Primitive Archer
Main Discussion Area => Bows => Topic started by: joachimM on March 10, 2016, 05:24:14 am
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There's been a lot of talk already on lower bow limbs taking more set in vertically stored bows.
I thought this might have to do with moisture content being higher close to the floor, and the lower limb therefore having a higher MC than the upper limb. Since compression resistance decreases with increasing MC, lower limbs of vertically stored bows might therefore be weaker in compression and take more set when exposed to the same strain.
So I started measuring ambient moisture, on the floor (insulated!) and at 2 m height. I live in a wet climate with a strong marine influence, similar to the UK.
The past few days, ambient moisture on the floor was on average c. 10% higher than higher up (74% compared to 64%). This translates into a marked difference in equilibrium moisture content from 11.7% to 14.1%
So it seems this is an important factor to consider.
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Interesting... I'll take some measurements on vertically stored timber off cuts.
Del
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I ain't buying it. I think it's due to other reasons.
I think it's mostly due to not making the bottom limb weak enough. Seriously.
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I don't think a 10% swing in ambient moisture would cause a weak limb. If that were the case I would be in big trouble. Our humidity changes by 40-60% throughout a year. I store my bows hanging vertically in my basement from the floor joists, the tips are about 24" off the concrete floor.
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I ain't buying it. I think it's due to other reasons.
I think it's mostly due to not making the bottom limb weak enough. Seriously.
Why would a bottom limb have to be strained differently from a top limb, especially in symmetrical bows? I always aim for a perfectly symmetrical tiller and don't leave the lower limb stiffer.
Now, if you leave one limb stiffer ("not weak enough"), it means the other (necessarily weaker) limb takes more than half of its share of the stress, and is strained more. More strain, more set. So shouldn't then the weaker (upper) limb take more set?
Pearl Drums: a 10% change in ambient moisture from 65% to 75% at 20°C gives a wood MC shift from 12% to 14% (see for yourself here: http://extension.psu.edu/natural-resources/forests/woodpro/equilibrium-moisture-content-calculator).
That seems like a big deal to me. See line C in this figure (Wood Handbook). For the example in the figure, the compression strength shifts from 55 Mpa to 50 Mpa due to this, which is roughly a 10% change. Any other change in MC of 2% will have a similar effect according to that graph. I'm not saying that, the authors of the wood handbook are.
This was an equilibrium MC difference across 2 m. In a bow, this difference will be more subtle. But maybe enough to make a noticeable difference in set.
Note, I haven't said moisture is the reason, rather a factor to consider.
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Interesting.
I check moister with a pin type meter and the bottom end of my bows and staves that are stored vertical is always 2-3% high on everyone I check no matter how old. I assumed it was caused by water weight and gravity. I try to keep the ones I'm shooting horizontal.
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Interesting but I am more inclined t believe bottom limbs take more stress because they are strained more by the split finger release.
Jawge
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thanks Emmet, that's very valuable information. 2-3% difference is even more than what I'd expect.
By the way, water weight and gravity is not sufficiently strong to overcome the forces of molecular adhesion of water to wood. That's why you'll never see moisture drip out of the bottom end of a vertically stored stave, not even if freshly cut.
Jawge: I'd think that the difference in strain caused by split finger (1 above 2 under) is less than the difference in compression strain caused by the moisture difference (5% for a 2% MC difference). But all this is conjecture, there's no hard facts nor data to support nor reject this hypothesis. It could contribute. I don't know if it does.
By the way, I store most of my bows horizontally, and I've never remarked such difference in set. But my lighter bows (<45#) I shoot with a two-finger split anyway.
What we do know, however, is that bows shot in too moist conditions (green bowstaves as the extreme example) take excessive set, and we know moisture is the cause. Emmet's findings support that bottom limbs of vertically stored bows have higher moisture content.
1+1=?
Joachim
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A 10% R.H. difference at that humidity range is certainly possible if the temps near the floor are only a few degrees different (centigrade). Heating with heaters vs central heat, or poorly circulated areas often exceed this difference from floor to head height.
http://www.coolerado.com/pdfs/Psychrmtrcs/0000Psych11x17US_SI.pdf
read the dry bulb temps (across the bottom) vs the RH lines (sloping lower left to upper right)
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That is very interesting. Much more difference than I would have thought.
