Stresses on a bow, reflex and deflex, wood capabilities

[RyanY]

I would guess that the reflexed bow is under more stress because less wood is being asked to do more work. If this worked without over stressing the bow then any bow could be made to draw any length till the string came off. But what we see is that they end up taking more and more set or breaking.

This is an easy experiment. Try it and let us know how it turns out. You’ll have to make sure the bow isn’t overbuilt though before reflexing.

[Allyn T]

You can't think of weight as the deciding factor in how stressed a bow is. Take your same thought but make it a 10" deflexed bow pulling 50lbs even though it is pulling the same weight it isnt stressed at all and won't shoot an arrow at hardly any speed. I think draw weight has more to do with arrow weight, heavier arrow heavier draw. Total tip movement has to do with stress but bow design and draw weight factor into that too

[beast]

yes allyn let me reply to what u said. u said a deflexed bow will be going under less stress right? but in fact because the deflexed bow will have to be nice and thick to reach  say 50 pounds compared to a d bow or reflexed bow which will be way thinner. and so allyn, if u pull a deflexed bow back, it will stack quicker. so the tips will do what we asked them to, so in a 30 inch draw, the 10inch deflex tips will now move 20 inches. but well find the bow will reach higher draw weight weigh faster past those 30 inches because the limbs are extra thick.
right now say we make a bow that isn't tillered and those limbs are a whole inch thick. now that bow wont be able to draw more than like 5 inches before exploding because the stress on the wood will be incredible, the draw weight of it will be very high.

right now I will test what I am thinking, right, now my intuition says obviously relfexed will break on me, but I dunno.

[beast]

well those mongols use sinew and horn to deal with their crazy recurves, well obviously they use those materials for a reason. but . eh. i . aw man this stuff is confusing me

[beast]

another thing, ya take a nice shaving of wood off the ground when ur working. you can bend that into a whole circle and it wont break because it is very thin

[bradsmith2010]

I have not read everything so if im repeating sorry,,
if your bow is reflexed 10 inches and you draw it to 28 inches,
it is bending alot further than a straight bow that is drawn to 28 inches,
so it is bending more ,, and stressed more,, bows are wierd, what seems to work in theory,, sometimes does not in reality,,

[bownarra]

Distill your question

[RyanY]

Beast, your use of terminology seems like it might not be consistent with most people’s understanding. Stacking comes into play when the bow becomes less efficient at storing energy later in the draw due to string angle. Bows can stack heavily without being over stressed. The wood stretching concept is useful here. You can have a very thick and narrow bow that pulls 50#@2” versus a thinner and wider bow that draws 50#@28” with the same strain/stress because the wood is being asked to stretch the same amount. The first difference is that the wood in the thinner bow is closer to the neutral plane and can bend to a tighter radius with the same amount of stretch as the thick bow. Second, the thick bow is storing significantly less energy as it is pulling the same draw weight over a shorter distance. The narrow short drawing bow will also have less mass due to the properties of a beam being stiffer by a cube of the increased thickness.

[beast]

right, well goodmorning.
ryan, i think ur wrong there. you said the thick bow and the thin bow drawing 50 pounds at 28 inches are under the same stress. no. the back of a wider bow has more area, as its wider, therefore those 50 pounds are spread out over a wider area.
the width makes a huge difference when it comes to stress.

right so ok, I thought stacking meant something else, I thought it happened when the material starts to fail, and the weight rises faster because of that. so forget everything I said about stacking.

ok, well I was only trying to bounce ideas off of you all, because all the recurves i made took so much set. maybe I need to stop working with whitewoods and start using wood that actually has decent compression strength.

right well what I learned is either I'm a knob or theory doesn't work with bows, like bradsmith said. but it should though. let me find new  wood to try a severe reflex on it, rather than the hickory I'm using.

ill let yall know when i finish some experiments. If i don't get blown up by an exploding bow.

[PatM]

White woods are fine.  Sounds like you are long on theory and terminology  and short on experience.   There  is no need to try to attain too severe a reflex with wood.   

