Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on May 31, 2018, 04:11:42 pm
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I'm close to finishing an OS d/r and I thought i could reduce the tips a bit and gain a few FPS. The bow, without string, weighed 625 grams. I took the tips down to about 1/4" wide tapering back up the bow about 6". Tidied up a bit and now it weighs 615 grams. Before the reducing the bow shot 169-170 consistently. After reducing, 169-170 consistently. No change at all. 10 grams is 152 grains, almost one third the weight of an arrow. I was surprised I got no increase in speed. I was using an old string that was 10 strands of FF. It weighed 64 grains. So I made a new 8 strand FF string. It weighed 42 grains. It shot 172 FPS consistently. Speeds are tested on my machine and they are very consistent. Now I understand that the string is a lot closer to the action than the tips so it doesn't take as much weight change to make a difference. But this just seemed to far apart. Roughly half the string weight is at the tips, too. Unfortunately I can't go back and redo the first test. Does this seem normal to you guys?
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What does the new string weight?
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What does the new string weight?
Oops 42 grains, I fixed the OP.
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That's why I quit making my tips ultra skinny. It just doesn't make that much difference. When you accelerate the string you loose all that energy. You get some of the energy back that you used to accelerate the limbs. The forward momentum of the limbs is being resisted by the arrow. I feel as long as they are trimmed up pretty good that's good enough, If you were testing very light arrows you might notice a little more difference.
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You guys are breaking my bias filled heart!lol!
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Don a few years back when I first got my Chrony I also did a very similar test on a yew bow. I don’t have any numbers or anything and I didn’t weigh the difference, just narrowed the tips and I was surprised that there wasn’t a noiticable difference. You’d probably see more of a difference going from fairly fat and chunky tip down to very narrow ones. I know your bows have quite narrow tips to begin with!
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I slim my tips because they look nice. I don't think it makes any difference in bow speed. You'd have to have some pretty ridiculously fat tips tip start losing fps. Tiller and design are WAY more important.
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I'm just surprised that the weight change of the string(20 grains) made a difference and yet 152 grains on the tips made little or no difference. I have to make the assumption that the weight(mass) of every part of the bow that moves will make a difference in the speed of the arrow. The tips are the most massive part that moves the most other than the string. I was sure it would have more effect. I think I will tie some weights to the tips and see if I can back this up a bit. Something I thought might be interesting will be to put unequal weights on the tips and see if the imbalance affects the speed. I'm fairly confident in the machines ability. I get the same number over and over and I have to move the target a bit once in a while because the arrow keeps going in the same hole. I'll use the Chrony because it reads out tenths of a fps, the Caldwell just does fps. Take a few days away from scraping and rasping :)
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D/C, the virtual mass of a well made bow is only about 150 grains and 1/3 of that is your string. So your talking 100 grains of virtual mass to move the limbs. You get most of that back at the end of the stroke when the leverage changes favoring the arrow. Contrary to popular belief the limbs do not slam home on a well made bow.
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Gotta remember the centre of the string moves at 3 times the speed of the tips of the bow.
That's why arrow and string weight are more important.
E.G at a 27 " draw the tips of the bow only come back about 9" but the string and tips both return to brace in the same time.
The logic of the experiment is also slightly flawed, reducing the tip mass must to some extent also effect draw weight.
A better test would be to add weights to the tips and see how much is needed to reduce speed by say 5fps. That way every other variable can be left the same.
Even if there is little measurable gain, reducing mass can't do any harm, and several small but barely measurable improvements can make a big difference.
I'm sure we've all re-worked a bow that isn't quite right and made it into a sweeter faster bow with a little work and some fiddling and fettling.
The other side of the coin is the sloppy approach of ... just leave it as it is, it doesn't make any difference. We know that's jut no good.
I often tell people, "It's easy to make bow, it's hard to make a good bow" ;D
Del
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Great thread! Always interesting to me that folks can come up with ideas about how things work, then over time people realize they made a mistake and revert to an earlier way of understanding things. String weight is definitely more important than the weight of tips. I generally make mine 10-15 mm wide where I carve the nocks. That seems to work fine. Not outlandishly heavy, but not outlandishly skinny either.
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A better test would be to add weights to the tips and see how much is needed to reduce speed by say 5fps.
That's the project for today.
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Tim Baker ran a test many years ago using lead tape on the tips. I don't remember much about the results except it took a lot to make a difference. I still keep mine slim at about 3/8 but the 1/4" tips I reserve only for light arrow flight bows. Lighter arrows do benefit more from lighter tips than hunting weight arrows.
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The thing that's bugging me is the amount of weight. Like Del said the string nock moves three times as much as the tips so you would expect 30 grains on the tips to have the same effect as 10 grams at the nock point. I wonder if there is some kind of exponential thing going on. Any engineers or physicists on board? I have to finish my coffee and get my hands limbered up and then I'll go play. :)
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DC, you are ignoring the biggest factor, you don't loose all the energy it takes to accelerate the limbs, most of it goes back into the arrow. I said a few posts up that a well made bow only has about 150 grains of virtual mass and about 50 of that is string mass. Your limbs weigh about 2,000 grains a piece, if it wasn't going back into the arrow your bows would have almost no power at all. 100 grains off your tip is more like 10 grains off of an arrow.
