Primitive Archer
Main Discussion Area => Bows => Topic started by: cdpbrewer on April 24, 2014, 04:47:46 pm
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I’d really appreciate the collective’s analysis of two jatoba/maple/hickory tri-lam failures.
Bow 1
37# @ 28”, 67” ntn, 18 oz.
1.25” at handle, 5/16” at nocks
Reflex/Deflex trilam: hickory back, tapered hard maple mid lam, jatoba belly
Belly ~0.21” thick at break
Bow 1:
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow1-HMJbow800.jpg)
Bow 1 Break
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow1-HMJbreak600.jpg)
Bow 2
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow2-HMJtrilambow800LQ.jpg)
Bow 2 Break
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow2-HMJtrilambreak600LQ.jpg)
Some more info:
1. The lams for both bows were cut from the same boards. The jatoba was pretty dense and bias cut. The grain is generally straight but has sporadic, isolated run-offs- e.g. the last pic on bow 1.
2. Almost all of the bows I’ve broken have been during construction. These broke after at least 300 shots. This puzzles me...
3. Have never had a osage trilam break like these two- even those with much worse grain run-off and less dense osage.
4. Two hickory backed jatoba bows (not tri-lams) cut from the same jatoba board (with similar run-offs) and of the same design as the second bow above are still shooting after over 500 shots. The belies of these are much thicker than on the two broken trilams.
5. Neither bow was chronographed but they seemed pretty fast.
What caused the breaks?
My GUESS is that they broke due to shear fractures along grain run-offs on the jatoba bellies coupled with the belly lams being pretty thin (4 above).
Meanwhile, although jatoba seems to make fast tri-lams and is easier to get and cheaper that osage, I think it’ll be osage for tri-lam bellies.
c.d.
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Looks like grain and/or combination of tillering issues.
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Jatoba is fickle. Steve (Badger) mentioned a time when all his Jatoba bellies were blowing out on him.
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No comment on why they broke, but if you can substitute ipe for the jatoba, you will have better success, IMHO.
Russ
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Looks like they broke in compression. I can see some grain run off in the jatoba, as mentioned. Jatoba is tricky in compression; it is dense and hard, but nearly as elastic as ipé or osage. It tends to chrysal easily and I can see some chrysals in the top part of "bow 2 break" picture. These chrysals probably formed AFTER the bow failed, correct? Did you see any chrysals prior to these bows' destruction?
I like the design you're using. I thick you'll be safe if you use ipé for the belly of such bows. If that's not possible, use a new board of jatoba and you might be fine as well. A slightly thinner core will be helpful as well, in avoiding compression issues in the belly.
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Both bows broke along a grain line in the belly. You can see the obvious diagonal grain break across the belly on both.
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Looks like that piece of jatoba didn't like that much angle of violation....the first one should've told you what would happen to the other one....next time find straighter wood IMO ......jatoba isn't osage even if it is denser....and its not a matter of use a different species to solve your problem, but choosing better grained stock in the first place IMHO
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I make alot of Jatoba bows and it can be very fickle as stated above. How thick was the belly lam? Did you taper it like you did the Maple? I've found that helps alot. I've made Jatoba bows with worse run-off than that and are still shooting years later because I tapered the lams. I will admit that the straighter the grain, the less likely it is to fail. Does this help you any? Anyone else try this? Just finished up a maple backed Jatoba tri-lam with Wenge handle riser. 68" overall and pulls 55#'s @ 28". Will post pics when I get the chance if you like.
G
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Definitely short grain along both the length and width as the major factor, despite the backing. A lot of belly staves survive some deviation along the length but not much deviation across the width, especially with narrow width limbs.
Try and get some straighter grained boards even though you will still use a backing.
Hamish.
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The only thing I can say to jatoba is that it seems to always chysal for me. Definitely will not make a warbow, I can say that much, :).
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THANKS for your thoughts and advice guys!
