Hollow Back experiments

[willie]

because there are not the advantages like on a hollow belly

Simson
As I understand the physics, your hld has the belly curl out, and Jans has the back curl in. Shouldn't both have the same advantage?

[longbow steve]

Turn the stave around and work with the natural curvature of the rings to form the back so no grain violation needs to occur. The outer surface of the stave now becomes the belly. Steve

[simson]

because there are not the advantages like on a hollow belly

Simson
As I understand the physics, your hld has the belly curl out, and Jans has the back curl in. Shouldn't both have the same advantage?

No Willie,
that's a totally different thing. Here is info: http://primitive-bows.com/hld-a-new-progressive-design-for-selfbows/

[Jan de Bogenman]

LongbowSteve. Exactly, the elm stave above was turned upside down. The ridges follow one ring.

Simson, very nice to hear from you on this topic. Love your HLD and other bows. The HLD triggered me to try this idea. So thanks!
I did not make a hollow belly yet, so I can not compare. But some thoughts...I believe, and you know, that a hollow belly saves mass and makes for a sweeter draw, if done properly. On the other hand, I think, the hollow back can enable more mass reduction, but will not have the sweet draw. And it is in greater danger.
this conclusion/thought/hope came after some testing with bamboo and pvc pipes and aluminium u profiles. When the belly is the round side, these half pipes carry much more load, but collapse very sudden. Upside down, they almost carry the same, untill the point where the ridges tend to break away. Maybe if one is able to avoid that, both are equally strong. My hope is to get beyond that point.
Logically, with hollow back, the edges are under great tension indeed. And I am not certain at all they will last. Some reports here make me very nervous!!

It's  great fun testing anyway, Hope to post some more results soon.

[Jan de Bogenman]

That would be interesting to measure, just how much shape changing is going on. One would hesitate to ask a guy to leave a bow at full draw long enough to take measurements, though.

Well what's a good experiment without some measurements? And is their any bowmaking without pain?
I have put the ash bow to the test and that was a little to much. But all for bow science!
Did some shape measurements before breaking. The draw curve however...

Some results:
- Very little shapechanging at full draw. Checked it several times and took my time. A more accurate vernier caliper is required. The small differences noticed, do not seem to indicate any flattening, but that the opposite is happening. That would match the bamboo test.
- The bow did not collapse completely! It was still braced. There is a safety aspect in here?
- Tension in the high edges is higher than I am used to....wood selection is more important....surprise!

So far for this ash bow. It has shot a few dozen arrows at full draw, set was within (my normal) limits. Mass was 15,7 oz for a 71" x 2 " pyramid flatbow.
The break can very well be due to the design, but certainly also to the bad, brown, brittle spot in the wood.

For me enough reason to try some more.
A better stave is selected and ready for the tillerboard!

[Philipp A]

certainly looks to me like some rot in the wood at that location! I think this break might even be reparable and you might still have a working bow after all is said and done.

Cheers,

Phil

[kbear]

 

[simson]

Jan, I don't want to affect you. Hope you don't mind, here are some thoughts from me:

1. Concave back is more mass saving than concave belly. > Not true! Mass saving is exact the same amount.

2. The cross section you have built is not able to spread/flatten out because of the geometry. In cross secction the thickest part is at the crown and tapers out to the sides. To avoid exessive stress in that ridges you have to allow them to flatten out. Or in other words work or load is not only in direction tip to tip, but also from ridge to ridge.

3. Wood selection: yes, only choose the best staves without imperfections.

4. If you have thick walled boo: Make your hollow back experiments with this stuff (split in half, inside is back). Look at Bhutanese bows!

5. Try a 'normal' HLD to get the idea and experience what happens when the the bow limbs flatten out.

Sorry for your breakage, in this case rotten wood.
Thanks for posting and experimentating.

[Jan de Bogenman]

Simson, thanks for your thoughts. All critical, but helpful contributions are welcome.
Yours is sure stuff to think about!

As for the bamboo bow. I made a few of them that way. Great fun! A good bow in very little time and tillering by scraping on the back side.
The third bow on this webpage is a take-down of mine. http://zelfbogenmaken.nl/?page_id=487. (Hope you like the website).

And I have two questions.
I dont hope to affect you.... All your bows look awsome, but maybe, somewhere you have a HLD bow that can be put to a test?? Somewhere, forgotten, in a corner?
Could be very interesting to pull it backwards and to see if there is any difference?
Maybe I should repair this Ash bow and do the same.

Second, can you give me an indication of how much the flattening out, in a good working HLD will be?

[willie]

No Willie,
that's a totally different thing. Here is info: http://primitive-bows.com/hld-a-new-progressive-design-for-selfbows/

Thanks for the link Simson,  Interesting and nicely made examples.   I am curious if your recent testing of HLD designs has changed your thoughts since the article was wrote.
 From the article.....

"The depth of the limb (measured over the hollow) compared to the width should be in the ratio of about 1:3."

Is that still your preference?


Jan,
if I understand correctly, your primarily measurements indicate the limb gets narrower and thicker when drawn?
that would be consistent with a poisson effect explaning the HLD advantage.


Interesting discussion, hopefully some force draw info could have been measured  , as it would be revealing to visually compare the curve  shape of various designs.

[Selfbowman]

I've built bows belly out meaning using the belly of the stave for the back. They worked and had good cast. To be uniform it needs to be a clean stave though. Arvin

[bootboy]

Holy Cow! Thats a novel new idea. I admire your creativity and courage in working on it.

[leonwood]

Awesome work Jan, and great discussion. Now to add something to this: Would it be possible to hollow the back in the same way as Simon does with his bellies? So that the ridges are thinner than the middle so the bow can flatten out during the draw and thereby spread the tension forces from the ridges to the middle and the sides. Will probably be pretty hard because there is a point just before flattening where all the stresses are on the two ridges but if anyone can do it it is you jan!

[Jan de Bogenman]

Leon, thanks, but to much honour really!! Actually, since you make this great looking, and performing, HLD bows, I had a little hope you could help me out on this one. If there is anybody.....(-:
Meanwhile, I am doing some little tests, and I have to warn all those who try thin edges on a hollow back...be very carefull!! Hope to post some results soon. (And to get some sleep!)

[willie]

For me enough reason to try some more.

maybe a gague sort of like this will help you not hold the bow at full draw so long?
Was the bow repairable?