Bottom limb getting weaker?

[YosemiteSam]

For my board bow, my limb tiller was perfectly even.  As I was shooting it in, the tiller would go off a little -- the bottom limb getting a little weaker over about 50 shots.  No worries -- just flipped it over & shot it the other way since I hadn't even decided which way was up or down yet.  Things slowed down.  Finished it up.  Now I'm seeing the same pattern, just happening a little slower.  How is this happening?  Am I pulling unevenly on the bottom limb?  Is this just part of the settling process?  I can shoot it upside-down if I need to.  But I'd rather see if anybody knows what causes this issue.  Thoughts?

[Bowjunkie]

Bueller...

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Bueller...

[George Tsoukalas]

I think there are some issues going on when we tiller.

It is pretty difficult to duplicate on your rope and pulley how you hold the bow...no matter where your tillering point is.

Some tiller from the center and some off to the side .

I tiller from the center. I was never comfortable tillering off to the side. I have someone snap a digi as I draw the bow at full draw.

The other issue is the way we hold the string while shooting puts extra pressure on the bottom limb.

I aim  for a slightly stiffer bottom limb at full draw.

But I check all through tillering to make sure handle pressure I draw is even...to make sure the limbs feel balanced.

After that, as long as the bow shoots well, I don't worry about it.

Jawge

[BMorv]

You pull the string from 2" higher than center then the bottom limb will be under more stress since it is seeing more leverage.  To counter this you can do a positive tiller where the top limb is weaker than the bottom or you can make the top limb 1 or 2" longer than the bottom.  Both do the same thing by balancing the forces.  
That being said, some people just tiller as you have and shoot the bow both ways.  Pick whichever way feels best in your hand, and the bottom limb should eventually settle in.

[BMorv]

But you already picked the top limb.... that bird can't fly upside down    :D

[YosemiteSam]

Originally posted by BMorv:
But you already picked the top limb.... that bird can't fly upside down     :D  

Ha!  It's a nuthatch.  It was already upside-down.  

[YosemiteSam]

Originally posted by BMorv:
You pull the string from 2" higher than center then the bottom limb will be under more stress since it is seeing more leverage.  
That being said, some people just tiller as you have and shoot the bow both ways.  Pick whichever way feels best in your hand, and the bottom limb should eventually settle in.

I don't quite understand the effect of the longer side of the string creating more stress.  It runs counter-intuitive in my brain for some reason.  But I've read it enough to believe it anyway.  I just can't quite wrap my head around it yet.

I positioned my tillering rope where my middle finger would hold the string (3-under).  But when shooting it, my groups were far more consistent, the shot was much quieter and the arrows flew straighter when I put the pressure on my index finger.  For some reason, all of the bows I've made have had this quirk.  Maybe that is creating the extra stress on the bottom limb?

[BMorv]

It's not a direct comparison because there's a lot of things going on when we begin to pull back a string and bend a bow but this may help you visualize what's going on:
Imagine that you have a stick.  Put that stick in a vise with the vise positioned directly in the middle of the stick.  Hang a weight from one end of the stick and measure the deflection.  Then move that stick a couple of inches off center and put it back in the vise with the same weight hanging from the longer end.  The stick will deflect more now that you have a couple inches more hanging out of the vise.  
Unless you grip and pull the bow in the direct center (which really never happens) you will have this imbalance of stress.  Split finger shooting puts more stress on the bottom limb than 3 under because you are pulling more off center with split finger.

[YosemiteSam]

Originally posted by BMorv:
It's not a direct comparison because there's a lot of things going on when we begin to pull back a string and bend a bow but this may help you visualize what's going on:
Imagine that you have a stick.  Put that stick in a vise with the vise positioned directly in the middle of the stick.  Hang a weight from one end of the stick and measure the deflection.  Then move that stick a couple of inches off center and put it back in the vise with the same weight hanging from the longer end.  The stick will deflect more now that you have a couple inches more hanging out of the vise.  
Unless you grip and pull the bow in the direct center (which really never happens) you will have this imbalance of stress.  Split finger shooting puts more stress on the bottom limb than 3 under because you are pulling more off center with split finger.

Great explanation.  Thank you.

[Roy from Pa]

I don't buy that stick theory. When the stick has a string stretched tight from end to end and you slide a weight one way or the other from center, you are more realistic.

[BMorv]

Your example of a string stretched tight from one end to the other and a weight being moved from center is a better analogy, I agree.  The point is the same, the farther you move that weight, the more stress imbalance you will have between the limbs.

[Eric Krewson]

I have had a lot of lower limbs go weak initially, I start a little over poundage and retiller after shoot in or cut an inch off the bottom limb and retiller.

[Roy from Pa]

I'm betting that when he shoots, he applies a lot of pressure with the heel of his hand, low grip. That applies more stress to the lower limb. I suggest leaving the bottom limb stronger when tillering. I dont suggest flipping the bow up and down to see which is going to be the top limb. Decide that when you build it and tiller for that.

I've had a buddy shoot bows in for me that are too heavy for my old butt to shoot in. I've had them tillered to equal limb timing and a zero tiller. Twice now after he shot a bow in, it took on a -1/8th tiller. I took pictures and videos of him shooting and I see he applies a lot of pressure with the heel of his hand. So now I leave the bottom limb a tad stiffer. This can be mimized a little by the grip design. In as making a slight pistol grip.

[YosemiteSam]

Didn't think about heel pressure.  That would certainly alter the balance point and compound the effect of pulling above center on the string.