## A Rule of Thumb for Selecting Spine Match

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# A Rule of Thumb for Selecting Spine Match

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Rod
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Joined: June 17th, 2005, 11:07 pm
A common method of roughly estimating a spine match is based upon a series of adjustments, starting from the measured draw-weight at 28" draw-length and assuming a standard target point of about 50-60 grains .

In its simplest form we only need to know the shaft length, point weight and draw-weight, but there are other contributing factors.

When we speak of shaft length we are measuring from the seat of the arrow nock to the base of the point.
In other words we are describing the "working length" of the shaft.
This does not include the ears of the nock or the length of the head or point.

At the selection stage with a bow we should consider the width of the bow at the arrow pass, the pass cross sectional shape and the depth of the pass since they all contribute to the degree of arrow offset at brace height and therefore influence the degree of disruption in paradox experienced by the arrow.

Given a bow that is not centreshot, minus 3 lb of spine for each 1/10" out from centre-shot at the arrow pass.
(Minus 10 lb from the draw-weight is the accepted practice based upon the old style target shooting longbow with a total width at the pass of approximately 1" which produces a stiffer shaft than the figure of 3lb per 1/10").

Plus 5 lb of spine for each inch of shaft length above 28"
Minus 5 lb of spine for each inch of shaft length below 28"
Measuring the working length of the shaft, from the seat of the nock to the base of the pile.

Plus 5 lb of spine for each 50 -75 grains of point weight above 50 grains.
Adding 5 lb for a 125 grain point, for example.

Brace height adjustment is the primary means by which we can slightly modify the effects of paradox of an arrow of a selected spine in any given bow.

Remember that there is always some degree of latitude in selection and use which can usually be modified by selective manipulation of the variables.

A practical example:

English single stave yew longbow.

Draw -weight:  54 lb @ 28"
Width at pass: 1 3/16"  (19/32" off centre)
Point weight :  125 grains
Shaft length/draw-length:  29 1/2"
Average speed:  155 fps
String type: Dacron B55, 15 strands
Brace height: 7 3/8"

Calculation.

Rule of thumb based on 5 lb increments

55 lb minus 10 lb for 19/32" off centre
45 lb plus 5 lb for 125 grain point
50 lb plus 5 lb for 29" of shaft length

Suggested spine of 55 lb

*** Provisional revision of the rule of thumb, based upon 3 lb increments.

This is included here because the 5 lb rule of thumb method is just that: a useful approximation, useful enough that it can be used succesfully. But having said that,  often fudge the calculation, being essentiaslly an instinctive rather than a scientific archer.
And knowing that I have habitually shot a rather stiff (prescriptive) spine for many years.
But in the knowledge that I could probably shoot at least one other, weaker selection, as well if not better.
It is only a matter of my choice and knowing my reasons for that choice and being comfortable with it.
The question should then be, does my preference accord with a methodical test of my shooting that choice.

If 3lb for each 1/10" (+ 0.59375") off centre-shot  (a deduction of 18 lb in spine)
36 lb plus 3 lb  in spine for additional 75 grains of point weight above  50 grains (1 lb for each 25 grains)
39 lb plus  3 lb for in spine additional 1" of shaft length

Suggested spine of 42 lb

I shoot these same arrows spined at 55-60 from four different bows, the other three of which are variously slightly less wide at the pass, of slightly heavier draw-weights and in differing degrees also a little faster.
This bow is the widest and slowest. The two fastest are a pieced yew target bow  and a short osage reflex/deflex recurve with tips off barely off contact at brace height and a fast-flight string.

I am drawing a 29 1/2" shaft at 29" to the finger using  55-60's which may be are somewhat prescriptive but shoot straight only with a clean loose.
This slightly stiff spine selection is made more acceptable by the higher than average brace height.

Someone else might find 50 - 55's, 45-50's or even 40-45's more to their taste.

I like the prescriptive match because it does not let me get away with a less than perfect loose.
If winning medals was more important to me than shooting well, I might well be shooting 45-50's to get a match that was more forgiving of any imperfections in my loose.

The ultimate test is how closely and consistently our arrows go where they should, as compared to where we too often make them go.

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