Arvins 62" osage design

[Selfbowman]

Ok this is where I get lost and y’all get frustrated with me.🤠if the bow has over a hundred % efficacy why does the arrow go slow? Do we need to change arrow weight?

[sleek]

How fast is it with a 450gr arrow? Also with a 150 gr?

more...
hmm the projected arrow velocity with 190gr arrow seems low. Should we look to tweak it up, Arvin?

[sleek]

Ok this is where I get lost and y’all get frustrated with me.🤠if the bow has over a hundred % efficacy why does the arrow go slow? Do we need to change arrow weight?

A bow can store all the energy it wants, statically. Its dynamic efficiency is what puts speed to the arrow. It seems that the dynamic efficiency and static efficiency are on opposite sides of a sea saw.  Its hard to make them play well with each other.

[willie]

Ok this is where I get lost and y’all get frustrated with me.🤠if the bow has over a hundred % efficacy why does the arrow go slow? Do we need to change arrow weight?

you must be looking at the 109 energy storage factor on the charateristics page? 

According to the manual

• Energy storage factor: This value indicates how good the shape of the draw curve is in
terms of energy storage. It is defined as the energy stored by the bow in relation to the
energy that would have been stored with a linear draw curve. The energy storage
factor increases the "more convex" the shape of the draw curve is.
• Factor < 1: The draw curve stores less energy than a linear draw curve
• Factor = 1: The draw curve stores as much energy as a linear draw curve
• Factor > 1: The draw curve stores more energy than a linear draw curve

a way of quantifying the "hump" in the force draw curve I think

so you went from carrying the 2.6" to 35% of the limb length to a Pyramid?  Is it more efficient?

I am not sure why the original plan included the straight section. Maybe as a precaution to handle the extra stress trpically induced by a recurved tip? 

I asked arvin if we should compare two designs with and without the straight section.  The elimination of the straight section seemed to reduce stress a small amount if the ramp down in thickness just past the fade was made a bit less abrupt, and it definately makes for a less complicated limb taper for the builder.

about the arrow speed:
Stressing the limb more will make the arrow go faster, all other things being equal,  stressing the limb less will avoid the possibility of set or unseen damage from compression. Determining a working stress for the materiel thats not too high or low seems to be the trick. but determining  what that ideal stress level is,  comes from experience and the quality of the wood.

[willie]

How fast is it with a 450gr arrow? Also with a 150 gr?

150 gr  237 fps
450 gr 178 fps

the 50# draweight is at 25" from the back of the bow and with a brace height of 7"
when the bow is built we plan to offer feedback to Stefan, the developer of the program. Hopefully he will comment about other observations of the accuracy of dynamic functions. 

[sleek]

How fast is it with a 450gr arrow? Also with a 150 gr?

150 gr  237 fps
450 gr 178 fps

the 50# draweight is at 25" from the back of the bow and with a brace height of 7"
when the bow is built we plan to offer feedback to Stefan, the developer of the program. Hopefully he will feed back with some other observations concerning the accuracy of some of the dynamic functions.

Make it a 26 inch draw from the back of the bow if you can. 450 grains us a benchmark I use, and if it doesnt hit 185fps minimum i wont shoot flight with it. But I also pull 26. So I need 26 to see a good comparison.

[mmattockx]

Hi Mark,
if you dont mind trying the calculator in autocad that would be nice.
The limb crossection at the fade the time looks like a segment of a circle who's radius is 3.5" on top of a rectangle, both elements  being 2.6" wide. (my sketch here shows the heigth of the segment to be about .28" and the "equivelent' rectangular section from Virtualbow is 2.6" wide by .42" thick, but that thickness is still tentative and may not work further down the limb as there is a gentle thickness taper in spite of the bow being a pryamid back, thus It would be interesting to see if that composite section can be compressed in the vertical direction without the radius of gyration changing.

I will try to get a look at this in the next couple days and let you know.


Mark

[Selfbowman]

What spine do we need on a 24” 150gr arrow for this bow. Flight arrow. Also if we where to shorten the working part of the Limb to 24” from center of bow. I think I use a 5”-6” brace or 6-1/2 -7” from back of bow.where did you measure brace from? I think in this scenario measure to back of the bow will be best.

[willie]

the program inputs brace from the back at the handle to the string, So with a 2" thick handle, 5" "inside to inside"

I  will try to anwser the part of the question that I think you were asking me, maybe the arrow part was for sleek?