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I never store my bows vertically but always get a slight difference in set. Then again my bow for the year is in AC for the summer. Jawge
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I think the split finger,, causes a bit more strain on the lower limb,,
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2% would be very significant. The problem I had with my lower limbs was storing them in a bucket with hardly any air circulation. Since I took them out of the 55 gallon drum I haven't had the same problem. I think I may have been getting more than 5% difference.
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It's very likely that there are multiple causes for this phenomenon, and neither of the proposed reasons being wrong in its own way. Always good to read different opinions and widen my own, sometimes narrow view.
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Anything is possible I suppose, but I'm not convinced that a 2.4% m.c. difference always happens, or when it does, that it's enough to cause more set in one limb verses another. In order for it to be true, many other things would have to be set perfectly in place. I'm prone to think its more about the way the limbs are stressed relative to each other and the shooter.
I would be a lot more intrigued if someone stored some of their bows with the top limb toward the floor and some of them with the bottom limb toward the floor, and all of the limbs nearer the floor took more set, regardless of whether they were top or bottom. That would make me sit up and take notice.
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I do, Jeff. I have about 30-35 bows hanging from my floor joists in the basement. Some top up and some top down. I suppose if a guy builds his bows right on the edge of explosion this type of minute climate change would change the bow in a visible way. I make a very slightly beefy bow and they don't move unless I screwed something up. i.e. Too short or too narrow for a given species or design, or both.
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JoachimM, to address you question, "Why would a bottom limb have to be strained differently from a top limb, especially in symmetrical bows?"
That's an easy one. In order for the limbs to share the load equally, one limb will need to be made weaker than the other because we hold a symmetrical bow at the center, but pull the string from almost 2" above that(shooting split finger). 'Simply' put, pulling the string above center means the distance from the string fulcrum to the bottom tip is longer than the string fulcrum to the top tip. This makes the portion of the string below the string hand a longer virtual lever than the upper, giving the LOWER portion of the bow a mechanical advantage. This advantage means it needs less strength to do equal work(flex) with the same load, i.e. it should be tillered to show weaker at brace height (very generally speaking of course) in order for the bow to be balanced at full draw.
Staying with symmetrical, even-tillered bows for this example... Moving the string hand above or below center on the string shifts the advantage. It's immediate and simple to feel... and obvious to see what it means/causes as a result. The slightest tug on the string tips the bow. Move the string hand up, we give the bottom limb the advantage and the top limb tips towards us.... move the string hand down below center, and the top limb is given the advantage, causing the bottom limb to tip towards us. The way to balance the bow relative to this, to cause equal amount of flexing in the limbs, is to weaken the limb with the advantage.
"I always aim for a perfectly symmetrical tiller and don't leave the lower limb stiffer."
I don't leave the bottom limb stiffer either. I balance limb strength at full draw relative to how the archer will hold and pull, then the tiller measurements at brace height are whatever they end up being... always a result, never a predetermined guide/guess.
You said, "Now, if you leave one limb stiffer ("not weak enough"), it means the other (necessarily weaker) limb takes more than half of its share of the stress, and is strained more. More strain, more set. So shouldn't then the weaker (upper) limb take more set?"
Yes, IF we pulled the string from center, but we DON'T. Since in reality we pull the string away from center, the answer is NO. Not necessarily. Your statement ignores the physics of levers mentioned above. In addition to that, stiffer as viewed how? At brace? What does equal(or ANY) tiller at brace mean when one limb has a mechanical advantage over the other?
The strain the limbs ultimately realize is due infinitely more to where and how we grip the bow, where and how we grip the string, and the strength of the limbs relative to one another WHILE those factors are in play at full draw, than how they look compared to each other at brace... static... yet folks insist on using the profile at brace as their unequivocal guiding light to gauge/assume relative limb strength. That can be a critical mistake, in my opinion.
That braced profile so many sink their trust in, is practically meaningless to me during construction. After the bow is finished being tillered on the tree and immediately prior to taking it out to shoot, I take the tiller measurements for the first time... just so I'll know if the tiller changes during shooting in... but it virtually never does... because, ah-ha, I tiller the limbs to see equal 'relative' strain.
