elm bow tiller check (repairing a broken bow?!)

[cadet]

What a superb job to have brought it this far!

[joachimM]

Did some flight shooting this evening with my repaired broken bow. I did cheat, I used carbon arrows. I just don't have my own flight arrows. My bamboo arrows all have spiral fletching for stability and are heavy (400-700 gr) so these just have so much drag they don't tell me a lot about my bows.
I noticed I rarely draw the full 24" of this bow with my sudden jerk release, but for flight I don't want to hold it to 24" either.
I was a bit disappointed with the heavier arrows (500 gr max 180 m), but the four lighter ones (240 gr) I all shot between 230 and 235 m far. And still I saw the arrows kick a bit sideways (not tuned at all to the bow). I did have some tail wind, and the landing spot is located 4 m lower than from where I shoot.

And it's still holding together after some 50 shots. Hopefully I'll be able to says this again after some 500 shots 
It's taken a bit of set (1" or so, none left after a few hours).

Time to sand it (2nd try) and give it a finish.

Joachim

[willie]

I have been following along to see what the difference between performance is with and without the sisal backing.

looking back at two threads and a number of posts, give me quite a few numbers to compare, but the gist of the comparison that I see, is that for equal weight and draw length....

the heavier arrows lost cast, while the lighter arrows gained cast? or are the differences too small, combined with the different shooting conditions, to make a confident determination?

Did the finished mass of the bows change much?  (added sisal on the back but subtracted belly wood when retillered)

In spite of the small differences, have you formed any hypotheses about the use of sisal for a backing?

[joachimM]

Bob,
Obviously, you can't compare performance on the basis of one bow first bare and then backed, especially when the first broke because it's back was overstrained and the bow was still fresh. That clearly wasn't a durable bow. Time will tell if the backed version is. I will try to chronograph it today. It still seems to be a faster bow than most of what I've built so far.

I have backed four bows with sisal so far, and they have never failed in tension. One I gave to a friend, one broke from the belly due to a dry fire, one is waiting for a stronger belly lam as it overpowers the light density hazel belly (chrysalled all over the place) and then there is this one.

my experience: it's cheap and easy to come by, it's easy to apply, it's easy to add weight to a sisal back and even do tiller adjustments by adding backing.
To me it combines some of the good features of both sinew (high strain) and bamboo (high stiffness) for backings. No need for power tools like band saw and belt sander, dozens of clamps like with a bamboo backing.
It doesn't have the obvious advantage of sinew (shrinking as it dries), so usually (didn't want to push my luck here) I reverse-brace the bow while applying the sisal, so as to pre-load the sisal in a bow at rest and take advantage of the fact that it can stretch more than 2%.

It mostly widens the toolbox for the bowyer

[joachimM]

Did some chronographing this afternoon. Meanwhile it has had >100 shots, no signs of degradation.

With a 460 grain arrow my 5 shot average was 168 fps with a max of 173. A 660 gr arrow shot some 145 fps. 240 gr arrows gave me readings from 192 to 196 fps.
I guess the overall performance is pretty comparable to what the bow would have given me if it hadn't broken and after being broken-in.

For a bow drawing only 24" I consider this to be a very good result. With the sisal backing I could draw it further, but it would only take more set (currently some 1" after shooting that returns to zero some time later) and that's not what I want.

Joachim

 

[joachimM]

Tampered a bit more with the sisal backing, and gave the bow a deeper heat treatment. Sanded it down a bit on the back to smooth it, but needed to retiller it because of that, re-applied some backing (that's the advantage of such backings: you can tiller both ways), and let it dry for a few days till the bow weight was stable (356 g).

I was a bit anxious that I would head again into a "good-better-broken" direction, but it's holding up more than fine. Set right after shooting is less than 0.5" and I'm getting readings of about 173-174 fps with the 460 grain arrows (my release has also improved, but I also feel it in my back muscles), and the light carbon arrows (240 grains) are giving me 205 fps now with a good release. With a decent string (the current one has quite some stretch) it could maybe become a moderate flight bow. Will try to make a flax string tonight.

