CP (Tourna Big Hitter Blue 17)

“Literally rips the fuzz off the ball. Terrific spin, though a little firm for first two weeks. Then, it was terrific. Balanced power and control until it became too loose.” 3.5 male baseliner with moderate spin using Prince O3 Speedport Black strung at 47/45 pounds CP (MSV Hex 1.1 19)

“Great playing string with excellent feel. Little or no vibration so it should be easy on the elbow. No string movement to speak of, which should contribute to durability.” 4.0 male baseliner with moderate spin using Wilson BLX Steam strung at 52 pounds CP (Wilson Sensation 17)

“Mains were interesting — looked like a 15 gauge triangular cross section poly. Crosses looked like a 16 or 17 gauge poly. This string had good power — almost too much. If I hit flat the ball was more likely to go out. However, this string really shone when I put topspin on the ball. Many times my opponents let the ball go by thinking it was out, only to see it drop in!” 4.0 male all court player using Wilson Steam S strung at 60 pounds CP (Ashaway Monogut 16)

“I’m totally impressed with the performance of this string in all aspects — cosmetics, performance, spin, and control. I suspect this is a premium string with a premium price, but well worth the cost due to its capacity to maintain consistent feel and power.” 4.0 male all court player using Head Youtek Graphene Speed PWR strung at 50 pounds CP (Gamma Moto 17)

“Really liked this string. Loaned my racquet to some of my younger, weaker students and they didn’t want to give it back!” 4.0 male all court player using Babolat Pure Drive strung at 60 pounds CP (Luxilon ALU Power 16)

“This string was unbelievable. I absolutely loved it. The touch and feel along with the control and playability were top notch. If you like a poly that holds tension and does not move, along with excellent spin potential, this string is for you.” 4.5 male all court player using Babolat Pure Drive strung at 45 pounds LO (Babolat RPM Blast 16)

“Although this string felt more on the stiff side when being installed, there was no issue with tying knots or any other installation issues compared to other poly strings I have used. I could really feel the angled texture of the string so I was anticipating great spin potential when I got on the court. I was not disappointed with the bite on the ball. I give this a 5 out of 5 for spin potential. Although the string was stiff, it still seemed to be able to pocket the ball a fair amount since I only strung this at 50 pounds. Although the playability didn’t seem to be up there compared to multifilament strings, this made up for it with the feeling of control and the spin.” 4.0 male all court player using Wilson Pro Staff 97 LS strung at 50 pounds CP (Solinco Tour Bite/ Head Fibergel Power Hybrid 16)

“A stiff durability string. Great for banging balls with not a lot of power. Good spin and control. String looser for elbow ease.” 4.5 male all court player using Wilson Steam S strung at 45 pounds CP (Wilson Optimus 16)

“Great string if you are looking for control and spin. This is suited for all-court players who like to control the ball in long rallies and with accurate passing shots. Must generate power for serving.” 4.5 male serve-and-volley player using Head Graphene Radical MP strung at 53 pounds CP (Luxilon ALU Power Feel 18)

“From the stringer’s point of view, there is coil retention. However, it was reasonable for a polyester or copolymer string. Some difficulty on the stringer’s fingers. The crosses were laborious to install. I liked the string somewhat. I was able to hit good serves and the topspin was good. Being a baseliner, the string was a little rough on the arm. Power was good from both the baseline and on overheads. Groundstrokes to the opponents backhands, along with the spin that I was able to put on the ball was through to be difficult for opponents to return. Played reasonably well with this string. Typical of many strings of this apparent type it is expected that the string’s durability will continue to prove to be good lasting 50, or 60 or more hours … as was perhaps indicated in the visual inspection of the bed.” 4.0 male all court player using Head Seven Star strung at 55/58 pounds LO (TierOne FireWire 17)

“I was convinced that I would not like this string before installing it in my racquet. I was wrong. The spin potential is off the charts. It is not as comfortable as my usual string, but it did not hurt my arm. Personally, I really like the 18 gauge cross string. For really big hitters, however, it probably won’t be durable enough.” 4.5 male serve-and-volley player using Wilson Steam S strung at 56 pounds CP (Tecnifibre Black Code/Tecnifibre NRG2 18/16)