By the way, since I do, my bows don't take more set in the bottom limb. If/when they take set, they do so evenly along the bow's length. I could show you dozens of examples. As a result, never once either have I had to flip a bow end for end because the tiller shifted.
It's things like this, that cause me to question the degree of relevance of m.c. differences between top and bottom limbs... in other words, I FIRST wonder how such bows were balanced relative to the shooter at full draw.
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Pearl, I agree. As I said earlier, other things would have to be perfectly in place. One of those would be pushing limits.
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DWS-
I don't leave the bottom limb stiffer either. I balance limb strength at full draw relative to how the archer will hold and pull, then the tiller measurements at brace height are whatever they end up being... always a result, never a predetermined guide/guess.
Do you balance the limb strength by making sure that the tips travel the same distance to get to brace, and then again to get to full draw?
thanks
willie
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Willie, the simplest answer is yes, pretty much... but there are requisites and qualifiers that should either be set forth in your question, or my answer.
They don't necessarily measure equidistant from the front plane of the handle, or the same distance 'down the tree', if that's what you mean, but instead, I balance the limbs in strength so their tips travel an equal distance and resist each other with equal force, or very nearly so, relative to the archer's holds on bow and string. This ensures the nock point comes straight back relative to the shelf.
This is done either with the handle supported in the tillering tree so it doesn't move/tip, while adjusting relative limb strength so the nock point comes straight down a line drawn perpendicular to the shelf, or by allowing the handle to tip in the tree on its dynamic balance point while adjusting limb balance/strength so it doesn't... at full draw.
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DWS
I do appreciate you taking the time to explain the finer points of your method..
just to clarify.....
or by allowing the handle to tip in the tree on its dynamic balance point while adjusting limb balance/strength so it doesn't... (come straight down a line drawn perpendicular to the shelf)...at full draw.
Seems a bit confusing to me (not unusual), but if the handle is free to rock in the tiller tree, then the nock point must travel in a straight line, therefore the tips will waggle?... or should come back evenly without a wagggle?
thanks again for your patience
willie
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Willie, in your last post, where you quoted me, and inserted your underlined statement that you assumed I meant... it was incorrect. The way I meant it to come across was...
... or by allowing the handle to tip in the tree on its dynamic balance point while adjusting limb balance/strength so it doesn't -TIP- at full draw.
Sorry for the confusion.
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DWS-
I appreciate the time you have taken to share your expertise. and have been looking harder, with new eyes, at the balance on some of my recent tillering work.
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DWS,
Thanks for your elaborate answer. You’ve convinced me that indeed lower limbs can take more set because of higher strain due to the way we hold a bow, and I can now imagine this difference in strain is substantial. Always good to learn new things here on PA. :)
Ain't saying that moisture differences are entirely unimportant (see Badger's remark), but there's clearly much more than that to it.
Joachim
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I have a video I made that shows this. I'm shooting 2 arrows out of my bow and to aim both I have to rock my hand on the grip as I draw. Each limb bending separately a noticeable amount when watched in slow mo.
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I try to match the tiller on both limbs.
I use a rope and pulley and watch that both limbs bend and return in unison.
I pull from the center but finish tillering with the bow in my hands.
Initially, I just draw the bow and pay attention to how she bends. Is it tippy or even?
I'll use a mirror or a window at night.
The best, is to have someone snap a digi.
I aim for the bottom limb to be slightly stiffer (1/4") at full draw but settle for even.
There are a host of reasons why the picture perfect tiller will not work for some bows.
Might be a topic for another day.
Jawge
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Dances with Squirrels ,, how do you know you have never had a limb shift, have you been able to monitor every bow you made for 5 ,, 10,, 20 years,,, has anyone else with a different hold and release ,,,shot one of your bows for 1000's of arrows,, I am just curious, ,, I have never heard anyone state that,,, have any of your bows been exposed to extreme moisture conditions,, such as hunting in the rain for a week,, or two,,or have they been shot in controlled environment,, I agree with your basic approach,,,, just curious as to how you have come to that conclusion,, :)
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Can you do it by seeing where the arrow hits, up or down at say 20 yards with your own standard arrow, and adjust the tiller of the limbs to shift it to where you like it at that range and that bow weight?
