Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on May 26, 2019, 08:55:23 pm
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Here is a FDC of my newest Boo Yew(pencil) and my old stand by (red). They weigh 453 grams and 461 grams respectively. The new one has a bulkier handle so I'm assuming the new one has lighter limbs. The new one is 50#@28", the old is 40@28. The new one shoots 10 gpp@ 183fps, the old one shoots 10gpp@191fps. I tried to arrange the beginning of the chart so things made sense. The only thing that makes this make any sense to me is the old ones FDC is slightly humped compared with the new one. Is that little hump enough "early draw weight" to make the difference. I shot a 300 grain arrow with both bows and the new one was 217fps and the old was 211fps but I think that's to be expected with 10# more draw weight. I can probably take some weight off the tips of the new bow so there may be some improvement possible. The draw length and draw weight are written in 5 # and " increments. Each graph line is 1 # or "
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Maybe its more about the qualities of the wood,.not so much the fdc,,I am just guessing,..a bit like the stave u are working on now,,,that seems to have different qualities in resisting bend.
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They are both edge grain yew possibly from the same stave. When I first got a bunch of yew it was in big staves that I split up and just stacked in the corner. The color and grain are very similar.
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Are they holding same reflex,,.the fdc seems to reflect the conference
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Ah, good point, that's probably it. The new one has less reflex even though it has bigger hooks. 1 3/4" versus 3/4". I'm having trouble guesstimating limb thickness before glue up. If the limb is thicker the pucker factor enters into it and it's harder to glue in more reflex. The new one started with 1 3/8" reflex and ended with 3/4" so it lost 5/8", I can live with that.
I'm sure happy with the old one. I've shot at least a thousand arrows and the speed dropped 3fps from 193 when very fresh to 191 now. i sure wish I could duplicate(and improve on) that.
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Seems like putting a bit more reflex,,,should help...it seems to me,,,the heavier the draw,,,the more difficult to get fps,,.?..maybe it just takes more skill,..like a 70# bow shooting a 700 grain arrow 190 fps,,,whew,..I am sure I read somewhere that as bows become heavier in draw they become less efficient...may not be true,..but I have read after 70# it is more noticeable..I think its in volume 1...I will look today
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Back to the original question, sorta. Is it the extra reflex that causes the "hump" in the FDC?
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im guessing yes,, (-P once you get above 190 fps, the speculations seems to drop off dramaticly,,im sure Badger will chime in,, ;D
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I know the one has a heavier draw weight but taking that into account would you say that the lighter bow has more string tension at brace?
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I've been meaning to make a jig to make measuring string tension easier. Maybe that's a job for today.
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cool (S)
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That was easy, a length of chain and a turnbuckle makes it infinitely adjustable.
OK this is interesting. The 50# bow has 76# of string tension. The 40# bow has 96#. Now if I could just figure out why. I made the test "string" the same length as the bows normal string. Brace heights are with an eighth inch of each other.
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The string tension might be a reflection of how little or how much set the bow took. I have seen bows with a lot of refex and really not that much string tension
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Do we know for sure that more string tension is better?
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I measured another 40# bow that shoots about 185 and it only has 70# of string tension.
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The biggest thing for high string tension is Lowe's string angles that brace, so the longer your low string angle the higher that brace tension
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In the TBB 1 it has graphs just like that in the bow design and preformance. bet you could find it there.
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Is that supposed to be "low string angles at brace" not "Lowes string angles that brace"? I did a google search and got garden tools ;D ;D Glasses still missing ;D ;D ;D
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In the TBB 1 it has graphs just like that in the bow design and preformance. bet you could find it there.
I've read that chapter at least 10 times ;D ;D
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I just started using a cell phone
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I just started using a cell phone
You will love it and you will hate it. It wants to make typos for you, but it's nice to have all the information access in your pocket.
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The top one has 96# of string tension and the bottom has 76#. Are you seeing a lower string angle on one? Sorry for the fuzzy pic.
PS I thought I would put another unbraced pic where we could see them together.
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Now this is just a guess but I would think the amount of reflex before the recurve makes for extra high braced tension.
I drew some lines on your pics to show you the top bow has a lot more of "pre-recurve" reflex. I know on the bows I did this with compared to just deflex and recurves it makes a lot of difference.
(https://live.staticflickr.com/65535/47950529137_fa6af3621d_z.jpg)
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I do see a lower string angle on the top one. It also matters how long the low angle is. You can see the dip in the Upper Limb even when braced. This would account for the extra tension.
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Now this is just a guess but I would think the amount of reflex before the recurve makes for extra high braced tension.
