I have been thinking a little about measuring the frame stiffness and would be interested in hearing you input. You can bend the frame and measure the deflection, as you do in the Babolat RDC, or you can look at the frequency of the vibrating frame. The latter is easier to measure without any special equipment: Record the sound and use a program like the free Audacity (PC and Mac) to calculate the frequency. I have also added frame frequency measurement to the latest version of racquetTune. To get the relation between the stiffness and the vibration we can look at simple beam theory. If we have a beam with a load in the middle we can define the stiffness as the load over deflection F/d: The Babolat RDC has this setup. The deflection is measured at one end, but the difference is only a question of a constant. The free vibrating beam has a basic frequency f: If we combine these two models using basic beam theory we can get a relation between the stiffness and the frequency: Where M is the mass of the beam. The constant 4 is a round off (it is actually 3.78 This formula is simple, but contains several approximations. The worst being that it assumes that the cross section of the beam is constant which definitely not is true for a racquet. However, since all racquets have a similar form it might serve to classify racquets. To test this assumption you can look at the frequency data compiled at: http://twu.tennis-warehouse.com/cgi-bin/vibfrequency.cgi If you take this data and plot the 4Mf^2 vs RA values you get: You can make two observations from this: 1. There is definitely a relation between the two stiffness expressions 2. The scatter is too large to make a direct calculation of RA from the frequency with acceptable accuracy. Which expression is "best"? Well here I leave it open for discussion. /Sten PS If anyone want to play around with the TWU frequency data I have put them in a spreadsheet that you can download here _________________________________________________ racquetTune, stringBed and swingTool racquet apps for the iPhone/iPad.