I prefer the dial indicator style with 28″ centers and 1.94lb weight because it compares directly with Easton/ASTM spine ratings that arrow manufacturers use. Cost is about $50 -$75 depending on materials used and quality of dial indicator. You’ll need a dial indicator with at least 1″ of travel … they can be had at Sears Auto for under $40. When taking readings with the dial indicator mounted under the shaft, ease the indicator tip to just touch the shaft in order to nullify the return spring’s pressure, or calculate the return spring’s constant and increase the weight accordingly (usually anywhere from 15g to 30g … 880g = 1.94 lbs).
Designs that are built on 26″ centers and which use 2.0 lbs weight are for Traditional arrows (wood) and correlate to the AMO Spine Chart … not the Easton/ASTM spine readings.
The reason for building a spine tester to the Easton/ASTM standards other than manufacturer’s spine ratings are bsased on the ASTM standard is for entering the data into computer arrow simulation software, like OnTarget2!.
The one I’ve made, shown above, has extra holes in the base to move the supports to 26″ centers and I have a 420-gr weight I add to the 1.94 lb weight to make 2 lbs, so I can build and check my Trad arrows.
I’ve checked the spine tester I’ve made against my RAM spine Tester and the readings compare very closely … within the third decimal place. The dial indicator on the RAM is about $150, which gives a very small increase in precision over the Sears Auto $37 indicator.
If all you want to do is sort and match arrow static spine and/or find the stiff side of the spine, then it doesn’t really matter what the distance between centers is or what weight is used as long as you can get a good reading.
Here’s one that I fashioned up a few weeks ago. It’s very good for testing for the side of an arrow with the strongest or weakest spine. I had to fashion a bracket that would hold the micrometer as well as allow me to remove it for other stuff. For testing actual spine, it’s very good but requires a little tinkering to get the arrow shaft to just make contact with the dead stop. Brian’s set up is fool proof due to how the mic functions with relationship to how and where it contacts the arrow shaft. Google “spine tester” and click “images.” There’s a lot to draw from there. Good luck.
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