Primitive Archer
Main Discussion Area => Bows => Topic started by: simk on January 09, 2019, 03:20:36 am
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Hi All
In 2019 one of my new projects will maybe a sinewed bow. Somebody now told me he did a well working sinewed shorty of ash. I was astonished heraring that, as compression ability of ash isnt top level regarding my own expieriences. Thinking about it I got stuck in the following question:
A) If you have two bows - both same length, wood, shape and drawweight and drawlength, but one is sinewed. Is there finally a difference in the compression forces on the belly? Does the sinewed one require a stronger belly than the other, or not?
A1) What if the sinewed one has 6" of reflex (but again same specs)?
Please please enligthen me 8)
Cheers
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There will be a definite amount of compression difference on the belly with sinew backing on any wood...The neutral plane shifts towards the belly...Proper design and wood type is important...I am not familiar with Ash...
Don
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Thank you Don!
On my side the hoped answer would have been the following: If its possible to build a 60" bow from x-wood with a draw weight of x-pound @ 28 it must be possible to build the same bow, same draw length, same draw weight with same wood - sinweed - without overstressing the belly.
I other words: Belly stress is only a function of bow length, draw-weight, draw-length and limb shape (width/thickness). The fact, that there is some sinew on the bow (and artificial reflex added) is irrelevant. If I can build a bow to a certain draw weight without chrysals, it should also be possible to sinew it, remove wood from the belly and retiller it again to the same draw weight without provocing chrysals. I'm not specificly asking about ash, but in general...
Am I completly wrong?
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Isn’t the neutral plane moved towards the back since sinew stretches more than wood. Wouldn’t the backing material have to stretch less in order to force the neutral plane towards the belly. Not sure, just what my thought experiments came up with.
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We had a good discussion about sinew here
http://www.primitivearcher.com/smf/index.php/topic,59835.0.html
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You can not make two bows in same shape. Sinewed bow must be reflexed or the sinew will not work. Sinew must be streched to give tension for a bow. I have sinewed ash and elm and those bellies crashed. You should use junniper, jew or osage for sinewed bow. Maybe some woods more im not familiar.
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sinew helps you make the same draw weight bow in a shorter length. all other things being equal.
one consideration that you might want to consider is the widsom of all the extra work and mat'l required to build a sinewed bow. would it not justify a primo piece of wood for the belly?
Sinewed bow must be reflexed or the sinew will not work.
"must" might be bit strong wording, although reflex certainly helps to get the full advantage from the sinew, but re flexing the design is one of the reasons a better belly wood makes sense.
Belly stress is only a function of bow length, draw-weight, draw-length and limb shape (width/thickness). The fact, that there is some sinew on the bow (and artificial reflex added) is irrelevant.
Simk
strain is a better choice of words than stress. when analyzing working properties. Stress causes strain (the elongation tor compression that can be measured), and the amount of strain any materiel can take before breaking down is what limits any design. In a composite design of two or more materiel, the amount of strain does change depending on how much of which materiel is present in the cross section.
it's typical that the neutral plane, (that part of the cross section that neither stretches or compresses) will move towards the stiffer of the two materiel's.
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Hi Guys
Thank you a lot for your contributions that helped my decisionmaking. For my first sinewing attempts i will get some of my yewie's, hollie's or laburni's out of the stash.
Basically my thought was only, that if a wood can resist a draw weight of 45# without sinew, it could also resist the same draw weight with sinew. But maybe it's not that easy...
Hope to present you a sinewed one sometime in 2019 :BB
Cheers
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Osage,Black Locust, Elm , Hickory, American HornBeam do,t need to be sinewed backed, but many very good bows have been made with these, and other woods by sinew backing.I agree you can,t back any wood with it, but you can back many woods with it, and get a fine bow.Juniper, and Eastern Red Cedar are tension weak, and compression strong, and with sinew backing go together like bacon and eggs.