Playtest: Isospeed Black Fire
By Greg Raven
Isospeed Black Fire is a modified co-polyester monofilament that is made using a triple-heating process to soften the stiff modulus somewhat. Isospeed tells us the string is produced very stiff for control, but shrinking the string in the last heating step reduces vibration on the arm. The result is a stiff string that still has power and comfort.
Isospeed has successfully tested Black Fire 17 with demanding professional tour players. It is designed to give hard-hitting college and pro players maximum control without sacrificing power.
Black Fire is available in 17-gauge (1.25mm) in black only. It is priced from $7.50 per set of 12 meters. For more information or to order, contact Isospeed at 800-883-6647, or visit isospeed.com/en/.
IN THE LAB
The coil measured 40 feet. The diameter measured 1.26-1.28 mm prior to stringing, and 1.22-1.23 mm after stringing. We recorded a stringbed stiffness of 73 RDC units immediately after stringing at 60 pounds in a Wilson Pro Staff 6.1 95 (16 × 18 pattern) on a constant-pull machine.
After 24 hours (no playing), stringbed stiffness measured 66 RDC units, representing a 10 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 Isospeed Black Fire 17 has a stiffness of 232 and a tension loss of 21.2 pounds. Black Fire 17 added 16 grams to the weight of our unstrung frame.
The string was tested for five weeks by 31 USRSA play-testers, with NTRP ratings from 3.5 to 6.0. These are blind tests, with testers receiving unmarked strings in unmarked packages. Isospeed recommends reducing the reference tension by 10 percent compared to a nylon string, and we so advised our playtest team members. The average number of hours tested was 23.9.
Black Fire feels thicker than its 17 gauge out of the package. The surface is smooth and cylindrical, and it feels “soft” during stringing. It is easy to weave and pull the crosses through the mains, but the string is slightly springy, so you have to pay attention during knotting.
None of our testers broke the sample during stringing, three reported problems with coil memory, one reported problems tying knots, none reported friction burn, and one reported other problems.
ON THE COURT
Isospeed Black Fire’s smooth surface makes installation easy, and seems also to contribute to the characteristic of aiding its resistance to movement. Our testers rated Black Fire the 15th best string in this category of the 174 strings we’ve play tested for publication to date. Isospeed promises control with Black Fire, and our testers agreed, rating Black Fire excellent in the Control category. Our testers also awarded Black Fire with well-above-average ratings in six categories: Durability, Playability, Power, Tension Retention, and Spin Potential. As a result, overall Isospeed Black Fire garnered an excellent rating from our test team.
None of our testers reported premature fraying or peeling, one reported buzzing, and four reported notching. None of the testers broke the sample during the playtest period.
Considering that Isospeed designed Black Fire 17 for players whose abilities are well above those of most of our test team’s, it speaks well of the Isospeed process that even lesser players found a lot to like in Black Fire 17. No string receives great scores in seven categories (including one in the top 20) — and an overall excellent rating — by accident.
“This is a softer feeling poly with great spin. The low power level lets me take big swings with confidence. Overall, this is a good, well-rounded poly.” 3.5 male baseliner with moderate spin using Prince EXO3 Tour (16x18) strung at 44 pounds CP (Gamma Synthetic Gut 17)
“I am surprised by the comfort of this poly. Even strung in the low forties, the control, spin and overall playability are exceptional.” 4.0 male baseliner with heavy spin using Babolat Pure Drive + Cortex GT strung at 43 pounds CP (Head Sonic Pro 17)
“Firm but feisty. This is a great string for the player who likes to hit hard and wants great tension maintenance. This one snaps back nicely. Despite feeling firm when installed, the overall playability should appeal to both singles and doubles players.” 4.5 male all court player using Babolat Pure Drive Roddick GT strung at 47 pounds CP (Solinco Tour Bite/Solinco Vanquish 18/16)
“This is a spin-friendly co-poly but with the comfort of a nylon multifilament.” 5.0 male all court player using Babolat Pure Drive Cortex strung at 60 pounds CP (Luxilon Alu Power 16L)