I drew some lines on your pics to show you the top bow has a lot more of "pre-recurve" reflex. I know on the bows I did this with compared to just deflex and recurves it makes a lot of difference.
(https://live.staticflickr.com/65535/47950529137_fa6af3621d_z.jpg)
I do see a lower string angle on the top one. It also matters how long the low angle is. You can see the dip in the Upper Limb even when braced. This would account for the extra tension.
You are both commenting on what I was thinking, I think. When I was tillering this bow on the long string when I got to about brace height that bend would kinda "pop through". Kind of an "oil can". I wondered at the time if that was a good thing. I'm thinking the high string tension is part of the "popping". Do we know if that is a good thing? It might give the arrow a last minute shove. Shouldn't we see some remnant of this on the FDC?
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That was easy, a length of chain and a turnbuckle makes it infinitely adjustable.
OK this is interesting. The 50# bow has 76# of string tension. The 40# bow has 96#. Now if I could just figure out why. I made the test "string" the same length as the bows normal string. Brace heights are with an eighth inch of each other.
I don't quite get this string tension stuff, how could a lower weight bow have more tension..? And how are you measuring the weight, ? with the luggage scale..? Got any photos of this weight testing.
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The scale is basically inserted in the string. I'll take a picture in a bit. I read fairly early on that string tension was higher than bow weight and I measured it on some of my first bows. It was a few pounds higher but nothing dramatic. It was when I measured this bow that I found that my luggage scales couldn't hack it. I had to use my 300 lb scale.
how could a lower weight bow have more tension..?
Haven't quite figured that out yet but I believe it's the mechanical advantage that the string has that raises it above draw weight. I have to do more testing but I think changing the brace height will change the string tension quite a bit. I have to make a jig that I can adjust under tension to check that. I do know that the string tension drops as you draw the bow. At full draw string tension and draw weight are the same or at least close to the same.
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so would raising the brace,, lower the tension, (--)
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I am pretty sure a higher brace height would lower the tension
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Strange, there seems to be a sweet spot. My string tension jig has a chain with 5/8" links so I raised and lowered the brace height by one link. So normal BH was 97-98#, shorter string(higher BH) was 88#, longer string(lower BH) 92#. I did it twice because I didn't believe it. Now I have to make an adjustable one so I can crank it up and down while I'm watching it. here's some pictures of the jig.
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Very counter intuitive. I'm interested in what the mechanics or geometry or vodoo witchcraft behind this is. Is string angle a player? String length? The string isn't feeling tension in just one spot, it feels it along the string. If you measured it in different spots on the string does it change? If you measured it in 2 spots at the same time are they the same reading? Lot's of questions....
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(-P (-P (-P
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it makes since to me,, that if the brace is higher, its like drawing the bow,, so the string tension would be lower,,and I hate to say this,, but maybe shoot slower,, (--)
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Yeah but shooting slower with a higher brace is a different matter in my mind. That can be explained by the distance the string travels before the arrow leaves the string. That makes sense.,,,to me....this doesn't. Not saying it's not accurate, just that it defies logic, or the logic I bring to the table, which is very non-mathmatical.
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I greased up the turnbuckle so I could move it under tension. I can watch the string tension go up as I lower the BH until it hits about 5 1/4" and then it goes down. This is just about the time that the string starts laying on the belly so I think that's the culprit. Shorter string or maybe the string angle goes up about then.
The tension is going to be the same all along the string and I don't have two scales anyway.
The string angle changing kind of explains it for me unless someone can explain why a shorter string(shorter bow) would drop the tension. I'll do some more testing when I get a jig that works better. The turnbuckle I have isn't long enough and a longer one would get more cumbersome to use. It's a real PITA the get this braced with all the gear attached.
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Yeah but shooting slower with a higher brace is a different matter in my mind. That can be explained by the distance the string travels before the arrow leaves the string. That makes sense.,,,to me....this doesn't. Not saying it's not accurate, just that it defies logic, or the logic I bring to the table, which is very non-mathmatical.
Maybe the brace height/performance thing just didn't explain it well. Maybe the real reason the you get more speed with a lower brace height is because of the increased string tension, not extra travel. Or maybe both or maybe it's the same thing just looked at sideways sorta.
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The test will be to do this on a straight bow. I'd bet a nickle that the ST (string tension) would keep going up.
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Maybe it is string tension, so why is string tension higher on a bow string at brace height than full draw when the draw weight is under10lbs at 6 inches and 50lbs at 26 inches? That's...baffling.
