I’ve been getting a lot of requests for string videos. I’ve also been getting a lot of questions about why a certain type of string broke or didn’t work, so this is the beginning of a small series on string building.
First things first, one of the most important numbers when building a string is the string’s breaking strength. This is the measurement in pounds of how much force it would take to break a string or rope. This does not take into account any friction or added abrasion, it’s simply a measure of how much a certain material can take before breaking.
Because a bow string is not just holding a certain weight but has to undergo the stress of snapping back with force, having a string with a breaking strength equal to the bow’s peak weight usually means your string will break. As a safety measure when dealing with fairly low-speed bows like longbows and PVC pipe bows (which essentially work like longbows) a minimum of four times breaking strength is advised for a string. This means that a 35 pound bow should have a bare minimum of 140 pounds breaking strength. I usually go by a factor of five for added string durability, which brings the strength needed for a 35 pound bow to 175 pounds breaking strength.
Below I have a list of the materials tested as well as a rough guide to how many strands are needed for a given bow weight. Also keep in mind that too thin a string or too thick a string won’t fit inside of an arrow nock properly. In the case of very strong cord, the string will be overbuilt to fit the nock. In the case of weak cord, a larger nock must be used. This chart does not go in order with the video, but is based upon weakest string material to strongest (based on string size not individual twine). The first line for each material states the name of the material and the breaking strength in pound. The second line goes over how many strands required for a particular weight of bow within reason. The minimum string diameter will be 1/16 of an inch and the maximum will be 3/16 of an inch.
Household Cotton Twine – 12 lbs.
3 strands up to 7 lbs. 5 strands up to 12 lbs.
Craft Hemp Twine -30 lbs.
2 strands up to 12 lbs. 3 strands up to 18 lbs.
Multi-Purpose Cotton Twine – 40 lbs.
2 strands up to 16 lbs. 3 strands up to 24 lbs.
Sisal Twine – 95 lbs.
1 strand up to 19 lbs. 2 strands up to 38 lbs. 2.5 strands up to 47 lbs.
Jute Twine (3 ply) – 105 lbs.
1 strand up to 21 lbs. 2 strands up to 42 lbs. 2.5 strands up to 52 lbs.
Heavy Duty Polypropylene Twine – 180 lbs.
1 strand up to 36 lbs (not advisable). 2 strands up to 72 lbs. 3 strands up to 108 lbs.
#1 Nylon Mason Line (8 strand) – 280 lbs.
1 strand up to 56 lbs (not advisable) 2 strands up to 112 lbs. 3 strands up to 168 lbs.
Polyester Kite Twine – 19 lbs.
20 strands up to 76 lbs. 24 strands up to 91 lbs. 28 strands up to 106.4 lbs.
Nylon Saddle Stitching Thread – 76 lbs.
10 strands up to 152 lbs. 12 strands up to 182 lbs. 14 strands up to 212 lbs.
B50 Dacron Bowstring Material – 75 lbs.
12 strands up to 180 lbs. 14 strands up to 210 lbs. 16 strands up to 240 lbs.
Braided Dacron Fishing Line (50 lb. test) – 85 lbs.
12 strands up to 204 lbs. 14 strands up to 238 lbs. 16 strands up to 272 lbs.
Keep in mind that natural materials tend to have very little stretch. This means that you can make a string for a bow very close to its breaking strength and the string will not be excessively stretchy. A linen string just strong enough for a 35 pound bow will stretch approximately as much as a Dacron string for a 180 pound bow, though the linen string is more likely to break since it has less of a margin of safety. Natural plant fibers are also more prone to wear and tear and wear out much faster than most synthetics. There are high quality hemp and linen threads that make fine bowstrings but I have not tested them today as they are exceedingly hard to find nowadays and most people will not have access to them.
Keep in mind that all synthetics stretch, so you will need a greater amount of breaking strength to avoid having a string that stretches like a rubberband. Such a string will not only rob the bow of power, but it will also cause the bow extra stress as the string will need to be made much shorter than normal, resulting in the possibility of the bow breaking while being strung. The main reason why the main bowstring materials are so low stretch is that their breaking strength is so high.
Hope this helps your bow building!