Primitive Archer
Main Discussion Area => Bows => Topic started by: Richard B on January 15, 2015, 04:17:30 pm
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Interested in opinions about desirability of set.
I have seen comments on how a little set (say 1-2") is "OK" and shows that the wood is working. Everything else being "equal" I guess a bow that has taken a little set will have higher stressed and therefore have lighter limbs and could be more efficient than a bow designed and tillered to have no set.
Do bows that have been tillered with no set last longer before they start to lose cast?
So far I have built one bow from elm that took about 1.5" of set and have not yet reached full draw tillering my second bow, so not much experience to go on so far
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Interested by this question also. I recently finished an Elm bow myself and it took a little set as well. keeping in mind I don't have much experience either, I would think it has to do with the species of wood and as long as it is not excessive string follow, and the bow does what it is meant to do, it doesn't matter. I think if I could make an Osage bow to be a perfect match to the Elm, it would take less set due to it being a better bow wood.
I'm rambling....I've had too many beers!
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You will get different answers on this from everybody. Personally, I like to build mine with the idea in mind of zero set. Never have achieved that, but the less the better. Once they are shot in however, those which took some set may be great bows, and only the maker knows how much set they took and only he may care about it. As a bow maker, I want as little as possible. As a bow shooter, it makes little difference to me. It also depends on where the set is. All in a 5 inch section? Near handle? Or spread out along the limbs. The latter being preferred.
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it really depends on the piece of wood,, 0 set could be overbuilt( mass of the bow would tell you alot)
but if it started with more reflex it might be just right,, just depends :) chronograph will tell you alot too,, if it is shooting slow for its draw maybe overbuilt,, if it is a rocket launcher then not
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I can't say I've shot any bow to know the lifetime of it with or without set. But in my opinion in terms of building it, a bow with zero set could have just the right amount of wood so that it doesn't take any set or it could be way overbuilt. The thing is is there's no way of knowing the difference unless you know how much wood you need like in the mass principle. I feel that set is a way of telling us when a bow is just right. A little bit of set won't hurt but you'll know for sure that the bow isn't overbuilt. I try and achieve around an inch of set or less since I haven't learned to use the mass principle effectively. With the use of the mass principle, a bow is overbuilt if it ends up way over the predicted mass and underbuilt if it ends up lower than predicted mass. I'm sure you can reliably predict if an underbuilt bow will bust depending on its mass but I don't have that experience.
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Tim Baker used to say wide enough at the handle for no set, narrow enough mid limb for a little set and stiffer outer limbs. I agree with that.
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badger-
could richard take advantage of your "no set tillering" method, to see of the bow is not too much overbuilt as ryoon suggested could be the case?
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I think it is always a good idea to use it. You may not get 100% but it does give you a lot of control over where you end up.
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If no set indicates overbuilt and a little set indicates "just right" you should go for 1/32 of set instead of a whole inch. >:D
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I try and achieve around an inch of set OR LESS
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A couple of inches of set is quite acceptable.
Jawge
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I try and achieve around an inch of set OR LESS
"Way less" would really show that you are walking the fine line.
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How the heck do you "overbuild" a bow ? I personally think that is nonsense. The less string follow the better! Id rather have a bow with no string follow than a highly stressed bow that will wear out and take lots of string follow.
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I think that most all experienced bowyers use a form of the no set tillering method without even realizing they are doing it. I kind of stumbled on to an explanation for it by accident. I have done very little tutoring or teahing on bow building, a couple of years ago I started teaching a few guys and kept finding myself at a loss of words when trying to explain something. I was showing a guy to excersize his bow and then double check the weight when it ocured to me that I could recheck it everytime I advance an inch. I have been doing it ever since.
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Interesting question. How about seeing it like this: a bow taking set means that back is overpowering belly. If the bow takes very little set, it means, that back and belly are balanced quite well. If the bow takes absolutely no set, does it mean that that belly is overpowering back? Of course there are other variables like elasticity, but as genereal rule?
Few examples: Juniper is known to be sensitive for back breaking, especially in very dry environments. Juniper takes very little set. I had one yew bow, little set, perfect back but it exploded from back. Many of my narrow white oak longbows takes moderate set, 1-2 inches but tolerates even over 40 inch draw, see for example this one:
https://www.youtube.com/watch?v=KdTK26j3WdM
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All springs take set.
The valve springs in your car engine take set.
