Primitive Archer
Main Discussion Area => Bows => Topic started by: 4est Trekker on July 27, 2011, 01:07:24 am
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I've been noticing several folks posting questions about how to correct limb twist. It seems like a simple concept, but it's deceiving. As such, I've photographed a little experiment to help illustrate the mechanics at work when the limbs are twisted, and thus how to correct it.
I took a simple pine slat and planed one face (the belly of the "bow") at an angle, thus creating a obvious weak strong side to the limbs.
(http://i984.photobucket.com/albums/ae321/isaacscr/Misc/HPIM5242.jpg)
I then strung up the "bow" and clamped it flat to the workbench. Because the back was left true, any limb twist would be easily seen.
(http://i984.photobucket.com/albums/ae321/isaacscr/Misc/HPIM5239.jpg)
I placed a square at the far end of the picture for easy reference.
(http://i984.photobucket.com/albums/ae321/isaacscr/Misc/HPIM5238.jpg)
The weak side of the limb has been marked with X's.
(http://i984.photobucket.com/albums/ae321/isaacscr/Misc/HPIM5240.jpg)
As you can see, the limbs are twisted TOWARD the weak side. Therefore, to correct the twist one must remove wood from the strong side, which is OPPOSITE the direction of the twist.
(http://i984.photobucket.com/albums/ae321/isaacscr/Misc/HPIM5241.jpg)
For me, at least, this is counterintuitive. As such, I have a diagram that hangs in my shop just to remind me of the mechanics at work. Although it might seem that the strong limb would pull the weak limb (creating a twist in the direction of the strong limb), the opposite is true. The weak limb doesn't have the strength to resist the strong limb, and thus twists under the load.
I hope this helps :)
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This is AWESOME!!! Thank you! Thank you! Thank you! I have always known the cause and the solution but always doubted which side was which due to the counterintuitive logic that you describe. This deserves an article in the magazine! Great experiment and well done!!! - Mike
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i wish i had taken a break from ruining a bow today to get on the forum. i had this very problem and removed too much material. what an ironic failure. won't make that mistake again. thanks
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Lovely set of photos. Very good illustration, as you say it's one of those problems which can be counterintuitive at times.
The centre line being off can also be both a problem and a syptom.
Nice to see well documented experimentation.
Del
BTW, How does the pine slat shoot? ;)
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Thank you im going to have to print this so I can hang it, keep the tips coming! :D
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How bout another test? I've always tried to fix twist by removing wood from the side of the limb towards the tip on the weaker side. Is that really correct? I guess what I'm asking is for you to use conventional techniques to fix that bow and document it. The last time I tried it I made it worse, so obviously I did it wrong.
George
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gstoneberg If you have your bow strung and see that it is pulling to one side or the other when looking down the limb you remove wood from the side that it is pulling towards of the side of the limb to bring the tips closer to center, this works well if the tips are not perfectly aligned with center of the handle. I have had to do this at least 50 times on laminated bows and has always worked like a charm.
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Thanks for posting this set of pics ans explanation, Curt. This will be very helpful to lots of folks.
I may sticky this at least for a while so it stays at the top. ;)
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I unstickeyed this. It got lost up there. Lets just keep it TTT for a while for everyone to see.
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Well done.
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This is a good topic. It got me wondering if maybe there was something in TBB that could explain this whole process better, and I was surprised that I couldn't find anything in any of the 4 volumes. The tillering chapter in volume one basically says just live with it :)
I think some of the confusion about which side to remove wood from is due to the fact that you actually CAN remove wood from either side. Let me try to explain my understanding, with some crude Homer Simpson type drawings I scanned.
Here's an example. When strung, this limb is twisting to the right of the imaginary center line. This means that the right half of the limb is the weak side, the left half is the strong side.
