Primitive Archer
Main Discussion Area => Bows => Topic started by: Kegan on May 18, 2009, 02:10:33 pm
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The ecent threads about selfbow speed mae me wonder: if an archer with a 28" draw is using a 6.5" brace height, and an archer with a 27" draw is using a 5.5" brace height, their power strokes are the same- so would their be the usual 5 or so fps difference in cast or not?
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With my limited experience in this subject.. I find that all things draw weight and arrow weight and bow design being equal.. there should be no diference..
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You will get a little more energy because the string is pushing the arrow a bit further.
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Gordon.. The power stroke is the same on the set up he is using...
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With my limited experience in this subject.. I find that all things draw weight and arrow weight and bow design being equal.. there should be no diference..
I have a notion that this might not be correct. If the bows are otherwise the same, they would have the same F/D curve but the inch in question would be on different parts of the curve.
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Good question kegan, me I have a 24" draw so at a 6" brace thats only 18" of power stroke :(
Maybe I should brace at 2" so I can get a 22" power stroke :D
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I have a notion that this might not be correct. If the bows are otherwise the same, they would have the same F/D curve but the inch in question would be on different parts of the curve.
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I was trying to say if they were built for the given draw leangth.. With that said even if they were not !! the diference of 1" of draw and droping the brace to make up the diference should not efect the force draw curve enough to be that noticeable..
There can be a Big !! performance diference in a bow designed for a 26" draw and one designed for 28" draw but only being drawn to 26".. I only draw 26" and I'm curently working on 2 or 3 design's specifically for the 26" draw. That I and others have.. And yes I'm changing the F/D curve to work at that draw leangth..
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If both bows had the same draw weight then the bow with the lower draw length and brace height will have the advantage
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Mark I agree.. But it isn't much of a advantage at the given 1". and no change of design..
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If both bows had the same draw weight then the bow with the lower draw length and brace height will have the advantage
Can you explain that a little further? Is it just matter of less stress on the bow wood?
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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
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Probably the only real way to test this would be to take the same bow and lower the brace height and shorten the draw length. If done with the same bow the longer draw would win as it would have a higher draw weigth and store more energy. If you simply lowered the brace height and kept the draw length the same it would pick up a bit of speed. Steve
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Mark I find that to not be always the case.. In allot of R/D bows the only thing that would change is the amount of string tension at brace but the bow will still only gain lets just say 2 1/2lbs per inch of draw. so the power stroke still remains the same..
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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
That makes alot of sense. Thanks Marc.
Steve- I meant two bows of same design, each built for the specific brace height and draw length.
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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
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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
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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.. ;)
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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?
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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.
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Yep.. but that higher weight is made up for in the bow befor it is gotten to brace height..
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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 :D)!
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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..
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Doesn't work that way David.
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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..
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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
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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 ? ;D
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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..
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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 :).
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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 ;D
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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
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I will just have to agree to disagree.. ;)
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:)
Here is a FD curve that shows what I am talking about. This is not one of my bows but rather it's a FD of a horn bow that I copied from another site. As you can see this bow pulls 80# @ 25" and the guy has overlapped 2 brace heights, a 6" brace and a 7" brace. Peak draw weight is still the same at both the 6" and the 7" brace height but at the lower brace height draw weight builds faster then tapers off while the 7" brace height climbs at a steady pace till both merge at 80# @ 25". The higher draw weight at the beginning of the draw boosted performance. The same principle would apply with a bow that pulls the same draw weight but a lower draw length.
[attachment deleted by admin]
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Thanks for the visual aid Marc :)
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David
I see from your deleted post that you still don't understand the way the energy is moved around through brace height and design. I have said before that to be able to see more clearly you need to look at some extremes. That 27" draw bow is very close to a 28" draw bow so the differences are very small therefore harder to see. If you were to take that 22" draw bow I mentioned above and make it so that you could shoot it with a zero brace height, this would give you the same power stroke as a 28" draw bow with a 6" brace height, then you would see a considerable boost in performance. That jump is due to the fact that the bow has to make the same draw weight inside a 22" draw, the draw curve has a much steeper climb than a 28" draw bow.
How would such a bow's force draw look like? The first inch of draw would show a tremendous jump in weight which would then level off for a few inches before starting to slowly climb again at a steady rate. I know this because I have done the test to prove it. This is how the 22" draw bow would make use of the stored energy in the limbs that is usually lost bracing the bow. In essence that 22" draw bow would behave much like a 70# @ 28" bow because the projected draw weight of this bow at 28" would be approximately 70#, figuring 2.5#/". A 55# @ 28" just can't compete with a 70# bow, everything else being equal
If you don't believe me then do the tests. It's not that hard to do
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It's not hard at all to do the test and I have on over 100 bows and 15 to 20 different designs.. On the subject of power stroke if you take the chart you posted and use the lower brace height but hit 80# at 24" this will change the curve to be the same as the higher braced bow at the 1" longer draw..giving the designs being the same
If you have done the test then you should also know that if you have 2 bows with same brace height and design with 1 reaching 50# at 27" and the other reaching 50# at 28" it is impossible for the shorter drawn bow to store the same or more energy than the longer drawn bow..
I know there are others who share the knowledge and have done testing.. That see your side and some who see my side..But if we didn't test and try new things we would we never move ahead..
P.S. I deleted the post because I was willing to Agree to Disagree..But a healthy debate is good sometimes..
