The following tabulation was derived using the Stringway conversion chart as a basis. The data points from the Stringway conversion describe a relatively smooth curve so I felt it reasonable to assume the smooth nature of the curve would continue beyond the 13.0 KG/CM shown in the published conversion tabulation and graphic.
I have Yamaha Secret 04 and 06 racquets which have KG/CM stiffness ratings above the 13.0 KG/CM. I measured the Secret 04 at 17 KG/CM and the Secret 06 at 15.4 KG/CM. Others on the TW forum have stated the Secret 04 had an RA ~81 which was taken from an old review of the racquet and the Secret 06 had an RA of 78 (but the reference was even more ambiguous).
I took the conversion graphic provided by Stringway and extended the line further, doing my best to match the slope and curvature. After having done this in Photoshop I pulled the graphic into the Dagra graphic digitizer. This software allows for pulling data from graphs which can be put into spreadsheet applications like Excel.
After exporting the data produced by Dagra I imported it into Excel and (not surprisingly) the graphic produced was extremely similar to the Stringway conversion graphic and was very smooth. I was pleased that the 15.4 KG/CM was very close to the 78 RA provided by another source and the Secret 04 came to 80 RA which is very close to referenced RA noted above.
The utility of this data is perhaps dubious because of the lack of concrete references but it is perhaps better than the nothing I had to use for my racquets previously.
I have absolutely no idea as to whether this is of any use to anyone but myself, particularly since modern racquets having an RA over 70 are not in abundance. I believe information such as this should be shared instead of sitting on a hard drive. I know there is an equation for this data because the graphic is smooth but it is currently beyond my ability to tease out. I've read several times that the RA scale goes from 1 to 100 which is good information but it does not assist me in deducing an equation for this data. Unfortunately, producing the equation is not in my tool kit but perhaps there is a physicist or clever engineer who could perform the task with little strain just for the fun mental exercise.
The KG/CM data is reproducible by anyone having a Stringway Lab 2 and I admit knowing the RA does little (or nothing) for my game or equipment maintenance but I'm just a curious guy by nature.
KG/CM RA
4.5 => 21.6
5.0 => 29.6
5.5 => 35.9
6.0 => 41.4
6.5 => 45.9
7.0 => 49.7
7.5 => 52.9
8.0 => 55.9
8.5 => 58.6
9.0 => 60.8
9.5 => 62.9
10.0 => 64.7
10.5 => 66.4
11.0 => 67.9
11.5 => 69.3
12.0 => 70.6
12.5 => 71.8
13.0 => 73.2
13.5 => 74.4
14.0 => 75.3
14.5 => 76.1
15.0 => 77.0
15.5 => 77.8
16.0 => 78.6
16.5 => 79.3
17.0 => 80.0
Update: With an assist from esgee48 I was able to produce an equation using Excel trend line analysis. If you find this Excel formula disagreeable the fault lies fully on my plate. esgee48 simply advised me on how to find a formula for the data shown above.
Thanks very much!! I won't be doing anymore on this curiosity quest (actually I did more) but I've seen a number of times where people have asked for this translation/conversion equation and have been told it doesn't exist. Well, it still doesn't (actually) but this is the best approximation I could do. While Excel's trend line utility will display an equation I found it to be erratic (could very well be me) so I used the regression utility on
http://www.xuru.org/rt/PR.asp#CopyPaste and increased the polynomial count to 8.
I'm very happy with the accuracy of the equation delivered but the accuracy of this equation and its utility is dependent the following: 1) the original accuracy of the conversion data from Stringway, 2) the assumption the data curve produced using the Stringway data continues smoothly past 13 kg\cm, and 3) my ability to accurately simulate the curvature of the graphed Stringway data past 13 kg\cm. Since the numbers are close to the Yamaha Secret 04 and 06 RA numbers I've seen from other sources I feel there is some hope of accuracy.
I used an 8th order polynomial and it’s within ~.1 RA for KG\CM values between 4.5 to 18. That is good enough for me but if someone wants it to be tighter they will need to increase the polynomial order until satisfied. Go outside the 4.5 to 18 range and the values are not reliable (producing ridiculous numbers). Inside that range the polynomial has a Coefficient of Determination: R2 = 9.999925589·10-1
The polynomial equation is: y = -2.493577256·10-7 x8 + 2.561143629·10-5 x7 - 1.144037698·10-3 x6 + 2.917184681·10-2 x5 - 4.678339064·10-1 x4 + 4.888608456 x3 - 33.22337151 x2 + 141.2515564 x - 239.9506264
Cut and paste the following Excel formula in cell B1 and put any KG\CM value between 4.5 to 18.0 in cell A1:
=-0.0000002493577256*(A1^8)+0.00002561143629*(A1^7)-0.001144037698*(A1^6)+0.02917184681*(A1^5)-0.4678339064*(A1^4)+4.888608456*(A1^3)-33.22337151*(A1^2)+141.2515564*A1-239.9506264
It will return a value within ~.1 RA of the values listed in the Stringway conversion chart.
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