Problem (Before Fix)
(http://i162.photobucket.com/albums/t253/cbergerman/640/problem.jpg)
Now, if you have extra width on the tips, you can do like George suggested, and remove wood from the right side, which will in effect move the string to the left, closer to the centerline. This moves the tip closer to center, at the cost of narrowing it (potentially causing a whip tiller) and not doing much to move the rest of the limb towards the center. Notice here you are removing wood from the weak side of the limb:
Solution #1 - Remove wood from the side of tip
(http://i162.photobucket.com/albums/t253/cbergerman/640/solution1.jpg)
If you can't afford to narrow your tips, you can actually bring the majority of the limb back to the center by removing wood from the belly on the strong half of the limb, further away from the tip, as shown below. All you have to remember is that the wood will bend toward the weak side, and to correct it you have to scrape the belly on the strong side. I realize this is just a drawing and not actually proof, but for those who are skeptical, just try it next time and you'll see for yourself :)
Solution #2 - Remove wood from the belly, midlimb on the strong side
(http://i162.photobucket.com/albums/t253/cbergerman/640/solution2.jpg)
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Great post, well explained.
It's the combination of these various problems and cures which confuses the beginer (and the rest of us at times :laugh:)
Del
BTW. On somehting skinny like an ELB sometimes the only cure (when all else has failed) is some heat bending.
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Just a quick clarification... Some staves are naturally twisted in that some may have a propellor twist. Literally, they look like propellors. If the twist is not severe leave it. If it is sever, heat it out. Other staves may have a slight twist on one of the limbs. I leave those. They add character. Such things are what set our bows apart from FG bows. I do know the trend in selfbowyery these days is to make our bows like FG bows. I'll pass on that. 4est, gave an excellent explanation on fixing a twist caused by the bowyer by uneven wood removal. Often, because of the way the stave is held in the vice, one side of each limb is favored while removing wood. Fix it as described. At other times, the bowyer may be diligent in trying to remove wood evenly from both sides of the limb and still the bow is a leaner (string off to the side). Fix it as described. Jawge
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We need to sticky this again Pat. I needed it this morning and it took a bit to find it.
Thanks, George
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This ain't rocket surgery or brain science, DUH!
Now that 4est Trekker has explained it in simple terms I get it. How bloody simple! It's obvious! And I must be dumber than a box of used post holes to have not figured it out myself!!! Why must I always complicate things far worse than they need to be?!?!?
Pardon me while I go out to my Corner of Shame and apologize to some unnecessarily wasted staves. Thanks 4est Trekker.
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Like others have been saying this seemed so counter intuitive to me, but after the visual demo it makes sense. I successful duplicated the experiment to see it first hand. Being new to making bows this will be a big help. Thanks 4est Trekker!
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I tried the little experiment on a scrap from a belly and it bent straight ::).
I'm looking at my bow and I had the twist going out... then I flipped it around cause I'm smart enough to remember which end is which dontcha know, and I made it worse.
So on the tree it's flat. I pull the cord nad the rt. limb (not marked with tape)flexes slightly away from me as it goes down and twists.
So to fix this I need to scrape either the SIDE that it is flexing toward on the belly a bit closer to the riser OR the side opposide the flex direction?
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I'm printing this. This has been a monkey on my back from the get go.
Thanks 4est.
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I am saving it to print also. I don't know how many times I've set in my shop staring at a twisted limb trying to remember which way I fixed the last one. Thanks !!!!
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haha, I'm glad I only just started bowmaking! I had encountered this on my first bow already. I felt I had to weaken one side of the limb. I had no idea wich one.
Thanks, that would have taken me a while...
Wait... which side do I have to weaken again?.. :P
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ttt good post and info, just found it, Ken
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I love this thread, every time I get confused I revisit it! My mental dexterity is shot,
and sometimes I have an awful tough time seeing my bow limb twist a little on the tillering tree and
for the sake of all that's holy wish I could keep it in my mind, what the dang thick side is !!
So, for once and all, to be 100% sure...
