Drawing core without corn/sinew (->Broken core?)
I have thought that a horn bow core is so thin that it allows it to bend to full draw, when the bulk of the energy is stored to sinew/horn.
I made some simulations using woodbear bow calc, about core without horn/sinew( it can't simulate those). It was very close to breaking point on itself using Moshcheveya Balka, which is not very agressive design.
(This results may also be due to the fact that the program was working beynd its limits, I mean the program is designed to model self bows, however i think it will show to optimistic results to these bows, not pessimistic.)
So, is it normal that a horn bow core can be drawn to maximum draw length,without horn and sinew? Or do they somehow decrease load from the core? I thougt the horn and sinew store the 99% of the energy and counter each other and core works exacly as if there was no backing, but obiviously it is not so.
I made some simulations using woodbear bow calc, about core without horn/sinew( it can't simulate those). It was very close to breaking point on itself using Moshcheveya Balka, which is not very agressive design.
(This results may also be due to the fact that the program was working beynd its limits, I mean the program is designed to model self bows, however i think it will show to optimistic results to these bows, not pessimistic.)
So, is it normal that a horn bow core can be drawn to maximum draw length,without horn and sinew? Or do they somehow decrease load from the core? I thougt the horn and sinew store the 99% of the energy and counter each other and core works exacly as if there was no backing, but obiviously it is not so.
Sinew is where most of the elastic energy tends to be. While sinew and horn carry the main loads the wooden core is under the same strain as it would be without the belly and backing, well almost as the neutral axis tends to shift a bit because of the horn.
Sounds like the program is used outside its application limits as you stated.
TM
Sounds like the program is used outside its application limits as you stated.
TM
So you have drawn a core to full draw without horn and sinew or know reliably that it should be possible on normal bow core?
Yep, the core can be draw full length. It's so thin that it almost can make a circle, it has no draw weight.
I did a bow that way, first a full tillering on core wood, then glue on sinew back and last the horn belly. All did work but not my glue between the horn and wood, so it broke.
I did a bow that way, first a full tillering on core wood, then glue on sinew back and last the horn belly. All did work but not my glue between the horn and wood, so it broke.
Hello all,
recently I decided too, to try personaly, whether horn bow core can be drawn to maximum draw length ( without horn and sinew ). Results were more than positive.
I used selected maple wood - harvested from the site near my home ( very probably Acer pseudoplatanus ) which was previously "simply bend tested" , if it is not brittle ( the wood bending propperties of this species can vary very much, as I ascertained "hard way" ).
I made only half of "generic" horn bow core ( from scrap-surplus wood ) with length of 60 cm and thickness from around 6,0 mm to 4,6 mm in the bending part, so it is not so thin . Width is 27 mm . Length of sal is 25 cm . It follows the surface of growth ring on the back side ( actually it is made of one growth ring in the sal part ). I tried to tiller it ( roughly ) for even strains in the central part of sal. I chose brace height of 18,5 cm and half of bowstring length of 54,5 cm and the core was drawn up to 81 cm ( 32 inches ) draw length. I did not measure draw weight.
I was very surprised by its capability to withstand bending strains of more than 2,5 % ( 2,7 % in most bending part of sal) - but without set subtraction . The core had considerable set after bending ( it was not completely straight from the start of testing - there was slight "positive bend" ).
Then I decided to test small sample ( with thickness of 5,9 mm ) for the breaking strain limit ( at 37 % RH / 21 deg of Celsius ) and it could withstand strains slightly over 6 % before breaking ( if my computations are OK ), which sounds almost unbelievable ( compared to published charts I have seen in books ).
Here are the pictures :
The core at standard draw length.
Even longer draw of 81 cm.
Breaking test sample.
Jano
recently I decided too, to try personaly, whether horn bow core can be drawn to maximum draw length ( without horn and sinew ). Results were more than positive.
I used selected maple wood - harvested from the site near my home ( very probably Acer pseudoplatanus ) which was previously "simply bend tested" , if it is not brittle ( the wood bending propperties of this species can vary very much, as I ascertained "hard way" ).
I made only half of "generic" horn bow core ( from scrap-surplus wood ) with length of 60 cm and thickness from around 6,0 mm to 4,6 mm in the bending part, so it is not so thin . Width is 27 mm . Length of sal is 25 cm . It follows the surface of growth ring on the back side ( actually it is made of one growth ring in the sal part ). I tried to tiller it ( roughly ) for even strains in the central part of sal. I chose brace height of 18,5 cm and half of bowstring length of 54,5 cm and the core was drawn up to 81 cm ( 32 inches ) draw length. I did not measure draw weight.
I was very surprised by its capability to withstand bending strains of more than 2,5 % ( 2,7 % in most bending part of sal) - but without set subtraction . The core had considerable set after bending ( it was not completely straight from the start of testing - there was slight "positive bend" ).
Then I decided to test small sample ( with thickness of 5,9 mm ) for the breaking strain limit ( at 37 % RH / 21 deg of Celsius ) and it could withstand strains slightly over 6 % before breaking ( if my computations are OK ), which sounds almost unbelievable ( compared to published charts I have seen in books ).
Here are the pictures :
The core at standard draw length.
Even longer draw of 81 cm.
Breaking test sample.
Jano
The sinew and the horn are needed for to enforce the tension and compression capablities of the wood, otherwise a hornbow could be done without any reenforcements.
So it is somehow useless to test the breaking strength of a wooden core.
I draw a wooden core about 50% of the final drawlength just for to pre-check the tiller.
Core- wood for hornbows has to be very elastic and open- porous at the same time. Mountain- maple is the best choice, it is the wood with the best glueing- capablities and it is very elastic.
Michael
So it is somehow useless to test the breaking strength of a wooden core.
I draw a wooden core about 50% of the final drawlength just for to pre-check the tiller.
Core- wood for hornbows has to be very elastic and open- porous at the same time. Mountain- maple is the best choice, it is the wood with the best glueing- capablities and it is very elastic.
Michael
Jano, this is very interesting. Years ago, I made a similar test of a wood lath only 3mm thick (not a bow's core) and equilibrated at 50%RH. trying to determine the breaking strains. The lath did not fail at around 3% surface strain. It was bent over round forms of known radius of curvature, progressively smaller. I also found it hard to believe. Laths with a rounded back/belly should probably withstand even larger strains.
Definitely something to think about and a puzzle for the engineers.
Adam
Definitely something to think about and a puzzle for the engineers.
Adam
Congratulation, Jano. Now you have your "test". I remember how we talking about year ago . You search some material for this test. Use original core material is best choice.