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There may be a couple of things happening here. You minimized mass in the tips of the bow which should help performance, but if it reduced the stiffness of the limb near the tips, which may may have losing most of what was gained by the reduced mass. I have experienced this many times. I even had a bow or two show worse performance after a tip reduction because the tips would start to flex and deform at the end of the shot, which interferes with the bow effectively converting limb kinetic energy to arrow kinetic energy at the end of the shot.
Alan
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Tim Baker ran a test many years ago using lead tape on the tips. I don't remember much about the results except it took a lot to make a difference. I still keep mine slim at about 3/8 but the 1/4" tips I reserve only for light arrow flight bows. Lighter arrows do benefit more from lighter tips than hunting weight arrows.
The test Tim did was nothing more than a rehashing of the test done in Archery: the technical side
I was going to say that light arrows benefit more from light tips
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I did some machine shooting.
Bare finished bow, no serving 5 shots average 174
11 grams of lead on lower limb average 170
11 grams on upper limb av 169
11 grams on each limb 163
Then I shot the bare bow again av 172
Served the center of the string, + 4 grains av 171
So the bow relaxed a bit while testing, it is still fresh, but at least I could see some change. You have to take off quite a bit of wood to make 22 grams. I only managed to take off 10 grams when I made the bow. I was pleased that I could see a 4 grain change. It was closer to 1.5 fps but it was definite. These chronos are amazing things. In all the five shot groups the biggest difference was 1.5 fps. I noticed afterwards that the fletching wrap was getting worn from where it was snagging on part of the machine. That may have caused a variance. Fixed now :)
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DC, you are ignoring the biggest factor, you don't loose all the energy it takes to accelerate the limbs, most of it goes back into the arrow. I said a few posts up that a well made bow only has about 150 grains of virtual mass and about 50 of that is string mass. Your limbs weigh about 2,000 grains a piece, if it wasn't going back into the arrow your bows would have almost no power at all. 100 grains off your tip is more like 10 grains off of an arrow.
I'll have to spend some time figuring this out. After my testing I can see that you're right, I just have to wrap my head around it.
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That was a cool test you did, simple enough but something most of us never get around to doing., Think of the arrow as getting heavier as the string angles get lower, not as much weight pushing against it but the now heavier arrow is more effectively slowing down the limbs, as it slows them down it sucks the kinetic energy back out of them.
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Ditto DC.....Your experimenting has shown something here for all to pay attention to.Thanks for doing that.Personally when I converted over to FF string from B50 I noticed the most significant raise in fps in the past.
Badger...Good analogy as to getting someones' head around what's going on.
I measured my serving weight once on the latest string[never did before]and it was 12 grains.Things dooo all add up when chasing fps.
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Well this is interesting but one thing for sure big tips equal shock
In my findings and that is not good to me. At some point mass has to count on the tips or the worlds slowest bow would not have won the contest. ;D arvin
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Arvin, I think the outer limbs do make a big difference if you are talking grossly overbuilt as opposed to already slim and then just making them scary slim.
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thanks for the info, really makes since the ways you guys described it,, (-S
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Yes Steve what I did when I first started was 3/4 inch wide tips a 1/2 inch thick. ;D my bows were slow and full of shock. That been said I often see bows built the same. We all start somewhere. I was enjoying the builds none the less and that is what counts when first doing this crazy hobby. As the works get better and appreciated by some folks you can now figure you are doing it right.
We don't ever quite learning and forums like this make the learning curve
easier. Don't know what this was like till about 3-4 years back. But it has been very
helpful to me. Arvin
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I just kept thinking about this thread,, back in the day, I would take off wood,, weigh it then see if the speed increased it,, as stated, on very wide tips it willl make more of a difference,, but as you get narrow seems there is a point of diminishing return,, just like when the arrow gets lighter, there comes a point where the bow wont shoot faster,,even with a lighter arrow,, thanks again for all the positive input,, I enjoy learning how to make my bow better or more effecient,,or at least having the options to try different techniques,, )P(
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I think you nailed it with the law of diminishing returns.
Del
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I just kept thinking about this thread,, back in the day, I would take off wood,, weigh it then see if the speed increased it,, as stated, on very wide tips it willl make more of a difference,, but as you get narrow seems there is a point of diminishing return,, just like when the arrow gets lighter, there comes a point where the bow wont shoot faster,,even with a lighter arrow,, thanks again for all the positive input,, I enjoy learning how to make my bow better or more effecient,,or at least having the options to try different techniques,, )P(
With wide tips you also have to factor in air drag, one thing narrow tips don't have a problem with. When we were kids we would snap off a twig and swish it through the air when going for a walk in the bush. The air drag was noticeable but leaving a leaf on the end would increase that drag considerable
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Maybe if I get bored I'll try taping cardboard to the tips and see what happens. I've always wondered about this.
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hmmm,, didnt think bout that (SH)
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I would have to come up with an amount of cardboard to tape to the limb. I doubt it would be much, maybe a square inch or so. If I tapered the last eight inches down from 1/2" to 1/4" that would be one sq. in. If I tapered 16" the same that would be two sq. in. Not but who knows.
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I'm steaming limbs so I had a bit of time. ;D I used DS tape to hold two pieces of 1/32 plywood to the tips. They added 1 sq. in. to each limb. Total weight of ply and tape 34 grains. Five shots with and five without. Averages with 185.6, without 186.66. Not a lot of difference but it's there.