Author Topic: The Mechanics of Limb Twisting Explained - An Experiment  (Read 20955 times)

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Offline 4est Trekker

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The Mechanics of Limb Twisting Explained - An Experiment
« on: July 26, 2011, 10:18:12 pm »
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.





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. 





I placed a square at the far end of the picture for easy reference.




The weak side of the limb has been marked with X's.





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.





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 :)   
"Walk softly, and carry a bent stick."

"And whatever you do, whether in word or deed, do it all in the name of the Lord Jesus Christ, giving thanks to God the Father through him."  Col. 3:17

Offline Almostpighunter

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #1 on: July 26, 2011, 10:59:46 pm »
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

Offline colt

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #2 on: July 27, 2011, 01:30:13 am »
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

Offline Del the cat

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #3 on: July 27, 2011, 01:59:18 am »
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? ;)
Health warning, these posts may contain traces of nut.

Offline soy

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #4 on: July 27, 2011, 03:39:20 am »
Thank you im going to have to print this so I can hang it, keep the tips coming!  :D
Is this bow making a sickness? or the cure...

Offline gstoneberg

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #5 on: July 27, 2011, 01:24:30 pm »
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
St Paul, TX

Offline Bow Nut

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #6 on: July 27, 2011, 01:28:38 pm »
 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.

Offline Pat B

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #7 on: July 27, 2011, 07:11:15 pm »
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.  ;)
Make the most of all that comes and the least of all that goes!    Pat Brennan  Brevard, NC

Offline Pat B

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #8 on: July 27, 2011, 09:56:36 pm »
I unstickeyed this. It got lost up there. Lets just keep it TTT for a while for everyone to see.
Make the most of all that comes and the least of all that goes!    Pat Brennan  Brevard, NC

Offline Gordon

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #9 on: July 27, 2011, 10:49:53 pm »
Well done.
Gordon

Offline Cameroo

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #10 on: July 27, 2011, 11:22:46 pm »
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)


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


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

Offline Del the cat

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #11 on: July 28, 2011, 03:48:57 am »
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.
« Last Edit: July 28, 2011, 04:03:04 am by Del the cat »
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Offline George Tsoukalas

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #12 on: July 28, 2011, 05:18:20 am »
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
Set Happens!
If you ain't breakin' you ain't makin!

Offline gstoneberg

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #13 on: August 06, 2011, 09:24:20 am »
We need to sticky this again Pat.  I needed it this morning and it took a bit to find it.

Thanks, George
St Paul, TX

Offline JW_Halverson

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Re: The Mechanics of Limb Twisting Explained - An Experiment
« Reply #14 on: August 06, 2011, 12:42:47 pm »
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.

Sawdust.  It's man glitter.