HHB Double Void Experimental Bow (3/4 finished)

[Philipp A]

Christmas Greetings from Canada,

I had this idea in my head for a while. I wanted to try a new style of bow that has a void each side mid limb. The idea was a further progression from an earlier bow where i tried to replicate the design of a turkey feather which has a deep groove on the back starting close from the base and fading out to the tip. I believe it allows for higher stresses similar to a double spar on an aircraft wing. I used it on a heavily crowned stave where I was afraid that I otherwise would break the bow on the back at the top of the ridge of the crown. It is actually a sweet shooting bow.

In any case I am 3/4 of the way finished with this experimental design and I am hopeful that I won't break it by the time I reach full draw. The two side limbs at the void are sloped at the belly to match the slope of the back of the bow. When drawing back I think I am getting some additional side deflection with the split limbs and I am hoping that I am storing some additional energy that way. The handle features a centre belly ridge and I will likely make the handle section a bit thinner yet to decrease the draw weight further and have it bend a bit more in the centre.

Here are the stats:

Wood: HHB stave seasoned for one year at < 9% moisture
Dimensions:

1) 67" n/n
2) 2.25" wide mid limb (including void)
3) each side limb at void 7/8"
4) width at handle: 1.25"
5) thickness from tip to void: 3/8"
6) thickness from void to handle: gradually increasing from 3/8" to 1/2"
7) thickness mid handle 3/4"
handle void transition concave at belly with belly ridge
9) voids transition with gentle slope to belly ridge and towards tip
10) tools used: bandsaw, hoof rasp, different files, drill to start voids and hand saw to cut voids
11) string FF with grey fox string silencers

I would love to hear what you think about it. I will post more pics once I reach full draw. I am aiming for a 70# draw weight.

I am posting the link to the google high res photos here and I am attaching a few low res photos with this posting (let me know if the link doesn't work):

https://photos.google.com/share/AF1QipNaeG7BrfDXdg1hoUTPVvfLC4V5CkcPhM1GoSRtngfe6z1pynOGlFYKPWe9voIqiw/photo/AF1QipPaknFtbUA-bO282TNrS1eRUZn4q0YzBzk7BAro?key=ZzRfWnliUGotN1VJZDNVWGk5TWlHZTc1T2VLODNB

[Badger]

    I think that it is really interesting and admire that there is some method to your madness besides looks. Generally speaking your energy storage is more a product of geometry that anything else. One place you might save a lot of energy or loose is in the vibrations at the end of the stroke. The majority of our energy losses come from limb distortion in the final few inches of power stroke and hysteresis. Seeing as how this is a sort of prototype I would back off the weight if it starts to take set and just try for as much as you can get.

[Philipp A]

Hi Badger,

Thanks a bunch for your advice! I think you are right and I maybe should not finish it with a high draw weight. I will take it slow until I get to final draw weight and length. I will continue to long string tiller the bow in order to avoid set. It has worked well so far. The proof will be in the pudding when shooting it. I am not yet sure what to expect. I am intuitively thinking though, that the bow should shoot straight (if I don't wreck it before), since I believe the double voids and sloped sides at the void should give it additional torsional stability.

This bow took me a while to make and I hope I can keep it together without destroying it. At lease I have some photo memories in case i do .

All the best, Phil

[barebo]

Personally speaking - I think that pursuing a concept that you had and following it through with an actual creation is spectacular!! Folks like Badger can give you all of the technical details to make your bow as good as it can be, but going with a design that is a departure from the classic styles usually seen is the beauty of this pursuit.
If new ideas aren't developed there can be no advances. I love the utter simplicity of a "Plain Jane" flat bow, or a well crafted self recurve, but seeing someone really pushing the limits is exciting and keeps things "fresh" if you will. Basically, I think your bow is pretty darn cool!

[ohma2]

Although you have gone past what my feeble mind can absorb i agree that experimenting is never wrong.folks like you badger and others are what make this bow  building thing so interesting ,wish you the best in your endeavor.

[Del the cat]

Cool 
Del

[Jim Davis]

That groove in the quill of a feather adds two webs, which help prevent the quill from collapsing.. But the gain is only because the feather is hollow--the webs and the edge surfaces of the quill bear the entire load.

In the case of a wooden beam, any material removed from the compression surface reduces the strength. So, your concave section removes weight where there is no measurable gain by doing so, and reduces the strength, unless you make the limb as much wider as the width of the "void."

For a given width in a wooden limb, making the void WEAKENS the limb.

There is no magic.

[Pat B]

Very cool bow. Hope it works out for you.
 In his book "Secrets Of The Omaha Bow" near the back, William Vonderhey shows a hickory selfbow he made that looks like 2 hickory poles attached at the handle and a few places along the limbs with voids between. I was amazed when I first got this book in 1992 that such a bow could be built. This bow is somewhat reminiscent of Vonderhey's bow. 

