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The Mechanics of Limb Twisting Explained - An Experiment

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4est Trekker:
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 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

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

Del the cat:
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.
BTW, How does the pine slat shoot? ;)

Thank you im going to have to print this so I can hang it, keep the tips coming!  :D


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