It is tempting to think that if you have a 50lb longbow
you need to buy arrows spined at 50lbs. This is not however the case. The measurement of an arrows spine is related to a recurve
bow and because the longbow arrow has to go round the handle of the bow rather than straight ahead as with a recurve, the
arrow needs to be more bendy. This is all to do with the archers paradox.
The best explanation of the archers paradox
on the web is by Nigel Cook and it can be found at: Nigel Cooks Longbow (see links)
I have included an extract:
The Archer's Paradox.
Any discussion concerning longbow arrows must begin with the archer's paradox.
The essence of it is thus. You draw your bow and sight the point of the arrow onto the target. For that moment that you aim,
imagine the line of the arrow shaft from nock to point. If you extend that imaginary line forwards you will go past the left
of the target (assuming that you are right eyed). Longbows are not centre shot bows, you lay the shaft against the bow, and
the arrow flies around the bow. That is the essence of the archer's paradox, that the arrow begins by pointing off to the
side but flies straight ahead.
The diagram on Nigel's site seeks to illustrate how this happens.
shows a through section of the bow, an exaggerated view of the behaviour of the arrow, and represents the string with a horizontal
1. The arrow is nocked. It lies against the bow, at full draw, and although the archer is aiming forwards,
the arrow points to the left.
2. At the loose the bow begins to straighten, and the kinetic energy which propels the arrow is introduced
to the shaft by the movement of the string towards the belly of the bow. The arrow flexes,
3. ...then straightens, before
4. ...flexing the other way,
5 & 6. and thus clears the bow.
This process takes a matter of about 25 milliseconds.
The arrow stops bending in 'flexural oscillations' as it flies towards the target, and will follow the line of the string,
ie. forwards rather than to the side.
In order for the arrow to flex around the bow shaft as it is loosed there must
be a precise degree of flexibility. If this is not the case the arrow will clatter against the bow, it will 'fish-tail' in
flight, and it will certainly not behave in a responsible manner. This property is known as the spinage of the arrow.
Calibrating a home-made spine tester
A spine tester measures the deflection of
a spine when it is supported between two points, 26 inches apart and a 2lb weight applied to the centre of the spine. The
resulting bend in the spine can be measured in inches. The deflection can then be converted into a spine value in pounds using
the following formula:
Spine (in lbs) = 26/d where d=the deflection in inches (or if your eyesight is keen mm can be
So, if the deflection is half an inch the maths is 26x2=52
In eights of an inch the approximate values
1 1/8 =23lbs
1 2/8 =21lbs
This means that an arrow that deflects the least is stiffer and therefore of a higher poundage that one
that deflects the post and is whippier and of a lower poundage.
Selecting the spine of your arrows
The values of spines are given in
pounds for a 26 shaft suitable for a recurve bow. Hilary Greenland recommends that you select arrows which are spined for
two thirds or three quarters of the actual draw weight of your bow at your draw length. Quicks have provided a table based
on the draw weight of the bow with values for different arrow lengths (nock to pile) from which the following is an extract:
length 24" 26" 28" 30" 32"
20/25 25/30 30/35 35/40 -
20/25 25/30 30/35 35/40 40/45
25/30 30/35 35/40 40/45 45/50
53-60lbs - 30/35 35/40
40/45 45/50 50/55
35/40 40/45 45/50 50/55 55/60
The values of the spines are given in ranges of 5lbs as this
is the usual accuracy at which they are sold. So if you were hoping to make a set of a dozen arrows between 34-36lbs, you
will need to buy two dozen shafts!
What else affects the choice of your arrows?
will also need to consider the weight of the pile, the diameter of the shafts and the size of the fletchings. These all affect
the flight of the arrow. I suggest you start with piles of 70-80 grains and feathers no more than 3 inches. For an article
on spine: http://homepage.ntlworld.com/joetapley/spine.htm
And this is not enough for you to be going on with,
you also need to consider the overall weight of the arrow. A heavy arrow will have a greater inertia than a lighter one. As
a general rule for the same bow a heavy arrow will need to be stiffer than a light arrow (or to put it another way- a bow
can shoot an arrow of any stiffness provided you get the weight right!).