“For a poly, there is a good balance of control, power and, surprisingly, feel. This string is very well rounded, with features that should satisfy many playing styles. It has the forgiving feel of a nylon but the durability and control of a poly. The power is very controllable. There is a also enough touch to hit drop shots. Let’s just say I am a fan.” 4.0 male all court player using Pro Kennex Kinetic Pro 7g strung at 55 pounds CP (Tourna Big Hitter Black 7 17)
“Kapow! Bam! Pow! Zap! I feel like Batman fighting crime with this weapon. The exceptional playability is there from the first to last hit. It has control, touch, and power. Every stroke feels good. Serves have nice bite and placement, and groundies have penetrating depth without much fuss. I’m not a ‘poly player,’ but this string has me considering a switch.” 4.5 male all court player using Babolat AeroPro Drive GT strung at 54 pounds LO (Tourna Quasi Gut 16)
“The high levels of comfort and power are very impressive (and rare) for such a durable poly. The playability and excellent feel will give this one broad appeal. It’s also got great spin, which helps with control.” 4.0 male baseliner with heavy spin using Dunlop Aerogel 4D 1 Hundred strung at 40 pounds LO (Luxilon XP 17)
“Despite feeling stiff during installation, this poly is very comfortable in play. Achieving depth is easy. This is one of the best polys I’ve tested. Truly wonderful.” 4.5 male all court player using Dunlop Biomimetic 400 strung at 50 pounds CP (Tecnifibre Pro Red Code/Gamma Synthetic Gut 18/16)
“This string has better tension maintenance than similar feeling polys. It makes it easy for me to control points with spin.” 6.0 male serve-and-volley player using Yonex V Core Xi (300 Grams) strung at 67 pounds CP (Yonex Poly Tour Pro 16L)
“This is very ‘stringer friendly.’ Installation is quick and painless with little coil memory or friction bun. On court, this plays like my favorite polys with high marks for control and spin. It also has average comfort and touch.” 5.0 male all court player using Head Youtek Speed Lite strung at 50 pounds CP (Polyester 16)
“This is the first poly that I don’t mind playing in a full stringbed. Spin and power were average for a non-shaped poly, but comfort and feel are better than expected. I still prefer the playability of a hybrid, but it’s nice to play with a full bed of poly that does not aggravate the arm.” 4.5 male all court player using Prince EXO3 Rebel (hole inserts) strung at 49 pounds LO (Solinco Tour Bite/Prince Premier Attack 18/17)
“Great sound from the first hit. When striking the ball I am rewarded with a very solid, authoritative pop. This is a surprisingly comfortable poly. While I could use a tad more spin, I am impressed by the comfort and playability.” 4 male baseliner with heavy spin using Wilson BLX Six One (16×18) strung at 50 pounds CP (Pacific Prime Gut/Genesis Black Magic 16/16)
“Very good touch for a durable control string.” 4.0 male baseliner with heavy spin using Babolat Pure Drive Roddick GT strung at 62 pounds LO (Solinco Tour Bite 16L)
“Strung in the mid fifties, the feel is stiff and the sweet spot feels small. After a short break-in, playability steadily improves and the strings loosen up. Serves have slightly more pop. Ground strokes have average spin and power with good control. Volleys feel crisp. I suggest a reference tension of 52 pounds for a midplus frame.” 4.5 male all court player using Babolat Pure Storm Tour + GT strung at 55 pounds CP (Tier One Tour Status 17)
“Very impressive playability for a co-poly. The balance of control and power is excellent. I typically hybrid polys, but not this one. It has more than enough feel and comfort by itself.” 4.0 male serve-and-volley player using Dunlop Aerogel 3 Hundred strung at 46 pounds CP (Wilson Sensation 17)
“This string plays with a little zing. Before I could swing confidently, I needed to adjust to the lively stringbed response. The spin is excellent, but off-center volleys feel a bit dead. It needs a bit more playability to work for serve and volley players.” 4.5 male serve-and-volley player using Head Youtek Radical MP strung at 54 pounds CP (Gamma Asterisk Tour 17)
“This is a comfortable poly. I found a groove from the first hit. Recommended to the player who wants a durability string with above average comfort and power.” 5.0 male all court player using Head Youtek Graphene Speed Rev strung at 62 pounds CP (Luxilon Timo 18)
“The feel goes down over time but the performance remains high. Nice combination of spin and power.” 4.0 male all court player using Head Youtek Prestige MP strung at 54 pounds CP (Gamma Live Wire XP 17)