It's a matter of degree...
Del
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I personally think that there should be some set, depending on design and wood and all, usually between 1 and 3 inches. On bows that draw nearly half their lenght, I'm absolutely easy with 2 inches of set. If you build a bow and it takes no set at all, something is wrong. Of course, a stave with 2 inches of relex that ends up straight has of course taken 2 inches of set.
Personally I think (with no prove on my side) that set is overestimatet. It's true that a reflexed bow has more energy stored, but you won't get that out until you unstring it. It certainly does not go into arrow flight. A lower brace will increase arrow energy, since ist's simply force * distance. What will also increase arrow energy is lighter bow limbs. And these will take some set.
I closely observe how a bow picks up set while tillering, because that way I know where the wood is working enough and where it isn't. I try to avoid set in the inner limbs, according to baker's suggestions, but mainly because extra wood in the inner limbs is more acceptable than further out. And yes, you CAN overbuild a bow by making it too wide and thin. Mass formula is excellent to get an idea, I use it on almost every bow I make, especially since I like using different woods.
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Draw weight is (I think) a big factor. If you're tillering a yew bow that draws 45# and pulls 28" having 3" of set is excessive.
If it's a yew bow of 160# and pulling 32" getting 3" is considered pretty good!
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Personally I think (with no prove on my side) that set is overestimatet. It's true that a reflexed bow has more energy stored, but you won't get that out until you unstring it. It certainly does not go into arrow flight.
Not true.
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If you are talking about making the most efficient bow from a certain piece of wood then, yes, zero set means overbuilt. To me, the perfect bow is the one that, after exercising it, shows a little "set" (what I call, stringfollow), but within a few minutes or an hour has regained its original shape. Still, I would rather have the overbuilt bow than the bow with more than an inch of set.
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Definitions are so important.
Just to clarify. There is a slight difference between set and string follow. Set is how much the limbs change from their original shape while string follow is the degree to which the limbs resemble a strung bow even when unstrung.
If the bowyer induces 3 inches of reflex and then makes a bow with 1 inch of reflex left then the bow has 2 inches of set and no string follow.
I have never built a bow with 0 set. I've built bows that started with a couple inches of reflex and ended even but that's still 2 inches of set.
Jawge
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Richard, I learned a long time ago not to use the word impossible but I've never seen a bow with 0 set or built one with 0 set particularly when the definition of set is considered.
Minimizing set is good. Proper wood choice, dry wood and easy does it tillering will help.
Right now with one bow completed set is not something you should worry about. Hone your technique and make a couple dozen bows and set won't be an issue.
BTW I measure set by putting the bow with the back against the wall and measuring tip deflection.
Jawge
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Not true.
You got someting more to convince me? Truth is, I'm just guessing, but I'd love to know, one way or the other.
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Obviously you can't use all the energy stored from bracing a reflexed bow but it boosts the early draw weight which adds to the total. As long as drawing it doesn't break down the wood to the point of negating those gains.
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Jacodus, when we measure the power curve on a bow we measure the work we are putting int it. If a bow has more reflex or is straighter than a bow with set it requires more work to pull it back even though the peek draw force may be equal. Some bows drawing 50# for instance may only store about 40 ft pounds of energy while another may stor slightly more than 50#. There is also good evidence that even tiny amounts of set will start to show in energy losses due to histrias ( internal friction).
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Steve is right about set. I think that set is the biggest problem in wooden bows, in view of performance.
Why wooden bow takes set? Because the belly of the bow can not take the stress of bending. There is few methods for relieving belly stress - for example trapping.
So, if the bow takes no set, it is not overbuilt but it is propably going to break on back. Try to make much trapped (almost triangle) bow, it takes no set but it will break on back. The, try trapping belly, the will take a lot of set.
There is no such thing than overbuilding, except making tips too heavy. Make wide and thin bow, it can bend more than narrow and thick, but it will take proportionally same set. Basically set depends on tension and compression strenght of the wood. For example, think about using g***s just the belly or back of the bow. It does not work.
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Personally I think (with no prove on my side) that set is overestimatet. It's true that a reflexed bow has more energy stored, but you won't get that out until you unstring it. It certainly does not go into arrow flight.
Not true.
I agree NOT TRUE
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If you build a bow and it takes no set at all, something is wrong.
How is there any logic in that at all? That is so untrue.
