Sinew backing preventing string follow questions...

[ssrhythm]

I tend to overthink things, but I don't think it's possible to overthink the physics that determines what you end up with when you set out to make a self bow.  I have read TBB vol 1 and use it like a field guide in the garage.  I have read Clay Hayes' bow building book.  I watch a lot of youtube also.  It is my understanding that the physical crushing of wood cells somewhere along the belly is what causes a bow to take take set.   Cells getting crushed could be the result of 1. pulling the bow when too wet (I assume the dryer the wood, the stronger the cells...why heat treating strengthens the belly and gives cells more resistance to crushing), 2.  pulling too far too soon during tillering, 3. having uneven tiller resulting in a weak spot or hinge that bends too much during tillering, or 4. having uneven limb thickness from one side to the other in an area of the limb. 

Things about this that make me scratch my head and make me think I'm not understanding what I think I'm understanding...or make me realize that I'm not fully understanding it...need more info.

1.  too wet...easy...don't start the process of tillering until the bow has reached moisture equilibrium.

2.  Pulling the bow too far too soon in the tillering process.  The idea here is never pull bow past intended draw weight and you will be good to go.  So, can you not build a 66" long, 1.75" wide flat bow to be 90#@28"?  We all see in various books and publications that you should build your bow to be at least twice as long as your intended draw-length.  You read and we know that weaker woods need to be shaped into wider, longer bows.  I don't think I've ever read the correlation between intended final draw weight and length/width.  Is an 80# at 28" bow of the above proportions destined to take a good bit of set vs a 60# bow at 28"?  How would you make a higher poundage flat bow design without crushing belly cells?  Another way to look at is....If I am after a 55lb flatbow and I never draw the bow past 55# thru tillering, and I get it bending evenly and beautifully and it pulls 55# at 28"....then I decide I want the non-stacking smoothness of a recurve and I recurve the tips resulting in a 68# bow...have I not basically done the exact same thing as I would have by drawing the bow (and bending the limbs) to much higher poundage than my intended weight during the initial tillering?

3.  Uneven tiller resulting in a hinge... I get this.  I get that this is why you never draw past where you notice any weak or strong spots in the arc.  Once you identify a hinge, you are limited to what poundage that hinge spot will give you once the stiff areas on either side are reduced and even tiller is achieved.  If you crush belly cells at the hinge, you are stuck with set or you will be limited to the poundage that hinged area will give you after reducing everything to proper tiller and removing the crushed belly cells...ie. its going to be a way light bow.

4.  Uneven limb thickness from side to side at any spot on the limb.  This one messes with me too.  I know this must happen from time to time (on osage at least) because of having to work with blemishes that happened during growth, but in an ideal world...Is there any situation where it is acceptable or desirable to have a portion of the limb cross-section that shows uneven thickness from side to side?  I ask because I have read  somewhere that you can use this technique in tillering to counter natural limb twist to get tips alligned.  It seems to me that if you did this and anytime you have uneven thickness from one side to the other at any point along the length of the limb, the weaker side would bend much more than the thicker...and cause the belly cells there to crush and take a set at that spot.

Any my last question...if my understanding of all the above is correct, then how is it that sinew will prevent string follow or how will sinew correct too much set?  I get that sinew will shrink and pull a bow into reflex.  Sinew backing also raises the tension/neutral/compression planes of the bow toward the back...so once the sinew is applied an you have a bow with really nice reflex...when you go through the tillering process, if the crushing of belly cells is what causes the set, then wouldn't any of the same issues cause the sinew backed bow to take set also.  If we are relying on sinew tension forces to actually overpower the crushed belly cells tendency to pull the bow into deflex, then shouldn't we always wait until a self bow has been shot in and taken the set its going to take before applying the sinew?

Any insight into any of these ideas that twist my brain will be much appreciated.

[bradsmith2010]

   ok im over simplifying ,, if you have an applied back,, some wood, and its too thick it will crush the belly cause it doesnt stretch enough,,
  sinew will stretch more than wood so it does not crush the belly in reasonable design,,
so you dont want to make the bow take set and then put it on,,
put it on so it reduces the crushing effect as you tiller,,
someone will chime in and explain it better,,
   if you take the sinew off a shot in bow, ,the wood part will not have take much set at all,,because of the way the sinew effects the bow,,
   that being said,, you can put sinew on a shot in bow, and get good results, but I think you will get better results putting it on before it is shot in,,there are just so many variable its hard to explain,, I have found that if you draw a sinew bow too far,, past the point it was designed for, ,and there is not enough sinew, it will take set too,,
   you can see it when you are test with a chrono,, for example if you draw to 25 inches and you get 170 fps,, and then tiller it to same poundage at 27 inches and get same fps,,,the bow is probably taking set and you have gone past the sweet spot,, or the set the bow is taking is keeping it from shooting harder at the longer draw,, these are just observations from my experience, and may not apply to some bows 

[ssrhythm]

 Well dang...you’ve now introduced a physical concept that I have not even thought of.  So...you are saying sinew back will stretch more...thus not result in as much compression force to belly.  That has to be due to the raising of the neutral plane and resulting increased overall mass of the compression plane or total amount of wood resisting compression...because regardless of how much sinew will stretch...won’t the old back bend approximately the same?  Wait...ok...nope, I’m still confused!😬

[PatM]

You've got the idea for sinew.

