Thoughts on pivot point location

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rodlonq
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Thoughts on pivot point location

#1 Post by rodlonq » Fri Mar 25, 2016 1:34 pm

On my earlier straight laid bows I had a fairly shallow grip and had the pivot point about an inch above the centre of the bow and the shelf another 1/2-3/4" above that. I note a lot of Hill style bows have the shelf about 2" above the centre of the bow.

Since I have been making more modern bows and particularly with high wrist grip I have moved the pivot point to the middle of the bow (geometrically). Now the shelf sits about 3/4" above the pivot point and I only have a smaller amount of positive tiller (1/16" to 1/8").

I can't say I have made enough bows with varying pivot point locations to draw a conclusion whether this is a good or bad decision. The question is, where do you like to place your pivot point and do you have a specific reason why you chose that location?

Cheers... Rod

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Re: Thoughts on pivot point location

#2 Post by bigbob » Fri Mar 25, 2016 4:09 pm

My D/R shelfs are generally cut in at 1 1/4'' but I have a jig with a 1 1/4'' dowel for the throat and a winch system where i draw back the bow and establish both the static and dynamic centres. Works pretty well.
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Re: Thoughts on pivot point location

#3 Post by greybeard » Mon Mar 28, 2016 8:10 pm

Hi Rod,

I hold a bow with a relaxed hand and have the fingers resting lightly on the back of the handle.

The draw weight of the bow is taken by the small area between the thumb and first finger of the hand.

I do the following to locate the shelf position;

By drawing the bow several times and slightly changing the bow hand position if needed I can ascertain where the balance point of the bow is and mark the shelf position as shown in the photograph.
Pivot Point Balance Point.jpg
Pivot Point Balance Point.jpg (80.64 KiB) Viewed 2878 times
By doing this I find that shooting off a shelf is similar to shooting off the knuckle.

This method may not work for everyone but it works for me.

Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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Re: Thoughts on pivot point location

#4 Post by greybeard » Tue Mar 29, 2016 4:06 pm

Rod, found this on the net ages back but can't find the author.

"Where should I locate the grip?

A question that appears often in many of the internet bow making forums is “Where should I locate the grip and shelf on my bow?” I’m sure there are as many opinions about this as there are bowyers, so I’ll give a simple explanation of what I think is important and you can decide what is best for you.

I think the two most important things that should determine the location of the grip and shelf are bow balance and shooting style…meaning whether the archer intends to shoot “split finger” or “three fingers under.” Putting the grip in the center of the bow (from tip-to-tip) creates a balanced bow that is easy to carry on the range and in the field. Holding an unbalanced bow for even just a few minutes can seem like agonizing long hours when you are trying to stalk quietly through the brush or stand perfectly still on a tree stand. A balanced bow is easy to carry and effortless to hold. Also, when it is time to raise the bow for the shot, a balanced bow comes up smoothly and effortlessly, without conscious thought or distraction.
There can be a difference between a balanced bow to carry at brace (let’s call this “static balance”), and a balanced bow during the draw (let’s call this “dynamic balance”). The dynamic balance of the bow is mostly determined by the bow grip fulcrum and the string grip location. This is where shooting style and tiller come into the discussion.

For the ease of bow making, I’ve typically just cut the grip at the center and put the shelf an inch or so above it for either split finger or three under shooters, then adjusted the tiller to balance the bow. On my designs, the tiller usually comes out pretty close to what most authors suggest (“1/8″ positive” tiller—lower limb stronger and upper limb weaker for split fingered shooters, and “even” tiller—both limbs of equal strength for three finger under shooters). However, after building bows for awhile and doing some experimenting with test bows on my tillering tree, I’ve learned to worry less about what everyone else is teaching about tiller and to be more concerned with how my bows balance on the pivoting tree. I’ve discovered that putting the grip at bow center usually works best for three under shooting and putting the grip slightly below center works best for split finger shooting. This places the bow grip and string grip locations so they create better dynamic balance in the bow. Once the grip is cut in, I use the tillering tree to show where the limbs need to be adjusted to create static and dynamic balance.

In my opinion, it really doesn’t matter what tiller measurement the bow ends up with. After all, it’s just a measurement. What really matters is that the bow is balanced. Dynamic balance means that, during the draw stroke, the limb tips pull back the same distance at every inch on the tillering tree, and during the power stroke, they return back to brace with exactly the same force. You will know the limbs are perfectly balanced when they just make a dull thump when you strum the string. The dull thump happens because the force vectors of the upper and lower limbs perfectly cancel each other out as the limbs come to a stop. If the limbs are out of balance, the forces will not cancel each other, causing vibration and making the string buzz."
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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Re: Thoughts on pivot point location

#5 Post by rodlonq » Tue Mar 29, 2016 5:32 pm

Hi Daryl,

Thanks for your input. The passage comes from Jim Thorne's "Build your own bow" website http://www.buildyourownbow.com/grip-and ... a-recurve/

I have read a lot of his writings and I think it is that same article that convinced me that the geometric centre of the bow is a good place to put the pivot point. t least for deep high wrist grips. If you have a Hill style grip it may be necessary to raise the shelf so the heel of the palm can sit comfortably on the grip without overlapping the grip.

I think for hunting bows finding static balance is probably more important because you will carry your bow all day and maybe draw it for a few seconds or not at all if you aren't lucky enough to find game. In saying that there is no point in producing a poorly shooting bow just so it is comfortable to carry around.

I will keep on experimenting and find out what works for me at least. There is no doubt that there is a large span of designs to cover the requirements of all archers out there.

Thank you too Bob, your input is also appreciated.

Cheers... Rod

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Re: Thoughts on pivot point location

#6 Post by greybeard » Mon Apr 04, 2016 12:46 pm

Hi Rod,

Some other points of view.

http://www.dickwightman.com/howardhill/ ... lgrip.html

Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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rodlonq
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Re: Thoughts on pivot point location

#7 Post by rodlonq » Tue Apr 05, 2016 11:35 am

Hi Daryl,

A very informative article. Need to read again and digest properly but it certainly highlight the differences in hand placement for different bow grip types.

Cheers... Rod

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Re: Thoughts on pivot point location

#8 Post by greybeard » Fri Apr 15, 2016 9:05 pm

rodlonq wrote:..........but it certainly highlight the differences in hand placement for different bow grip types....
Rod, I think grip is also determined by the style and draw weight of the bow.

The photo depicts a Tibetan female archer flight shooting in 1938. Notice the finger placement on the back of the bow.
Female Archer Tibet 1938.jpg
Female Archer Tibet 1938.jpg (85.29 KiB) Viewed 2784 times
I do however doubt the suitability of this type of grip on heavy bows such as war bows.

Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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