I've started to work on the sister stave from the same log. Trying a similar design but with skinnier tips (didn't dare this here, as one tip had split and broken off the back portion), and hopefully push it a bit further in a simple composite flight bow direction.

Thanks for reading. Will ask my kids or wife to take a few flight shooting pics when the weather permits, cos a thread without a picture now and then can be a bit boring.

[willie]

Quite a bit of experimentation going on with that bow. I will bet that you have a few new ideas to try with the next one. be looking forward to seeing the new one.

Btw , you mentioned earlier that you were replacing the belly on a bow with something better.
Do you have some new candidates for belly wood to combine with your sisal?

[joachimM]

Thanks Willie.

Well I need to cope with what I have available here for belly lams.
I have a board of unknown dense (SG .90-ish) and hard dark tropical hardwood that I might try, but I also have a few Opepe/Bilinga boards (a SG .74 tropical hardwood that's become very popular for outdoor furniture, mostly due to the increasing scarceness of better woods like Jatoba, but with terribly wavy grain).

I could also try black locust (anything is better than SG .35 hazel...), but maybe black cherry with its good compression properties would be the most logical choice. In fact, I might just try to decrown a 2" wide BC sapling and back it with sisal. I have a dried and reduced reflexed stave ready for floor-tillering that I could use for that.

Joachim

[Jim Davis]

I like your attitude toward salvaging a broken bow.

I do wonder about the various stretch, strain, etc. numbers you toss out. How do you come up with the amount of strain the back of a bow will have? Of course we want stress, but we don't want any strain, if possible.

[joachimM]

Hi Jim,

Yes generally I don't try to repair broken bows. I usually do keep the non-broken limb for an eventual splice against another broken bow :-)

In my book, strain is just the amount of stretch the bow back gets.
Basically, when a bow bends, the neutral plane (supposedly in the middle of the bow between back and belly) still doesn't undergo any stretching, whereas the back is stretched and the belly compressed. The thicker the bow, the more a certain bending of the limb stretches the back and strains it.

I I explained that a few weeks ago here, with a spreadsheet for easy calculations: http://www.primitivearcher.com/smf/index.php/topic,56310.msg769864.html#msg769864

You can fit a circle with a known radius to the most bending portion (sometimes the entire limb in pyramid bows), and calculate the length of the arch (the section of the circle covering the bend). I do this with a vector drawing program (inkscape)
When you know the thickness of the bow and the length of the arch in the neutral plane, you can calculate how much the wood on the back has stretched compared to the neutral plane.
Very few woods can take a strain larger than 1% before breaking.

As for the amount of strain plant fibers, sinew and other materials can take: all of that is in the "public domain", with lots of papers covering the subject. I compiled this into a database, which is also available here: https://drive.google.com/open?id=0B3YYA3Sr_3gqcDF6VHFaeHFlSnc

[willie]

Joachim-

I have been scratching my head a bit about wood properties, and have been wondering about how to best estimate strain.

If wood is stronger in tension than compression, (sometimes by a factor of 2-3 times stronger),

can we still suppose that the neutral plane is in the center of the limb between back and belly?

willie

[joachimM]

David Dewey (aka Woodbear on PP) did some work in a tillering spreadsheet where the cross-section of the bow is taken into account for the positioning of the neutral plane.
Tension twice as strong as compression doesnt move the neutral plane that much off center, just a bit above.
But yes essentially you're right.
In belly-tempered wood both balance out quite evenly, however, or so I believe

[willie]

Joachim

If we consider a simple rectangular cross section, The text I have been reading, supports the idea that the distance that the neutral axis moves is in direct proportion to the moe(s).
two to three times would seem to be quite significant. That would be quite a bit of belly tempering.

For the self bow, maybe these considerations are a  bit academic, but I am wondering, when we go to design a composite, might there might be some advantages to matching materials by looking at the moes of the possible backs and bellys?


willie