Bow Mechanics 3: String Theory

Yes, I know I said that the next post would be about arrows but you shouldn’t believe everything you read online.  I want to deal briefly with strings before moving on to arrows.

First and most importantly, you must check with your bowyer before you change your string.  Use of an unsuitable string can break your bow beyond repair.  Be warned!

There are many different kinds of string.  In particular, there are various string materials and two main ways of turning the threads into a bow string.  I am not going to go into much depth but I want to discuss two main features of strings that can affect performance: stretch and mass.

String Stretch

You might think that a nice elastic (stretchy) string would give some extra pace to the arrow.  You’d be wrong.  If you use an elastic string then at the end of the power stroke it doesn’t stop dead, leaving the arrow to fly off.  Instead, the string slows down as it stretches.  By the time the arrow flies off it has lost some of its speed.

Studies in fact show that the various modern materials are unlikely to differ enough in their stretch to make a significant difference to arrow speed.  Materials such as B50, Dacron, FastFlight etc have different elasticity but they are close enough that the difference in arrow speed from stretch will be no more than 1 or 2 feet per second (fps).  This is about the same as increasing your draw weight by 1 or 2 pounds.

The way the string is put together will also affect stretch.  There are two basic methods of making a string: Flemish splice (also known as laid in) and endless loop.  You don’t need to know the technical differences but it is worth knowing which you have.  As a general rule, Flemish splice strings have slightly more stretch to them.  Endless loops have higher performance: they don’t stretch as much and they can be made identical more easily than Flemish strings (allowing you to maintain performance the same with spare strings).  The Flemish splice string will also “creep” more.  This means that once the bow is braced the string will stretch so that it becomes slightly longer.  This, of course, lowers your brace height over time, making consistency harder to achieve.

So why would anyone choose a more elastic string than necessary?  Because they are more forgiving on the bow.  At the moment that the string snaps taut there is an awful lot of force going through the tips of the limbs where the string attaches.  If there is no ‘give’ in the string then weaker tips will break at this point.  This is the main reason why you should check with your bowyer before using a new type of string.  In particular, bows made from traditional materials such as horn, wood and sinew may well not be able to cope with modern low-stretch strings.

String Mass

String mass generally has a greater impact on performance than stretch.  A high-performance string material such as FastFlight or Dyneema weighs much less than something like Dacron (which is what most “off-the-shelf” strings are made of).  To take an example, I have in front of me two strings.  One is a Dacron string that weighs (including the serving) 151gn.  The other is FastFlight and weighs 63gn.

So what?  Well, the string, like the arrow, is a mass that needs to be driven forwards by the bow’s stored energy.  The heavier the string, the slower the arrow will be.  According the Traditional Bowyer’s Bible, Vol.1 (which admittedly deals primarily with straight limbed wooden bows), a 20gn increase in string mass will slow the arrow by roughly 1fps for a bow of around 50lbs draw weight.  The effect is greater on lighter bows (because a greater percentage of the stored energy is needed to move the string).

This assumes that the added string mass is evenly spread.  Mass added to the centre of the string has about 3 times the effect (as does mass added to the arrow, but we’ll come to that in a later post).  Those little brass nocking points clamped to your string weigh 5gn each.  If you have two of them on your string and are shooting a relatively light bow then you could be losing about 2fps just from them.

All this may not seem very much but there are good reasons to get it right.  One is that the string is the cheapest way to gain arrow speed.  Bows and arrows are expensive but a decent string is not.  If you can reduce your string mass by 100gn (which you could easily do if your current string is one of the big heavy horrors that you sometimes find) and replace your brass nocking points with a dental floss wrap or similar then you can make considerable gains in arrow speed.  Further gains can be made with your arrows, to which I shall turn next.

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