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as there seems to be temporary kind of set that is noticeable before it becomes permanent set, does anyone try to watch the "return to zero speed" of the limb while tillering? I try to watch the last quarter inch of relaxation by sighting to a line behind the tillering tree
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Willie that is a very interesting observation you made. Set is very progressive. Lets say you took the string off the bow and were somehow able to instantly measure set in less than 1/100 of s econd from wehn you took the string off. You might find it has something like 4". Within 1 second it might be to 2" within 5 seconds it might be to 1" and within 1 minute might be at 1/4". Thats sounds crazy but it illustrates how set affects performance. It can be even more dramatic after a full draw and a complete shot sequence.
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I doubt it's anywhere near that level or at that speed, Steve. The limb would return too slowly to push the arrow if you start talking more than even fractions of a second.
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Pat, you atr talking in thousands of a second. The healing process on the limbs starts occuring the very second you let go of the string. If there is no healing to do a bow has no histrias. How fast that bow recovers it original shape is what concerns us, any recovery that takes place after the arrow has left the bow is set regardless of what we see 2 seconds after we unstring it.
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well i was remembering something I watched when the bow was still on the long string. now you got me thinking .....
you could make a string that you could instantly change from long to short, then you might be able to watch the bow relax as you tiller out further.
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I think you have to consider the momentum of the loose. If you break a string on the loose the limbs almost certainly instantly go past the resting unstrung position.
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patm,
i think we are talking about different things
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Not at all. A broken string represents what happens with a quick unstringing better than anything.
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i am thinking of how to monitor set while tillering a bow.
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I am going by the thread title and the main portion of the discussion.
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Willie, you can easily monitor set while tillering a bow. Find out what your bow is pulling at say 12", evrey time you go past 12" come back and recheck the weight to see if it has changed. Any change in the weight is set regardless if you can see it or not. Of course if you remove wood to adjust tiller you can expect to loose weight so would need to go back and restablish a new benchmark.
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going back to the thread title as Pat suggests, I think a zero set bow probably would be overbuilt but not neccessarily have to be. If the mass of the bow was overiding the positive effects of low histerias then it would be overbuilt
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Overbuilding a bow is not necessarily a bad thing.
Read The Bent Stick by Comstock.
Jawge
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I agree with that Jawge. Over building a bow would mean either making it longer than is needed for the intended draw length or wider than is needed for the intended draw weight. One would first have to know what the "ideal" would be for each individual bow stave. That alone is more of an educated guess than anything else. Certainly experience gets you in the ballpark on those numbers, and using the mass principal gets you even closer, but I am still making some assumptions after a certain point. That being the case, and this pertains to my level of experience only as others may have narrowed the field even tighter, the question really is how long and wide should I make THIS particular bow. An Osage self bow puling 50 lbs at 27 inches, stiff handle...64-66 inches long and 1 3/8 inches wide...some of mine will take very little set at those dimensions, and others will have more noticeable amounts. I would not classify the former as being "over built", just well built for that piece of wood. Over built might apply if you instead were making those same Osage bows 2 inches wide or 70 inches long. They may show very little set but you clearly have more mass than is needed and that mass will have a more negative effect on the cast than the lack of set can make up for. Just my rambling thoughts on the matter this morning.
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Slimbob,
That's why making self-bows is an art.
I basically let the stave determine the design.
Let's take osage, since you mentioned that wood. Let's say I want 45-50#. The stave has no knots, relatively thick rings, and good early to late wood...nice and orange.
Then I'll go 1 1/8" wide and 64-64" long ntn. I have a 26" but I like smooth all the way back.
If there are knots, poor early to late wood ratio, or thin rings then I'll go wider...say 1.5 inches.
What if that is too wide for the stave? It will let me know. The stave will begin to get too thin to bend it safely, risking a serious plunk in the head. Been there.
Then I begin to narrow it to bring the tiller home.
Jawge
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When I think about optimizing a bow I look at it in terms of thickness and width. Thickness is what determines the bend radius at any point along the limbs. For optimization this means we want the maximum thickness the wood can bend at without taking set. Once we have the correct bend we can adjust width for our desired draw weight. With maximal thickness for the correct bend and the minimal width for draw weight we get the smallest cross sectional area which means the least amount of mass necessary for the bow. Length comes into play by adjusting thickness where a longer bow can accommodate a larger bend radius and therefore the thickness will be greater before taking set. When making a bow there is a lot of factors in the building process that can effect how it takes set. An optimal method would start with wood wider and thicker than necessary and adjust both width and thickness as we assess for set. If the bow is bending 20" for a 28" draw bow and has taken no set than we can reliably reduce width as the thickness is at a point where it is not stressed. I think for many of us we start at a width that we feel will work for the bow and go from there. I'm sure this is what people like Steve do in his no set tillering. It is an interplay of width and thickness to achieve the lowest cross sectional area and therefore mass.
