Interesting HLD bow explosion (Pic heavy)

[leonwood]

That is a good report. I have always wondered of the hollow limb design might be less prone to vibration in the last few inches of the power stroke. Flat bows have some distortion here that looses energy.

Hmm, that sounds like a good explanation, maybe I will do the test and try to arrange an ultra slowmotion camera to see the difference in vibration when the string slams home

[simson]

Sorry for you lost, Jaap.
I really admire what you've done, assuming this is your first HLD it is good work – even it is blown up.
You and Leon should examine exactly why it exploded. Looking at the fd, I think you are asking a lot from this bowwood. The break is at an area where the limb seems to be thinner. It started as a tension break, after that it splitted (IMO).
I personally would use a risky wood like this with all the knots. A HLD requests the best stave ever, no knots and evenly high crown.
I like the tools you've shown, but I don't think you can get with them the right depth. Wood is not an industrial product, so we have to react on the changes at the back. If there is a hump, follow the geometry on the belly. This can only be done with the scraper (at least in the final tillering stadium) and a caliper.
Back to the fd:
I couldn't find the specs, how long was the bow, how long non bending areas?
And when you draw the bow on the tiller tree, be careful to hold not that long!

I'm convinced elder is good bow wood, not osage – but good! It just needs to be a bit longer, or a bendy handle.

My HLD theory and experience:
A HLD needs less mass compared to a flat or a recurve (same weight). So there is more energy on the arrow.
Meanwhile I've made a bunch (at least about 20) of HLDs, and my experience is they all outperformed bows of same drawweight with common designs.
Example: a little elder bendy handle HLD (http://primitive-bows.com/elderberry-hld-4028-no-37/) shoots 10fps faster than a really well made osage recurve (http://primitive-bows.com/bowquiverarrow-set-for-a-my-wife-4028-no-27/).
I have some of them chrono tested, unfotunately I don't own a chrony myself.

Leon: „Take two staves from the same tree an tiller them at the exact draw weight and mass, one hld and one flat. Would that work???“
IMO: No, assuming the have same length and same length bending parts, the HLD is the lighter bow for sure.


Now let's have a look at the cross section pic:
What you've created I would call a semi-HLD. Will say, the 'moon' is too thick, the limb cannot flatten out. So you will loose same advantages from a real HLD. For further info:
http://primitive-bows.com/hld-a-new-progressive-design-for-selfbows/
Your cross section is a good mass saving design, but provokes a lot of stress in the ridges.   
It wasn't obviously not a problem on your bow, but I found some minor bow woods couldn't withstand without starting to chrysel.

At least:
Don't give up making HLDs. Lot of work – I know, but a successful is a really fun to shoot.
And you've done already great work.

[Weylin]

I think the mass is one area that there can be a somewhat objective comparison. If HLD bows are consistently coming in with less mass for the same specs then that might be some indication of an advantage. That seems to be the argument for HLD being efficient, needing less mass to do the same job because of the shape. 

[BoltBows]

Sorry for you lost, Jaap.
I really admire what you've done, assuming this is your first HLD it is good work – even it is blown up.
You and Leon should examine exactly why it exploded. Looking at the fd, I think you are asking a lot from this bowwood. The break is at an area where the limb seems to be thinner. It started as a tension break, after that it splitted (IMO).
I personally would use a risky wood like this with all the knots. A HLD requests the best stave ever, no knots and evenly high crown.
I like the tools you've shown, but I don't think you can get with them the right depth. Wood is not an industrial product, so we have to react on the changes at the back. If there is a hump, follow the geometry on the belly. This can only be done with the scraper (at least in the final tillering stadium) and a caliper.
Back to the fd:
I couldn't find the specs, how long was the bow, how long non bending areas?
And when you draw the bow on the tiller tree, be careful to hold not that long!

I'm convinced elder is good bow wood, not osage – but good! It just needs to be a bit longer, or a bendy handle.

My HLD theory and experience:
A HLD needs less mass compared to a flat or a recurve (same weight). So there is more energy on the arrow.
Meanwhile I've made a bunch (at least about 20) of HLDs, and my experience is they all outperformed bows of same drawweight with common designs.
Example: a little elder bendy handle HLD (http://primitive-bows.com/elderberry-hld-4028-no-37/) shoots 10fps faster than a really well made osage recurve (http://primitive-bows.com/bowquiverarrow-set-for-a-my-wife-4028-no-27/).
I have some of them chrono tested, unfotunately I don't own a chrony myself.

Leon: „Take two staves from the same tree an tiller them at the exact draw weight and mass, one hld and one flat. Would that work???“
IMO: No, assuming the have same length and same length bending parts, the HLD is the lighter bow for sure.


Now let's have a look at the cross section pic:
What you've created I would call a semi-HLD. Will say, the 'moon' is too thick, the limb cannot flatten out. So you will loose same advantages from a real HLD. For further info:
http://primitive-bows.com/hld-a-new-progressive-design-for-selfbows/
Your cross section is a good mass saving design, but provokes a lot of stress in the ridges.   
It wasn't obviously not a problem on your bow, but I found some minor bow woods couldn't withstand without starting to chrysel.

