Arrow Spine: Determining Factors

Now that we have established what spine does, you need to know how to change it. The following factors affect the spine of arrows:

    Shaft Stiffness (Spine)

There are far too many possible jokes for me to bother making any.

Fairly obviously, the most important factor in determining how much an arrow bends is how bendy the arrow shaft is. This will be determined by the material and the thickness of the shaft. The bendiness of the shaft is generally referred to as its spine, but it is important to remember that it is not the whole story. I tend to think of the bendiness of the shaft as “arrow spine” or “static spine”, as opposed to “dynamic spine”, to which we are coming.

Arrow spine is measured in pounds of draw weight but I hope that by now you realise that draw weight is not the only factor. If you have not grasped this point, you might like to read my earlier posts on bow mechanics. Arrow shafts are sold by spine weight, usually in bands of about 5lbs. For wooden shafts you will simply see a box labelled “35-40” or similar. Carbon and aluminium shafts have a slightly odd system of coding, unique to the type of shaft, which tells you the wall thickness, diameter and mass per inch, as well as shaft spine. There are tables, known as spine charts or spine tables that will help you to find the right shaft.

For a given shaft spine, however, you can make the arrow more or less stiff by adjusting the following factors. They all affect how much the arrow flexes when subjected to the force of the bow’s shot. This is sometimes called dynamic spine (i.e. spine when moving).

    Length

Longer arrows are more flexible. Take a long stick and waggle it. Then snap off a 6″ section and try to waggle that. See?

In addition, a longer arrow should denote a longer draw length (you should not have a large amount of arrow sticking out in front of the bow at full draw) and longer draw length means more stored energy and, all else being equal, more force on the arrow, resulting in more bend.

Having said that you should not have a lot of arrow sticking forward, there is nothing wrong with a couple of inches. Leaving the arrows a bit long may therefore be used to lower the spine of the arrow.

    Point Mass

Heavier points make at arrows bend more. When the bow first applies force to the back of the arrow it will bend the shaft until enough force travels up to the point to move the point which, being heavier, needs more force to move. The heavier the point, the harder it is to move and so the more the shaft bends before it moves the point. Put another way, take a stick and waggle it, then attach a weight to the end and waggle again. See?

    Brace Height

Lower brace heights increase the length of the power stroke, thereby increasing the amount of energy available for pushing the arrow. This makes it bend more. Take that stick again and waggle it. Now waggle it harder. See? The same applies to string mass: it changes the force applied to the arrow.

    Overall Adjustment

It is possible, by means of changing length and point mass and brace height, to change the dynamic spine of an arrow quite considerably. Don’t.

Changing your point mass or arrow length affects arrow mass as well as spine. Point mass will also affect something called FOC, which is the proportion of arrow mass that is in front of the middle of the shaft’s length. This affects flight characteristics. Changing your brace height will change the energy available to the arrow and can produce handshock, poor arrow flight and so on.

What you should do is brace your bow at the correct height as recommended by the bowyer, then select the correct arrow spine. Get your arrows about 4″ too long. This gives you a decent starting point for bareshaft tuning, which is what we shall cover next time. Point weight, arrow length, brace height and string mass should be thought of as things you have to keep constant in order to maintain consistent spine once you have found it. In selecting and tuning your arrows in the first place, they are for fine adjustments, not major changes.

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