With the work beginning on the arrow shaft reviews I thought it would be good to go through the methods I will be using to make the measurements. I have been making mechanical measurements for many years and there are always tough challenges to make sure that the data taken is accurate and meaningful. Arrow shafts are not simple to measure accurately and it’s taken me some serious trial and error along with consultation with different measurement experts and arrow manufacturers.
Arrow Shaft Weight Measurement
Weight is fairly straight forward to measure but in order to get the results needed for meaningful comparison between shafts, dozens of shafts, and different models to their respective specifications, a high accuracy scale is needed. My current grain scale is great for weighing shafts and components on a macro level for use in basic calculations, but not accurate nor repeatable enough for the shaft reviews here. I’ll be using an analytical lab balance that is good out to .00001 grams that is calibrated every six months. Because such balances normally run around the $4000, I’ll be borrowing the one from my day job (pictured to the right.) This scale is definitely overkill, but will give much better results than a small desktop scale.
For normal, everyday use there are many great grain scales on the market. If you are looking to buy something to check your overall arrow weight and arrow to arrow consistency, most of the grain scales that measure down to 0.1 grains will be sufficient (don’t go super cheap, get something at least in the $25-$30 range). Scales will all go out of calibration over time, so if you are concerned about keeping records of your arrow weight over time with a good degree of accuracy, buy a scale that has a calibration function and comes with a calibration weight. Some scales will have the function but the weight has to be purchased separately, so make sure you know what you are buying first.
Arrow Shaft Spine Measurement
Spine measurement is a degree or two of magnitude more difficult than weight measurement. Getting consistent results requires a lot of patience, time and a repeatable test fixture. Patience I don’t have a lot of, time is always scarce, but I have a built a fixture that I feel is very repeatable and gives good results.
The measurement is made by spanning the shafts between 28″ centers made of Teflon. A 1.94 lb. brass weight is suspended from the center of the shaft and the deflection measurement is made with a digital linear gauge. This is per the AMO standard.
One very important thing to note is that when taking precise measurements, results should always be correlated only to other results taken using the exact same measurement system. The results from my fixture, though using the AMO standard, may not be exactly correlated to another measurement system including those at the manufacturer. When looking at spine measurement results, the key is not necessarily the final number itself, but rather the consistency of each shaft to each other.
Arrow Shaft Straightness Measurement
Apparently there is not just one method that the various arrow/shaft manufacturers use to measure straightness. I have heard of measurements spanning 8″ of the shaft, some 28″, some that are double supported and some that are cantilevered. Because there are differing methods, my measurements may not be directly correlated to what specific manufacturer specifications may say. I decided upon measuring the straightness at the center of an arrow supported between two blocks 28″ apart and is the AMO standard (similar to the spine method.) I chose this method because it uses a long span and is more likely to catch inconsistencies in straightness.
The measurement is made by placing the arrow onto the Teflon supports and rotating it very carefully and slowing. A dial test indicator that measures to .ooo5″ is located at the dead center of the supports. The maximum and minimum readings from the indicator are compared and the difference is the straightness.
I will continue to try to get answers from individual shaft manufacturers on how they make their measurements to get a better understanding of how to relate the different measurements. For now all my measurements will continue to use the 28″ span.
Arrow Shaft Inside and Outside Diameter Measurements
The inside and outside diameter measurements are important because they determine how well other components fit and are a good indication of the overall consistency of the shafts. For the outside diameter measurement and micrometer is used and for the inside diameter hole gauges are used.
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{ 4 comments… read them below or add one }
When trying to spine an arrow what can be done for an arrow that’s less than 28 inches. I shoot 26.5 inch arrows. I shoot 3555 gold tips. I have my poundage set at 55- 57 lbs and get reasonably good arrow flight with both broadheads and field tips @ 100 gr. I’m just curious as to how stiff my arrows register on a spine scale. Would it be possible to suspend 2lbs of weight between 24 inches and measure the deflection or would this figure just be a relative comparison number ?
Thanks, Bill
Bill,
Unfortunately measuring the spine while trying to scale the test system won’t quite work. You might be able to figure out how to get the scaling close, but it would vary from arrow type to type. Also remember that using this method is only measuring the static spine which is only a starting point. Once you cut the arrows and attach various components to them, the dynamic spine will vary greatly from bow to bow. Generally speaking, the manufacturer’s data on static spine is fairly close, but only shooting and tuning your individual setup will get you where you need to be.
The Measurment is taken @26 inches using two pounds of weight. Whle it would still be a comparison it will give you a close approximation of AMO spine.
Spanning 28″ and a 1.94 lb. weight is the current standard used for aluminum and carbon arrows. The spec of 26″ and 2.0 lbs. was what AMO initially spec’ed for wood arrows.