ACS Recurve Tech Talk

[Sixby]

Storage is important to a degree but ultimately it boils down to efficiency at the intended draw length. Thats my singular input on this subject.

God bless you all, Steve

[JJB2]

MAT, if the overall bow length is the same, the one with the longer the working limb will be the better performer from a speed standpoint, but depending on your draw, the riser design, etc. the longer power stroke may result in less stability.  @ 26" though I would say the shorter riser / longer limbed bow would be your best bet.

[John Havard]

Hi Mat,

The energy storage efficiency per pound of draw will always be maximized with a shorter riser & longer limbs (based on the data I've developed through the years).  So, let's say you're choosing between a 13" riser & long limbs for a 58" bow versus a 15" riser & medium limbs for a similar 58" bow.  The shorter riser & longer limbs will store more energy at 24", 26", 28", and 30" of draw (SE/PDF - Stored Energy per Pound of Draw Force is the way to describe energy storage efficiency) than the longer riser & shorter limbs, ceteris paribus (same BH, same design, etc).  The longer set of limbs will be more efficient at storing energy at any practical draw length than the shorter limbs.  And since the dynamic efficiency is the same (within accuracy measurement limits) then the bow with the longer limbs will give slightly better performance.  

Picturing a Force/Draw curve in your mind quickly points out the reason.  An XL limb that has a LONG hump/pre-load section on the F/D curve and doesn't begin to incrementally start to increase until you go beyond 32" AMO draw length essentially has only a pre-load F/D hump out to 26".  A limb that's designed to start stacking at 27" ("optimized"?? for a 26" draw) has a F/D pre-load hump that ends at 22" or thereabouts.  It stands to reason that the longer limb will store more energy per pound of draw.

Again, the difference is relatively small - perhaps 1-2 fps difference at the max.  But the nod for maximum performance has always gone to the longer limbs for every bow design (mine and many other bows from other companies) that I have tested.

I'm the first to admit that I don't know everything, and a bow that does not bear out what I said above could exist.  But I'm also pretty confident that few (if any) other folks have tested as many different stick bows as precisely and methodically as I have.  Not a brag - just a testament to my nerd belief that I don't know anything until I prove (or disprove) it through objective testing.

[MAT]

You have a much better way of saying what I think than I do.  I figured it out when I drew 2 D/F curves on some scratch paper as you described.  

Here is another question; you seem to be saying that SE/PDF has a bigger role than DE on bow performance.  So shouldn’t we calculate what the actual delivered SE would be by multiplying the SE/PDF by the DE?  For example, the ACS recurve SE/PDF of 0.97 x the DE of 81.1% is 0.786.  I’ll call that dynamic SE (dSE, or more proper dSE/PDF).  For the ACS longbow the dSE is 0.742, thus showing what you said about the recurve being a bit better due to the higher SE, and the lower DE didn’t have that much effect.  

Thus my recurve with its higher DE (84.5%) only has a dSE 0.768, short of the ACS 0.786 so they offer a bit more.  But the test of my bow was with a 18” riser 60” AMO and the ACS was 15” riser 62” so these are not quite apples to apples. Cool - I think this conversation just eliminated a misconception I had about shorter limbs.