Adjusting limb twist on a laminated bow

[Patrick McCormick]

How does one adjust minor limb twist when making a laminated longbow?

[PV]

If you do a search for limb twist there's a picture in one of the threads that explains it far better than I can.

[BMN]

Like this?

 

[Shaun]

I personally believe that drawing is incorrect. The string travels towards the stronger side since this stronger side bends less.Therefore, you need to sand the side where the string is too close. I had to call Charlie Lamb to ask him about this when I was building a glass bow.

Think about it. You sand the side that the string is tending towards and it will weaken and relax allowing the string to move towards the center.

The part about deepening the nock is correct if you have room to do this. Its a good idea to leave the tips a little wide to facilitate this adjustment at first bracing.

[tim-flood]

Cut a piece of wood with a bevel about 2' long & 1.5" wide and clamp it to your table and bend it up and see what way it bends!!!

[Shaun]

Then what Tim?

[BMN]

First let me start by saying that I have absolutely no experience with this. The above drawing was sent to me and I filed it away for future reference for when I start making laminated fiberglass bows.

That being said, here is how it was explained to me. The drawing applies to laminated fiberglass bows only. With a selfbow, there is a chance that one side of the limb could be stronger than the other due to thicker material on one side of the limb (as in Tim's experiment) or maybe just different material properties from one side of the limb to the other. The chances of a laminated fiberglass bow being stronger on one side than the other are slim. Fiberglass bows use uniform thickness lams across the width of the limb and material properties are uniform throughout. The limb doesn't twist because one side of the limb is stronger than the other, it twists because the limb is not being loaded properly. The forces bending the limb are not being applied symmetrically because of misalignment or layout problems. The goal is to have the string centered over the centerline of the limb. To do this, you can move the string towards the centerline of the limb (deepen the string groove) or you can move the centerline of the limb towards the string (sanding the side of the limb).

That's how it was explained to me. Is it correct? I can't say for sure. Like I said, I have never tried this but at the time it made sense to me. I'd like to hear from someone who has actually made this adjustment before and how they corrected the problem. Maybe the drawing is incorrect and if so I need to get rid of it.

Bill

[TradBowyer]

Shaun is correct, the drawing is backwards regarding the side of the limb to sand. the limb will twist towards the weak side so the string will track to the strong side i.e. the side that the string is closest to is the strong side and needs sanded to bring the string center.

[Holm-Made]

The picture is correct.  String tracks toward the weak side.  If you look at the picture on the left in the above drawing where the string is tracking left...  the left edge of that bow is higher(curling up toward the string)It is weaker and that is why it twists in.  
Hope that explaination makes sense.  I cut my bows out a little wider then needed because about half of them need to be tweeked one way or another after the initial string up.  It may be different for wood bows but the drawing is correct for glass bows.  I also narrow the tip and deepen the nock from the weak side to bring the string more "in line" with the center of the bow limb.  Chad

[Holm-Made]

I just read BMN's post and he is on the right track.  It isn't twist in the limb that necessarily needs to be straighten out but the limbs are not laid out perfectly straight, (and if they are not laid out straight then the adjustments pictured above will straighten them out)  That being said...if the limbs are not layed out perfectly straight you will get a little twist in the limb.  

Sounds to me like you need to use 2 opposite techniques for glass bows vs. self bows.  Chad

[Steve J]

Wow!  Who's right?

Sad thing is, I have a Tri-Lam that I'm working on now and I'm having a little limb twist, and I haven't built a bow in almost 2 years.  The technique I use is like the picture above.  It was being pretty difficult.  It gave me fits compared to the glass bows I've built.

[tim-flood]

When you bend up the beveled piece it is apparent which side is thicker and which way it bends.  It seems to me that when you bend up a limb like when it is strung that it will bend toward the thick side the thin side is making a smaller radius than the side it is bending toward which is a bigger radius or thick side or if not thicker stronger.  IMHO

[tim-flood]

how bout this, if you clamp two pices to a table one 1/8" thick x 1/2" wide next to a  1/4" thick and 1/2" wide, now pull them with a string around them with 10 lbs which one will bend more,which will be straighter now if they were attached they would curl toward the thicker one , Right??

[Holm-Made]

It's getting confusing around here.  Let me explain it like this.  On a glass bow you need to have equal amounts of material on either side of the string when viewing it as we are in the exagerated picture above.  If there isn't equal amounds of material on either side of the string, when you draw the bow the limb will twist.  On a deep cored longbow it will only twist slightly and you may not even notice it.  On a recurve it will twist noticeably.  As it is drawn the limb twists and the string will continue to move toward the side it is already tracking toward, which is the weak side because there is less material there.

Think of it this way...remove material from the places on the limb that will center the string on the limb, just as the picture directs us to..
Clear as mud, right, Chad