Power stroke?

[Marc St Louis]

David
All things being equal the bow with the same draw weight at a shorter draw length will have a steeper climb in its force draw curve, it has to.  You can alter the profile of a bow to affect a change in where the increase in draw weight per inch will be higher

[knightd]

Mark It doesn't have to on all bows..on some of my R/D designs they gain the same amount per inch the only thing that changes is the amount of force it takes to get it to brace but after braced the F/D curve still climbs at the same rate.. That is the reason behind me building a bow designed specifically for shorter draws.. And it has also been a headache of mine for years.. but I just about got all the bug's worked out.. 

[Kegan]

Mark It doesn't have to on all bows..on some of my R/D designs they gain the same amount per inch the only thing that changes is the amount of force it takes to get it to brace but after braced the F/D curve still climbs at the same rate.. That is the reason behind me building a bow designed specifically for shorter draws.. And it has also been a headache of mine for years.. but I just about got all the bug's worked out.. 

I thought that a bow that was tighter at brace shot better? Comstock wrote about it in The Bent Stick that he, Baker and others found that out? Somethng baout storing more enerrgy and transfering it at the end of the release?

[Marc St Louis]

David
If both bows pull the same draw weight then the bow that pulls that weight at 27" has to make up the difference somewhere along its power stroke

Kegan
Generally speaking a bow that has a higher string tension will shoot harder.

[knightd]

Yep.. but that higher weight is made up for in the bow befor it is gotten to brace height..

[Kegan]

Yep.. but that higher weight is made up for in the bow befor it is gotten to brace height..

How do you mean?

And thank you Marc (again )!

[knightd]

In short it means that the shorter drawn bow at 1" of draw would be lets say 37# instead of 34# and only gain 2 1/2# per inch of draw. where as the longer draw bow would be 34# at 1" and still only gain 2 1/2# per inch of draw.  Essentially both gain the same per inch of draw..

[Marc St Louis]

Doesn't work that way David.

[knightd]

For one thing the bow with the shorter draw for same draw weight will have a more aggressive increase in draw weight per inch.  That means there will be a bit more push to the arrow throughout the power stroke.  Then there is the lower stress to the wood factor as well but that will not amount to much but the 2 combined will give the shorter draw bow a definite advantage
 
 
Nope don't work that way Mark but it does this way....If the shorter bow was built to be the same draw weight at 27" instead of 28" both bows would be the same draw weight at 1" of draw because the shorter draw length bow has a 1" lower brace height so the leangth of the force draw curves are the same. which in turn makes the  force draw curves are exactly the same..

[Marc St Louis]

David
Good retort but even though the power stroke is the same the shorter draw bow still has to make the peak draw weight at a shorter draw.  That means that inside that power stroke there will be an increase in draw weight disproportionate to the longer draw bow.  Depending on the style of bow that increase may be at the beginning of the draw or anywhere along the draw length.  The closer to the beginning of the draw that increase is the more performance will jump up

[NOMADIC PIRATE]

Kegan, my lifetime fastest bow had a draw length of 26 3/4" and a brace height of 5". It started breaking down the wood as I increased the draw length past 27" and slowed down considerably after that. Steve

In my limited experiance I found the same, wood breaks down, ..so I would say the shorter draw,...but who knows ? 

[knightd]

Yes on paper if you look at the chart it does look like that but only becuase the chart starts at 0 and goes to 28" by moving the brace down 1" and drawing it 1" shorter The power stroke within the chart is exactly the same pound for pound on both bows. If the draw weight is the same at every inch of draw and the leangth of the power stroke is the same on both bows they will be even.. there is no gain or loss..
 
Kegan
And as for a bow with a shorter draw and a shorter power stroke storing more energy.. If you measure the poundege at each inch of draw this will give you stored inch# at every inch then you add them togather and it will give you the total stored energy expressed in inch pounds of the bow. take that total and divide it by 12 and that will give you the total amount of stored FT# of energy in the  bow.. With a  1" shorter power stroke you will have 1 less # to add and divide that number on a 50# bow would be 50 so it would leave you with 4.somthing pounds less FT pounds of stored energy.. So it is not possible to store more energy with a shorter power stroke at a given draw weight..
 It is possible to store the same amount with the same leangth power stroke by droping the brace and having the same draw weight at final draw leangth..
  