“I found the hybrid to play much more comfortably than expected. Both the triangular mains and round crosses seemed soft to me. This is a full poly set up that I could actually see myself using in the future.” 4.5 male all court player using Prince Tour strung at 49 pounds CP (Solinco Tour Bite/Prince Premier Control 18/17)

“Initially I found the string to be dead and a bit boardy, but as it broke-in, I found I liked it more and more. Although the power level was a bit low, I did enjoy the spin aspect of the string. My kick serve certainly hopped a bit more, and I could place the ball just about anywhere I wanted. Liked playing with this string, but not so much teaching as it was tough on the arm during long periods of feeding.” 4.5 male all court player using Babolat Aero Pro Drive strung at 54/57 pounds LO (Babolat RPM Blast 16)

“Shots were lacking power and harder to penetrate court. Huge spin from this string! One of the best in this category. Touch and feel were well above average for a polyester. I like the idea of a two-polyester hybrid. The characteristics of each string provide a solid overall stringbed that ranks up there with the best in terms of spin, feel, and durability.” 5.5 male baseliner with heavy spin using Wilson BLX Pro Open strung at 58 pounds LO (Luxilon ALU Power 1.25 17)

“This string was stiff but I had good control and good bite on the ball. Overall, I would say it is a good to very good string.” 5.5 male all court player using Babolat Pure Drive strung at 60 pounds CP (Babolat VS Gut 16)

“Thick gauge triangular-shaped mains offered some spin potential, but lacked crispness and resilience. A thinner gauge main would improve playability and spin. Durability would be a high point of this string.” 4.0 male baseliner with heavy spin using Dunlop Aerogel 4D 100 strung at 40 pounds LO (Volkl Cyclone 16)

“I’m not a huge fan of polyester strings, but this was very good. Excellent tension holding, good spin potential, and it didn’t hurt my wrist, elbow or shoulder. Very good all-around performer!” 4.5 male all court player using Head Graphene Speed S strung at 44 pounds CP (Tourna Big Hitter 7 17)

“Definitely more spin produced when hitting with either Eastern or Semi-Western grip. Same for backhand. More bite on slice and pop on topspin. Nice string!” 4.0 female all court player using Babolat Pure Drive strung at 50 pounds CP (Natural Gut 16)

“It lasted longer than my usual string, but didn’t give me the feel and bite I usually get. I’d recommend it for heavy topspin, 4.5 plus players who break string often.” 5.0 male all court player using Babolat Pure Drive with Cortex strung at 41 pounds LO (Luxilon ALU Power/Babolat Pro Hurricane 16)

“This was the first hybrid I have experienced. I liked how soft the strings felt. Stringing the racquet was easy and having the thinner gauge offered more feel. String had good ball pocketing since it was soft, which concerns me on durability.” 4.0 male all court player using Pro Kennex Kinetic Pro 7G strung at 55 pounds CP (Luxilon Big Banger ALU 16)

“One of the better strings I have playtested out of the box, but I lost it about a third of the way in. At that point it no longer held the same pop or feel that it had in the first 10 hours or so. Pretty common characteristics for a 17 gauge poly, great on control, allowing some really nice racquet head speed, lacking somewhat on the touch, although not enough to take it off my Christmas list and I do a great deal of net play. Nice spin potential with the rough edges, though I am not a huge fan of the value such features generally. Perfect for those looking for a playable thin poly and wanting to feel the emotional confidence that the sharp edges are providing them with additional bite.” 5.0 male all court player using Wilson Blade strung at 55 pounds LO (Wilson NXT 16)

“The white main string was so thick it took a long time to pull it through the holes, so stringing time was increased. I would not want to string a lot of these. I lost my ERT, but it seemed as though tension loss was very slight. Played well, and I would choose to play with it if it was faster to string.” 4.5 male baseliner with moderate spin using Head Graphene Prestige MP strung at 50 pounds CP (Head Sonic Pro 17)

