Category Archives: Tension

What’s The Difference?

As tennis players, you must constantly ask “what’s the difference” when it comes to tennis racquets and string! Well, as racquet technicians we ask the same questions!

This post is intended to showcase the differences of string in testing, not playing, however, some of the data may be noticeable to the player in certain situations.

What this graph shows us, in addition to our trying to save a tree by printing on the back of previously used paper, is that each of these stings will provide almost the same performance. This is indicated by the curve and how closely related the strings are.

Tensile Strength Comparison

The differences you do see here can be attributed to the gauge, or diameter, of the string, with the largest diameter (Tour Bite) having the highest tensile strength.  Down in the “hitting” displacement range (way below the 39.9mm!), there is very little difference.

The tensile strength can be a factor as the string begins to “notch” or otherwise come apart.  Each of the strings in this graph is monofilament so notching would be the failure mode in a racquet.


What Can Pictures Tell Us About String?

It is said that “a picture is worth a thousand words,” which may be true as long as the picture tells a story.

This story is about natural gut string, and the pictures will show what we can achieve, informationally, with our testing equipment. The two (2) strings are Babolat VS Touch 17 and Luxilon Natural Gut 125.

Both are awesome products, and we use a lot of both of them, but for different reasons, we may be able to understand by the pictures!

Our Questron software scales the images to suit the data so the graphs will not be the same size.

Babolat VS Touch 17

Our testing loads the string from 0 pounds force to 50 pounds and back to zero then up to 50 pounds three (3) times.  This is the “stress/strain” curve.  Fifty (50) pounds is arbitrary and because we are using the same methodology for all string materials it is a good number.  The closer these lines are together the better.

The farther it takes to reach 50 pounds the “higher elongation” the string is.  In this case it is about 44mm.

The important property of this string is the linearity!  That is the “straightness” of the line from beginning to end.  This indicates predictability, stability, and consistency.

This picture tells us the tensile strength and the knot strength.  In this case the knot strength is 52.3 pounds and takes a stretch to very close to 60mm before failure.

Luxilon Natural Gut 125

What is, probably, the first thing you notice about this “picture”? Could it be the squiggly lines? What could be causing this?

We believe it is the “break-in period” players associate with Luxilon gut! We have heard it from the players but have not been able to “see” it! It could be the bonding agents realigning as the load is applied.

You will notice a couple of things: the similarity of the “stress/strain” curve and the displacement to “knot strength” of the two strings. This string will feel a little less “soft” than the Babolat VS Touch 17. The linearity is quite good up to failure.

So, based on these pictures, could you make a recommendation? If so, let us hear them!

And the winner is…Head FXP 17


Head FXP is a string that seems to be largely overlooked and I am surprised after seeing the results of the installation and testing!

String NameCategoryGaugeUltimate Tensile Strength/LbsKnot Strength/LbsCountry of Origin
Head FXPMultifilament - Nylon17165.993.4Japan

Both the tensile strength and knot strength are exceptional and the graph shows that this string is very “elastic” which is a good thing for tennis racquet string but sometimes just not considered.

Head FXP 17

The area under the solid red lines is considered a stress-strain curve and indicates the “elasticity” of string by how close the lines fit from 0 to 50 three (3) times.  A perfectly elastic material would be one line even though it went through three (3) cycles.  What you see here is quite good and one of the reasons it is the winner!

Prince Vortex 300

The new Prince Vortex design is a new offering of an older string pattern with 14 main strings and 21 cross strings in a variable taper beam…whew!  You can see by the throat design that the main strings are longer that a conventional throat design.  The longer the main strings the more energy they can return…sort of, and in some cases!

Vortex 300

The beam starts at the grip with a strong 25mm then tapers into a 23mm at the throat and then back to 25mm at the tip.  Do not let the beam dimensions fool you!  The RDC flex of 59 after stringing is quite acceptable for most every player style.

