Category Archives: shoulder
In the words of Lord Kelvin (May 1883) “When you can measure what you are speaking about, and express it in numbers, you know something about it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts advanced to the stage of science.”
That is why every racquet we do has over fifty (50) numbers attached to the finished data. Most of these numbers will remain unknown to the client, but for us, it is imperative that we know them.
Which leads me, again, to this very important discussion.
Every day we see a statement from tennis string manufactures claiming, or suggesting, their string is the “softest ever tested” and other claims. What the heck is “soft” anyway? There is a lot more to it than meets the eye so we have done significant analysis on bunches of string and can now quantify “soft” as it relates to tennis string.
What is “soft”?
In 1994 I did a presentation for the USRSA in Atlanta. What was the topic?
It is now 2016, and we are still trying to understand string! Especially “soft” polyester based string.
In 1994 PolyStar was the only polyester based string I was familiar with. Since then there are dozens of offerings from anyone that can afford to purchase from manufacturers and market the string. If you have a desire to do it, I applaud you!
In 1989 I started testing string and calculating “power potential.” Why “power potential”? Because “modulus,” “elongation” and “elasticity” didn’t get to the bottom line of string performance quickly enough! The steps to arrive at power potential are many.
For the testing, several calculations take place including “stretching” the string as in a ball impact. The difference between the first calculation and the “stretched” calculation is the power potential!
I have calculated hundreds of power potentials but have not until now quantified “soft.”
I think now is the time!
Under the direction of Dr. Rich Zarda, we have done a tremendous amount of work on this issue so we can now distill this work into the following explanation.
So, what is a “soft” tennis string?
Strings in a tennis racquet carry the ball impact load in two ways:
1) Via the pre-load string tension placed in the strings caused by a stringing machine (and the racquet frame “holding” those tensions in place) and
2) Via additional tensions that develop in the same string caused by the elongation of the strings as they deflect with ball impact.
Both of these conditions occur simultaneously and contribute to the string bed stiffness (SBS, units of lbs./in). Racquet technicians measure SBS by applying a load to the center of a supported string bed and measuring the resulting deflection. Dividing the load by the deflection provides the SBS (lbs./in). The lower the SBS, the more power you have (power here is the ability of the ball to easily rebound from the string bed), but the less control (presumably); the higher the SBS, the less power you have but, the more control you have (presumably).
One more point about SBS: the lower the SBS, the less the load your body will feel for a given swing. But for an SBS too low (less than 50-80 lbs./in), balls will be flying off your racquet going over the fence; and for an SBS too high (greater than 200-240 lbs./in), the racquet will hit like a board with significantly less ball rebound. So the most common SBSs are between 100-200 lbs./in: a balance between control and power.
As already expressed, SBS is a function of the pulled string tension and the string elongation. Here is what is interesting: For large string elongations (for example, greater than 15%) and reasonably pulled string tensions (greater than 30-40 lbs.), SBS only depends on the pulled string tension, and it does not depend on string elongation. Additionally, for this condition, SBS, for these high elongation strings, does not change as a ball is hit with more impact.
But for a string bed with low elongation strings (less than 5%) under low pulled tensions (less than 20 lbs., or tensions that have been reduced due to racquet deformation and/or string tension relaxing with time), the SBS additionally depends on the string elongation and will significantly increase, in a nonlinear ever-increasing way, for harder ball impacts.
In order to achieve a repetitive feel for a player when hitting with a racquet, it is best to have an SBS that is independent of an increasing ball impact force. This will lead to a more consistent playability of the racquet, which includes a more repetitive feel. This desired “feel” implies using high elongation strings (greater than 10%). If low elongation strings are used (less than 4%), the SBS will significantly increase as the ball impact force increases, resulting in a racquet feeling “boardy” for higher impact loads. And low elongation strings will cause un-proportionally increasing load into the body.
As you can see by the graph, elongation contributes to SBS in a big way. The red line indicates a stiff string, about 4%, and the blue line indicates a “soft” string, about 15% elongation. You can see the loads increase dramatically as the impact increases. So the harder the hit the higher the loads on the body.
So to the question asked at the start “What is a soft tennis string?” In the context of the SBS discussed above, I would suggest that a soft tennis string is one whose elongation is 10-15%, and a stiff tennis string is 4-6%. And any string under 4% should be categorized as ultra-stiff.
String elongation (soft, stiff, ultra-stiff), stringing machine strung tension, and string pattern(s) all contribute to SBS and SBS is an important measure of how a racquet plays and should be adjusted for an individual player, stiff and ultra-stiff strings can lead to less-repeatable racquet performance and player injury.
