Mike, I'd say that Robert has the answer, spot-on. Any time you add weight to the shaft (anywhere) you alter the dynamic spine that particular shaft will show from a given bow; and that, in turn, alters the quality of flight.
When developing my extreme FOC arrows, I start with a pre-determined point weight setup I want to use (tip, adaptor, insert, and IF, if I'm using one). This is then mounted on a full length bare shaft. I progressively shorten the shaft (cutting from the shaft's rear, usually shortening by 5mm increments each time) until I have the shaft's dynamic spine correct for that point weight.
In other words, I don't start out to reach a pre-conceived shaft length; I'm seeking-out whatever shaft length is required to gives the correct degree of flex (dynamic spine) for that point-weight from that particular bow to achieve the best possible bare shaft flight.
If I can't get the dynamic spine stiff enough by the time I reach my minimum usable shaft length, then I start all over, using the next stiffest static spine shaft.
I begin the tuning at close range; 10 meters or so; and shorten the shaft until I have shots showing very slight weak-spine. Then I move back to 20 meters, again shortening the shaft by tiny increments until only a very slight weak spine shows. Then back to 30, and then 40 meters, repeating the process at each distance. I try to leave a VERY SLIGHT DEGREE (only an inch or two) of weak-spine bare shaft impact at 40 meters. Adding fletching has a slight spine-stiffening effect.
During this bare-shafting process, I'm not concerned with the direction the nock shows after impact; though I do initially 'rough-in' the nocking point to remove any marked up-down knock kick. I find it helpful to use a vertical 'wand', or strip of tape as my tuning target - and I align it 'true vertical' using a plumb-bob.
Basically, I'm only concerned with the left-right POINT OF IMPACT; and I do the shooting with my bow in a vertical position, not the canted position I normally shoot. I judge point of impact based on the group-center of a number of shots at each range. Especailly as you near the correct spine/shaft length, don't make the shaft-shortening cut until you are ABSOLUTELY POSITIVE that the dynamic spine is still showing weak. You want to avoid crossing over to strong-spine.
Next stage is to fine-tune the nocking point to eliminate any up-down kick. I do this at close range, and double check it further back. Then I compare the bare-shaft IMPACT against a matching fletched shaft (with field point). Generally the points of impact match perfectly on the first try. If there is any impact disparity, normally only a slight bit of nock-point tuining is required to remove it. As a final step, I check a fletched shaft with a BH; against both the fletched field point and a bare shaft. I do this at 10, 20, 30 and 40 meters.
The alternate technique; using a fixed or pre-determined shaft length; requires you to tune the shaft's dynamic spine by altering the amount of weight at the arrow's front (and elsewhere along the shaft too). This method limits point setup weight to whatever it takes to balance the spine correctly to the bow.
The technique becomes a bit more complex if you're working with bows that can be adjusted to beyond center-shot and with 'forgiving' arrow rest, but can be used. Just keep in mind that changing the rest-tension, elevation or in-out alignment (changing the degree of center-shot) can drastically change the shaft dynamic-spine required.
During all tuning, NEVER change more than one single item before re-testing the setup. That's the only way to see what that change did to the arrow's flight. Change two, and you can't know which change caused what.
Hope that helps,
Ed
Topic: Dr. Ed, ????????? (Read 455 times)
