Finnie,
Here are my thoughts on your problem with porpoising of your overspined arrows.
The picture of your braced bow shows that the lower limb (on the right hand side of your picture) is slightly stiffer than the upper limb on the left hand side of the same picture. Ordinarily, that would pose no shooting problems. However, in your full length drawn bow picture, something else is happening which one would not expect from the shape of the braced bow.
- Draw-2.jpg (121.1 KiB) Viewed 1612 times
I have rotated your picture in Photoshop to place the line of the arrow in a dead horizontal position in the picture. I have over-drawn the drawn bowstrings in white for clarity and I have drawn a vertical line in red from bowtip nock to bowtip nock. This red line is at 89.7º from dead vertical which is ordinarily very good for evenness of the transfer of thrust from both limb to the arrow - meaning that both limbs are transferring thrust in the same horizontal plane.
However, I compared the shape of the drawn limbs by making a duplicate of the lower limb (white outline) and superimposing it over the upper limb.
From your stance and hold, I am making a presumption that the bow is held vertically in the fashion of a compound bow, so there is very little foreshortening of either limb in the picture due to canting of the bow. The close range of the picture should obviate most of this phenomenon anyway.
The superimposed lower limb shows that the
upper limb when drawn is actually slightly stiffer than the lower limb in the outer 1/2 of that limb - that is, it is bending less - making it
stiffer even though it is slightly longer. It is also more whip-ended than the lower limb which you can see by the tigher arc of bend it has in the outer part. These two phenomena will cause it to behave differently to the lower limb, even though the thrust alignments are very good.
My opinion is that the whip-ended stiffer upper limb is whipping forward more quickly than the lower limb tip causing the arrows to be driven down onto the arrow shelf and causing them to bounce and hence porpoise. Their overspine should not be a porpoising issue. The excessive amount to which you need to set your nock point above the normal range is a consequence of your bow pushing the arrow nock from a higher position on the string so the arrow is lifted up and away from the shelf as it passes and doesn't bounce off it during the forward movement.
Sanding the belly of the
outer part of the upper limb to weaken it slightly to match the curve of the lower limb will most likely get you over this problem. With the bow on a tiller at full draw, trace the inside shape of the lower limb onto your driveway or somewhere flat as well as the centre line of the tiller so you can place the tiller in the same position each time. As you sand more from the upper limb, keep flipping the tiller over so the upper limb lies over the tracing of your lower limb until its shape matches that of the lower limb.
At that stage, nock and shoot an arrow from reasonably normal position and see if the problem persists. I don't think it will. If the draw weight drops too much, you still have enough length in your bow to pike it an inch at each end and bring it up a couple of pounds again.
By the way, I have made 3 or 4 very light weight bows from Bunningswood with success so long as they are kept below 30lbs at least shoulder height, widish and with very flat bellies and well rounded edges. This stuff does not take kindly to compression, so you need to keep as much wood in the bow as you can where the bending is greatest and keep the tips as fine as you can to keep outer limb mass low.
Now I'd like to get the tiller to working standard, finish it, hang it, and then move on to the next one.