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So if it is string length (lift off) then the longer the recurves (more contact) the higher the tension,? So we need to be making the recurves as long as possible to capitalize on that? Has to be some other factor that mitigates that...I think.
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We're still not positive that higher ST improves performance, at least I'm not, yet ;D. I don't think ST and draw weight are really dependent on each other althought it is hard to separate them in your head.
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I’m not a scientist lord knows,but there is a relationship between stored energy, and kinetic energy. Seems reasonable that string tension would fit in there under stored energy.
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I think it's definitely the higher string tension that's making the difference.
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Would you venture a guess as to whether it's actually the higher ST or is the high ST just an indicator that you got something else right? It just seems to me that the ST is kind of tied up with itself and I'm not sure that it could affect anything else. Just thinking outloud ;D
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ok its over my head now,, :BB (lol) but a very interesting way to look at it,,
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The string tension is higher at brace because you have a very low string angle. As the string angle increases, leverage increases against the tip. As leverage increases, the required force to do work decreases, which is why the string tension drops. Its literally stacking in reverse.
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Sleek that was so confusing,, (-P
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Let me translate...........................no idea.
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The string tension is higher at brace because you have a very low string angle. As the string angle increases, leverage increases against the tip. As leverage increases, the required force to do work decreases, which is why the string tension drops. Its literally stacking in reverse.
Basically, think of it like this. How hard do you have to push straight down on a bows tips to get it to bend an inch? If the bow is perfectly straight, you can't push down hard enough, its impossible. Put a little bend in it, ( changing the angle of force on the tips aka string angle ) and it becomes possible to bend it that one inch. The more bend you put in the bow, the easier it becomes to bend it an inch more. This is because your angle of applied force gives you leverage. The angle of applied force is why a braced bow has more string tension than a drawn bow.
So, why doesn't a bow become easier to bend as you draw it then? That would be because of the bend in the string at the arrow nock point. As the string comes back, that angle becomes less, and when combined with your tip string angle it decreases your overall leverage. The combined angle of your string at the tips and the arrow nock, added to the load of the bow, gives you your final draw weight.
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That's a pretty good explanation sleek
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I've been wondering about how to roughly calculate the strings mechanical advantage. I looked at bunch of websites and got a lot of formulae that I didn't understand. I wanted something like figuring the MA of a lever or gear, something I can get my head around. Something in Sleeks post triggered a thought. If you use the limb length as one arm of the lever and the distance from the centre of the handle to the string as the other arm it seems to work. As the string gets closer to the bow the MA starts to get close to infinite and vice versa. Thanks Sleek, that gets me closer to understanding how these things work.
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I don’t believe higher string tension is a useful indicator of performance.
At very low brace with a longer string, the bow limb has a lot of leverage on the string which will drive a very high string tension. But ignore you look at it from the string’s perspective, it has very poor leverage on the limb. This becomes important when shooting the bow. If the string doesn’t have sufficient leverage on the bow limbs, then the string will just stretch at the end of the shot, and it will not be able to effectively transfer and slow the forward momentum of the limbs to the arrow. Efficiency will be reduced, and the limbs will have a lot of post shot vibration.
Raising the brace height improves the leverage the string has over the limb, but reduces the power stroke, or amount of energy that can be stored in the draw. There is a happy medium where the bow has the best combination of efficiency and stored energy, and that depends on the properties of the string and design of the bow.
I was chronograph testing a new set of bow limbs over the weekend and had some interesting results that are related to this discussion. I had two strings. One was longer and the brace height was just under 6” and had very high tension at brace. The second string was shorter for a brace height of just over 7” and had lower tension at brace. When I shot a very heavy arrow, the lower braced configuration was faster. But with a lighter 3 grains per pound arrow (this is for flight shooting), the higher brace height string (with lower tension at brace) produced faster arrow speeds. And much lower post-shot vibration too.
Alan
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I've been wondering about how to roughly calculate the strings mechanical advantage. I looked at bunch of websites and got a lot of formulae that I didn't understand. I wanted something like figuring the MA of a lever or gear, something I can get my head around. Something in Sleeks post triggered a thought. If you use the limb length as one arm of the lever and the distance from the centre of the handle to the string as the other arm it seems to work. As the string gets closer to the bow the MA starts to get close to infinite and vice versa. Thanks Sleek, that gets me closer to understanding how these things work.
The string mechanical advantage is the ratio of the horizontal brace height to the neutral plane at the bow center, to vertical lever arm from the center of the bow to the string contact point. I hope that makes sense.
Alan
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Perfect sense, it is a simple machine after all :) :)
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ok I can understand that, trial and error to find best brace,, ;D