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Here is how I minimize set.
First, moisture content is vital. I use a moisture meter throughout right down to the stave's initial stringing. I look for the wood to be at 8-10% except for hickory at 6-8%.
Second, I floor tiller getting the limbs to bend a few inches.
Third, I long string tiller out to 10 inches of string movement (for a 26" draw) looking for target weight or a little more and good limb bending. Don't force it to 10 inches. Easy does it. My aim is to be no more that 15# above target weight.
Needless to say I use a rope and pulley.
Fourth, then I string it at 2-3".
Fifth, let's assume I want 48#. I never draw more than it takes to expose a problem. As I increase the draw length, I increase the brace height. I look to get in the high 30's for a draw weight at 20". I look to hit draw weight at 25 inches and that's for the first time. I shoot it in to 26".
Since I've used a scraper-like tool since I string it little sanding is needed. I avoid holding for more than a second when shooting.
Jawge
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For the sake of a theoretical comparison a simple spreadsheet model of a bow assuming some basic published data for yew (Modulus of proportionality of 6.48 e7 MPA and elastic modulus of 9.1 e9 MPA) suggests that a bow that is 70" NTN 1.44" wide at the fades and 0.5" at the tip should take about 0.75" set if perfectly tillered at 52 lb at 28 inches.
To keep the strain in the yew below the proportional limit and achieve zero set, at the the same draw and weight, the bow would need to be 81" NTN 2.25" wide at the fades and 0.5 " wide at the tip, which seems a bit over the top.
Clearly all staves are different (even around the same trunk). I quite like the approach in TBB vol 1 where bend tests are done on wood close to (or part of) the stave being used so that a judgement can be made about achievable draw weight and profile before its too late.
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For the sake of a theoretical comparison a simple spreadsheet model of a bow assuming some basic published data for yew (Modulus of proportionality of 6.48 e7 MPA and elastic modulus of 9.1 e9 MPA) suggests that a bow that is 70" NTN 1.44" wide at the fades and 0.5" at the tip should take about 0.75" set if perfectly tillered at 52 lb at 28 inches.
To keep the strain in the yew below the proportional limit and achieve zero set, at the the same draw and weight, the bow would need to be 81" NTN 2.25" wide at the fades and 0.5 " wide at the tip, which seems a bit over the top.
Clearly all staves are different (even around the same trunk). I quite like the approach in TBB vol 1 where bend tests are done on wood close to (or part of) the stave being used so that a judgement can be made about achievable draw weight and profile before its too late.
Wow............
DBar
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Richard, that sound pretty accurate, I like to go about 1 5/8 for a 68" yew.
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For the sake of a theoretical comparison a simple spreadsheet model of a bow assuming some basic published data ....
Where can I get this model?
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Badger,
The precise numbers I gave in my last post might be a bit misleading....
My spreadsheet model can never be that accurate, because of the uncertain nature of wood. Even if you have measured the properties of a sample of wood close to the stave you are using, that actual stave will be different (wavy grain, knots, ring density etc. etc.) so any theoretical model will only be an approximation at best. You will still need to feel your way with you particular bit of wood as you shape it and tiller it.
What I use it for is establishing a first guess at what draw weight I can achieve with my stave and producing a target tillered shape that should give me approximately the target weight at the target draw weight, with even distribution of stress down the limbs (so should, in theory, give me even set). It helped me achieve my target draw weight and draw length on my first (elm) bow and I have posted screen shots of the target tiller curves superimposed on the tillered shape of my current bow.
http://www.primitivearcher.com/smf/index.php/topic,45456.msg684371.html#msg68437 (http://www.primitivearcher.com/smf/index.php/topic,45456.msg684371.html#msg68437).
My spreadsheet is based on one by David Dewey, which you can find on google and download. I produced my own because I wanted to understand what is going on and would like to improve the way it is modelling set (which adds a lot of complexity and uncertainty), as well as extending it to look at asymmetric bows, and make it a bit more user friendly.