Soft = 10 -15% Elongation Power Potential Range = 10.0 – 16.0
Stiff = 4 – 6% Elongation Power Potential Range = 4.0 – 7.0
Ultra Stiff = Less than 4% Power Potential Range = .65 – 3.96
I spend hours each day dealing with tennis racquets, strings, machines and questions of all sorts!
By doing this I am learning what is important to tennis players but it should not require a one-on-one discussion to learn this, in my opinion.
So, what is important to you? Here is what I am discovering.
Comfort. It goes without saying that you don’t want to play tennis if you are hurting! Players are requesting racquets that are more arm friendly. But wait, the racquet really holds the string which has a huge impact on comfort. So should we begin with string? I think so!
String. Every string I have has undergone a comprehensive testing procedure to determine elongation which in turn is converted to Power Potential. The higher the elongation the higher the power potential and the less stiff the string bed will feel when the ball is hit hard, all other settings being equal. If you have a stiff racquet it is important to select a string and tension that will mitigate the racquet stiffness to some extent. Every racquet we do has the “effective stiffness” calculated which is the combined stiffness of the racquet and string bed. Once we have the preferred effective stiffness for a customer we can achieve that even if a new racquet is added to the mix.
Durability. We try to associate the cost of racquet stringing to “cost per hour” of play time. What is your threshold? $1.00 per hour or $10.00 per hour? When considering durability do not confuse “performance” with “durability”! There are several strings that may not fail for several months however the performance is gone in a few hours. This is typical of polyester based strings. So, even if the string is still intact the performance is way gone!
Cost. The cost of tennis racquets is increasing, sometimes justified, sometimes not but are rising none the less. If cost is your “driver” some navigation around the market is important, however, we do not suggest you buy the “cheapest” thing you can find without a thorough understanding of what you are getting. We can assist you in evaluating racquets from any source.
When the discussion is about stiff polyester string, it will always include the word “hybrid”! Typically this word is used to convince players that by putting a “soft” multi-filament string in the cross position the string bed will be easier on the wrist, elbow, and shoulder.
Intuitively this makes sense, but in reality, the reverse could be true!
I began analyzing hybrid string beds years ago and did many just to test the theory. At the time it did not seem so important because, frankly, the use of polyester based string did not approach the usage of current times.
I have nothing against the polyester string(s)! I do have an issue with bad applications of polyester string(s).
I am bringing this up again because recently an “interviewee” stated that that replacing the polyester cross string with a multi-filament would cure the ills of a very stiff string bed.
The bottom line:
A high elongation string of any material can increase the string bed stiffness of a hybrid string bed!
How can this be?
Stiff (polyester) strings are “stiff” and the tension applied to them during stringing is low. However, high elongation (multi-filament) strings will be influenced more by tension and become “stiffer”. The cross stings are typically shorter, and there are more of them, so the combined affect is stiffness.
The initial reaction to this conundrum is to automatically reduce tension on the cross string by a certain amount. Again this raises another issue, and that is racquet distortion.
During the installation of the main strings most stringing machines will allow the racquet to become wider, sometimes a lot wider! So, reducing the cross string tension may not return the racquet to the designed shape. What happens then is the racquet will continue to move around trying to find a “safe” place and therefore the string bed stiffness changes.
In summary, the hybrid string bed will not be statistically different than the full string bed of polyester. This is even truer if the initial string tensions of the polyester are very low, such as 35 to 40 pounds.
So if you feel the need to use polyester just go with lower, lower, tensions.
As you know I do a lot of string evaluations for myself, my customers and some manufacturers. I do this to have a clear understanding of what a string does at various tensions in various racquets ,and, also in a “controlled” environment!
So, if you ask me for a recommendation my answer will based on data, and, of course some anecdotal evidence. I know most manufacturers try very hard to place the string into the correct category but sometime they simply miss!
There is an ongoing conversation(s) regarding the categorization of polyester based strings relative to racquets and player stature. This may, for example, look like; “If you use Racquet “X” and are under fourteen (14) years old do not use “XYS” string at tensions higher than 40lbs (18.1 Kilo)”.
It is well known that it is very “tricky” to use polyester based string for most younger players that are experimenting with stroke production and still do not have the physical strength to really take advantage of what polyester may offer. For the record I do not recommend it.
Durability is always an issue so when I ask for “playing time” it should be in hours, not days or weeks, but hours. It is a big help to know what portion of those hour are training or playing. It is obvious that one (1) hour of training will be more “destructive” than one (1) hour of tournament play.
The more we know about string the better the choices can be. It is my imperative that the string matches/enhances the application. Tennis Warehouse, the premier online source for tennis stuff, is also very active in the effort to enlighten players in the selection of the string they order. We can do this!
What do you think?