Category Archives: Kinesiology
Here is where it gets interesting! This racquet, the Gravity S, is a realtivley thin beam 104 square inch racquet!
It has been a while since Head has presented this head size in a player oriented racquet! This 296 gram (10.44 ounce) racquet fills a serious need, in my opinion, for a “players” 104! The 24mm beam allows the stiffness to reflect the new “Spiral Tech” fiber placement.
I believe this racquet will appeal to serious adults seeking a comfortable racquet (RDC 56 strung) that can elevate their game.
This racquet has plenty of “cutomization” potential which makes it great choice for quickly advancing players.
Stop by and feel this combination for yourself, and take the demo for a real test on the court!
|Racquet Model||Head Graphene 360 + Gravity S|
|Reference Tension||55 lbs - 24.9 kg|
|String||Head Velocity MLT Black|
|Machine Used||True Tension Professional|
|Racquet Flex, RDC||55 - After stringing|
|Racquet Flex, FlexFour||45|
|Racquet - In Plane Stiffness||304.6 lbs/Inch|
|Head Area, cm2||678.4|
|Head Area, Sq. Inch||105.2|
|Number of Main Strings||16|
|Number of Cross Strings||20|
|Main String Grid||7.25|
|Cross String Grid||9.87|
|Density (% of head filled with string)||.680|
|Average Cross String Space||.495|
|Average Main String Space||.453|
|Dynamic Tension, Kp, ERT||37|
|Dynamic Tension, Lbs/in||206.9|
|First Moment, Nm||.752|
|Swing Weight, Kg/cm2||292|
|Swing Weight, Ounces||10.30|
|Swing Weight Calculated||320.4|
|Head Weight, %||48.0%|
|Center of Percussion||20.9|
|Dwell Time, ms||8.50|
|Efective Stiffness - lbs||27.7|
If you read “Play Like a Girl,” you will have a good idea where this is headed! This part is intended to make it possible for you to experiment without feeling “forced” so it is based on “numbers” not “feelings.” Of course, feel is relevant to tennis players.
Two events this week make it easy to compare numbers and feeling.
- A racquet came in from a tournament playing junior that had the good fortune of picking up a competitors racquet and recognize the difference immediately and wanted to try it. His racquet is a Babolat Pure Strike 16×19 with a swing weight of 302! The racquet he picked up has a swing weight of 341! His revised swing weight is 325.
- A really good young lady is switching racquets and string setup, so there is a four (4) inch strip of 1/4″ tape on the inside of each side of the string bed in the 3 and nine o’clock position — total weight of about 5 grams, for a swing weight of 321. During a training session, a coach said the racquet was too heavy and removed about 2 inches total of tape, maybe 1.5 grams, and everything was fine! This player is strong enough to play with a 335 swing weight so was this a “visual” suggestion, and the process of removing tape convinced the payer that the racquet was now much lighter?
In case you don’t remember there are 28.35 grams per ounce. So you can see that 1.5 grams is quite small!
Did you know that a dry overgrip is about 5 grams and a wet over grip can be as much as 12 grams? The location of an overgrip (under your hand) has virtually no effect on swing weight so it generally goes unnoticed.
Unless agreed upon in advance most weight can be removed or re-located so don’t fear your friend…weight!
We were thrilled to have Dr. Brad Goodman visit the World Headquarters to produce a session for his Doc-Talk-Live program!
Dr. Goodman is a tennis player who wants to know more about equipment in an effort to protect his body and beat his opponents. You can catch this episode here: Doc Talk Live
In addition to the session Dr, Goodman was given the opportunity to “stretch” both a very stiff string and a very “stretchy” string, something that he, and many others, have not done. Needless to say he was amazed at the difference.
Dr, Goodman’s visit was a great opportunity to have a real conversation about tennis equipment. Please let me have your comments!
In our “Recommended Stringing Frequency” calculator we state that this frequency is to get the best possible performance from your racquet. But, what exactly is “best performance”?
To establish this we need to take you, the player, out of the picture for a moment and concentrate on the racquet and string setup. the reason is simple: no two players strike the ball the same way.
We start with “Swing Weight” which is the most important dynamic property of a racquet. So a higher swing weight will contribute to power and stability, thus performance.
Overall weight is important because you need to be able to get the racquet to the court and out of your bag! Heavier racquets contribute to energy, stability, and comfort.
Effective Stiffness is important because it represents the stiffness of the string bed (SBS) and the racquet stiffness (xxRA, or something like that). This number represents the impact each time you strike the ball. The higher the number, the stiffer, of course.
Of the four (4) things mentioned above we can control the string bed stiffness with ease. One of the easiest methods is to string your racquet regularly to maintain the effectiveness of the elongation of the string being used. Elongation relates to energy return in a string and while strings will stay resilient for a long time a well-worn string takes a while to return energy to the ball.
You, the player, of course, determine performance so when using the SFR you can enter a high UTR or Style rating or a low UTR or style rating if maximum performance is not required.
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