Spine issues

[Formby]

I have a Joe's Traditional XRD longbow. It is 52#@28" I draw about 28.75". The bow is 62" long, centercut with a fast flight string. There is a thin spacer behind the rest to put the arrow slightly before center. I have been shooting Dark timber shafts 400s cut to 30". I have tested multiple point weights and everything shows weak until I drop down to around 140gr. The 140 is slightly weak but not much. I want to shoot around a 200-250 grain point. Should I up my spine?

[Orion]

When you say the shelf is center cut, do you mean it's cut to center or past center so the arrow is centershot?  If the latter, a better option might be to build out the side plate a little more to bring the shaft a bit more proud of center.  Wouldn't take much.  Of course, could also try I a spine range higher.

[Formby]

Its cut to center so the arrow is left of center.

[Bisch]

Seems like you have an inch or so of length to play with. I would put the weight I want to shoot on the front, and try cutting down the shaft 1/4” at a time till they either worked or I got too short. If you get too short, then you will probably have to go to next higher spine, and likely end up longer than 30”.

Bisch

[Trumpkin the Dwarf]

I agree with Bisch that you could trim it down a little bit. You can also play with brace height. Try going up in braceheight a 1/4" first, then drop it 1/2" and see if it gets you closer. My personal tuning order of operations is to change point weight first, then build out side plate, then play with brace height, then arrow spine (I draw 32" so I've never trimmed an arrow).

I get weird looks every time I say this, but I find going up in brace height will weaken an arrow shaft, while dropping it will stiffen the shaft. That sounds backwards as lower brace makes a faster arrow, but the bareshafts don't lie.

[Orion]

Doesn't sound backward.  Increasing the brace height increases the draw weight, which would make the shaft act weaker.  Performance is a wash, however.  Any increase in speed one might get by the increased poundage is lost due to the fact that the arrow doesn't stay on the string as long, ie. a shorter power stroke. 

[gnome]

Doesn't sound backward.  Increasing the brace height increases the draw weight, which would make the shaft act weaker.  Performance is a wash, however.  Any increase in speed one might get by the increased poundage is lost due to the fact that the arrow doesn't stay on the string as long, ie. a shorter power stroke. 

I agree with Bisch that you could trim it down a little bit. You can also play with brace height. Try going up in braceheight a 1/4" first, then drop it 1/2" and see if it gets you closer. My personal tuning order of operations is to change point weight first, then build out side plate, then play with brace height, then arrow spine (I draw 32" so I've never trimmed an arrow).

I get weird looks every time I say this, but I find going up in brace height will weaken an arrow shaft, while dropping it will stiffen the shaft. That sounds backwards as lower brace makes a faster arrow, but the bareshafts don't lie.

OK... now i'm confused...
Brace height incresses draw weight?  ..... I;m going to say draw weight at xx"s remains the same regardless of the resting height of the string.   What does change is the power stroke .    A  higher brace height puts less energy in to the arrow, allowing for a weaker arrow to be shot.  A lower brace height is a longer power stroke, transferring more energy to the arrow, thus requiring a stiffer arrow.    Exact centershot with a finger release is bad.  Because of rotational torque applied to the string as it rolls off the fingers, a true center shot setup will most always show weak. 

[Orion]

Not quite gnome. Brace height does affect draw weight by putting more or less initial stress on the limbs.  It's not a lot, usually less than a pound either way within reasonable brace height range. If you increase the brace height, you reduce the power stroke, which is the time the arrow spends on the string after release.  The effect of shortening the power stroke is less energy/speed imparted to the arrow. But that doesn't mean that a weaker spine is required.  Just the contrary, because the initial thrust at the higher brace height is greater, it would theoretically require a higher spine, but because the arrow doesn't stay on the string as long, some energy is lost through the shorter power stroke. Just the opposite occurs if you lower the brace height.  It lowers draw weight a tad (and the initial thrust applied to the arrow), but because the arrow stays on the string longer, more energy is transferred to the arrow. Which of these forces wins out, if any, depends on bow design.  In most cases, one cancels out the other. 

Regardless, we're talking about very small differences here.  Most folks wouldn't notice the difference one way or another, though some claim to fine tune their bare shafts with a twist or two of the string one way or the other. 

[gnome]

Not quite gnome. Brace height does affect draw weight by putting more or less initial stress on the limbs.  It's not a lot, usually less than a pound either way within reasonable brace height range. If you increase the brace height, you reduce the power stroke, which is the time the arrow spends on the string after release.  The effect of shortening the power stroke is less energy/speed imparted to the arrow. But that doesn't mean that a weaker spine is required.  Just the contrary, because the initial thrust at the higher brace height is greater, it would theoretically require a higher spine, but because the arrow doesn't stay on the string as long, some energy is lost through the shorter power stroke. Just the opposite occurs if you lower the brace height.  It lowers draw weight a tad (and the initial thrust applied to the arrow), but because the arrow stays on the string longer, more energy is transferred to the arrow. Which of these forces wins out, if any, depends on bow design.  In most cases, one cancels out the other. 

Regardless, we're talking about very small differences here.  Most folks wouldn't notice the difference one way or another, though some claim to fine tune their bare shafts with a twist or two of the string one way or the other.

Thank you for that...I'm going to chew on it for awhile and see if I can get my head wrapped around what your saying...  

[gnome]

Back to blowgunners original question.... try padding out the side plate...

[Formby]

This is the shelf of the bow.

[Orion]

I agree with gnome. Looks like your bow is cut to, not past, center.  Building out the side plate some should make your 400s work.  If that works, it would also indicate that 340s would likely work as well if you did not build out the side plate. Then it's just a matter of deciding which you want to do.   

[Formby]

I agree with gnome. Looks like your bow is cut to, not past, center.  Building out the side plate some should make your 400s work.  If that works, it would also indicate that 340s would likely work as well if you did not build out the side plate. Then it's just a matter of deciding which you want to do.   

I have been palying around with point weights all day and still no luck. I didnt have any carbons in 340 but I did have some aluminum Easton game getters in 340 so I tried them out. The 29.5" 340 with 250gr point flew perfectly bareshaft.