[RyanY]

right, well goodmorning.
ryan, i think ur wrong there. you said the thick bow and the thin bow drawing 50 pounds at 28 inches are under the same stress. no. the back of a wider bow has more area, as its wider, therefore those 50 pounds are spread out over a wider area.
the width makes a huge difference when it comes to stress..

Yes under the same stress because they are being asked to lengthen the same amount. Width has to do with more stored energy. You are mixing up stress (how close it is to breaking) with stored energy. A beam of any given thickness can only bend to a specific radius before breaking regardless of width. Say it is 1/2” thick and can bend to a radius of 28” with 50# of force. If you double the width it’ll take 100# of force to pull to 28 with the same amount of stress/strain. I think you haven’t grasped that with differing thicknesses and widths that we are asking the wood fibers to stretch (or compress) as much as possible with each given bow without taking set.

[Allyn T]

yes allyn let me reply to what u said. u said a deflexed bow will be going under less stress right? but in fact because the deflexed bow will have to be nice and thick to reach  say 50 pounds compared to a d bow or reflexed bow which will be way thinner. and so allyn, if u pull a deflexed bow back, it will stack quicker. so the tips will do what we asked them to, so in a 30 inch draw, the 10inch deflex tips will now move 20 inches. but well find the bow will reach higher draw weight weigh faster past those 30 inches because the limbs are extra thick.
right now say we make a bow that isn't tillered and those limbs are a whole inch thick. now that bow wont be able to draw more than like 5 inches before exploding because the stress on the wood will be incredible, the draw weight of it will be very high.

right now I will test what I am thinking, right, now my intuition says obviously relfexed will break on me, but I dunno.

You are still using just draw weight as the only measure of stress. Thicker or thinner wood doesn't mean more or less stress. Thinner wood can bend further without taking as much set but that doesn't change the amount of stress. Wider limbs distribute stress and so do longer limbs. If the stress on a bow was just based on draw weight no one would have to change bow design for weight and wood species. If you blow up two ballons to 50psi but one balloon was smaller than the other before the air, they both would have the same amount of pressure inside but the smaller ballons would have thinner walls and be closer to popping(more stressed)

[RyanY]

I’ll add that a specific amount of wood is needed to do the work being asked of it. If you make a bow thinner (less mass) and expect it to do more work, it’s going to take set or break.

[diliviu]

More bend means more strain. And more strain leads to more stress (higher force per area in tension and in compression). For two 50 lb bows the tension force on back and compression force on belly are not the same if strain is different. The 50 lb bow that is more strained will deal with much higher tension and compression forces compared to the 50 lb bow that is less strained. The poundage of the bow (as a math function) is not a direct, linear result of tension and compression forces that appear when the bow is drawn.

[beast]

White woods are fine.  Sounds like you are long on theory and terminology  and short on experience.   There  is no need to try to attain too severe a reflex with wood.

but there is a reason isn't there. you want faster speeds, you need high early draw weight.

i think I'm being misunderstood here ngl.
forget reflex now:
think about this as Bow A and Bow B
Bow A is a normal bow with given thickness
Lets say Bow A pulls 50 pounds at 28 inches. alright now we reduce that thickness. now the bow A will pull like 35 pounds at 28 inches. lets call this new bow: bow B.

Ok. now lets think of bow A as being pulled from 6 inches of brace height to 28 inches.

And bow B, lets think of it working from 10 inches of brace height to 32 full draw rather than reflex because its pretty similar.

Sweet. now This bow B, pulled 32 inches back, should be under the same amount of stress as bow A pulled back 28 inches. please correct me if I am wrong.

we are not changing width or length here, only thickness.

again correct me if I'm wrong, but what determines a woods breaking point is its modulus of rupture.

hold up pause i just got a brain wave.

so is the reason why reflexes are under more stress because our back of a reflexed bow that is doing work is much thinner and therefore we have less volume of wood under the same amount of stress. because tension isn't spread across an area, its across the back of a bow which isn't just the back, its from the back to the middle of the thickness of the bow?? and now that volume is smaller because its thinner?