When my straight stick with no propeller gets a little twisty as the string is pulled
down on the tree (belly pointed down, back is up), is it safe to say that the the edge thats
higher is the thick side ???
Cleaning up and putting the bendy stick away till i know Im not making it worse!
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Like GEORGE said we builld wood self bows not FG bows. Where a stwist or knot means something to the man thats made it.
I use'lly do not have that problem to much. I pick good staves but I know eveyone can't do this.
As long as the swist is'nt to bad. I correct by removeing wood from the strong side. or if it's a little worse. When I cut in the nock I'll leave the strong side hight high in the string gruve.
If it's any worse than that since I don't heat moveing parts. I'll find another stave. Heating a bow 10 times might make the profile look good. But it won't stay there with any time. Every time woods heated it changes cell strucker at that spot. If it's at a stress spot it won't stay very long at the spot where you bent it.
Even hard woods like osage that takes heat well. The bows still not as good as it could be.
I'm not say you have'nt built a nice looking bow you have. A slower process you could do the same with out heat. I just ruff out to bow form green and put in a caul while still green. With a 2 2 1/2 inch reflex added. I can keep most of my bows to 1/2 reflex to straight to 1/2 string follow.
NO HEAT.
CROOTETARROW BUILT SELFBOWS FOR OVER 50 YEARS. The last 20 he did like this. YOU CAN'T BUILD A BETTER SELF BOW FOR EXDURANCE.
I have 3 personal bows14,17,18 years old. Built like this I've retired, I shoot now and then. There the same as when they were built. They had at least 50 to 100,000 shots through them.
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If you change the shape of the bow while green you're changing the structure too.
Heat is fine. We have heated bows shooting just fine years and thousands of shots later as well.
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pat, the bow isnt green..
crooketarrow
you remove wood from the strong side, which side of the limb is that (Higer or lower) ?
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thank you for this...
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pat, the bow isnt green..
crooketarrow
you remove wood from the strong side, which side of the limb is that (Higer or lower) ?
That was a response to crooketarrow's ranting.
Your question is answered very clearly on this thread. Very clearly.
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Yep, something about heat he don't like, ::)must have had a bad experience with it somewhere in his passed. ;) Yes this is a very good thread that explains it well. :) Thanks.
Pappy
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thank you :)
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Thanks for putting this where I can find it. Dam dyslexia! I can never remember.
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I had two twist on me when I made them so I went and got a calliper boyh limbs were the same thickness and it ended up been the wood I used. It never did um twist for me during the tillering
It shoots fine and straight
And it looks fine till it gets drawn then you can see the twist in the limbs.
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First time to view this, very helpful post.
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I'm rather late commenting to this good thread but I like it.I'm sure it'll help many future bow makers out there.
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Wow, good thread. This is really helpful.
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With board bows, limb twist can also be caused by the wood fiber not following the center of the limb as it would in a split stave. Sometimes this is hard to see by visual inspection of the original board as it can occur in boards where all the growth ring lines appear straight. This is caused by the tree twisting as it grows. In tropical hardwoods it twists in reverse for half the year. The fix for this is to laminate the bow with two consequent slices of the same board and flip one over so that when the two lambs are laid side by side they are mirror images. Tiller by filing scraping sanding--no planes. Try to plan your design so that each lam is a similar thickness and the limbs will bend straight with some use of the above method.
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hopefully i can be helpful this time. edited the quote w/ imgur links
I've been noticing several folks posting questions about how to correct limb twist. It seems like a simple concept, but it's deceiving. As such, I've photographed a little experiment to help illustrate the mechanics at work when the limbs are twisted, and thus how to correct it.
I took a simple pine slat and planed one face (the belly of the "bow") at an angle, thus creating a obvious weak strong side to the limbs.
(http://i.imgur.com/qJAwaBq.jpg)
I then strung up the "bow" and clamped it flat to the workbench. Because the back was left true, any limb twist would be easily seen.