Also if we where to shorten the working part of the Limb to 24” from center of bow.

Shortening the working part of the limb means some other part of the limb has to work harder if you want to keep the the draweight at 50#.  Kinda like playing whack-a-mole if you are concerned about stresses getting too high someplace else.

right now, the limb is bending significantly 5" out from center and is fully working 6" out from center.
 the limb starts getting thicker 22 inches out from center but is still working.

the first screenshot below shows the stress at brace, and the red line is drawn at the maximum stress (just under 10,000 PSI as read on the scale on the left hand side.

the same line is shown in the second screenshot of the limb at full draw. The maximum stress has risen to 16,000 PSI and the area above the red line represents some of the area where the limb is doing the work that will be returned to the arrow upon release. If I could over lay one pic on the other you would see a better representation.
The green line shows how much the stress has risen out at 24 inches from the center, but the limb is only an inch wide there, so the work contributed at 24 inches out will be just over a third of the work done at the fade which is 2.6 times wider.

Normally, when one tillers a bow and starts to see set or loose early draw weight, one just reduces the weight goal and calls it another 35 pounder for the ladies.
what do you think about if you can come in at 50# without set, or said another way, the bow is basically overbuilt.....

putting some more hook into the recurves and taking a few scrapes to get back don to 50#, untill the arrow speed comes up?
I dunno how many times you can rebend the tips but you are our master osage bender......

[Selfbowman]

Willie I have not reached the full recurve design and yes I can heat it up a few more times to get there. I’m waiting to see the final design thickness to proceed in that recurve section. Right now we have 56 in thickness 26” from center of bow . that is the thinnest part of the limb.

[willie]

that thickness should be enough, maybe too much ,but I would not do any thickness reductions anytime soon.

the program has no way to account for any loss or addition of stiffness if we heat bend the wood or heat treat to set a bend.  being too thick out there in the non bending section is not going to change the limb design elsewhere on the limb appreciably,  so leaving it "as is" will be good.

maybe you typically set the bend with a scorching heat treat with most bows?, but maybe not doing that right away would be a good idea if you want to increase the bend later in the build.

taking any excess mass off the tip to reach a final highest possible arrow speed should be the last thing done, after (or before?) the hotter?  heat treat to set the final bend.

just to get ahead of ourselves, I will see what a 3" hook does to the design as it stands

[willie]

to anwser Arvins earlier question about what happens if we make the outers work less and become more like levers, I adjusted the model. and might have to change my earlier opinion.

Directly below is the stress plot shown earlier with more bendy outers,  and below it is the stress plot for the same design with stiffer outers. the stress increased 1/2 of 1% but the arrow speed went up from 222 to 236.  quite an improvement for a modest increase in stress.
The difference is not easy to see at a glance, but a careful inspection arount the 24" mark will show less stress (and bending) and a straighter slope to the nock.

[willie]

bending the recurve to 3" behind the handle from the 1.5" in the previous improved design raised the stress and increased the poundage from 50.9 to 55.9.  But Arvin needs to stay at or below  50# to stay in the class. 

Taking a few scrapes to bring it down to 50.1#  and yields a bow with 10% more stress than the 1.5" recurve and the arrowspeed increases from 236 to  ummmm... 229.

there could be any number of explanations. off the top of my head is
I am not doing something consistently with my models
I need to input a more realistic wood density
something with the new string angle is not helping
the tips are too thick and hence too heavy.

below is the stress plot for the 3" recurve and the arrow velocity during the powerstroke for both recurves. 

the 190 gr arrow shot with the smaller recurve seems to leave the string sooner and comes up to speed quicker. 

note how at the point of the highest velocity the curve seems to go flatter. this would be consistent with the observation that its hard to eek the last few fps out of a light arrow, and there are considerations that the program does not account for in addition to hoping the program for arrow  velocity dynamics is accurate at the edge of the envelope.  interesting to play with tho.

[Selfbowman]

Willie try a 150 gr arrow though the new design please.Willie I’m curious about if the design has a specific need for a certain weight of arrow to have the most speed from the bow. It would be a pain probably to do but try 140-200 gr arrows through the design . Any design. Do it in 10 gr increments to see at what weight the arrow needs to be the most efficient.  Please explain to me in simple terms if this would or would not be useful.

[Selfbowman]

I’ve been using my trusty contour gauge and I have all 4” increments the same on both ends of the bow. Well the one right at the fades have a slight difference. All the rest match. So if I can get the thickness from each 4” increment right we should be good to go.