BEYOND THE BEGINNER'S COURSE 1
Adrian Tippins looks at essential arrow literacy
I've always thought that the hardest thing for novice archers is attempting to understand and navigate the technical minefield that is unfortunately an essential part of archery. It's really easy to feel overwhelmed and incompetent. Whilst you can still shoot good scores without having technical knowledge, there are situations where an understanding is essential – especially when it comes to making a purchase. Most archers at some point or another have bought an incompatible item or something that's the wrong size or weight. Purchase mistakes are especially prevalent when it comes to arrows, as there are multiple weights and dimensions to get right. It's normally pretty straightforward if you’re using parts native to the brand and model of the arrow shaft, but there are lots of occasions where you may well be using a third party component, such as one of the excellent products from Tophat or Bohning for example. The method of sizing & labelling varies greatly within brands – with a little knowledge, research and the application of some very basic maths you can make sure all of your new parts fit.
The aim of this article is to arm you with basic arrow knowledge. There's more to learn - there always is - but. By the end you'll hopefully understand exactly what you have and what parts fit it. There's material here for beginners, novices, and experienced archers alike, and this knowledge should inspire a lot more confidence when it comes to making a purchase.
WEIGHTS AND MEASURES
The first thing to understand about arrows is how they are measured. The correct form of measurement is the AMO measurement. The length
is taken from the nock groove to the end of the shaft EXCLUDING any points or inserts. If you're buying arrows online or in store you'll need to communicate the required AMO measurement. Over the years I've seen way too many people buy arrows too long for them because they have given an end to tip length rather than the AMO. The purpose of the AMO method is to put everyone on the same page and gain an industry standard method for consistency of product.
All modern shafts will have numerical information printed on them. Deciphering this information is the vital first step. You may have noticed that aluminium arrows have a four digit number printed on them. An example of this may be something like 1616 or 1916. This number is often incorrectly referred to as the “spine” of the arrow. It is in fact the tube measurements. There are two measurements contained in this number. The first two digits are the external diameter in 64ths of an inch. The last two digits are the thickness of the tubing wall noted in 1000ths of an inch.
We'll now look at how this number and some basic maths combine in order to make a purchase. Let’s say you needed to buy some glue over nocks for a a 1616 taper swage arrow such as an Easton Jazz or Tribute. We know the size of outside diameter is listed in 64ths and that 16 is one quarter of 64. Therefore the correct size nock for this arrow is the ¼. If the first two digits of the number are 18 the correct size nock would be the 9/32. This number scaled up would give you the 18 64s. If the number is 20 the nock would be 5/16. This principle will ensure glue over nocks, inserts and screw in points will be as flush to the shaft as possible. Aluminium arrows that are over a 12 and under a 20 tend to be fitted with a Uni-bush at the rear. This is a little adapter that narrows the aperture to accept standard nock fittings. Uni-bushes slim the aperture down to 12/64ths. Aluminium arrows of 20 and over will normally have a ‘Super Uni-bush’ taking the aperture down but to a much fatter 19/64ths. These numbers of 12 & 19 are magic numbers in arrow building. Companies like Beiter will actually size their nocks with these prefixes. Easton size the 12 as ‘G-nocks’ and the 19 a ‘Super Nock’.
It's a really good idea to know the internal and external diameters of any arrows you might be using. This information can normally be gleaned from the manufacturer's website. The dimensions are normally listed to three decimal places. A common example of an internal diameter of thin full carbon or hybrid arrows is 0.166”.
Ballistic items including arrow building components are weighed using the grain system of measurement. This system has been around since the Bronze Age. The weight of one grain is nominally based on the weight of a cereal grain believed to be that of a grain of barley. Points etc. are weighed as a total mass such as 120 grains. Shafts are listed in GPI – grains per inch. They are listed this way as people will cut them to different lengths, thus altering the overall weight. Simple multiplication of the length x GPI will give the total shaft weight without the need for physical weighing on scales. One grain has the modern day equivalent of 64.798 milligrams.
FRONT OF CENTRE EXPLAINED
In simple terms, front of centre is the arrows balance point once the arrow is totally configured. Sometimes FOC is shown as a percentage figure which refers to the percentage of the arrow's total mass which is located in front of the central point. This is probably the most overlooked element of arrow configuration and shot tuning. It does however have a major influence on arrow flight characteristics and trajectory.
FOC is a very important consideration when shooting longer distances but less so for shorter indoor rounds. An arrow has a nose, a tail and flight stabilisation surfaces, usually vanes (fletchings) or spinwings. In avionics, a flight configuration would be referred to as the trim setting.
If the mass and balance of a projectile is placed too far to the rear it would turn around in mid flight. A darts player can hold a dart point backward and throw it a the board. The weighted rear would catch up and overtake the front and land point first into the board. If an arrow’s balance is too far back it will have a wobbly flight that will be hard to tune out. If the weight is too far forward the projectile will have poor trajectory and will want to nose dive. Stability requires weight, but good long range trajectory requires the container or packaging to be aerodynamic and lightweight. Therefore it's really important to strike a healthy balance between the two.
Everything that gets added to to the shaft affects the FOC. Points have the biggest influence on FOC as they are a heavy item compared to other arrow products. (Later on we will be taking a more in depth look at points and how we can use them to manipulate FOC.)
Here's a more scientific explanation of FOC for those of you that are mathematically minded. FOC is a measurement of where the centre of mass is relative to the actual centre of the arrow. Because of this, adding weight can be an advantage. For example, installing a pin nock adapter in the rear can neutralise the affect of selecting a point with an extra 10 grains at the front. The FOC and trajectory will stay pretty much the same even thought the overall mass of the arrow has increased. Altering FOC can either improve or worsen sight marks depending on the position. Most of my recurve customers prefer the weight further back to maximise sight marks. My compound customers tend to favour nose-heavy arrows for stability as sight marks are less of an issue with compound shooting.
SPINE
Another number to be found on most shafts in the spine rating. In simple terms the spine of an arrow or “Static Spine Deflection” is the amount of flexibility an arrow shaft has or how stiff it is. A shaft’s length in combination with shape, material, diameter and wall thickness will