Category Archives: Knot Strength
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!
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.
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…
The winner is Tecnifibre MultiFeel 17!
This string, MultiFeel, is a very “soft” polyurethane bonded construction that offers playability and a good value!
Tecnifibre MultiFeel !7 Gauge
This graph shows the high elongation and nice elasticity of this string. The fairly linear slope up to failure would indicate some consistency of shot all the way up to breakage!
The knot strength is pretty good for a thin multifilament and we can say from experience that the know is going to be fine when property “tightened” and large enough to not slip bak into the grommet barrel.
If you are considering a multifilament this would be a good one to try!
Racquet Quest, LLC