[Philipp A]

Thanks guys for all your encouragement and thoughts, it is much appreciated.

@ Jim Davis: I also appreciate your feedback and wanted to add some more colour to my post and see whether this makes sense to you. In regards to the turkey feathers when I dissect them the two "beams" on the back that are joined on the belly side have actually a rubber like mass inside of them and the end of the feather is the only thing that is hollow. So in reality I believe these feathers are a composite structure. I am not sure whether the function of the rubber like substance is to provide additional elasticity, I certainly would be interested in the answer since these feather are not just very flexible but also incredibly strong.

I agree that the void will have to be compensated by additional width which I have done (each side of the void the spars are 7/8" in width and I have carried through the void until the 7/8" width of each side meet up again towards the outer limb and the handle. I think that way I should not get a stress riser if I fade the void into the outer limb and the handle. One could argue I could have made due with just a narrower stave and gotten to the same place. That would be a fair point. I think though that what I have done might be a useful way to take care of a more heavily crowned stave without getting the extra stress in the centre portion on the back. By sloping the belly to match the slope of the back I am hoping that I again equalize the stresses over the width of both the back and the belly.

Whether I get any type of performance gain is still an outstanding question and at the end I might break this thing and it turns out an exercise in futility but in that case it sure was fun to make

[Philipp A]

Hi Pat,

I forgot to reply to your post about the bow made by William Vonderhey, that is very interesting. Were there any type of further comments on it as to its performance and reasons for the design?

Carson Brown has told me that my bow reminded him of some illustrations he had seen once of some Egyptian bows. They had two square shaped limbs running parallel and then bound together at handle and tips, but free and unbound in the limb, so a similar concept. Maybe they did this just due to availability of suitable bow wood?

Cheers,

Phil

 

[Jim Davis]

Philipp, you are right a bout the quill being filled. That does prevent it from collapsing.

Since you have added width to compensate for the void, you have dealt with that concern.

As far as dealing with a high crown by making the void, that is an extreme version of a concave limb bow someone posted recently.  This geometry does put another force, as you have mentioned, into springing the belly edges away from the center line. But I'm not convinced that energy is more efficiently stored there than in a conventional geometry. If it takes 25 pounds to move the limb tip 15 inches, 25 pounds is all you've got whether you have a flat plain limb or a concave limb or a limb with a void in it.

Lot's of us worry about a high crown lifting a splinter and breaking. But in fact, that almost never happens with a perfect growth ring end to end.

I made a toy indoor bow (don't look at the wall by the quilt rack!)  that is about 3 feet long, 3/16" thick in the limbs and 3/4" wide at the fades. It has a 1/16" thick strip of hickory that is 3/8" wide end to end on the center line of the back of the bow. That little strip carries pretty much all of the tension forces and the bow flips a little blunt arrow out there hard enough to dent sheet rock (I ASKED you not to look at that wall!). I think the high-crowned back boogey man is not so scary as he seems.

[Philipp A]

Hi Jim,

I hear you about the high crown concern, I did however break 3 HHB bows with heavy crowns without having violated any growth rings. These were also earlier bows of mine where my tillers were not much to write home about, so these more heavily crowned bows were likely even less forgiving if you had any resemblance of a hinge. Your indoor bow sounds like a lot of fun though. I am not sure whether I would be allowed to use one at my place. The dog would want to play catch and I would likely have to sleep in the dog house later if I make a hole into the dry wall .

To be honest, I can try to rationalize my design all I want, at the end it is really just an idea I had and I wanted to see whether I manage to make it work. That question is of course still outstanding since I have not yet shot the bow. I still have a few small knots on the belly side (they don't go through to the back) that give me a bit of cause for concern and I am still wondering whether I should just work with them or carve / drill them out and fill them with HHB filings and 5 minute epoxy mix just like what I have done with a nasty knot on the belly side closer to the handle. They are just after the transition from the void to the outer solid limb a tricky spot for sure. Any advice would be appreciated!

[Jim Davis]

Philipp, if the knots are solid wood and tight, leave them. They are probably harder than the surrounding wood and will be fine.

Here is a picture of the back of that little bow I spoke of. The strip on the back does taper down at the ends to the width of the nocks.

Contrast is not so good in this photo.

[Philipp A]

Hi Jim,

I see your point now that you shared a picture. Is it a board bow other than the strip? Thanks for sharing, I still think I should have one of those for my home to play with in the basement.

Cheers,

Phil

[Jim Davis]

Yes a "board" bow, though I cut the wood from a log, ripped it on a bandsaw and planed it to 1/8th thick myself. Board, but not from the BORG.