Ok. Back from work.
dj,
If you look at the web page that you are talking about, there are actually MULTIPLE charts for different draw lengths and they all differ by five pounds. More on that later, but this is one thing I don't like about the plans on that site or the orginal plans in PA magazine. These charts can be very confusing for beginners if they aren't very knowledglable about spine. The author of the article did a great job in explaining how to make this thing, but not a very good job as far as how to read it and use it, IMO.
Before I go any further, let me say I have confidence in my chart for two reasons. Number one: I based it on the AMO standards. You can find the AMO standards on the Rose City website. Number two: I tested it against aluminum arrows that were spined on a good factory spine tester and my chart jived. See the Arrows by Kelly website for the spine values of different aluminum arrows.
There are two kinds of spine. Static and dynamic. Static spine is the measurement you get when you put an arrow on a spine tester. Dynamic spine is how the arrow actually acts when it is shot. If you have a 26" arrow and a 32" arrow that are both spined 50lbs, they will fly differently when shot. In other words, their dynamic spine will be different. BUT..they will both still read 50lbs on the spine tester.
There is an old rule of thumb for determing spine. For every inch of drawlength over 28", add five pounds of spine. For every inch of drawlength under 28", subtract five pounds of spine. I believe the author was trying to take this into consideration when he came up with multiple charts for different drawlengths. But he seems to have gotten that wrong because the one chart that DOES jive with the AMO standards is the chart for a 26" draw length, when it should be the one for a 28" draw length.
All of this puzzled me years back when I built one of these spine testers. Plus...I wanted to be able read arrows that were stiffer and weaker than the ones he has in his charts. His chart only goes up to 70lbs, and it keeps dropping the longer the drawlength is. He tells you how he came up with these values, but you have to know something about triginometry in order to understand what he is saying. I didn't know anything about trig, but my brother did so I went to him.
He also found the author's methods confusing. He asked me if I had the AMO standards in hundredths of an inch. I said I did and he said it will be real easy to figure out. He showed me how to do it on a scientific calculator and I did it. After I did it, I found out that my values converted from the AMO standards lined up with the author's values for a 26" draw length. If you would like to do it yourself, shoot me a pm. I can tell you how to spine every arrow to the pound ( not just the five pound groups ) if you are interested. It's very easy. All you need is a scientific calculator and you probably have one on your computer.
Out of all the homemade spine tester plans, this is the simplest and most inexpensive one to make and it's just as accurate as the other types. I believe it would be more popular if the charts in the article weren't so limited and confusing. I tried to simplify the chart by making it jive with AMO standards and by expanding it a bit.
If you are interested in how I did it, shoot me a pm or email me at
[email protected].
Hope this clears up some confusion.
Austin
Topic: Protractor spine tester confusion? (Read 1683 times)
