Category Archives: Technology

What Can String Failure Tell Us – Part Deux

In Part Un we discussed the difference between shanking (mis-hit) and friction failure.  It was obvious that the string was broken.  But what happens when it is not so obvious?

Part Deux, this part, will examine the frictional notching failure of monofilament string and how we can be prepared for it!  To further refine this discussion we will be comparing PET polyester has PEEK monofilament string.  The reason is that each material while both will notch one requires more time to reach the critical dimensional decrease that is a failure!

In almost every Racquet Quest Podcast we talk about tension v string diameter and agree that once 50% of the string diameter is notched away the string is vulnerable!  So a .050 (1.27mm) diameter string that has a tensile strength of 120 pounds at 50% notching will have 60 pounds of tensile strength remaining.

Notched v un notched string

This graph is a string that was broken during use.  The string was removed from the racquet.  The top line is the tensile strength in the area of no notching so you can see that it is pretty strong still and has stabilized due to use.  That stabilization is indicated by the very tight stress/strain grouping.

However, things go sideways when the notched area of the string is put under stress.  The string failed at a force of 63.8 pounds, or about 59% of the used tensile strength.  Not bad!

So, notching is failure-inducing but how long it takes to create the fatal notch differs with string material.  This particular set of strings had about six (6) hours of play.

In Part Trois, we will look at PEEK material under the same conditions!

 

What Can String Failure Tell Us?

Well, in the simplest terms, failure tells us it is time to have the request strung! However, there may be subtleties in string failure that can help us in our quest for tennis racquet performance.

Such as?

Is the failure shear related or tensile strength related? Was friction the major contributor to the failure? Where did the failure occur (on the racquet, not the court)? Was the failure during play or in the bag?

Shear-related failure is when the string breaks very near the racquet frame. This failure is called a mis-hit or shank! It is like cutting the string with a pair of scissors!

Shear Failure

Friction Failure

Friction failure is caused by just that, friction!  Friction is caused by the string moving on each other. That rubbing creates friction and notches the string where it will fail.

If the racquet failed during play and it is not shear-related, the tensile strength of the string was exceeded. If a string has a tensile strength of 120 pounds and the tension is 60 pounds leaving 60 pounds to be used to hit the ball. Some big hitters can generate at least that much force on a solid forehand!

The graphs show the tensile strength and relative elongation of different material.

This graph shows the tensile strength of the string to be about 115 pounds.  Given the movement of this string-on-string, the frictional notching can contribute to relatively early failure based on the hitters force.

This graph shows the tensile strength of the string to be about 155 pounds but it has to travel (stretches) further to reach that force.

So, you can see, with this information we can make better decisions when asked to suggest a string, or strings, for a client!

Our Mission

Which Comes First!

We all have heard the question “which comes first the chicken or the egg”?  However, my question is “Which comes first the game or the string”?

I believe they happen simultaneously.  But first a quick story.

In 2005 I was attending a Head product introduction on the island of Mallorca, Spain,  Yes, that one!

The product introduction was exciting but what I am going to tell you about now was even more meaningful.

The Director of one of the top US Tennis Training organizations, at that time, was there and we were discussing teaching techniques and what he said after being in this part of Europe was “we need to start teaching our players how to hit this way!”  Well, “this way” was the way of low-powered strings that were popular in Europe but not so much in the US, yet.

So, it began!  The players could not hit harder, like the Europeans, unless they used the same string material as the Europeans and that was very stiff and mostly PET polyester.

So, the idea was the “egg” and the string was the “chicken”, sort of!  I guess the feeling was that “if Americans are going to compete we must use the same equipment”.

Our history confirms that almost no one plays better with stiff string and durability is suffering!

Now, I believe the professional game can go on about its way but otherwise, we need to consider changing the game by returning to a combination of comfort and playability.

Our history shows us that the “high performance” life span of many polyester strings is about 2-3 hours, or less, maybe about 10-12 games.  We don’t believe this is quite long enough for most players.  But, how do you quantify “performance”?  It may be different things for different players.

There are many components to performance but what if it was associated with UTR data?  Racquet Quest can track UTR numbers and make some determinations based on that data.  If a UTR is stable or increasing it is a good bet that the performance of the player and equipment is OK.  However, if the UTR is slipping it is a good indication that something is not working as it should…but what?

We have found that, in some cases, it is injury or discomfort, that is causing the slippage!  Stop it!  The following data is for a 12 month period and acquired from the UTR website.  Even small positive changes are tough!  But negative changes seem to have an enormous impact more quickly than positive changes!

For example:

PlayerRacquetStringUTR1UTR2Delta
AHead Speed PEEK12.8412.86+ .02
BBabolat Pure AeroPolyester10.919.56-1.35
CHead Radical MPAPEEK4.505.61+1.11
DWilson Pro Staff 97PEEK5.07.03+2.03
EBabolat Pure AeroPEEK3.85.64+1.84
FWilson Blade 98 Polyester10.09.41-.59
GHead Radical ProNatural Gut3.75.15+1.45
This information is provided as a small sample comparison instrument and is not intended to pry anyone away from their favorite setup!  Even if it hurts!

 

 

 

 

Bolt 100 v2

Bolt tennis racquets have been around for a few years and we have reported on them in the past, however, the new V2 of the Bolt racquet lineup is very impressive!

If you are not at all familiar with the Bolt technology it is the “Zip Strip” that makes them unique.  The “Zip Strip” is a carbon fiber component that looks like a miniature tennis racquet cross-section that fits into the sides and top that actually do the “bending” during string and racquet impact!

This “bending” can mitigate harshness associated with very stiff strings or a very stiff racquet!

Arm problems are no match for the ZipStrips!

Before we look at the specifications you can visit the Bolt site for more information.

Bolt 98L, V2 
ManufacturerBolt
Racquet ModelBolt 100, v2
Reference Tension57 lbs - 25.9 k
String: Main/Cross
Head Reflex
Machine UsedTrue Tension Pro
Static
ASPS, RDC56
ASPS, FlexFour64.5
Racquet Flex, RDC64 - After stringing
Racquet Flex, FlexFour46
Weight, Grams313
Weight, Ounces11.06
Balance, mm337
Balance, Inch13.27
Length, Cm68.5
Length, Inch26.98
Head Width9.589
Head Length13.13
Head Area, cm2665.1
Head Area, Sq. Inch103.1
Beam Width, mm, Shaft, Center, Tip24.5, 24.5, 23.8
In Plane Stiffness, Pounds/In454 Lbs/In.
In Plane Stiffness, Kg/cm178.7 Kg/cm
Number of Main Strings16
Number of Cross Strings19
Ratio Cross/Mains.642
Main String Grid7.37
Cross String Grid9.62
Density (% of head filled with string).688
Average Cross String Space.506
Average Main String Space.461
Dynamic
Dynamic Tension, Kp, ERT36
Dynamic Tension, Lbs/in201.35
First Moment, Nm.822
Polar Moment350
Torsional Stability19
Swing Weight, Kg/cm2331
Swing Weight, Ounces11.68
Swing Weight Calculated356.6
Power, RDC48
Control, RDC53
Manueverability, RDC66
Power, Calculated 2183.7
Head Points1.42 (negative = head heavy)
Head Weight, %49.3%
Center of Percussion21.4
Dwell Time, ms, No Swing8.50
Effective Stiffness - lbs29.9
K, Lb/In (SBS) RDC179.49
Recoil Weight157.0
Twist Weight234.11
End Weight118.7
Tip Weight196.1
Total Weight314.8
9 O'Clock103.0
3 O'Clock98.9
Butt Cap110.3
Total Weight312.2