“Middle of road string. Notched right away. Did not generate as much spin as anticipated given edges. Interesting look. String visibly edged. Not much feel or power. Good wearability, good tension maintenance. I would string 15% looser to start next time.” 6.0 male all court player using Wilson Steam strung at 56 pounds CP (Wilson NXT 16)

“I’m predicting this well be popular with players, not so much for stringers. My fingers still hurt! At least the nine I have left!” 4.0 male touch player using Head Graphene Extreme MP strung at 55 pounds LO (Genesis Hexonic 16)

“This hybrid shines from the baseline. It has a lot of power and enough spin to keep the ball in court. Unfortunately, performance fell off dramatically after about 8 hours, to the point where I was relieved when the string broke. I wasn’t expecting to enjoy the remainder of the playtest. I’m not sure I’d recommend this string to my customers. The durability is poor enough that it wouldn’t be a good value to most players.” 4.0 male baseliner with heavy spin using Wilson SixOne 95 S strung at 50 pounds CP (Luxilon ALU Power Spin 16L)

“Not a string I would use.” 4.0 male all court player using Volkl strung at 52/48 pounds CP (Wilson Synthetic Gut 16/17)

“Overall, not really impressed with the string. Average on comfort. Lacked the feel and touch I need for my game.” 4.0 male all court player using Head Graphene Speed PWR strung at 51 pounds CP (Solinco Tour Bite Soft 17)

“I found this string interesting for about two hours. After that, all I had was the pain. This string just doesn’t fit my game. I had some great serves and groundstrokes at first, but then the string seemed to lose its pop and I couldn’t find anything to like about it. I question the theory behind a triangular string. The white mains are laying on one of the three sides and then slightly tip to one side or the other as they follow the rise of the cross string. I’m not convinced of the purpose and effectiveness of that design, or that it works any better than a five-sided string. I struggled to make myself play 15 hours with this. It felt like a board eventually.” 3.5 male baseliner with moderate spin using Wilson Blade 98 (18x20) strung at 50 pounds CP (Solinco Tour Bite/Gamma TNT2 17/17)

“Stringbed wrapped the ball, but felt mushy and forced me to swing harder to get depth. I felt like a sloppy player with these versus my normal string. Expected more bite with the shape of the white main, but didn’t feel it.” 4.0 male all court player using Wilson BLX Six One strung at 58 pounds LO (Ashaway Monogut ZX Pro 17)

“Sub-par string. The triangle shape made it difficult to tie knots. Limited feel of the ball when struck, the string itself felt plastic-y and cheap compared to the average poly string. Upon breakage, the string splintered/split at the ends. Low cost poly feel with low durability. One star.” 4.5 male all court player using Wilson Steam strung at 58 pounds CP (Luxilon 4G Rough 17)

“Thickness and stiffness of mains made it difficult to string. From a player’s perspective, not responsive. The mains did break during play. Break occurred at L2 in middle of racquet.” 3.5 male all court player using Head Youtek Graphene Speed PWR strung at 54 pounds CP (Babolat RPM Blast/Wilson Sensation 17)

(Strings normally used by testers are indicated in parentheses. For the rest of the tester comments, visit

Playtester Ratings

Ease of Stringing (compared to other strings)
much easier 0
somewhat easier 1
about as easy 18
not quite as easy 13
not nearly as easy 6
Overall Playability (compared to the string played most often)
much better 1
somewhat better 10
about as playable 10
not quite as playable 13
not nearly as playable 3
Overall Durability (compared to other strings of similar gauge)
much better 7
somewhat better 10
about as durable 14
not quite as durable 3
not nearly as durable 3
Rating Averages
From 1 to 5 (best)
Playability 3.4
Durability 3.9
Power 3.5
Control 3.5
Comfort 3.1
Touch/Feel 3.1
Spin Potential (9th overall) 4.0
Holding Tension 3.6
Resistance to Movement 3.8



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