Fanto,
I hope i am not annoying you with all this discussion about bow setup! I dont just sit and meddle with these bows and nothing more - I do shoot huge numbers of arrows each day and im striving for better and better shooting.
Absolutely not. It seems to me that there is a great deal of information 'out there' which, for some reason, makes the fundamentally simple business of shooting an arrow at a mark into something fraught with complications.
The science surrounding the how and why of archery is detailed and fascinating, but somehow, many people seem unable to distil basic principles from that knowledge and end up not being able to see the forest of information is made up of individual trees. In the modern enthusiasm for acquiring huge banks of knowledge, people seem to lose sight of the fact that actually shooting a bow is a fairly simple affair requiring a fairly basic knowledge set.
Very few archers these days understand the basic principle of controlling the paradox phenomenon by having something against which the passing arrow must push itself away. The basic principle requires that for an arrow of given spine, the degree to which the arrow plate is positive (wide of the bow's centreline toward the arrow) is what actually controls how the arrow leaves the bow. It is the combination of spine AND positive arrow plate which controls arrow flight, NOT spine alone which is the mistake commonly made.
They go to the spine charts, read up on the required spine for that draw weight/draw length/arrow head mass combination and completely ignore quite innocently, the crucial importance of how much side pressure the arrow plate puts against the arrow as it launches and instigates the paradox bending. That side pressure is completely passive because in a conventional trad bow, it cannot be altered easily short of packing it out or rasping some off.
That spine jig article I posted is based on an arrow plate cut no deeper than the string line which means that the arrow point on such a bow will always lie outside the string line at brace height. Hence, the principle that for every 1/8 inch further outside the string line (away from the arrow plate), the required spine of the arrow is reduced (bigger number) by one spine grading (5 lbs).
After that beginning, for every 10 grains extra head mass greater than 125 grains, the required spine must be increased (smaller number) by one spine grade.
For practical purposes, each spine grade is equivalent to 50 thou.
So, the process is -
1. You select the nominal spine grade from the charts I supplied above which are based on a 125 gn head mass and a 28 inch draw length;
2. Using the figure worked out from 1. above, you then assess how much the arrow plate of your bow is outside of centreshot and allow one spine grade for each 1/8 inch (about 2mm) outside centreshot;
3. Using the figure worked out from 2. above, you then increase or decrease that spine rating by one spine grade for every 10 grains greater or less than 125 grains head mass.
So, applying the above for an exercise - Joe Blow has a 50lbs long bow and draws 28 inches. His bow is outside centreshot by 1/4" and he wants to use a 200 grain head (because he is a great fan of high FoC), so
1. Nominally, he requires and arrow which bends 650 - 640 thou on the spine jig (assuming the arrow has a 125gn head).
2. Because his bow is 1/4 inch outside centreshot, he will need to drop his required spine down by 2 spine grades (100 thou) to approximately 750 - 740 thou.
3. But because he wishes to use a 200 grain head, he will then need to increase the spine rating of his shafts calculated at 2. by around 7 spine grades (350 thou) to around the 400 - 350 thou mark.
TO INCREASE SPINE GRADE - SUBTRACT THE NUMBER OF THOU. TO DECREASE SPINE GRADE - ADD THE NUMBER OF THOU because more thou equals more bend or less stiffness and fewer thou equals less bend and greater stiffness.
If Joe was to alter his draw length different to 28 inches, that would have to be taken into account at stage 1. of this calculation. Every inch of draw length change requires one spine grade decrease (bigger number) for every inch shorter than 28 inches and one spine grade increase (lower number) for every inch longer than 28 inches.
This process may spin your head a bit at first, but once you get the hang of it, it is not very difficult. BUT, SO YOU DON'T CONFUSE YOURSELF, YOU
MUST FOLLOW THIS SAME PROCESS FOR EACH AND EVERY BOW. You also need to get your head around the concept that increasing the spine grade means that there is less deflection, which means that the spine number is smaller, not bigger.
If you do a few hypothetical calculations as an exercise, you will soon get the hang of it, but I repeat, you must follow the same sequence in the calculations or you will get nowhere except profoundly confused.
All of this above applies to a minimum required spine rating for a bow held vertically. If you cant your bow, then you will find that you can use shafts greatly in excess of the above without problem. I use my spine jig these days only to see what is going on or for testing purposes or to ensure that all of my shafts are the same stiffness. Long ago, I realised that so long as my shafts could stand in my bow, I had no problems.
(I need a cup of tea.)