At least:
Don't give up making HLDs. Lot of work – I know, but a successful is a really fun to shoot.
And you've done already great work.

Simson, thanks for your answer! let me try to reply some of your questions!
- I will check if the pieces are still around in my dad's workshop, so I can examine it again more thoroughly. You might be right about the tension break, I hope I can find out if was indeed too thin.
- yep it was a risky stave, especially with the big knot in the middle of the left limb and the ones on the sides of the right limb, but amazingly they held their own!
- The moulding plane I use is only meant for roughly shaping the bow. I don't use it when tillering and like you said, I follow the geometry of the backing with scrapers and relatively flat gouges.
But it definitely speeds up the process of shaping, it's a replacement of what I would normally do with a drawknife.
- About the specs: Handle area is definitely 5'' long, and the fades 2,5'' each. At the fades it's about 45mm broad and midlimb about 35mm. The length when it broke was 164cm (64,5'') and it would've been about 46lbs @ 28''. I'm not entirely sure about all the measurements, but I will check soon.
- I never ever hold my bows on draw long, simply because I don't have the balls for it! Although if you make an under strained bow, with almost no set, it should be able to withstand at least a few seconds on full draw, shouldn't it?
- About the cross section: Ahaaa that makes sense! I guess I didn't dare to hollow it out too much, but I will do a better job on the next one!
- About your elderberry HLD, that one is a work of art! It's really a masterpiece to die for! I also really wonder how fast it is, considering it out shot that osage recurve by 12fps! 

Thank you so much for your information, I will continue to build HLD's, because the virus has caught me. Currently I'm working on two HLD deflexed recurve bows. One of yew, and one elderberry (the latter is a pretty clean stave, but is also quite flat, so it will be a flat HLD.)

Cheers,
 Jaap

[Jim Davis]

One thing seems to be missing from this post mortem. When a half cylinder is bent, there is more happening than simple longitudinal tension and compression. Bending opens the radius so that the inside radius is in tangential tension---and has almost no strength to resist that tension.

Try bending a long piece of PVC that has been sawn in half lengthwise. It probably won't break easily, but it will have a nearly flat cross section where it kinks.

O.L. Adcock tried to revolutionize laminated fiberglass bows by making the limbs concave. Lots of hype, but only a memory now.

[BoltBows]

One thing seems to be missing from this post mortem. When a half cylinder is bent, there is more happening than simple longitudinal tension and compression. Bending opens the radius so that the inside radius is in tangential tension---and has almost no strength to resist that tension.

Try bending a long piece of PVC that has been sawn in half lengthwise. It probably won't break easily, but it will have a nearly flat cross section where it kinks.

O.L. Adcock tried to revolutionize laminated fiberglass bows by making the limbs concave. Lots of hype, but only a memory now.

Hi Jim, you have a good point on the tangential tension, something I’m aware about. I think the question is if it adds something to peformance or doesn’t. For sure it shouldn’t be overbent because like you point out, it will just flatten and break. However I think if the flattening effect is kept to a certain maximum, it might even benefit the bows peformance. If I’m not wrong it will even smoothen the draw a bit.
Enough stuff to experiment on! That’s what makes bow building fun!

Ps: please don’t take this the wrong way, but Da Vinci also tried to build flying machines. Not everything will work the first time. Maybe this also won’t! But I want to find out.

[Jim Davis]

Boltbows, I am sure such a bow will hold together if the stresses are limited to what the wood can bear. Don't know what to think about a performance gain. I think any gain would be due to reduced limb weight and perhaps less hysteresis.

[leonwood]

My experience is that HLD bows have less set also, so the combination of lower mass, less set and les hysteresis (which are all performance killers!) when added up it is a good possibility that HLD does enhance the performance of a bow

[Badger]

   I am curious about the mass, are there any examples that show the mass and dimensions?

[leonwood]

   I am curious about the mass, are there any examples that show the mass and dimensions?

I can measure the dimensions, draw weight and mass of my black locust bow if that helps?

[scp]

would it be wrong to argue that a HLD profile in general needs less wood/mass to generate same strain/draw weight than other common cross-section designs and therefore should be more competitive? just asking...

It appears that concave belly would make wider portion of crowned back work in storing energy. For small diameter staves, this might be a better way to make the whole back work than de-crowning the back, by making the narrow limb work like a much wider one. I don't think the concave belly cross-section is more effective than the flat one. It would be more intuitive for the concave back cross-section as in Bhutanese bamboo bows should work better.

[bradsmith2010]

if the concave belly was superior in perfromance, I think we would see more test results,,they are beautiful,, but I doubt they are superior in performance,, I am just guessing,, 

[bownarra]

I think these bows may store a little energy by trying to 'split' the belly in half. Interesting to think what the poisson effect is doing here
I highly doubt they will work out faster than an optimised flat belly.

[BowEd]

The age old proven process of keeping the majority of the mass on the inner limbs wide enough for less set with power and speed on wood bows I think holds true for any design.
These HLD bows are very interesting.Never devoted the time into one though.I feel they could get the best out of a highly concave sapling bow from less dense second string wood avoiding any decrowning to keep their width.