Trad Gang
Main Boards => The Bowyer's Bench => Topic started by: mwosborn on March 27, 2016, 09:28:00 PM
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Which side of the limb is the "weak" side?
I am working on a trilam bow. I have it braced up to about 5". The limbs have opposite twists. The string tracks pretty well centered. It does have a tiny bit of twist unbraced. Must of happened in glue up.
Here is a picture of one of the limbs. You are looking at the tip of the bow and I colored one half of the tip red. The first picture is belly down, the second picture is belly up.
I have read several topics on limb twist - including the "the mechanics of limb twist" by 4est trekker. I would still like some help here.
Thanks!!
(http://i.imgur.com/AUKcYQe.jpg)
(http://i.imgur.com/cC6iVnr.jpg)
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The red side is stiff. The other side is weak.
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The string always drifts towards the weaker side. So remove wood from the opposite side. You need to get the sides evened up also. Since it's twisted unbraced too, that happened at glue up. First thing, I would unstring it, apply some pressure/weight to the limb to get it straight, even go a tad past straight, then heat it with a heat gun. Heat the belly more than the back.
(http://i47.photobucket.com/albums/f165/ROY-CHRIS/limbalignment.jpg)
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Thanks Ben and Roy. Roy, I have that diagram hanging on a door in my cabinet. In this case I can not tell which way the string is drifting. That is why I took a picture of the tip so that the twist shows.
Which is the weak side? The one I marked red or the other?
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The other side, not the red side. Also are the string grooves both the same depth?
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Roy - I have not cut string grooves into the sides - only in the overlay at this point. They are cut even across and the same depth across.
Thanks -
I just went out and performed my own test - will post the result for critique!
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Ok, I don't cut grooves in the side either.
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Ok - for what it is worth, here is a little test I did this morning. Limb twists really irritate the @#$% out of me and I just needed to try this.
Pictures have the explanations.
As far as for the bow I am working on - I concluded the twist is not due to uneven limb thickness. I have measured them twice and the thickness is even all the way down the limb. I caused the twist at glue up. You can see it when unbraced and when under stress of being braced it is just more obvious. I might try heat to help as Roy suggested - or I may just tiller it out and live with the twist - and learn from the experience.
Thanks for the help!
(http://i.imgur.com/luu74NH.jpg)
(http://i.imgur.com/HBRXv8t.jpg)
(http://i.imgur.com/ppFunSw.jpg)
(http://i.imgur.com/HHto8Ft.jpg)
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That's odd and it looks like the string loop on the far side is almost off the wood? And what is the black string in the center for?
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Take another picture just like the last one, except this time line the string up with the center of the handle area and see if the near limb looks like it's pointing toward the right... the weak side.
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Does it look like the string at the far end is way to the right?
That end wasn't scraped.
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Camera angle can affect how things look - I went back out and took a few more.
Roy - the black string is just the tail from the knot when I tied a loop. I just grabbed a piece of string I had laying around and tied a couple of loops in it.
The string is no doubt moving toward the weak limb - which we know it does. I was wanting to know what the very end of the bow looked liked when it was bending toward the weak side. It appears to me that the end of the bow tip is slanted down toward the weak side.
macbow - I only scraped on the near camera limb - the far limb was not scraped and is still the same thickness throughout.
As previously mentioned, I concluded the twist in the trilam I am working on was a glue up induced twist and not caused by limb thickness being different from one side to the other.
Always learning!
(http://i.imgur.com/0zmUrj2.jpg)
(http://i.imgur.com/w1xt2dg.jpg)
(http://i.imgur.com/5TjHgNK.jpg)
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These diagrams work if your wood is homogenous! Or glass!
Does the grain on your bow in any of the lams run off sideways.
Your pine slat is nicely grained but do the same test with a piece with non-straight grain and see what happens....that's when it get confusing ;)
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Mike - that is true, never took grain into consideration - it is already confusing enough! The grain on my 3 lams were all very good. For this bow anyway, I am pretty confident that it was my glue up that caused the problem. Next one will be better. :)
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What did you use as a form. Kinda like a house, if the foundation is off the whole thing is off.
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Yep - you are correct. Didn't use a form. Glued the lams using post and clamps and then glued the riser on afterwards. I thought the riser fit was "close enough" . It was not! If I make additional trilams I think I will make a form and glue up all at once using my air hose.