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Brad, I can only speak for the bow's I can keep track of. But I have never had someone contact me and say the tiller shifted on their bow. I don't often make a bow for someone else, but when I do, I get as much info as possible regarding their shooting idioms, replicate them on the tillering tree as closely as possible, and balance the strength of the limbs relative to them. That said, I didn't mean that NONE of mine ever have, but that those since I have began balancing dynamically haven't. That has encompassed most of my bow making, btw. I understand that may have sounded like a bold statement, but it's true near as I can tell. Maybe I've just been lucky. I'm about to kick off some critical testing to try to better understand and explain the intricacies and will put findings out there as they become available.
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We would appear to be comparing apples and oranges.....
Of course we need to make a bow correctly balanced to accommodate how we pull them.....pulleys, various tillering methods etc are all fine and dandy BUT if you take that perfectly tillered bow and alter the mc levels one limb relative to the other you don't have a perfectly tillered bow anymore now do you?!?
JoachimM is dead right about the change in mc levels. If anybody thinks that a difference of 2% m.c. in a bows limbs doesn't make a difference then well......they are wrong and that's just a plain scientific fact. Easily substantiated by anybody who cares to do a little testing. Common sense can be used as well!
The fact is that that we need to look at ALL factors involved when making bows. The more experience we get the more different factors we should notice. If however some people think they have everything figured out then good luck to them. The truly wise man knows how little he knows ;)
A bow should always be laid flat if at all possible. How do you sleep? Standing up?
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Yes, common sense should be used.
"BUT if you take that perfectly tillered bow and alter the mc levels one limb relative to the other you don't have a perfectly tillered bow anymore now do you?!?"
Well, maybe. It depends, I think. Common sense tells me any potential effect of moisture differences in a bow's limbs would be relative to the actual moisture content(s) within the bow... meaning, the 2% difference between 12 and 14% may show a noticeable effect while a 2% difference between 7 and 9% may not. Common sense also tells me that any such difference(and/or higher level of actual average m.c.) might only have a noticeable affect on a limb if it was also already borderline overly stressed due to things like design shortcomings, or limb balance disparity... again, relevant to degree. I think common sense helps me remain objective about this as my bows are always stored vertically and their bottom limbs seem no worse for it.
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Assuming you're not flight shooting, wouldn't tillering for balance and storing consistently the same way keep a bow shooting consistently and reasonably for a person? I'm not talking about an extreme moisture differential or one that causes permanent additional set for a shot-in bow. Or commercial bows.
Wouldn't changing a storage method be more likely to change the balance?
Just in real terms under reasonable conditions a bow seems to adapt to the bowyer's preferences automatically, since how he tillers ends up fitting the relative stiffness of the limbs under his storage conditions. If a bow doesn't do that he will likely re-tiller or prefer and use another bow he made that does. In the end it's the survival of the fittest -- and most adapted.
Not saying here that horizontal storage might not be ideal. I'm sure it is. But not always practical.
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Jeff, I may have missed it but do you use equal length limbs?
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Pat, I usually make the top limb about 1 1/2" longer.
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Being a relatively newcomer to the building of selfbows I am very interested in any of these types of discussions. I value all the different opinions and try to learn from them.
Of the bows I have built. I have hand rubbed 5 to 6 coats of Truoil on them. Realizing that no finish is 100% waterproof, I wonder how much difference a 10% RH difference would actually make in a bows MC between lower and upper limb storing in a.verticle position?
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One thing I've seen happen to bows that are left alone for 6 months or so is they actually increase in poundage at tillered draw length by drying out more.Even up to 4 to 5 pounds more. It's really an opportunity I think to heat treat reflex /remove material and retiller the old bow.
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Of the bows I have built. I have hand rubbed 5 to 6 coats of Truoil on them. Realizing that no finish is 100% waterproof, I wonder how much difference a 10% RH difference would actually make in a bows MC between lower and upper limb storing in a.verticle position?
The finish just slows the moisture movement. The bow will still change MC, it's only the time that changes. Your results WILL vary :D