String Bed Stiffness Linearity
What is it and what does it mean?
String Bed Stiffness, sometimes seen as SBS, is the stiffness of all the strings acting in concert with every other string. When you apply a load to one sting it “pulls” along the rest. That is the String Bed Stiffness. This is different than “reference tension”!
It means when you hit the ball you are, in affect, hitting all the strings. The dynamics of this impact are very convoluted and only the few longer main and cross stings contribute much to the result. The strings around the perimeter need to be there but play a limited roll in performance.
String Bed Stiffness Linearity is another useful piece of data especially with the increased usage of very stiff polyester based string.
Here is how I test for linearity. I have “control” racquets, which, at this time, are Babolat Pure Drive’s. For most of my testing I will use the same racquet. Not just the same model but the same racquet. You know what that means don’t you? After every test the strings are removed and a new, and different, set is installed. This practice eliminates any variation in racquet dimensions and assures meaningful data.
After the racquet is strung and other important data is recorded the string bed is subjected to deflections of .100, .200, .300, .400, and .500 inches. At each deflection the force required is recorded. For example, the force to deflect the string bed .100 may be 20 pounds. That means it would require 200 pounds to deflect the string bed 1inch, (10 *20). The next deflection of .200 may result of 40 pounds. That means it would require 200 pounds to deflect the string bed 1 inch. You can see where this is going, can’t you?
So, a perfectly linear string bed would see the force increasing 20 pounds for each .100 of deflection. For example the force would be 20, 40, 60, 80, and 100 pounds.
With many strings, and especially very stiff strings in tension, they will not be linear. This is, for example, one of the reasons natural gut continues to be the easiest on the arm, and other body parts. Natural gut may (probably will) have a “positive” linearity which means it required less force as the deflection increases.
Polyester based strings may have a “negative” linearity meaning, simply, the string becomes “stiffer” has the load is increased. Why is this important? Many, if not most, polyester based strings do not have enough elongation to create “power”. That means you, the player, must produce most of the power. That in turn requires you, the player, to swing harder. Swinging harder puts more loads on the strings, which makes them stiffer!
Simply put, the ranking of strings, in terms of linearity looks something like this:
- Natural Gut
- PEEK (Zyex™)
- Aramid (Kevlar™)
Remember, each string has a specific construction to mitigate some of the “problems” while creating a string that offers some advantage.
If you are seeking a string that will be easy on your body look for a string that has a “profile” that turns downward as force increases. If you are looking for a string that allows, or forces, you to swing hard look for a “profile” that turns up as the force increases.
An upward turn requires you hit harder which, if the racquet is going from low to high, will generate more spin.