Hollow Back Bow no.4

[willie]

I believe the compression is more pronounced on the belly with Jan's design than in HLD

Philipp, can you explain better about "pronounced"? Spreads it out across the belly better?

[Philipp A]

Hi Willie, I could be wrong but I am assuming that the belly is slightly rounded. My logic would tell me that the belly would get more compression than in the HLD design where the sides of the belly get compressed first before it starts to flatten. In Jan's design the centre of the belly would compress over a wider cross section early in the draw and keep on compressing while flattening out. I might of course be out to lunch on this, but that is one more reason to get the comparison done I was suggesting earlier.

The numbers Jan published are so impressive that there has to be merit to this design. After all nature is usually never wrong, since his design resembles that of a feather which is light weight and can tolerate high cyclical loads.

[DC]

This would be the same as highly trapping a bow, wouldn't it? say a 1 1/4" belly and a 1/2" back. Same loss of wood just the trapped back is split into two strips. Not trying to take anything away from Jan's bow, it's a beauty

[Philipp A]

@DC, I think the difference between what your example (if I understood correctly) and Jan's is that in a trapezoid bow with narrow back you would not get any flattening when you draw. I believe Jan tried to measure the flattening on one of his earlier versions of this design and I don't recall what the results were. I also think more wood is engaged in active work than in a trapezoid bow since you end up loosing the wood in the neutral zone in contributing to spring loading the bow. I believe the double spar design with the curvature to the middle engages the entire cross section with little "inactive" wood left in the neutral zone if I make sense to you?

[willie]

Philipp, thanks for the clarification. It's always good to hear what others are thinking.

I also think more wood is engaged in active work than in a trapezoid bow

I also had the same thoughts as DC about a comparisons to a trapezoid crossection, and I am not sure what might be responsible for any crossection deformation with the hollow back. At first, I suspected Possion effect was at work, but I was never able to find out how that might change for differing crossections. Could deformation be caused by a uneven distribution of forces due to the asymmetry of the crosssection?

BTW,even in a round or rectangular, and most likely trapezoidal section there are possion effecr forces making the limb crossection squish and stretch, not exactly "inactive", but its just enough that you have to look very hard to see much movement, and radiusing the edges of the back of a rectangular limb has been shown to be necessary, because the back does raise an edge enough to lift a splinter.

[BowEd]

Interesting bow and very nice work finishing your bow.

[Philipp A]

Question for Jan: are only the belly edges or is the entire belly rounded or is it entirely flat? It is hard to see from the pictures.

[Jan de Bogenman]

Here are a couple of turkey feather pics that show the resemblance.

Thanks for the picture Philipp. We do not have turkeys overhere. But lots of swans. Their feathers have a groove too, but it is not open as in this picture. This is how turkey feathers are naturally made??
Inspiring!

[Badger]

  As for the mass, you are well below projected mass and the bow didn't take much set so I would conclude the design does save mass. 17.5 oz is what I would have projected

[Jan de Bogenman]

Thanks for feedback Badger. Hopefully the bow will hold up and gets some companions. so we can see if it is viable.

[Philipp A]

Jan: In regards to feathers, i believe the turkey wing feather experiences tremendous loads because of the weight of the bird and their almost vertical take off when flying to a tree. It is unbelievable how much lift they can generate with the shape of their wing and their strong wing muscles. I think it is the perfect design for extreme cyclical loading while saving weight.

I believe your bow design (which comes very close to the turkey feather in style) is a testament to how much weight reduction you can achieve while at the same time storing the energy very efficiently based on the arrow cast you managed to obtain.

Getting back to one of my earlier questions: is the belly rounded?

[Jan de Bogenman]

In the picture below is a cross section of the bow. figure B. (in mm.)
As you can see, the belly is nearly flat.
In a 'normal' flatbow (figure 'A') the dark zone in the belly is taking the pressure. My consideration was, to prevent set, one should not reduce this area.
Previous tests did show that there is no flattening out. Moreover, a hollow back tries to curl up. (Untill severe bending forces it to flatten out...very suddenly and with a lot of noise and splinters. But that is far beyond draw length.)
And no flattening out means that there will not be an escape for the pressure like in, for instance, a HLD. So I wanted the belly to be able to take the full load and, on the other hand, the idea was to save mass.
So it became a flat but thin belly. A little less than half the thickness of a normal flatbow, because the outer fibers are doing most of the work. The belly is about 5 mm thick. I hope this thin strip also allows the limb to do some curling and to get rid of the forces involved with that.
No flattening out also means that the ridges on the back need to take the tension all the way. Tests show that reducing these edges too much brings the bow in great danger. I see them as two cables, under high tension, which are holding everything together. If the limb is bending these cables want to get in horizontal line with the belly. But the curling up effect and the high tension gives them no room to move sideways. I made them about 10 mm wide on top and they seem to hold up.
The result is a kind of U-profile. Well known to cunstrutors, engineers, architects and construction workers!

No flattening out would also mean that there will be no extra smooth draw. Moreover, curling up would possibly mean that the draw is more tough. I think this is the case.

[Badger]

  How does it respond if the belly is hollow? Does it flatten out at all?

[Philipp A]

Hi Jan, thanks for the design clarifications and drawings. Based on your explanations and drawings it appears that the design mostly reduces weight while maintaining performance. Are you maintaining the same thickness of the spars throughout the U profile? In a feather it is both reducing in width and in thickness over the length of the limb to the tip.

[Jan de Bogenman]

  How does it respond if the belly is hollow? Does it flatten out at all?

I did not make a "grown up" hollow belly myself, Simon Siess is the expert on this I think. As I recall it from his website, in a comment on his atricle he answers that he estimates the flattening out is about 3 mm sideways. Don't know if there are any measurements yet.

Did some testing wih bamboo slats and little bows.
In a hollow belly construction the ridges are like coloms under pressure. They tend to break away sideways.
Its quiet clear in the pictures below, In the first picture a hollow belly. The slat is bending towards the camera. Note the spot near the vise, appointed by the round file. The bamboo splits open. Also visible is that the ridges move sideways.
Second foto is taken while bending the same slat the other way, so with a hollow back. The split closes again. The ridges are straightening up.
Considering this, I think a well made HLD will flatten out.