Stack weight

[sleek]

I wish people would call it string tip angle instead of string angle, the latter of which makes no sense at all since it leads newbies to believe it is the angle of the string at the fingers. I was wracking my brain trying to figure out why the math behind what the TBB said wasn't making sense; when you plot the inverse of cosine of half the angle at the finger tips you see a clear hyperbolic graph that shows the mechanical advantage of string tension is highest at brace and gradually lowers as you draw, but nothing that would indicate a sudden increase in draw weight at any point. I had to go back and re-read the TBB carefully to see that it was the angle of the string relative to the limb tips that they were talking about.

You sir have a VERY valid point. I will call it String Tip Angle from now on. Has a good abbreviation as well... STA

[SLIMBOB]

I can’t argue the mechanics of stack as I don’t have a good understanding of it, so I take your explanation at face value. My point is more directed to the results of stack. I’m with Sleek in that it is all about string angle (or mostly). If that is correct then it is the leverage the angle provides that determines it all. So the energy in energy out ratio changes as a result...as the stack increase it takes more energy to draw the bow a given distance, so the energy out does not drop, it just takes more to deliver the same results as earlier in the draw. If the angle was better it would take less energy in to arrive at at a given FPS. So a higher draw weight would be the same energy in (Round about) resulting in a faster arrow than the previous. What am I missing?

[sleek]

If the angle was better it would take less energy in to arrive at at a given FPS. So a higher draw weight would be the same energy in (Round about) resulting in a faster arrow than the previous. What am I missing?

I dont think you have it quite right here. You are not acknowledging the fact that different weight arrows will gain from different style bows. Also, if a higher draw weight is required to get the same energy in, the FPS will not be higher than the previous unless you have a different weight arrow. It will actually be slower. Higher draw weights with more anorexic fd charts favor a lighter arrow.

Unless I missed what you saying?

[willie]

Diminished returns are still returns.   The Turkish flight bows max out the stack weight.

A lot of flight bows have been stackers

Steve,
 would this be more true about unlimited weight class bows or bows that are shot in specific weight classes?

[Stick Bender]

I have this FG bow in theory the results should be the same  I would think with the caveat that the FG taking more stress but this bow is 58"  46lb @31  at  31" it will do 197 fps  with a 9.7 gpp arrow  that's the closet I could get to 10 gpp  so on average it's 6.3 fps per in. If my math is right  it just starts to stack at 31 1/2 "  when I draw it to 32" it only gains about 2 fps for the extra 1"  & the limb is still working threw out the whole curve , I think the same would happen with wood ?  I have not done the test on any of my wood bows for fear of destroying them !

[Badger]

  It just depends how the wood is bending, you still have decent string angle there very close to 90 degrees, if the outer limbs were bending more it would stack sooner.

[DC]

I'm not sure if this ties in much but here goes. I made this 57" OS straight bow, tillered it to 40#@26"(I was too chicken to go more ) did a FDC and speed test. Pencil line and 166fps @26". Then I recurved it and did the same tests without retillering. It was now 39#@24. Red line and 161fps@ 24". Then I retillered it to 40#@26". Green Line and 170-171fps@26.
You'll notice the dip in the red line at about 35#. I think the bow took a little set right there. Or it could have been one of the tips straightening a bit. I immediately redid the FDC and got the black line. The pencil line shows a little stack at 24". The tip string angle is getting pretty close to 90° there. None of the other lines(now recurved) show any stack if you ignore the dip in the red line.

[Badger]

Once you pass 90 it noticeably starts stacking. You hadn't passed it yet.

[SLIMBOB]

Unless I am misinterpreting DC’s chart, it confirms what I said...I think. The straight limb bow shoots 166 FPS at 26”.  When recurved it shot 170 FPS at 26”. The bow had to be retillered so wood was removed to accomplish that. So bow with less wood shot faster and I will assume with the same arrow. String angle changed, energy in was less, energy stored and delivered was higher. I know that early string tension has some effect on this but this again tends to confirm my point. What am I missing?

[Badger]

  It looks to me like it just stored more energy with a better force draw curve. The mass removed to retiller I am pretty sure was small

[DC]

I think it was about 5 scrapes on one limb and 10 on the other. It's too bad it didn't hold the red line. I've glued a bit of a belly splint on the suspected weak spot so it may recover. Then I'm going to try to get 28"@40# out of it so I may scrape most of the splint off or it may be a garden stake.

[DC]

I smoothed the splint out. It is about 1/16" thick and about 5" long right at the base of the recurve. Did another FDC and the line dropped about halfway between the black and red line. Speed went up to 175fps@26". Worth doing . I'll do some chores and then tiller it to 27", do an FDC etc and then tiller it to 28"

[bradsmith2010]

thanks DC,, thats so interesting,

[PatM]

Your results fly in the face of  the thought  that recurving causes more stress and ultimately slower performance from the same bow.

[DC]

Your results fly in the face of  the thought  that recurving causes more stress and ultimately slower performance from the same bow.

That's one reason I'm going to try to get it to 28". Just to see if it does fall off with more stress. I'm going to guess that it's much like stack, limited gains but gains just the same.