[Kegan]

This thread is becoming quite interesting and informative, even though it's beyond my scope of tests and building prowess.

However, I have gleaned that the shorter draw, shorter brace bow will work just as well, if not better, and also last a little longer- all of which I'm pleased with knowing .

[Jesse]

Yes on paper if you look at the chart it does look like that but only becuase the chart starts at 0 and goes to 28" by moving the brace down 1" and drawing it 1" shorter The power stroke within the chart is exactly the same pound for pound on both bows. If the draw weight is the same at every inch of draw and the leangth of the power stroke is the same on both bows they will be even.. there is no gain or loss..
 
Kegan
And as for a bow with a shorter draw and a shorter power stroke storing more energy.. If you measure the poundege at each inch of draw this will give you stored inch# at every inch then you add them togather and it will give you the total stored energy expressed in inch pounds of the bow. take that total and divide it by 12 and that will give you the total amount of stored FT# of energy in the  bow.. With a  1" shorter power stroke you will have 1 less # to add and divide that number on a 50# bow would be 50 so it would leave you with 4.somthing pounds less FT pounds of stored energy.. So it is not possible to store more energy with a shorter power stroke at a given draw weight..
 It is possible to store the same amount with the same leangth power stroke by droping the brace and having the same draw weight at final draw leangth..
  

Thats exactly what I was going to say

[Marc St Louis]

Sorry David but the fact remains that the shorter draw length bow has to get to the same draw weight but at a shorter draw length.  The power stroke has nothing to do with this because draw length is measured from the back of the bow to the nock of the arrow.  The power stroke reflects the energy the bow is storing and that is why the 27" draw length bow with a lower brace height can store as much, or in this case more than the 28" draw bow.  A 27" draw is still a 27" draw no matter how you look at it

Here's another way of looking at it. 

Let's say you have a bow that pulls 55# @ 27" with a 5.5" brace height and a bow that pulls 55# @ 28" with a 6.5" brace height.  Both bows have an increase of 2.5#/inch at the end of their draw.  If you raise the brace height of the 27" bow to 6.5" does this mean that it will only pull 52# and if you raise the brace height again to 7.5" then will draw weight goes down to 50# @ 27"?  Or take the 28" bow and lower the brace height to 5.5" does that mean it will pull 57.5# @ 28" or lowering the brace to 4.5" mean it will pull 60# @ 28"?  No.  Lowering the brace height frees up energy tied up in the limbs and raising the brace height ties up energy in the limbs.  This is the energy that is used to push the arrow and is reflected in the force draw curve.  Changing the brace height merely makes the bow more or less efficient.  Lowering the brace height plumps up the force draw curve making more energy available to the arrow and raising the brace height ties up energy in the limbs making the bow less efficient.  It does nothing to the peak draw weight at max draw length.  The energy is always there it's just a matter of whether it is available to the arrow or not

Take as an extreme example a bow that pulls 55# @ 22".  No matter what you do you cannot lower the brace height enough to get the power stroke to equal that of a 28" draw bow so the force draw will reflect that in stored energy.  It still has to make that 55# max draw weight inside it's 22" draw so the increase #'s per inch will be much higher than a 28" draw bow.  That means the push to the arrow in every inch of travel will be significantly higher than a 28" draw bow but the short power stroke doesn't allow the arrow time to accelerate.

How will this show up on a scale?  If you alter the brace height of a bow by lowering it 2" you will get a bump in draw weight at the beginning of the draw then farther up in the draw there will be a less dramatic increase per inch than you would get with the higher brace height, you still get the same overall peak draw weight.  That is how you get that increase in efficiency.  By moving some of the stored energy down at the beginning of the draw you get more push to the arrow at the end of the stroke.  Making a bow more efficient is how you get a higher string tension at brace height.  Higher string tension usually means better performance

Anyway if you can't see that then I guess we will just have to agree to disagree