The Mechanics of Limb Twisting Explained - An Experiment

[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    

[Chris P. Bacon]

Great demonstration! I really want to build my own bow and this is great to know          

[Chris P. Bacon]

sorry, double post    

[AKmud]

Thanks for that 4est, every time I deal with a twist I have to stop and think it over for an hour till I remember what exactly is going on.  Your photos and explanation will be printed out and in the shop!

[wildbill001]

That's really cool to know.  Thanks for posting that.

Bill

[Shaun]

since you did not have a before... I did my own experiment with a piece of screen molding.


before



after removing wood, string travels towards weekend side



therefore: to center string in handle - remove wood on the side towards which you want the string to move (strong side).

[George Tsoukalas]

Good tutorial, 4est. Jawge

[4est trekker]

Thanks, Shaun!  I guess I should have included a before shot.  I bet that pine slat really slings an arrow!  

[jcrafferty]

Best explanation on limb twist I've seen yet. thanks

[SEMO_HUNTER]

Yep, very informative Curt thanks for sharing.

Now would you mind showing how making one side of the nock groove deeper can also effect string position as in getting it to track through the center of the bow? That's the one that always gives me a hard time, especially if you have a compounded problem with the nocks being off on both ends. I hate cutting nocks in a bow! That's my least favorite part of making them.

Thanks

[SportHunter]

Another great post, thanks for making it easy.

[SportHunter]

Another great post, thanks for making it easy.

[SportHunter]

Another great post, thanks for making it easy.

[Bow-n-Head]

Thanx for the post. That took the twist out of my counterintuitive brain. I think. Ray

[bjansen]

Excellent data!

[Buemaker]

TTT  

[Roy from Pa]

Good thinking, Bue.

[Crooked Stic]

The above pic showing recurve limbs probably can be corrected most time by deepening the weak side string groove. Leaving things a bit wider than projected width until you get the string centered up.

[Stagmitis]

Nice post!

If I may add a couple more suggestions to getting the limbs straight.

Sometimes if my belly proportions are exactly where I want them (Trap etc) and I suffer a twisted limb it can be corrected by sanding the edge on the back of the bow.

In 4est example you would sand the SAME side of the weak limb on the back. So, it would be the OPPOSITE in the back of the bow from the limb you would sand on the belly to correct twist.

And btw, dont have too much fun correcting opposing twist on upper and lower limbs