SBS is “String Bed Stiffness” and is the stiffness of the entire strung area of a tennis racquet. SBS is not the same as “tension” and this is important to understand. Why?
When you talk to your racquet technician about “tension” it is, normally, about what number to set on the stringing machine. This number is usually called “reference tension” and every stringing machine has a way to set how many pounds, or kilo’s, it will pull each string before it stops.
Here is the problem with “reference tension”. It means different things to different machines! A “reference tension” of 55 pounds will result in a different SBS when set on different machines. If you take your racquet to technician “A” who uses a lockout machine it will have a different SBS than technician “B” that uses a constant pull machine . Over time, perhaps, each technician will arrive at the perfect machine setting to satisfy your requirements.
Why do all of this? You need to request a SBS! When talking to your technician you will talk about resultant SBS. So each time you have the racquet strung it will have the same SBS regardless of what machine it is strung on. The SBS number will be based on the diagnostic device the technician uses to collect data. There are three (3) or four (4) devices that will be familiar to all technicians:
The two Beer’s devices will return virtually the same number since they use the same technology.
The Babolat RDC uses, as far as I can determine, a combination of deflection and voltage. The FlexFour uses a deflection and includes the racquet stiffness..
So, the numbers may be different but it is the variation between tests that are crucial. If you are serious about your SBS I suggest you get a Beer’s ERT300 (about $180.00) and keep it in your bag. If the racquet has a SBS of 41 right after stringing you should consider having it strung when the SBS has decreased to about 33. For any device you should consider a reduction of twenty (20%) percent a reminder to have the racquet strung, soon!