Zion:
I came to this design myself by doing experiments with different things. But everything was on the planet before we thought on it. Once I saw an archeological yew bow found from Switzerland, which had a furrow in the middle of the belly. Long time nobody could explain the reason for it, maybe this was the beginning of hollow limb design. Perhaps the future brings new finds ….
Btw. Some Indian arrows had grooves to stiffen the shaft and prevent twisting, that is the same principle.
Squirrel:
As explained, this design is mass saving and has other pros. So, more energy goes into the arrow and mot in the limb. See answers above and below.
Newindian:
Yes, you are right! So probably not every wood can withstand this (is to be checked out). I have done it (ok, a mild version) even with hazel (not good in compression) with very good result. My thoughts on this point: at Brace you will have a given thickness of the bow limb (measured from the crown to the belly walls). What happens when you draw the bow? The hollow limbs flatten out, the thickness of the limbs are now less than at brace. It’s like drawing another bow, one with thinner limbs. This affects a comfortable draw, the increase in the f/d isn’t that high than normal and, main point, that thinner limbs creates lower stress on back and belly.
The optimum is probably when we can create a limb design that can match the horizontal and the vertical forces at a specific drawweight. Will say, a good bow is built near the break. Hollow limb design should do the same also with horizontal forces, the flattening out should be that much as possible.
Here is a quick sketch to make it a bit clearer.
Ssgtchad:
Thanks for the word (goatleg = bent gauge)
Time spent: about 1h for debarking, 0,5h layout and bandsaw run, 1,5h first session, same for second session, will watch the upcoming time if you are interested
It takes me more time to do that buildalong, taking pics, photo bucket, computing, searching words and so on …
Fiddler:
Mike, thank you for your trust in me, I will do my best.
Reason to do the hollow limb design here is because I see no alternative. A conventionally design on this stave would end with a 15 pounder, because so much wood has to be taken off. The invention of hollow limb design: please see answer given to Zion, above and to Cody (missilemaster) on page 1.
I also have some elder staves with big pith (need still seasoning), they are candidates to hollow out for sure.
A theme, I think about, is Bhutanese bows. You know the Bhutan archers use bows made from a half split of a thick wall bamboo calm. They use it inside out, the inside of the calm is the back and the outside is the belly. This is the similar principle, but there is a big difference in the horizontal forces: in Bhutanese bows you will get tension on the back and compression on the belly when the furrow flattens out – with hollow limb design you will get the exact opposite. Please see sketch above.
Jodocus:
Thanks please see answers above.
Autologous:
Yes, Hollow limb design only makes sense at high crowned limbs. And yes, some of the bows strength comes from the resistance from flattening out. This load must not come from limb thickness like in a rectangular cross section. So less wood is necessary and the result is less stress. See sketch above!
All:
Again thanks for interest, this is a good discussion. And it seems there is a lot to discover.
Session 3:
I have marked some areas in red where wood is to be scraped off. I have meanwhile mostly changed over from the bent gauge to the gooseneck. I control thickness with my fingers, but have to react every inch on other problems – this stave is really an adventure!
Upper limb
lower limb
near fade, heavy destroyed wood ….
working out knots/holes
knot/dead branch near fade
also fine correcting side taper
I did some floortiller, I can bend that thing to a straight line (all the amount of reflex). Sorry no pic, would need an assistant. Two things are evident: first the stave tends to warp to the right (seen from archer, upside up) – so need to bend in the handle section to the left side; and second floortiller shows a much more stiffer lower limb (that one with the dogleg) – so needs to be scraped down and reduce the too high reflex.
Let’s begin with the last, reducing reflex. I will do that in two portions, one at the dogleg and then one near the fade.
That dogleg in the lower limb causes most of the uneven reflex in the limbs. I will try to heat it out a bit to bring the side profile more in line
here is a sketch what I want to do
now here is the heating setup
For heat bending I use paraffin oil (cheap at Ikea). It damps (and stinks!), when the heat gets too high. Good control mechanism.
The stave is clamped in the vice and secured with an additional clamp (left from vice). As weight I use a canister, bound on the stave. Of course this done before heating. Now measure the distance to the floor. As you see, I have a plastic clamp on the tip and a ruler leant upon. With this setup I can easily control the bending process, without interrupt the heating up.
Another good trick is an old can with a cutting lengthwise, this allows the heat going round the whole limb. It is held in place with pliers, simple and super effective!
I got the tip moving about 3” to the string. That is good, but not enough. Take a look at the pic, handle is still not parallel to my bench (both tips are laying on the edge of my bench). I leave it for now at it is; perhaps I will give it later another try.
Aren’t that beautiful colors on the oiled sections? That’s why I like that wood so much.
Now is time to bring the string in line. The string is about 1,5” off.
Noticed the handle is heavily damaged by the fungus, more than I thought. Problem here: It should be stiff and I need to heat bend the stave here for string alignment. I will do that with a longer run and less heat than normal. Perhaps we will need additional reinforcement here, that should be a matter for later.
This way it is clamped in the vice, notice the melting glue stick for preventing damage on the limb
how the can is held in place while heating
I heated down the 1,5” offset carefully with low heat (way lower than I normally do).
After cooling down, I saw that fret ( see earlierpic) on the handle had opened a bit. Well, I have expected it and it happened. No nerves for now to get on, will think over a repair. Any suggestions?
the leftover from session 3