“This is one of the softest polys I’ve hit. Great pocketing and very easy spin production for a round, non-textured string. This is recommended to players who want the durability and control of a poly, but with less shock to the arm.” 4.0 male all court player using Head Youtek IG Prestige Pro MP strung at 52 pounds CP (Head Synthetic Gut PPS 17)
“The pocketing and spin potential are exceptional. This poly does not ‘play dead,’ nor does it feel like a trampoline. I’m able to teach longer than usual with it. No pain when feeding balls.” 5.0 male baseliner with heavy spin using Prince EXO3 Rebel Team strung at 52 pounds CP (Prince Beast XP 16)
“Excellent tension maintenance. Very good control and spin potential. No string straightening necessary.” 3.5 male baseliner with moderate spin using Prince O3 Speedport Black (port inserts) strung at 58 pounds CP (Zons Polymo Hexplosion 17)
“The string pocketing is better than expected. This is a great option for non polyester players who want to make the transition to polyester.” 4.5 male all court player using Wilson BLX Blade strung at 56 pounds CP (Luxilon 4G 16L)
“Plays with a lot of power ‘right out of the box.’ This string retains its comfort after many hours of play. Nice action on groundies and serves, and good control on volleys. It would work well in a hybrid.” 4.0 male all court player using Babolat Pure Drive strung at 54 pounds CP (Natural Gut 17)
“This string lacks the ‘wow’ factor. There isn’t enough of an upside to justify switching from my typical string. It is just an average string.” 4 male all court player using Volkl V1 Classic strung at 52/54 pounds CP (Wilson Hyperlast Spin 19)
“These strings hold tension well and never move, even when hitting heavy spin. However, there is no outstanding quality that makes me want to switch from my favorite string. The industry has to stop making so many black strings. Give me some color please.” 4.0 male baseliner with moderate spin using Babolat AeroPro Drive Cortex strung at 54 pounds LO (Tourna Big Hitter Black 7 17)
“Very easy installation, with low coil memory. Tension maintenance is decent for a poly. Comfort is average. Control is adequate. Spin is not up the level of my shaped poly. Power, however, is good, especially when freshly strung.” 4.5 male baseliner with heavy spin using Babolat Aero Storm Tour GT strung at 56 pounds LO (Babolat RPM Blast 16)
“This test string has average playability. It would be good in a hybrid with a multifilament or natural gut cross.” 4.0 male baseliner with moderate spin using Babolat AeroPro Drive strung at 56 pounds CP (Babolat RPM Blast 17)
“I expect to get more pop from a poly like this, but the sweet spot is too small. It could be that I am in a little slump; I am not in a hurry to try this one again. My wrist and hand are not in a hurry either.” 3.5 male all court player using Wilson kBlade strung at 53 pounds CP (Solinco Tour Bite/Gamma TNT2 16L/17)
“This is a dull string overall. Nothing stands out. All elements of the string were average or below average. It might play better if textured or shaped. I want my string to snap back or trampoline on flat serves. This one fails the liveliness test.” 4.5 male all court player using Wilson BLX Steam strung at 59 pounds CP (Luxilon Savage 17)
“This is an average polyester with decent power and spin. If it is priced less than ten dollars a set, I will recommend it. But it’s not for me.” 5.0 male all court player using Babolat Pure Drive + Cortex strung at 43 pounds LO (Luxilon Alu Power 16)
| EASE OF STRINGING
(compared to other strings)
|Number of testers who said it was:|
|about as easy||23|
|not quite as easy||4|
|not nearly as easy||0|
| OVERALL PLAYABILITY
(compared to string played most often)
|Number of testers who said it was:|
|about as playable||11|
|not quite as playable||13|
|not nearly as playable||1|
| OVERALL DURABILITY
(compared to other strings of similar gauge)
|Number of testers who said it was:|
|about as durable||16|
|not quite as durable||0|
|not nearly as durable||0|
|From 1 to 5 (best)|
|Resistance to Movement (15th overall)||4.1|
See all articles by Greg Raven
About the Author
Greg Raven is an associate editor for RSI magazine and technical writer. He is certified as a Master Racquet Technician by the U.S. Racquet Stringers Association. He can be reached via e-mail at firstname.lastname@example.org, or through Facebook, LinkedIn, and Twitter. He plays tennis five days a week, and is turning into an avid cyclist.
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