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Wow, You read the guys deleted post! Then used it in your post, to argue after he agreed to disagree, and let it go. I thought that the reason to delete a post was because you decided to NOT post that, and wanted to retract it, for whatever reason. Its kind of disturbing that you can read the deleted posts at all. Are p.m.'s really private messages?
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Y'all are making my little pea-sized brain hurt. If power stroke is so important, why do we stop at 28"? We could all learn to draw 36" and have better F/D curves. ;D I think we already determined that 26" is probably a more realistic average draw length. My draw length is what it is, so I just try to make decent bows that shoot good for me and shoot them how I shoot them and don't worry about it too much. :)
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David
I see from your deleted post that you still don't understand the way the energy is moved around through brace height and design. I have said before that to be able to see more clearly you need to look at some extremes. That 27" draw bow is very close to a 28" draw bow so the differences are very small therefore harder to see. If you were to take that 22" draw bow I mentioned above and make it so that you could shoot it with a zero brace height, this would give you the same power stroke as a 28" draw bow with a 6" brace height, then you would see a considerable boost in performance. That jump is due to the fact that the bow has to make the same draw weight inside a 22" draw, the draw curve has a much steeper climb than a 28" draw bow.
How would such a bow's force draw look like? The first inch of draw would show a tremendous jump in weight which would then level off for a few inches before starting to slowly climb again at a steady rate. I know this because I have done the test to prove it. This is how the 22" draw bow would make use of the stored energy in the limbs that is usually lost bracing the bow. In essence that 22" draw bow would behave much like a 70# @ 28" bow because the projected draw weight of this bow at 28" would be approximately 70#, figuring 2.5#/". A 55# @ 28" just can't compete with a 70# bow, everything else being equal
If you don't believe me then do the tests. It's not that hard to do
Marc, would the leverage of the limbs explain some of that early build up for lower brace bows (as in, the stirng has to pull the limbs from striaght- more leverage than having to pull on already bent limb tips)?
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It's not hard at all to do the test and I have on over 100 bows and 15 to 20 different designs.. On the subject of power stroke if you take the chart you posted and use the lower brace height but hit 80# at 24" this will change the curve to be the same as the higher braced bow at the 1" longer draw..giving the designs being the same
If you have done the test then you should also know that if you have 2 bows with same brace height and design with 1 reaching 50# at 27" and the other reaching 50# at 28" it is impossible for the shorter drawn bow to store the same or more energy than the longer drawn bow..
I know there are others who share the knowledge and have done testing.. That see your side and some who see my side..But if we didn't test and try new things we would we never move ahead..
P.S. I deleted the post because I was willing to Agree to Disagree..But a healthy debate is good sometimes..
David
Much easier to move the higher braced curve over to a 26" draw at the same draw weight. Wouldn't prove very much though because the whole curve would be affected, as in depressed. Still if you do that you would see that the lower braced bow would have a fatter FD curve but it wouldn't be by much.
But we aren't talking about 2 bows with the same brace height. We are talking about bows with the same power stroke. That's why I used the 22" draw bow with a zero brace height as an example.
Debate can be good
Bill
I don't know if a PM can be read by a mod. All deleted post can be read though as they are temporarily stored and a post can be read by anyone before it is deleted as well.
Kegan
Good question. Possibly. On the other hand a bow limb that is already bent has energy stored in it so it will be harder to bend it more.
Anyway I can see that this debate seems to be disturbing to some so I won't make anymore commenst on it.
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Bill-no, PMs cannot be read by moderators.
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POWER STROKE??? Well hell, I've got an osage longbow that's 42 inches long....and I draw that little booger 32 inches!! It pulls about 45 tons. Talk about a power stroke! James Parker, The CIA, even President Obama are constantly calling my cell phone trying to see it so they can buy it off me. Hell no I say! I use it to shoot at the space shuttle when it comes in for landing, or jetliners that are cruising by at 30,000 feet. Obama doesn't like it when I send arrows screaming by the cockpit of Air Force One....cause my arrows are flying Mach 32,000 and the shock wave always cracks the plane's windshield.
One time I shot it at an oak tree....the tree instantly vanished cause my super warp speed arrows turned it into powder...instantly! I can't shoot at deer cause it vaporizes them. One time the neighbors below me were raising all kinds of hell, making noise, partying, and smoking the funny stuff. Well, I'd had enough so I just shot an arrow down through the floor, it exploded when it got below my floor and all the people who were making noise suddenly disappeared...no blood, no fingerprints..nothing. Just VAPORIZED THEM!! The feds could never get me for homicide cause there was absolutely no evidence. Yeah, that's right ya govt yo-yo's...ya'll couldn't get me and you never will! Hahaha
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This is a good debate guys, I can't contribute to it but I am learning from it especially since a I have a short 24" draw which
makes it tough to build a bow matching the performance of a longer draw.
billy thats funny but somehow I think someone will find it offensive :o Go figure eh ;)
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Marc, and others, have helped me through being entrenched in horribly muddled thinking on a number of topics. And I would hope my comments over the years have helped foliks see things from a different perspective. It's very difficult sometimes to really entertain a topic thoroughly and objectively while trying to support an opposing view at the same time. Usually a break from it for days or weeks can break the log jam.
Such things as the physics of the bow and arrow in this context need not be based upon conjecture, and certainly not by popular vote or "who is right." There's only one "right" answer, from the scientific pov, but unfortunately infinite ways to describe it in laymans terms.
More often than not when discussions go on for days and days something is being missed, one side or both aren't actually commuinicating to the other. In such cases it's pointless to keep drumming away. Better to hang up and dial back later, perhaps securing a better "connection."