The Mechanics of Limb Twisting Explained - An Experiment

[James Cargile]

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.

[neuse]

First time to view this, very helpful post.

[BowEd]

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.

[Ippus]

Wow, good thread. This is really helpful.

[Ballasted_Bowyer]

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.

[loon]

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.





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

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

[DC]

Good one, thanks

[gfugal]

Thank you loon! saved this post, since the images are now deleted.

[gfugal]

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".

[Taxus brevifolia]

I'd love to see the pics, Photobucket has choked the feed

[Kidder]

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.

[ssrhythm]

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.

[Doug509]

Way I think of this is like looking up at belly of an airplane as if it was the belly of a bow on the tiller tree.  Am I correct that the weak side lifts away from the earth and strong side dips towards the earth like a plane going into a barrel roll?

[JW_Halverson]

Way I think of this is like looking up at belly of an airplane as if it was the belly of a bow on the tiller tree.  Am I correct that the weak side lifts away from the earth and strong side dips towards the earth like a plane going into a barrel roll?

Yup