Primitive Archer

Main Discussion Area => Bows => Topic started by: Selfbowman on May 19, 2022, 05:56:59 pm

Title: Performance?
Post by: Selfbowman on May 19, 2022, 05:56:59 pm
Is there a formula  that determines mass of a wood to energy achieved? I would guess it would depend on design for sure. Also the mass theory might be what I’m thinking. ?????
Title: Re: Performance?
Post by: BowEd on May 20, 2022, 07:31:15 am
It could be.All I know is that I try to keep it wide enough inner limbs from fades to avoid set with the rest of the limb narrowed quite a bit.Whatever that weight turns out to be it is.At last with the designs that I like and what I want from a bow.
I seem to come in with bows that are at the lower end of the mass theory even with reflex.
Title: Re: Performance?
Post by: bradsmith2010 on May 20, 2022, 03:45:18 pm
yes make alot of bows,, and you will have the formula,,, ;D
Title: Re: Performance?
Post by: Selfbowman on May 21, 2022, 11:00:04 am
yes make alot of bows,, and you will have the formula,,, ;D

I’ve made close to 300. And the thing Ive noticed is the closer to perfect  diminishing mass I get the faster and smoother they are. Leaving very little extra on the ends. But yes Brad you have a very good point. I think more of a science question that I probably would not understand the answer anyway .
Title: Re: Performance?
Post by: RyanY on May 21, 2022, 04:00:04 pm
This could be measured in the same way as modulus of elasticity where you get the properties of a piece of wood and know how much energy it can store per mass at a given percent strain.
Title: Re: Performance?
Post by: Selfbowman on May 21, 2022, 06:18:37 pm
I was right I would not understand the answer!🤠🤠🤠
Title: Re: Performance?
Post by: bradsmith2010 on May 21, 2022, 06:25:36 pm
me too,, if it shoots good, I go with that,, :D ;D
Title: Re: Performance?
Post by: Jano on May 22, 2022, 08:16:46 am
You already get similar ( like from RyanY ) answer from Avcase in your thread "Is the early wood the weak link to cause set?" on the bottom of the page 20 : "The amount of energy that a material can store in bending is a product of the elastic modulus and amount of strain or deformation it can handle before taking a set.  It can be taken a step further by throwing in the density in order to get a property that rates materials as ft-lb of energy per ounce of material.", which is probably more understandable     :-)     .

IMHO - for rapid and good repeatable results - one have to start with such measurements of the appropriate sample of prepared wood stave/billets, then using these values to design your bow in some software ( Supertiller, VirtualBow ... ) to achieve desired properties and continue with shaping the bow by this model measurements ( with reserve ). As wood is not very homogeneous material, you cannot rely completely on theoretical dimensions and have to continue in tillering by bendmetering, to be sure you are keeping the strains in optimal ranges. ( Good description of this method is in this book : 

- please, see the pictures slightly above the bottom of the page - BTW I advice to rather buy - more handy - digital dial indicator for this purpose . Free quote of homebody's contribution somewhere : “Tillering Gizmo is only simplification of a bow-making bendmeter, first devised by Russell Willcox in the 1920’s. Russ and others refined it over the years. He used it in tillering his Duoflex bow … " )

Bendmeter can be very precise, hence you can determine exact strain values at every “station” on the bending limb and “tiller” it to be precisely equally stressed ( or what stress distribution you choose ) along its length in accordance with the diminishing thickness ( elliptical tiller ). At the beginning you have to measure initial offset ( on each station along the limbs ) on non stressed bow, then the deflection and the thickness on actual station on ( partially ) stressed bow and then compute the actual strain value ( in percent ) at this station. And so on for all stations and tillering stages .

 I used bendmetering several years ago on my wooden bows . I measured the bow only at about half-draw to eliminate string follow caused by long time spent at full-draw position, when measuring it by bendmeter ( shape of limb bend and strains at this half-drawn position can be resolved through mentioned software ).

With true bendmeter it is possible ( I hope ) to tiller the bow by amount of set/string follow in each station along the working limbs ( and by this way eliminate long time measuring on drawn bow ) , because it is so precise ( I never did it ).

Or, maybe ( only my fantasy ), if you want to stay on primitive way, you could observe the chrysals on intentionally highly arched belly of the bow and when they are placed everywhere evenly at full draw, you could remove the damaged wood ( slightly more ) without changing the drawn shape of the bow and you will obtain "evenly" strained bow. 

