How brace-height affects a bow’s energy and why a recurved stores more energy
Originally published in Archery Focus: USA’s official olympic magazine.
What gets accomplished when an amateur bow-maker and a techno-weenie get together during the cold Christmas holidays in Canada? Well in this case, they designed a device that accurately computes and plots interesting bow performance characteristics from one draw of the bow. More specifically, draw-weight (force in lbs) and the corresponding draw-length (displacement in inches) are recorded approximately 200 times throughout the draw of the bow. So accomplishing this, we grabbed the first dozen bows within reach and printed each bow’s performance. It did not take long before we started doing experiments. From some of the data, here is an explanation of how brace-heights affect a bow and a little comparison between self-bows and recurves.
First of all, let us get the word brace-height out of the way. Brace-height is the distance from the string to the bow (when the bow is strung). Draw length is the distance from the bow to the string at a full draw. Many bowyers once assumed that an increased brace-height would shoot arrows faster because of increased poundage and limb strain. This seemed to make sense, but actually was a myth.
Whether a recurved or a non-recurved bow, increased brace height actually slowed the arrow speed down because the bow stored less energy. When a bow’s brace-height was small, the string actually traveled further when pulled to full draw. This stored more energy then if the bow was braced higher, resulting in the string traveling less distance. How is energy calculated? The energy of the bow is a function of the draw length and the draw weight, throughout the whole draw. When plotted on a graph, the area below the plotted line can be calculated to determine the energy (in inch-pounds). Higher available energy is the key.
Before going any further, it is important to be aware that certain bow designs better utilize stored energy upon release of the arrow. A lot of the stored energy can be wasted by extra end-tip mass, bulky or long limbs, string diameter and the list goes on. However, knowing the energy a bow stores is still a very important part of a bow design.
|In figure 1, a 30 lb, 62″ recurve was plotted comparing three brace heights. At a glance, it became clear that when the bow’s brace height was less, the area (energy) under the line was greater, hence a potentially faster arrow speed. Increased brace heights raised the poundage of the bow, but stored less energy because of shorter string travel.Tests were performed on a variety of bows with the same results. In short, less brace height allowed for more energy storage. Of course if a bow was braced too low, there would be other problems , such as accuracy. Ideally, the feathers should not hit the bow before leaving the string.
I mentioned something interesting about energy earlier. The bow’s potential “energy is a function of the draw length and the draw weight throughout the whole draw”. That is why a recurve of less weight can hold its own with a non-recurve of more weight.
In Figure 2, a 62″ recurve with a draw weight of 30 lbs, is compared to a 62″ non-recurved bow with a draw weight of 35 lbs (both bows had a draw length of 26″ and a brace height of near 5″).
The non-recurve pulled somewhat evenly throughout the draw. The recurve showed different characteristics during the draw. The reason that this 30 lb recurve offered as much energy as the 35 lb non-recurve was due to the higher weight during the earlier stages of the draw. In the non-recurved bow’s defense, its limbs were not working (being strained) as much as the recurved limbs.
To prove a point, the straight bow from figure 2 was increased to a 7 inch brace-height (see figure 3). As expected, the higher brace-height (in red) held less energy. As far as energy storage, the recurve held the advantage over the non-recurved bow.
As a bow-maker, it was satisfying and educational to have acquired these statistics on my bows. In the future, we hope to measure new (or uncommon) bow designs, as well as creating an interesting little device that measures arrow speed.
By Chad Clifford and Doug Marsh.