Category Archives: Testing Devices
To Pre-Stretch or not to Pre-Stretch?
What is Pre-Stretching, how is it done, and what does it do?
What follows is a brief, limited, and non-technical explanation.
- Pre-stretching is simply applying tension to the entire length of a tennis string.
- The best Pre-stretch is a tension of 25-30 pounds for 30-35 seconds.
- Pre-stretching stabilizes the string and reduces short-term creep (tension loss).
Over the years, especially in the last decade, concerns have been raised about the potential damage pre-stretching can cause to certain string materials, mainly polyester. We have found that this is not the case, and most strings, including polyester, benefit from a well-done pre-stretch.
Two material properties important to this discussion are elasticity and elastic limit.
- Elasticity, a fundamental material property, refers to the string’s ability to return to the original length after applying a load (tension). Understanding this concept will give you insight into the string’s behavior. An Elastic Limit is when the string has passed the point of no return…in other words, the string has been stretched too much and will never return to anything close to the original length. When the string has reached this spot, it has entered the “plastic” (opposite of elastic) stage.
We test every string for several properties, one of which is UTS, Ultimate Tensile Strength, which ranges from 120 to about 200 pounds. Not once have we experienced the string reaching the elastic limit before failing!
When pre-stretching is properly administered, it poses no danger to the string.
Pre-stretching offers numerous and significant advantages. These include better tension retention, stability, consistency across the string bed, and ease of handling a string with a high recoil memory.
As a player, you can ask your racquet technician to use or not use pre-stretch. It should be your decision based on performance and benefits.
It won’t hurt the string!
Power vs String Bed Stiffness
We have been testing strings for power potential since 1984! Currently, we have over 500 strings registered on our website, and we will put a link in this post that will take you there. Over the years, we have tested that several times that number but need to remove some strings that simply do not need to be in this data while keeping “legacy” strings for reference to today’s offerings.
What does the power potential of string have to do with String Bed stiffness? While a single strand of string tells us a lot about the performance characteristics of any string, it is the total string bed that you play with, not a single string!
We test for String Bed Stiffness in four (4) different ways: Babolat RDC, Flex Four, Master Tensometer, and our proprietary String Bed Master.
What we want to clarify is the power of the string bed stiffness, and to do that, we need to include the tennis ball, which is, by design, not very efficient. I believe the rebound percentage is about 53 to 58 percent. If the rebound percentage were closer to 100 percent, the ball would not stay in the court!
The graph below helps us understand what String Bed Stiffness will work in any given player situation.
Power vs String Bed Stiffness
I know there are a lot of lines, but I am certain you can understand them. We will help you along and explain why this is very important to you and us!
The vertical axis (Y) is Power, and the horizontal axis (X) is String Bed stiffness in pounds per inch. Every device we use for string bed stiffness presents the force in pounds per inch. The green dotted box represents the “normal” range (≈ 160. to 210.0) for string bed stiffness.
If we want to maximize the power of the ball contact and string, we would need a string bed stiffness of about 100 pounds per inch, which is achievable but not “normal.
The upper curve (brown or red) represents the sting bed only. So you see a power range of 20.0 down to 6.67. The lower curve (blue) introduces the ball and, due to the ball design, does not add any power but detracts from power. So you see a power range of 13.7 down to 8.38. Because we don’t know how hard the ball is being struck, we prefer to use the string bed-only curve. It is, however, critical for us to know how the ball is participating.
An interesting and important value of the graph is the power value. As the string bed becomes stiffer than the ball (≈ 210.0 pounds per inch range), the ball will provide more power than the string bed. Not much, but at least the ball will not detract from the power!
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.
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!
Racquet Quest, LLC
