# String Bed Linearity…What is It?

String Bed Linearity is how the stiffness of the entire string bed reacts to increased loads.  A load would be how hard the racquet hits the ball, or impact.

I collect this data on virtually all racquets.  My methodology  involves applying a measured load to the string bed and recording the force required to deflect the string bed that amount.  The load is increased five (5) times and the corresponding force is recorded.

The data shown here is for a racquet that was strung at a low reference tension.  Increasing  reference tension will, of course, increase each load number, however, the “percent linear” should be proportionally the same but the non-linearity may occur at a lower load.

If, for example the first load requires a force of 13, like the chart shows, then a linear string bed would increase by 13 for each deflection with a final number of 65.

Then, the first deflection for both “perfect linearity” and “Measured Linearity” will be the same, in this case 13.

To better understand why this is important it is necessary to know that polyester based string, such as this one, exhibit very low elongation, somewhere around 5% at 60 pounds.  This requires more force by the player to return energy to the ball and take advantage of the string movement that is thought to “snap” back and create ball spin.  The potential issue is that this is the force area that begins to go non-linear and stiffer!

String Bed Linearity Example

I prefer to use polyester based sting only for players that have developed strokes, and, developed strength in the wrist and shoulder.  There are many players that are not able to realize any advantage of polyester, and , therefore, should be introduced to alternative strings.

In summary, then, it makes sense to find a string that exhibits “perfect linearity” or slightly below perfect.  This means the string bed is getting “softer” as the ball is hit harder.  As you would expect string bed linearity depends not only on the string but the string pattern, tension set, the machine, and the person installing the string.