Bendmetering is quite “boring” and “longlegged” method of tillering ( nowadays facilitated by spreadsheets on computers ) , but gives very reliable results ( even for novices ) and should be used at least to enhance flightbow making if too laborious for common bows.

Guess, I was not very boring ( sorry for my English and lengthy contribution ) and hope, that more contemporary bowmakers will be interrested in this “old masters technique” again.
Title: Re: Performance?
Post by: Selfbowman on May 22, 2022, 11:36:54 am
Good read Jano!  So keep practicing Arvin .🤠🤠
Title: Re: Performance?
Post by: Jano on May 24, 2022, 06:48:51 am
Thanks, but someway I am not ( able to ) making bows for longer time already    :-\    .

I would like to mention, that one can use bendmetering without previous steps. Only scribe front profile on the back of the stave, reduce it to this width and by reducing the thickness by this method ( it is enough to start measuring when the bow is half tillered - but first step of initial offset measuring must be done on unstressed bow of course ) one will obtain tillered shape of the limb automatically ( no hesitations whether "is the shape elliptical enough " or so ).

At my beginnings I used simple "triangle ruler with perpendicular scale scribed on its center" - which was quite cheaper, than dial gauge - for measuring on the straight lines scribed ( before stressing the wood ) on both sides of the bow blank reduced to right width ( front ) profile. But by this way the calculations were quite imprecise - anyway, better than my untrained eyes.

I am still quite curious, if there are some people who are using this method ( I supposed, that majority of experienced bowyers know about it ) ?
Title: Re: Performance?
Post by: BowEd on May 25, 2022, 03:31:34 pm
To back what Arvin said it seems to me the more I can get shorter working limbs wide enough to handle the strain and less wood where it's not needed for power the more efficient cleaner shooting bow I get.
It's a fine line to accomplish I might add.
This seems to oppose the thought of getting the full length of a working limb to do it's fair share of the work,but in reality the outer limbs are just shaped differently with less mass and still will share the load enough to relieve the inner limbs.
I would'nt say this without pulling a lot of shavings and making a lot of saw dust and testing.
I can only imagine yet the amount of wood and bows Badger went through to get where he's at.
Title: Re: Performance?
Post by: bradsmith2010 on May 25, 2022, 05:51:24 pm
when you start shooting through a chronograph,, some things that work in theory,, kinda work,, but not as well as some would think,,thats a great example,, making the wood work where it needs to ,, not necessarily, all "evenly",,,but getting good performance from the bow,, or as you mentioned before, taking off mass where its not needed,,,
Title: Re: Performance?
Post by: RyanY on May 26, 2022, 08:12:13 am
Ed, often that distribution of work and mass correlates to less working limb and therefore less limb vibration as well.
Title: Re: Performance?
Post by: BowEd on May 26, 2022, 09:48:08 am
Yes...It's a win/win formula.
Title: Re: Performance?
Post by: Jim Davis on May 26, 2022, 11:04:52 am
Jano, I am glad to find someone who knows the wheel (or the bend meter) was invented a long time ago! I made a dial bend meter a couple of decades ago and used it successfully. You are right that it is boring work!

After that period, I have made nothing but "pyramid" bows and find them exponentially easier to make and good performers. Cut the back profile, band saw to about 9/16" thick, belt sand and or scrape to even tiller and it's done. For a stronger bow, start wider. to reduce weight, narrow by the percent of change you want (that last works for any design).

Robert Elmer's writings are among the best. He was a very intelligent and careful man. I'd have been glad to have had him as my doctor.
Title: Re: Performance?
Post by: Jano on May 28, 2022, 04:20:55 am
As I wrote before, one can scribe whatever front profile and choose any stress distribution and then make the bow after this model. But everytime you will get charts of data ( with achieved strains along the limbs ) useful for comparing with following bows. After an experience with bendmetering I felt quite "blindly", when not using it in  bow tillering. 

Jim, I am glad too. My last selfbow tillered by bendmetering was made in 2014. These bows never broke   :)  , hence I had enough of them for use.  From this time I am interrested in ( studying the theory mainly ) hornbowmaking. 
Title: Re: Performance?
Post by: Selfbowman on May 28, 2022, 08:56:27 am
Jim I agree about the pyramid. It’s easy to distribute the mass evenly. This puts all mass where it needs to be. Yes m a firm believer that perfect diminishing mass will be the fastest smoothest shooting bow you can make. Now you throw recurve or flipped tips into the picture and it changes the mass distribution some but the gains are more beneficial than the extra mass to a point. And yes it’s a fine line where the benefits stop .