I have on my bow-weighing machine an attachment for breaking string.
I would be glad to make tests of strings for anyone who might desire to mail me a test sample as outlined below, the results to be published in A. B.-R. identified by number only but with each contributor knowing his own number, which should be of interest. It is a test of material, evenness of strands, design and construction of loop, etc.
I would suggest that any ambitious string maker prepare a double loop string 21½ inches to 22½ inches long with the loops desirably served and of a size that will go over a ½ inch pin. The winner will be chosen as the man making the string having the highest strength weight factor. This would simply be the pounds' breaking strength of the string divided by its weight in grains. If the men would send a thread of which the string is made, we could also arrive at the efficiency of his product, which would simply be the pounds' pull of breaking strength divided by the number of strands times the strength of each strand. I would need about 10 feet of thread to determine this.
Just a hint in this connection: bees wax does not increase the strength of a linen string but merely makes it more durable. Water does increase the strength of a string,-at least a perfectly bonedry string is very weak and by an amount more than is accounted for by the percentage of moisture re-moved.
The string should desirably be under 200 pounds in total strength, that is no more than 40-5 pound strands or 20-10 pound strands; in other words, an ordinary target string would serve.
—Forrest Nagler, 7428 Oakhill Ave., Wauwatosa, Wisc.
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The response to the invitation on Bow String Testing has been, to put it mildly, ample. My living room has at times resembled the bottom of a bowl of noodle soup.
The attached table shows the result to date. The best string is No. 21. The best thread was No. 13. A sample of this thread was later sent to the maker of No. 21 and he produced No. 25: something went wrong. The second best string is No. 19, from a maker whose average by the way is as high as anybodys and whose results are very consistent.
It is interesting to compare the strength of the strings with the product of the number of strands and the breaking load of the thread. No. 16 got more than the thread tested, but the reason was that when separate strands of a salmon twine are tested, they do not do as well as the assembled salmon twine. This explains the apparent indication that the string is stronger than the sum of its assembled strands.
No. 5 and No. 6 are cotton, which shows two or three times the stretch obtained with linen. This is very undesirable in bow strings. No. 14 and No. 15 are a black product. Nos. 5, 6, 7, and 8, show low results because the loops and splices are too heavy.
In further explanation of the accompanying table, the strings are 21½ inches long with double loop. The value of K is simply the breaking load in pounds divided by the weight in grains. The stronger the string the higher becomes the value of K: similarly, the lighter the string the higher becomes the value of K.
On the thread, K for thread is labeled KT and is really the pounds per square inch breaking load divided by the specific gravity of the string. As pointed out by Maurice Taylor and as has appeared in previous letters, this factor, i.e., strength divided by specific gravity, works out without measuring diameter of the string in the following form:
|----------------------||= KT =||----------------|
|Specific Gravity||Wt. Grains|
All I do is to take a piece of thread, weigh it, measure its length and its breaking load, and figure KT from formula. The very best of steel gives a figure of about 30,000: a good piece of wood goes 40,000 or 50,000: linen as shown in the attached table may go as high as 82,000. By the way, the piece that did that is labeled "3-cord #3 5 reverse twist" and it came from a firm in Boston. I don't waste any time making strings of thread that tests under 50,000. No. 27 is of interest as it is a Sears Roebuck twisted paper string.
|double loop strings||thread|
|No.|| Wax or |
| Weight, |
|Strands|| Fail., |
|K||Length|| No. |
| Weight, |
|25||Not||23.2||Spl.||154||6.65||Same as #13||79,000|
|5-6-7-8 Ends too heavy.|
|* 16 Probably too low. 2 strands of 14 cord salmon twine.|