 As far as 2 goes if a bow is relatively overbuilt or at least optimally built for a higher weight the whole don't pull past target weight doesn't really apply.  It does still get tossed out there as if the bow "knows"  I have addressed this on here before and  even seasoned gurus just repeat the mantra and ignore the original point.  I felt like I wasn't able to make them understand the point and gave up.

[bradsmith2010]

Pat would you explain that to me in a different way,, I want to understand your point,,

[PatM]

I presented  it more or less the same way as the OP.   

   A wide limbed longer bow can handle being made to high weight so it doesn't really matter if you draw it past target weight

[ssrhythm]

I’ll give it a shot...and might talk my way into understanding it.  The rule of thumb is “don’t pull the BP past intended final weight and everything will be cool regarding not inducing set.  I think this is misunderstood...it makes sense not to stress wood any more than necessary, as this would result in less failures of various sorts all things being equal.  That said, if a bow you are tillering is bending without hinges or overly stiff areas...if it’s bending well, then you should be able to pull it to 28” at 75# without it damaging a thing.  After all, I’ve never seen anyone say that you can’t make a 66” long 75# longbow.  Right?  If you are after a 55# bow, you should be able to reduce the weight to 55# by taking even scrapes in belly and keeping the tiller good.  It seems to me that it would be much easier to overshoot wood removal snd end up with a light bow if you never pull it past  intended draw weight or past intended draw length....aiming to fix tiller issues and end up at 55# at 28”.  Seems like it would be better to try to get the tiller correct with an overbuilt bow then slowly reduce the weight with conservative scraping while checking the tiller often making sure you maintain the bend profile you want.  Heck...IDK. All I know is tag I’m going to make my next bow 1/4” wider than I’ve been shooting for and work like crazy to have a perfectly uniform side to side thickness with a consistent handle to tip taper...I may sinew it.  Beyond that, I don’t know what else to do to minimize set.

[Badger]

   I agree with the above posts. An easier way to comprehend what sinew does is to maybe think of yourself making a 40# bow and then adding sinew for another 10#. The belly will have somewhat less strain on it. Sinew can hide a lot of set simply by pulling the bow back into shape.

   As for your other questions I think they are all good well thought out questions. I never could answer the width length question so I started using mass as my guide. I foind it works out very well at guiding me into the right widths I need to work with but does require an educated guess getting started and I have also found some unexplained contradictions to it that I doubt I will ever delve into and try to solve. One of them is making a bow extremely wide which is something we usually assume will add more mass but in many many cases the extra wide bows are coming in at lower mass and much thinner than I would have calculated. My first educated guess on that is we are doing more damage than we think when making a bow. That is the only explanation I can come up with. A wider thinner bow will obviously take less set but also seems to be an efficient use of wood.

[willie]

my understanding of "overbuilt" is that the bow is not working as hard as the wood is capable of being strained.

I’ve never seen anyone say that you can’t make a 66” long 75# longbow.  Right?  If you are after a 55# bow, you should be able to reduce the weight to 55# by taking even scrapes in belly and keeping the tiller good.

in this case, the 55# bow would be overbuilt. (presuming the bow never took any set at 75#)

if you follow the "never pull past the intended weight rule", and you have correctly estimated the capability of the wood when setting the draw weight goal for the design, and the bow just begins to start taking set at your draw length, then you have done well.

if you can draw it past your desired draw length without taking set, it is overbuilt for the lesser drawlength

if the bow takes more set the further you draw it, then it needs to be thinner for that longer drawlength.

having established an optimum thickness for a particular drawlength, the only way to have a higher poundage bow without set is by making it wider

Beyond that, I don’t know what else to do to minimize set.

from what I have experienced, not pulling past the intended weight during tillering is just a matter of being careful. The part that takes patience and requires an eye only developed by experience, is following the admonition to not pull past the point where the bend is uneven. A pyramid limb shape bends in an arc, and is an easy design choice to learn with. The gizmo or an arc traced on a piece of cardboard behind the bow may be used for reference,

[bradsmith2010]

or longer??

[willie]

yes, or longer

Overbuilding by going longer than necessary gives an option to pike if no set is taken before attaining the draw length goal, whereas reducing width once the desired drawlength is attained requires you to tiller for a higher draw weight as ssrhythm proposes.

My preference would be to tiller for a desired weight at a longer length, as setting a weight goal on the high side could be more problematic.

[druid]

Irregular limb cross section is thing I use very often to balance some things- twist, crooked limbs etc. Always work good but you have to be careful.