(http://i.imgur.com/bM2gogp.jpg)
I placed a square at the far end of the picture for easy reference.
(http://i.imgur.com/EyL8KrU.jpg)
The weak side of the limb has been marked with X's.
(http://i.imgur.com/MsakvJR.jpg)
As you can see, the limbs are twisted TOWARD the weak side. Therefore, to correct the twist one must remove wood from the strong side, which is OPPOSITE the direction of the twist.
(http://i.imgur.com/644zY6a.jpg)
For me, at least, this is counterintuitive. As such, I have a diagram that hangs in my shop just to remind me of the mechanics at work. Although it might seem that the strong limb would pull the weak limb (creating a twist in the direction of the strong limb), the opposite is true. The weak limb doesn't have the strength to resist the strong limb, and thus twists under the load.
I hope this helps :)
This is a good topic. It got me wondering if maybe there was something in TBB that could explain this whole process better, and I was surprised that I couldn't find anything in any of the 4 volumes. The tillering chapter in volume one basically says just live with it :)
I think some of the confusion about which side to remove wood from is due to the fact that you actually CAN remove wood from either side. Let me try to explain my understanding, with some crude Homer Simpson type drawings I scanned.
Here's an example. When strung, this limb is twisting to the right of the imaginary center line. This means that the right half of the limb is the weak side, the left half is the strong side.
Problem (Before Fix)
(http://i.imgur.com/3aFTtVr.jpg)
Now, if you have extra width on the tips, you can do like George suggested, and remove wood from the right side, which will in effect move the string to the left, closer to the centerline. This moves the tip closer to center, at the cost of narrowing it (potentially causing a whip tiller) and not doing much to move the rest of the limb towards the center. Notice here you are removing wood from the weak side of the limb:
Solution #1 - Remove wood from the side of tip
(http://i.imgur.com/IxCY50c.jpg)
If you can't afford to narrow your tips, you can actually bring the majority of the limb back to the center by removing wood from the belly on the strong half of the limb, further away from the tip, as shown below. All you have to remember is that the wood will bend toward the weak side, and to correct it you have to scrape the belly on the strong side. I realize this is just a drawing and not actually proof, but for those who are skeptical, just try it next time and you'll see for yourself :)
Solution #2 - Remove wood from the belly, midlimb on the strong side
(http://i.imgur.com/IbcH6WM.jpg)
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Good one, thanks
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Thank you loon! saved this post, since the images are now deleted.
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I did the same experiment and got the same results http://www.primitivearcher.com/smf/index.php/topic,61383.0.html. I also show how the back raises on the strong side (or belly raises depending if you're looking at the back (string down) or belly (string up). So remember, if you're looking at the belly during tillering, DON'T take wood off the side that raises up, no matter how tempting it is to "try and flatten it down".
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I'd love to see the pics, Photobucket has choked the feed
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I can’t seem to find the answer, but it would make sense that wider limbs are more prone to twist than narrower limbs - is my assumption correct? Thanks in advance.
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Lots of folks get what’s already been said, but peoples’ brains are different. I would just confuse myself more by reading this. That said, I’m going to write this solution to a problem the way my brain beats understands it in case someone else out there has the misfortune of processing things like I do.
I experimented with a broken limb to KNOW what I was about to do would indeed fix my issue.
If a limb is twisted and the string is tracking to/favoring one side of the belly of the limb, remove wood from the side of the belly that you want the string to move towards.
In my case, I usually have to deal with this when I have a recurve bow that has limb twist somewhere, and when it’s braced, everything looks good except for the string not tracking exactly down the center of the recurved limb where it touches the limb. On my trade bow build, the string was slightly off to the left looking at it from the belly. I took a few scrapes from the right side of the belly, and the string moves that direction.
I know this is redundant for those who’s brains work correctly, but this is the way I remember and understand it.