AVCase design Arvin bow build

[bradsmith2010]

   when you glue lams together,, it is very different than a self bow,, perry reflex is a good example,, the glue enables the bow to hold a reflex a self bow would not,, and if the backing is too thick it will crush the bow,, the glue lines and lams are different than a self bow, even though wood is wood,, the glue changes that,,
   its seems to me if you pull a bow on a pully there is one pressure point on the string,, if you shoot the bow then there are usually three fingers on the string,,, that makes the pressure different on the limbs,,, and the tiller may need to be adjusted for that,, I think thats why when the bottom limb is a bit stiffer it compensates for that and makes  good arrow flight easier to achieve,, if the shooter does not have a good release,,,the tiller wont make much difference,, but as the shooter becomes more proficient,, the difference is more clear,, especially on shorter bows,,,

[Selfbowman]

Hey Arvin, how wide and long is that bow?
[/quote

1-1/2 to mid limb or there abouts. 67” ntn

[mmattockx]

the glue lines and lams are different than a self bow, even though wood is wood,, the glue changes that,,

I keep seeing people say this, but no one has ever convinced me this is correct. Glue lines are very thin and the glue should be as strong and stiff as the wood if the bow is to survive, so why would it change how a limb bends? I agree using lams allows techniques that aren't available with a solid piece of wood, but if you cut strips off a board and then laminate them back together why do you think the properties have changed?

   its seems to me if you pull a bow on a pully there is one pressure point on the string,, if you shoot the bow then there are usually three fingers on the string,,, that makes the pressure different on the limbs,,,

I agree. How a particular archer applies finger pressure can affect the tiller a bit and a bad release really does screw everything up.


Mark

[avcase]

The difference between a 50# design and 35# real bow is too great to easily explain away with a typical amount of variation in material properties.

Difference #1:  Engineer used wrong material properties (-6 pounds draw weight)
I went back through my tests of various osage material samples, and compared those material properties to the material properties I actually used. Guess what I found?  The engineer (me) was not using the same properties in my model as I had measured! This explains a little under 6 pounds draw weight difference.

Difference #2: The real bow had about 2” reflex instead of 2.75” in the design. (-2 pounds draw weight)

This explains about half the discrepancy. I would have expected the real bow to be around 42# at 28” after correcting for these discrepancies.

So, where is the rest of the lost draw weight coming from?  We will know more when we measure the material properties of the pieces from Arvin’s stave.  I don’t expect this to be a major contributor.  I’m sure it would be very noticeable if the stave was significantly sub par.

A couple more pounds may have been lost if the bow took some set.  There is still at least 5 pounds missing draw weight to account for.

Alan

[PEARL DRUMS]

The missing 5# you are trying to account for is on his shop floor.

[Pappy]

  now that’s funny Chris. Pappy

[Selfbowman]

I’m sure pearl is right cause I missed the measurements! 🤠🤠🤠🤠

[PEARL DRUMS]

Anytime I've missed weight it was easy to find the missing poundage. Either on the floor or stuck in my Shinto still

[Selfbowman]

I think in glue vs natural growth rings is the density of  the early ring which is where the sheer is happening. The glue joints don’t have sheer. I know that lams have early wood But they don’t run the entire length of the bow. So if we did not have early wood where would the sheer happen Arvin??? Good question?🤠🤠

[mmattockx]

The difference between a 50# design and 35# real bow is too great to easily explain away with a typical amount of variation in material properties.

I agree. Wood is inconsistent, but not that inconsistent. I wasn't really suggesting all of the difference was in the wood.

A couple more pounds may have been lost if the bow took some set.  There is still at least 5 pounds missing draw weight to account for.

That's 10% of your target weight and I have seen different pieces of the same species of wood vary that much in MOE, so some of the rest of your answer may be in there, along with any dimensional discrepancies. I wouldn't be surprised if the wood turns out to be a bit lower MOE and the limbs a touch thin to get you that last 10%.

The missing 5# you are trying to account for is on his shop floor.

  That's very possible.

Losing 5% off the target weight only requires being under in limb thickness by 1.7%, which is around 0.010" or so on this weight of bow. Combine that with the wood being a bit less stiff and you get to 5# pretty easily.


Mark

[willie]

The missing 5# you are trying to account for is on his shop floor.

lol, but dont most bowyers find that some woods "loose more poundage" than others when tillering, even without taking visible set due to the compaction or hidden set that happens as wood is worked at stresses near the max?

I don't expect this behavior to be well quantified in typical wood strength of materiels studies, as wood is almost always used at much lower stresses. Even airplane fuselages and sailboat masts have higher saftey factors in their designs than most bows, let alone bows for flight shooting. 

[avcase]

For those who are curious, here are the correct osage material properties, based on the average of a number of samples I measured a couple of years ago.

Elastic modulus (bending): 1.87 Mpsi
Bending stress at “yield”: 15000 psi. This is approximately the point where higher bending stresses produced some noticeable set.  Strain at this point is about 0.79%.
Ultimate stress at failure: 20000 psi
Density: 50.5 lb/ft^3

Alan