We have been taking coefficient of friction (COF) data for years  but have not included it this review format, until now.  The 14×21 string pattern is unique enough that it is worth including.  You will see the numbers on the review specs so we won’t go into them now, however the numbers are interesting.

The 14 main strings contribute to a very “open” area right about where most extreme “spin” shots are hit according to our data of string failure positions.

The frame geometry is sort of “Aero” like.  The shaft is trapezoidal that transitions into a “reverse” trapezoid around the head.  It makes the racquet look stiffer than it is!

Take a look at the specs then pick up a demo to see what the numbers mean to you!

Racquet ModelPrince Vortex 300
Reference Tension56 lbs - 25.4kg
Victrex 7718
Machine UsedTrue Tension Professional
ASPS, FlexFour58.5
Racquet Flex, RDC59 - After stringing
Racquet Flex, FlexFour40.0
Racquet - In Plane Stiffness526.3 lbs/Inch
Weight, Grams322
Weight, Ounces11.36
Balance, mm320
Balance, Inch12.60
Length, Cm68.6
Length, Inch27.008
Head Width9.94
Head Length12.94
Head Area, cm2652
Head Area, Sq. Inch101.1
Number of Main Strings14
Number of Cross Strings21
Ratio Cross/Mains.512
Main String Grid7.30
Cross String Grid9.50
Density (% of head filled with string).686
Average Cross String Space.452
Average Main String Space.521
Dynamic Tension, Kp, ERT32
Dynamic Tension, Lbs/in176.98
First Moment, Nm.790
Polar Moment336
Torsional Stability17
Swing Weight, Kg/cm2319
Swing Weight, Ounces11.25
Swing Weight Calculated329.7
Power, RDC47
Control, RDC52
Manueverability, RDC74
Power, Calculated 1921.1
Head Points7.24
Head Weight, %46.6
Center of Percussion21.7
Dwell Time, ms9.48
Efective Stiffness - lbs30.2
K, Lb/In144.23
Recoil Weight165.41
Twist Weight237.56
End Weight 134.5
Tip Weight 189.0
9 O'Clock96.2
3 O'Clock96.1
Butt Cap131.4
Coefficient of Friction: M.400
Coefficient of Friction: X.262



Head Speed Pro Black

As you probably know by now Novak Djokovic plays with a version of the Head Speed but in black, not black and white! So it was only a matter of time until the all-black Speed Pro became available.

Until now we only had the black and white Speed Pro demo but now we have the black one!

This review will include the white and black version of the Speed and we will include a comparison to the Wilson Blade Pro!

These are each extraordinary racquets!  You will discover subtle differences in the specifications, especially the Wilson Blade Pro being a 16×19 pattern and the Head Speed Pro, in this case, is 18×20.  The geometry and frame of the 16×19 and 18×20 are the same so the numbers are meaningful!


Racquet ModelHead Graphene 360 + Speed Pro Black
Reference Tension54 lbs - 24.5 kg
Head Reflex MLT
Machine UsedTrue Tension Professional
ASPS, FlexFour66.5
Racquet Flex, RDC57 - After stringing
Racquet Flex, FlexFour50.5
Racquet - In Plane Stiffness325.2 lbs/Inch
Weight, Grams334
Weight, Ounces11.78

Balance, mm325
Balance, Inch12.80
Length, Cm68.6
Length, Inch27.008
Head Width9.618
Head Length13.10
Head Area, cm2636.1
Head Area, Sq. Inch98.6
Number of Main Strings18
Number of Cross Strings20
Ratio Cross/Mains.662
Main String Grid7.565
Cross String Grid10.30
Density (% of head filled with string).7735
Average Cross String Space.513
Average Main String Space.413
Dynamic Tension, Kp, ERT35
Dynamic Tension, Lbs/in195.7
First Moment, Nm.836
Polar Moment341
Torsional Stability15
Swing Weight, Kg/cm2326
Swing Weight, Ounces11.50
Swing Weight Calculated352.8
Power, RDC42
Control, RDC59
Manueverability, RDC69
Power, Calculated 1832.1
Head Points5.51
Head Weight, %47.0%
Center of Percussion21.016
Dwell Time, ms8.502
Efective Stiffness - lbs28.25
K, Lb/In179.49
Recoil Weight159.3
Twist Weight231.4
End Weight 134.5
Tip Weight 198.0
9 O'Clock99.0
3 O'Clock99.6
Butt Cap134.0


