Tillering help needed please with sinew-backed bow.

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Tillering help needed please with sinew-backed bow.

Postby docmann » Wed Jul 15, 2015 1:22 am

This is a problem that I've had to a lesser extent before, but with this bow, I'd like some input please.
This is a sinew-backed bow that was originally built with a small amount of reflex in the limbs (which I now regret). As you will notice, the upper limb is stiffer and appears less flexed vs. the lower limb. Interestingly, the bow has a fair cast, and at full tiller, the limb tips ultimately match up but the lower limb is a bit more "whip-ended" and arcuate vs. the upper limb (which makes more of a gradual arc). While both limbs are no-doubt storing the same amount of energy at full draw, I'm wondering what I may be compromising with differences in how the limbs are flexed. The bow doesn't have much hand shock, but I have to believe it would shoot more efficiently with similarly-tillered limbs.
I can modify the stiffer limb to match the lower one but of course, I'll reduce the weight of the bow in the process, and it's just barely pulling 50 lbs. now. I could proceed in that fashion and shorten the bow to regain draw weight but it's already at 60". I could apply more sinew to the lower limb 2/3 out and reduce belly wood closer to the handle on the bottom to attempt to match it with the upper limb, but I hate to think of the extra bulk on the back, and I've waited 2 months for current sinew to dry. I will be backing it further with rawhide which, if stretched taut, might help out the lower limb. I've considered attempting to carefully build in more reflex to the bottom, but I cannot believe heat applied to a sinew-backed limb is well-advised (although I'd like to know if it is possible without degrading the already applied sinew). I know that at boiling temperatures, sinew denatures to a lump of goo, therefore any heat applied would have to be low and slow.
I could simply leave it alone and just concede that the bow probably performs sub-optimally.
In closing, I'm considering cutting another set of nocks about an inch in from the existing nocks thereby having double-nocked limb tips so as to increase strength without cutting it down any further (I already have horn overlays on the tips and would like not to have to shorten and reapply). Has anybody done this (double nocks), and are you happy with the results?
Sorry so long, but I'd like to get some responses to the fundamental problem of limbs that are pulling the same weight, but are flexing at dissimilar parts of the limbs to get there. If it's more of a cosmetic issue, then I'll leave it alone.
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Re: Tillering help needed please with sinew-backed bow.

Postby KsBow » Wed Jul 15, 2015 8:50 pm

Wow Doc, you're in a pickle this time my Friend. I can see that the upper limb has a negative tiller compared to the lower limb. I would not heat a limb with sinew already applied. Someone else might, but not me. You have plenty of length to shorten the bow. Sixty inches is plenty of length.
Perhaps you could shorten just the bottom limb to stiffen that one up. All that would mean is a new string. Finally, this whole scenario is a testimony as to why I don't cut a shelf into a bow. If you're gonna sinew a bow, do so after it's tillered with no shelf, then sinew it, Check the tiller and then cut the shelf if you must. If there were no shelf, you could just turn it over and you'd be real close without having to do much at all.
Good luck, keep thinking about all the possibilities and try one thing at a time. Kinda like the Scientific Method. lol
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Re: Tillering help needed please with sinew-backed bow.

Postby docmann » Thu Jul 16, 2015 1:34 am

Thanks Ralph,
I rarely use a shelf. Likewise, I very rarely back a bow with anything except air. This was one of those whim projects. I've put the darn thing down and walked off a dozen times, only to pick it up again investing another 30-45 minutes thinking I might be able to pull it from the jaws of death. It's full of "I wish I would have done that differently" items, but at least it has been a good project in attempts at "poor design remediation". I guess my ace-in-the-hole is that with the strong sinew backing, I do have the ability to cut it on down, and I'm thinking with the shorter limbs and increased weight, I might just muscle out the difference. I'll keep you posted. If it simply won't behave, I might have to just wrap it up and give it to my wife for a Christmas present, or perhaps anniversary. (Boy wouldn't that make for a cold New Year's Eve!)
Thanks again Ralph,
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Re: Tillering help needed please with sinew-backed bow.

Postby McClura » Thu Jul 16, 2015 8:06 am

I would back it, string it and shoot it. See how the sinew reacts and if the shape changes. I have several osage bows that doesn't look too good unstrung. One has a reflex in top limb like yours and the bottom limb is deflex a little. When strung, tiller is good, measuring every 6" from center to tip on each limb and it shoots great. No hand shock and it has good cast. It is the second bow I made before I learned of heating limbs and moving them during the build.
If, you are wanting more draw weight, the extra nock grooves work well.

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Re: Tillering help needed please with sinew-backed bow.

Postby docmann » Fri Jul 17, 2015 6:34 pm

Thanks for the input fellas.
I ended up cutting a second set of nocks which shortened the NTN by about 2" (now at 58"), but with the sinew backing and the rawhide that I'm about to apply, I'm not afraid of it.
The extra ump seemed to pull out most of the defect at full draw. The photo bekow is pulled to 24" and is getting close to symmetrical (my draw length is ~29" +/-).
By shortening it, I got the weight up over 50# easy enough.
Mike, I'm just resigned to the fact that when unstrung, it won't be the prettiest girl on the block, but if it'll hurl the wood through the air, that's no big deal.
Thanks for the advice on the double nocks. Glad you gave me permission to do that as I've become a little tentative lately, especially this far along in the build.
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Re: Tillering help needed please with sinew-backed bow.

Postby KsBow » Sat Jul 18, 2015 7:59 am

That puppy is looking good Doc. It also looks like it'll spit an arrow out. After all, the way it and it's maker shoot is all that matters.
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Re: Tillering help needed please with sinew-backed bow.

Postby Se1f6ow22 » Mon Aug 03, 2015 6:42 pm

I know I'm late to the party. But for what its worth, I always apply the sinew before the tillering process, when there's about 3/4 inches of wood on the limb. This way when the sinew is drying it cant pull the limbs in any funky direction, it dries the way the bow is suppose to look. And I always weigh out the sinew so I know each limb is getting the same amount.
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Re: Tillering help needed please with sinew-backed bow.

Postby docmann » Tue Aug 04, 2015 2:26 am

Looking back, I think I erred in building in the reflex. With the sinew, I probably didn't need it. While I built in just a small amount, one limb, at brace height, just wanted to flex more than the other. Like Mike observed with some of his bows, when the bow is drawn, the limbs equal out (mine look fairly symmetrical at about 24" and beyond). But at rest and with it strung, the bow appeared to have a sub-optimal tillering job. There is a "rest of the story". When I added the rawhide to the sinew, the asymmetry became even less. Ultimately, I guess as I add strength to both limbs, the asymmetry becomes less pronounced as the added horse power overcame the greater flexion of the upper limb. Incidentally, shortening the nocks and adding the rawhide added about 6 lbs to a 29" draw length. I wouldn't have guessed that. Mike Yancey supplied a very nice set of rawhide strips that are probably a bit stiffer than average. They were easy to work, and I am very pleased with the way they applied, and even more so with their appearance.
Good lesson in physics and patience. Also a lesson in not always having to be satisfied with a lighter draw weight if one over-shoots in the tillering process.
Thanks again for the input fellas. (Ralph, you could've just told me how to do this instead of grinning like a opossum waiting for me to finally figure it out!)
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