Category Archives: Stress/Strain
An Inside Look at String Evaluation
In this series, we will look at the various characteristics of tennis string without the tennis racquet!
Racquet Quest evaluates every string we use plus strings submitted from other sources. These evaluations are “No Prep” and “Prep”, which simply means the Prep string has been pre-stretched in the entire length. It has not been through a tennis racquet…yet!
The following images represent a string that has not been “prepped.” After these plots are fully understood we will do the same string in the”prepped” format to see if there is a difference.
Ultimate Tensile Strength
What we notice immediately is the string is very “soft” as seen in the deflection of 47.8 mm to reach 50 pounds. Also, the elasticity, the ability to recover to the original length, is fairly low.
The area under the 50-pound and 47.8 mark is the stress/strain curve that shows how the string behaves in three (3) cycles. The load and number of cycles can be changed.
The long run (linearity) from the 50-pound mark to failure is quite good and suggests consistency.
The UTS, the ultimate tensile strength, of 127.1 pounds is good for this thin string.
The following plot shows the knot strength of this string.
Knot Strength
This plot is overlayed on the previous image so a quick comparison can be made. It is common for knot strength to be lower than un-knotted string. Knot strength is primarily important to the person tying the knot. Knots rarely fail after they are successfully tied. Pulling a knot too tight or jerking can break a knot.
This plot says to us that this string needs to be handled carefully when it comes to knots! We would expect a knot strength of at least 80 pounds for typical monofilament strings, however, if this string exhibits extraordinary play characteristics anyone can learn to tie the knots!
Based on this information we would suggest this string for a player looking for extraordinary comfort, power, and relatively short life!
Pro Stock Limited Reserve
This is a high-performance string that has gone unnoticed for a while and I don’t know why!
Pro Stock Limited Reserve is a string we purchased because it has interesting characteristics that appealed to us however the client base is small. Recently, however, with the number of players that are moving away from a polyester string, this has become a popular alternative!
Pro Stock Limited Reserve
In a word, this plot looks very much like that of natural gut, and whatever you think natural gut is still considered the best performance material for racquet string!
What are we seeing here:
- Linearity, the more linear the string the more consistent (predictable)
- Elongation, at 33.1 mm at 50 pounds
- Elasticity, 71.9 %
- This is the area where the advantages of pre-stretching will be seen.
- The curve will become essentially one line meaning the string returns to a nominal length after stretching.
- Ultimate Tensile Strength, 163.6 pounds to fail (high), @107.2 mm deflection
- Knot strength, @102.4 pounds, and surpasses natural gut in this property.
What we can’t see in the plot is the construction of this multifilament string. Each strand is a thin, flat ribbon of polyolefin material. The ribbons are much like the natural gut.
The plot below is a comparison of natural gut string and is included as a visual to compare to Pro Stock Limited Reserve and show how much natural gut fibers are the same for any string manufacturer. Of course, manufacturing techniques, bonding agents, and coatings make the difference between a good gut and a not-so-good gut!
If you compare the Pro Stock Limited Reserve to natural gut you can quickly see why it may be a good string to try!
The Same but Different!
How can two totally different things be the same in so many ways?
Here is a good example:
Wilson Sensation 16, natural v Wilson Sensation Plus 17, black.
Same but Different!
Looking a the stress/strain portion of the graph, it is nearly impossible to see any difference!
Both strings exhibit good elongation and elasticity.
Finally, when it comes to UTS the Sensation is a little stronger, as you would expect, for a 1.33mm string.
The Sensation Plus measures 1.26mm! So, the UTS is pretty good!
If you have been using Sensation but would like a black, thin string from Wilson simply use Sensation Plus!
Our Questron in Action!
As you know, Racquet Quest is a data-driven business, and data requires numbers. To generate those numbers, we have designed and built several devices.
One device is the Questron!
The Questron is used to test every string we receive, and the data is compiled to understand where that particular string fits.
So, instead of talking about it we have included a short video!
Thank you for watching our Questron in Action! If you have a question, or a particular string of interest, please let us know. We may have already taken the data! On GASP.network there are many graphs of previous tests. GASP.network is a membership ($40.00 one time) site.
SBS…what is it and why should we care?
We have made several posts regarding SBS, which is “String Bed Stiffness” and this is another one!
If you read this post we really need your comment(s), really!
String bed stiffness is the “feeling” when the ball hits the string on a tennis racquet. Due to the various string materials there will be “soft” and “hard” feelings. But wait, there’s more!
The string bed is made up of several strings, some longer called the Main string (M)and some shorter called the Cross string(X). Using “reference” tension each of the sets of string will be pulled at the same machine setting! It the machine is set at 50 pounds the tension head will stop pulling when it feels 50 pounds of resistance, regardless of what he tension inside the racquet head may be.
Let’s say you come into the world headquarters and we ask you what SBS you would like to have? Would you know? Probably not and not many would! We have grown up using the term “reference tension”, not SBS.
Reference tension is “number” you would ask your racquet technician to set the stringing machine tension system on. That number will probably be between 30 and 60 pounds (≈13 to 26 Kilo).
So, depending on many other variables, such as string material, string pattern, stringing machine, stringer technique, etc., you can end up with may different versions of the same “reference tension”.
A better way, and one we have been using for over thirty (30) years, is SBS but not everyone has bought into the concept, even though a qualified racquet technician will have a way to measure SBS! Maybe because it is too much trouble to figure out what your desired SBS from machine X would be from machine A! It is not!
There have been several really good SBS data collection devices but they have been difficult to use, and pricy! Not to mention gigantic!
Would you purchase a SBS data collection device?
How much would you pay for such a device?
Would you prefer a mechanical device or an electronic device?
The device must be portable, that is easily carried in a racquet bag or backpack
Yes or It doesn’t matter
It must be easy to use.
If you use an SBS device would you use a racquet technician that did not know what SBS is or how to measure it?
Thank you for adding your comments to this discussion! It is important stuff!
Racquet Quest, LLC
