Measuring Up

I have commented more than just a few times about the importance of having arrows matched to you and your bow. I was working with a newish recurve student regarding how to do just this an I realized that many archers do not have the required equipment. Here are some “workarounds” you can use to get the information you need to order the correct arrows.

What You Need
Every manufacturer provides a “spine chart” which allows you to select which of their arrow shafts you need for arrows to fly well out of your bow. These charts require your draw length and draw weight to make the selection. I will start with how to get these numbers for recurve bows and long bows.

Measuring Your Draw Length
There are such things as “draw length arrows” which are arrows with printed scales on them. You simply draw back into good form and somebody else reads the scale at the rest/plunger button and you add 1.75˝ and voila. If you don’t have such a device you can use an ordinary arrow. Draw to good full draw position and have your helper by a mark on your arrow right at the arrow rest hole/plunger button. Repeat this a number of times (3-5). All of the marks should be within a quarter inch of one another.. Measure from the bottom of the nock’s groove to the center of the array of marks and add 1.75˝ and voila.

It is important that you be warmed up before doing this and have shot in good order. It also helps if your “helper” can see if you are in good alignment at full draw because you will get a false reading if you are not.

There are many “formulas” that determine draw length from other body measurements. I have collected about a dozen of these. None of them are as reliable or as good as the technique I describe above.

Measuring Your Draw Weight
I have a hand-held electronic bow scale that just requires me to attach it to the bow string and pull an arrow the proper distance and it records the maximum value within 0.1#. Of course, you probably don’t have a $150 bow scale in your pocket, so a bathroom scale will work. An alternative is if you have a large spring scale, you can mount it up high on something sturdy and then hook the string on the scale and pull down (the correct length—you can use an arrow with a mark on it for this) and then read the scale). There are commercial scales sold for just this purpose, although any spring scale is inherently somewhat inaccurate, it is close enough for our purposes.

Easton Spine Chart PageYou can estimate your draw weight by taking the nominal draw weight value of your limbs (24#@28˝, 38#@28˝, etc.), correct this value for any adjustments at the limb pockets (ILF limb pockets allow about 10% of the draw weight to be adjusted off by backing out the limb attachment bolts (typically 4-5 turns max.). Then you need to adjust this value for difference between your draw length and the nominal one (@28˝). To do this you add 2# for each inch over 28˝ or subtract 2# for each inch under 28˝ (use 3# per inch if your limbs are 40# or heavier).

If you want an actual measurement of your draw weight rather than an estimate, you need a stiff stick, at least 3 feet/1 meter long, with a notch in one end. From the bottom of the notch measure down the stick and place a mark at X˝, X being your draw length measurement but without the 1.75˝ extra. The stick is then stood on its end on a bathroom scale and the bow placed upon it with the string (where the arrows attach) in the groove and the riser hanging down. You then press down on the riser until the arrow rest hole/pressure button is even with mark on the stick and read the scale. Of course, this is no more accurate than your bathroom scale, but it is something.

Special Considerations for Compound Bows
Compound bows are different. The draw length of a compound bow is a setting on that bow which may or may not be correct for an archer. It is correct if you can get into correct full draw position with a straight bow arm and your draw elbow lined up with the arrow on the bow. When this is so you can use the techniques described above. If it is not, you need to adjust your bow until it is.

The spine charts for compound bows do not list the draw weight in hand, they list the “peak weight” which is the highest draw force encountered throughout the entire draw. (The draw force reduces to a small fraction of the peak weight at full draw, which is called the holding weight.) If you use the bathroom scale technique described above you have to carefully watch the scale reading as it rises, reaches a peak, and then drops off. You want the highest reading to use in the arrow selection. This is why “digital” scales are not as useful for this task as “analog” scales. The numbers on the digital scale often change so fast as to be a blur and you cannot move the bow back and forth past the peak to clarify anything, otherwise you will get a false reading. You must “pull” the bow down in one continuous stroke will doing this.

My $150 gewgaw registers both the peak and holding weights with one pull of a compound bow. It is a shame those devices are no longer available.

I hope this helps those of you who do not have the tech services of a full archery pro shop nearby.

PS Wood arrows can be quite different but both draw weight and draw length are considerations.

 

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3 Comments

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3 responses to “Measuring Up

  1. Chris

    Hi Steve. What advice would you give for an archer that falls into the boundaries between spine groups? Maybe they have a draw length that puts them on the border of two arrow shafts. Or a bow weight which is right at the upper boundary of one shaft, but not quite into the next? Thanks, Chris

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    • Hi, Chris,

      The answer depends. If you were young and on the way up (as you are), your draw weight would be inclined to increase and you should go with the stiffer shaft (cut a bit longer to compensate for current DW). This gives you the option of trimming a bit off of this shaft when the DW is inched up. If you were suffering from a chronic injury or getting old (like me) your draw weight would be inclined to decrease, so you might want to go with the weaker shaft and tweak the DW down to tune it in.

      Since an inch of shaft length corresponds to a whole spine group (or 5# of draw weight) you probably can’t go too wrong with either shaft choice. Unfortunately, considering the prices of today’s high performing arrows, I guess you can’t afford to make a mistake. ;o(

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  2. Pingback: My Archery Experiences

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