Blacky's Limb Test.. and the winner is:

[Blacky]

...the 60" limbs. Those limbs were the fastest limbs of the four sets PSA X Bocote limbs, that I tested.  


I tested four sets of PSA X limbs from 56" to 62". They were all shot at the same brace height of 8 1/2" and with the original 14 strand Dyna 97 strings that BW supplies with the bows. No string silencers were installed. The bow was shot with a Hooter Shooter shooting machine and mechanical release. The arrow weight was 9 grains per pound at all the different draw lengths that I tested. 6 arrows were shot at each draw length and the average speed was rounded to the next half digit.

To save me some writing, I put all the measurements and readings in graphs. By looking at the graphs, you'll see the following:

best speed goes to the 60" limbs

best energy storage goes to the 62" limbs

best dynamic efficiency goes to the 56" limbs

You can also see that at any given draw length, the difference in speed from the shortest to the longest limb is less that 2.5 fps maximum.

The 60" limbs are the fastest at any draw length, even at the short draw lengths.

Keep in mind that these numbers cannot be applied to any bow of another manufacturer, since the BW limbs are optimized for their length.

I also tested my 60" PA X (one-piece)Tulipwood bow for cross-reference and got pretty much the same numbers as on the 60" PSA limbs. Since the PA and the PSA have the same geometry and limb set-up I think it will be a safe bet, that the PA bows will perform pretty close to the PSA numbers.

That means you can pick your PSA bow length to suit your shooting style and your needs. Choose the one that will give you the best accuracy. You'll definately will have a better release with a longer bow, if you have big hands. Don't worry for speed, since the difference from longest limb to shortest limb is minimal.

Hope the numbers are interesting to read. To be honest, I didn't expect the numbers like they are. But what do I know?      

Enjoy the graphs:

   

[doug77]

great my new 60" PMA should be hear in about 2 week's.

doug77

[O.L. Adcock]

Good stuff Blacky....Have you questioned why the DE goes up and down so much? Never seen DE's that non-linear?....O.L.

[Buemaker]

Thank you, very interesting.
I have one question, why does the draw force curve start at 10 inches on the graph ?
I have tried to measure my own bows, but have been uncertain about how to do it.  Bue--.

[Blacky]

Bue,

brace height plus 1 3/4" to meet ATA standards will be 10 1/4". So since I'm going in 1" steps when measuring, at 10" it's 0 and at the next inch (11") it's around 5 lbs.

[Blackhawk]

Very interesting...especially the part that shows the speed is only increasing 3-5 fps per extra inch of draw. (Did I read that correctly?  :confused:  )

If so, we can expect about 3 pounds of pull increase per inch. That comes down to a little over 1 fps increase for each pound increase.  

[bentpole]

Thanks for the info!  

[TNstickn]

Awesome Blacky, appreciate your test!!  

[Benny Nganabbarru]

Good on ya, Blacky!

[WESTBROOK]

Great info Blacky, your effort is appreciated!

O.L.  What could we expect to see change if that same test was performed with 11-12 gpp arrows? The 62" has the most SE but the lowest DE, the 56" is just the opposite and neither one is the fastest. Would the 62" move towards the top of the DE as the arrow weight goes up?

Eric

[O.L. Adcock]

Eric, The 62" being slower then the 60" is a fluke that could be for many reasons. Glass has variations especially if not from the same lot, the cores have variations, and if they were trying hard to get them all the same draw weight, I'd suspect significant width and thickness sanding occured to marry them up.

Bows all function the same from green willow selfbows to the latest compounds and they are suprizingly linear so changing arrow weights wouldn't change the overall relationship between them. When we see something that appears to not follow the "book"...Either our understanding of the book is flawed or another variable is involved we failed to account for.

Take the idea that shorter bows are better performing choices for those with shorter draws.

1. Limbs have less mass..True..Question is how much and how much difference does it make?

2. The 56" bow has 10% less string to carry.

3. Some say you are bending the shorter limbs farther therefore working them harder and storing more energy.

All that sounds reasonable but real world testing doesn't bear this out. On average the longer bows will store more energy and be more efficient even with all the theorized advantages of the shorter bows. The difference is the shorter the bow is the further the limbs move for any given draw length. That increased distance the limbs travel hurts efficiency far more then the lower limb/string mass can counteract. This is seen in the first few inches of the draw where the limbs may only move 1" for every 5" of arrow power stroke while toward the end it more along the lines of 1:2. With all that the basic design has far more bearing on how well they'll perform then length does.

Lon, you say only 3-5 fps as if it's not much? Keep in mind engines put out horsepower or torque in ft/lbs...Not RPM...Bows put of KE....Not FPS..That 3-5 fps per inch is with a heavier arrow as well. With a 20" power stroke, 1" is 5% of total power (SE), minus 20% efficiency that's around 4% you'll see in the arrow. At 9gpp every fps is about 1% in total power. So it's not just 3-5fps it's 3-5% more power.

Some turn up their noses at tests done at 28" or 30" when they have a 25" draw...Note the fastest bow at 30" is so at 26" as well. These bows were VERY close together, not the 15-20 fps differences you see when testing a wide varity of bows. When you see 5-10 fps difference at 28-30" between 2 bows, that difference will be vary close to the same at 24" as well.....O.L.

[seabound]

Thanks very much for taking the time to test these limbs and graph the data, it's always interesting!

[Blackhawk]

OL, for me the 3-5 fps per inch is not much when I relate it to my 27" draw vs. my buddy's 28" draw. My point was that I was always in envy of those with longer draws, even an inch.  

I suppose I was just trying convince myself that being an inch shorter is not that big a deal. My arrow going 170 fps can be just as effective as another going 175 fps if they both hit the intended target.  Can a whitetail tell the difference?  

BTW, I appreciate and understand your rationale completely.  You're the master...I'm the grasshopper.

[Ham]

I expected all of the arrow speeds to be very close since all of the bows were of the same design and all arrows were 9 gr/#.  I think we might have seen similar varience if all the limbs were the same length.  I would have guessed that the 62" limbs would have had a slight advantage because of the lower string to limb angle.  I'm glad OL is following this thread; because I would like his thoughts on limb design.  I have always equated the recurve's advantage to my boxing experience.  That is it takes less energy to throw a left hook to deliver a compareable blow with a right cross.  The recurve seems like a hook.  The R/D's move toward the recure.  I think the ACS can equal the recurve because of reduced tip weight.  I know you can't apply the same profile to a recures limbs through out.  However, why couldn't you apply the concave shape at the recures to gain a weight advantage where it could make the most difference?

[Blacky]

ttt

[James Wrenn]

Well my money was on the 62".  

Still they were close enough the differnce in the material between the limbs could easily make those results.For 4 sets of limbs of different lengths to be that close on the same riser speaks well of the quality of the process used building Black Widows.