And now for the white and black version:

Racquet ModelHead Graphene 360+ Speed Pro
Reference Tension56 lbs - 25.4 kg
Victrex PEEK fiber Experimental 7718
Machine UsedTrue Tension Professional
ASPS, FlexFour66.5
Racquet Flex, RDC60 - After stringing
Racquet Flex, FlexFour43
Weight, Grams327
Weight, Ounces11.53
Balance, mm323
Balance, Inch12.72
Length, Cm68.5
Length, Inch26.97
Head Width9.69
Head Length13.06
Head Area, cm2641.2
Head Area, Sq. Inch99.4
Beam Width, mm, Shaft, Center, Tip23, 23, 23
In Plane Stiffness, Pounds/In335.2 Lbs/In.
In Plane Stiffness, Kg/cm 152.0 Kg/cm
Number of Main Strings18
Number of Cross Strings20
Ratio Cross/Mains.668
Main String Grid7.62
Cross String Grid10.37
Density (% of head filled with string).768
Average Cross String Space.513
Average Main String Space.414
Dynamic Tension, Kp, ERT37
Dynamic Tension, Lbs/in206.94
First Moment, Nm.812
Polar Moment336
Torsional Stability16
Swing Weight, Kg/cm2320
Swing Weight, Ounces11.29
Swing Weight Calculated341.2
Power, RDC45
Control, RDC57
Manueverability, RDC73
Power, Calculated 1908.2
Head Points6.14 (negative = head heavy
Head Weight, %47.2%
Center of Percussion21.2
Dwell Time, ms, No Swing8.58
Efective Stiffness - lbs28.7
K, Lb/In (SBS) RDC176.28
Recoil Weight159.71
Twist Weight229.85
End Weight 133.4
Tip Weight 192.8
9 O'Clock97.1
3 O'Clock97.5
Butt Cap131.0
This Wilson Blade Pro was not strung by Racquet Quest. The specifications are included as a comparison only…not a string recommendation.

And now for the Wilson Blade Pro:

Racquet ModelWilson Blade Pro
Reference Tension54 lbs - 23.6 k2
ALU Power Rough
Machine UsedUnknown
ASPS, FlexFour53
Racquet Flex, RDC62 - After stringing
Racquet Flex, FlexFour48
Racquet - In Plane Stiffness387.1 lbs/Inch
Weight, Grams337
Weight, Ounces11.89
Balance, mm327
Balance, Inch12.87
Length, Cm68.6
Length, Inch27.008
Head Width9.6
Head Length12.80
Head Area, cm2623.3
Head Area, Sq. Inch96.6
Number of Main Strings16
Number of Cross Strings19
Ratio Cross/Mains.634
Main String Grid7.37
Cross String Grid10.40
Density (% of head filled with string).7934
Average Cross String Space.547
Average Main String Space.461
Dynamic Tension, Kp, ERT27
Dynamic Tension, Lbs/in151.0
First Moment, Nm.850
Polar Moment358
Torsional Stability16
Swing Weight, Kg/cm2342
Swing Weight, Ounces12.06
Swing Weight Calculated360.35
Power, RDC55
Control, RDC42
Manueverability, RDC59
Power, Calculated 2069.0
Head Points5.04
Head Weight, %47.07
Center of Percussion21.535
Dwell Time, ms10.755
Efective Stiffness - lbs22.37
K, Lb/In112.18
Recoil Weight170.8
Twist Weight234.0
End Weight 135.0
Tip Weight 202.9
9 O'Clock100.8
3 O'Clock103.7
Butt Cap132.2