Okay, confused. I read through the calculations page, did your calculation examples therein to make sure I was using the calculator correctly, and plugged in my values. However, my results differ significantly. I assume I am doing something wrong. Here is what I got (in italics): T=(fld)sqrd/K Using .116" diameter: T=(78*80cm*.295cm)sqrd/19400 T=174.67 Now, using 70% of this T, calculate diameter: 70% of 174.67# = 122.27# d=sqrt(K*t)/f*l d=sqrt(19400*122)/78*80 d=.247 cm = 2.47 mm = .097" Assuming that 308# tension is needed to bring the .118" string to pitch (though I don't know how), using 70% of 300#: d=sqrt(19400*210)/78*80 d=2018.4/6240 d=.323cm = 3.2 mm = .127" Interestingly, the example on the Calculation webpage of yours is very near my case, only it is a half-step higher and 10 cm longer. The resulting diameter in your example is 2.63 mm, or .104", which is about half-way between your recommended diameter of .093" and my current diameter of .116". Also, using a lower tension of 190# (like in your example) yeilds a diameter of 3.08 mm, or .121", only slightly lower than the above diameter. Note that in either calculation, the resulting d is above what I was going to order (.116"). Questions: why did our calculations differ? How do we know what T to shoot for? I apologize if any of this is less than spectacular. I haven't done math like this in many years and apparently that part of my brain has atrophied, <div class="gmail_quote">On Tue, Jul 12, 2011 at 5:25 PM, John Delacour <<a href="mailto:JD@pianomaker.co.uk">JD@pianomaker.co.uk</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> At 16:19 -0400 12/07/2011, Noah Frere wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> ...I'm thinking of ordering from a different company, being super careful while replacing, and seeing what happens. If it works normally, I could call Schaff and ask for a refund. If they refuse, no big deal. </blockquote> Did you tell Schaff which note the string came from? If not then they had no alternative but to make the string to pattern, since they lacked the data required to calculate whether the tension was too high.<div class="im"> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> FYI the strings that broke - and yes I mic'd them before installing - were .040 core; .118 winding; hitch to winding 5 1/8"; winding length 30 3/4". The new string will be the same lengths but .039 core and .116 winding. </blockquote> </div> So you have a core of 17.5 or 18 with the length of winding 77.1 cm. Let's add 3 cm to get the speaking length (which is the only relevant measurement) and we get 80 cm. O.116" (your lower figure) is 2.95 mm. The tension you require to bring this string to pitch is 308 lbs. which is miles too high and the string is bound to break. Use a value of 19400 for K in the formula you find here: <<a href="http://pianomaker.co.uk/technical/string_formulae/" target="_blank">http://pianomaker.co.uk/technical/string_formulae/</a>> and work out what diameter you need the string to be, considering that for mwg. 17.5 you should not exceed 70% or 300, ie. 210 lbs., 220 lbs. for mwg. 18. You will find that the proper overall diameter for the string is 0.093" or less if you use a #17.5 core. Do this every time you order an odd string and you will get no breakages. Otherwise you have nobody to blame but yourself. JD </blockquote></div>