DarkSoul is right- the crystals in the "Bow 2 Break" pic occurred when the break happened since they only run up to the break. Other than that, there are no crystals on any of the jatoba belly bows. Just after stringing a bow and again after unstringing, I check it for over pretty good and noted nothing amiss with the ones that broke, well, except I didn't have to unstring the ones that broke! My tillering is likely fair at best so the lack of crystals must be dumb luck.
Warpath: Thanks for the tip on tapering the belly of tri-lams. The belly on neither of the broke bows was tapered but I'm definitely going to try it on the next trilam. The belly on the first bow pictured is ~0.21” thick at break. I forgot to post the specs for the second bow:
33# @ 28”, 64” ntn, 16 oz
1.25” at handle, 1/4” at nocks
Trilam, most of reflex is close to handle
Hickory back, tapered hard maple mid lam, jatoba belly
Belly ~0.15” thick at break
I’d sure like to see photos of your maple/jabota trilam.
Darksoul & RBLusthaus I've tried bellies cut from one batch of 4 IPE boards (via internet...) on trilams and simple backed bows similar in design to those pictured and always got a bunch of set. Maybe IPE was the one of the ~100 species of IPE that takes set :o
Below are pics of the other jatoba bellied bows (not tri-lams- just hickory backed) that are still shooting. Both have had many more arrows through them than the ones that broke. The bellies were cut from the same jatoba board and were glued up on the same form as the Bow 2 tri-lam that broke.
For both bows:
Belly thicknesses at mid- limb: ~0.26”, 1.25” wide at fades
Top Bow in pics:
34# @ 27”, 67” ntn; 0.23” at nocks, 1.5” reflex
One of my favorite bows- has been shot a bunch.
Bottom Bow in pics:
37# @ 27”, 64” ntn, 0.32 at nocks, 1” rested reflex
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/HJReflexBows13-813-9-1.jpg)
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/HJReflexBows13-813-9-2.jpg)
Here’s pics of the worst grain run-outs on the bellies of the two bows:
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/HJReflexBows13-813-9-3.jpg)
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/HJReflexBows13-813-9-4.jpg)
The grain run outs look pretty bad so I’m rethinking continuing to shoot them.
c.d.
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Maybe the belly was just too thin on the trilams?
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I don't see any grain run out on the belly of either of those bows. If you've been shooting them, and they're fine, I would continue to do so. I've used jatoba successfully on a few bows... one a backed ELB and a few flatbows. Don't ask too much from it. I've never made a jatoba bow over 50#. It's fickle, and tiller must be perfect. It does, however, make a decent core wood for a tri-lam, and I've done that more than once. Go easy on highly stressed designs, like reflex or recurves. Jatoba's not my favourite belly wood, but it does work within limits.
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I can see the grain runs snaky on the belly of both of those bows.
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I can see the grain runs snaky on the belly of both of those bows.
Yah, a bit, but nothing that would raise red flags on the belly of a backed bow for me.
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Also, I totally think bow break number 1 was caused by a tension failure. Catastrophic failure square across the back... looks like tension failure to me... and it's hickory... which does that sometimes.
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Again, I really appreciate yall's thoughts!
adb, I never suspected the hickory back failed first instead of the jatoba belly on bow 1. All but one of the backing and bend test failures I've had with hickory have been splintering type failures- they looked kinda like those the single small hickory splinter on bow 2- which was ripped from hickory board as bow 1.
Some better pics of the bow 1 break in hopes that other folks will weigh in:
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow1-HMJr2back.jpg)
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow1-HMJr2side1.jpg)
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow1-HMJr2side2.jpg)
(http://i1067.photobucket.com/albums/u439/cdpbrewer/broke%20HMJ%20trilams/Brokebow1-HMJr2belly.jpg)
None of the glue lines failed. A good bit of the maple core is still intact and it's splintering on one side of the break is definitely due to tension. Maybe that points to where the failure started?
toomanyknots, I sure want to think the thinness of the belly lams had something to do with the breaks but I can't rationally come up with a reason for that conjecture/hope.
c.d.