ACS Recurve Tech Talk

[MAT]

I wanted to start this tread to discuss some of the technical aspects of the ACS recurve limbs now being sold, and some of the newer materials (like carbon foam, which have some of the same advantages).  I am not an engineer, but I have a physical science background and the testing data fascinates me.  I still don’t understand all the physics involved but here are my observations and discussion points based on Blacky’s published testing data.  Note - John Harvard also has an excellent summary of the ACS design on the Dryad website.

The ACS design is based on producing limbs with less physical mass (weight), which improve the dynamic efficiency (DE).  This is really why the ACS longbow performance is on par with all but the best recurve limbs.  Efficiency is expressed in %, which is the energy imparted on the arrow divided by the stored energy. Energy is lost accelerating heavy limbs, thus the lighter the limbs the better.  Energy is also lost in heat (friction) from flexing the limbs, which is based on the material used.  I suspect the ACS might have an advantage because the limbs are not a thick as a standard longbow, and carbon is better than fiberglass.  It would be nice if someone who knows would weigh in on this.  There is also air drag moving the limbs through the air, so the shorter and narrower the better.

Longbows can’t match the stored energy of a recurve limb though, which is why the ACS longbow can’t exceed the performance of a well designed recurve.  This is expressed as stored energy per pound of draw force, or SE/PDF.  You can see this in the draw force curve, the SE is really the area under the curve, and so the more the curve bows out to the left the more energy is stored.  The ACS recurve has the best draw force curve I’ve ever seen, with SE/PDF of 0.97.  That is huge.  But it only has a DE of 81.1%. I think this might be related to the fact these are x-long limbs (15” riser, 62” AMO).  

If they can match the DE of their ACS longbow limbs they will have the best performing bows on the planet.  I would like to know if shorter limbs (all else equal) will have a higher DE, which wasn’t discussed on the Dryad site.  I would think that shorter limbs would be lighter, but I don’t know if bending them more decreases DE or improves.  Shorter limbs move through the air more, but they have less area than longer limbs.  My favorite bow, which is considered a high performer, has a SE/PDF of .91, and a DE of 84.5% with a 18” riser and 60” AMO (much shorter limbs).  I suspect the ACS recurve would be no better than my bow, given the higher DE.   But, if it could match the DE then its higher SE/PDF would make it better (more KE).

Before anyone gets on my case about performance/speed, realize I only draw 26”, so I am giving up a lot of KE to you guys with monkey arms.  All the bow testing to date was done at 28” with bow lengths 60"+ AMO, but you can still see the differences between bows at my 26” draw by looking at the draw force curves.  What is not known is the DE at 26”, or the effect of shorter limbs with a shorter draw.  So what affect does a shorter bow with a short draw have on performance?  It’s not apples to apples comparing them to longer draw lengths.

I also can’t draw that much weight, and because I hunt elk and moose I need all the KE I can get.  I shoot heavy EWF arrows so this is not about speed, even though speed is a measure of performance (and KE).   Something else not mentioned is the affect of brace height (BH).  The shorter the BH the longer the power stroke and the more KE stored.  For me, having a bow with a designed lower BH is an advantage.  Shorter bows have a shorter BH.  Let the conversation begin!

[Blaino]

It's my belief that momentum if far more important then kenitic energy.    

[atatarpm]

I really wish I knew enough about what you said to speak will to the subject although one of the major things that I have learned is that the efficence of the limbs holds the most value to the cast of the arrow. In that statement we maybe saying the samething. Driad also has ACS limbs and I think you should check out their site on their new recurve bow. It looks truely impressive. I believe that I will enjoy watching this thread and hope you kep your chin up.

[Wickles]

I havn't heard much about the acs recurve since they came out. How do they shoot?

[Wickles]

[calgarychef]

Shouldn't be long before someone comes on and says "speed isn't important."

[Wickles]

[John Havard]

I'm at the PBS convention in Portland but will offer a couple of comments.

First of all, the ACS recurve now being built out performs our ACS longbow limbs by a tad.  Independent testing confirms this fact.  I struggled long and hard to get a proper balance between maximizing energy storage and dynamic efficiency in these recurve limbs.  They do not match our longbow limbs' DE but they greatly exceed the SE/PDF values - hence they are faster/perform better.

I'm very proud of the ACS recurve - it's performance and stability are the result of over 5 years of development.  In designing it, a blank sheet of paper was the starting point and every design feature was tested and made as optimum as possible.

Typically longer limbs out perform shorter limbs all else being equal.  Longer limbs store more energy and will slightly out-perform their shorter brothers.  However, the difference is fairly small so bow length should be chosen based on draw length and what the archer needs in overall length.  Personally I detest short bows but I also draw 30"+.

The best core material we've found through extensive testing is our dirty bamboo but that doesn't mean there might not be something out there that's better.  Always looking!

John

[WRV]

I sure would like to see one in action

[Wickles]

Me too!

[wingnut]

We have five sets going out in the mail on Monday and five more going each week for the forseeable future.  So some feedback should start trickling down.

It's tough sometimes in a small shop to hold the build schedule by we are busting it trying.

Mike

[ChuckC]

John. .   we hear a lot about the longer draw length and the boost that the longer power stroke provides.  Is that relative ?  

I mean,  if you shorten the bow. .  
build it    for    a shorter draw. .  would you be able to achieve similar results in terms of arrow speeds attained ?
ChuckC

[John Havard]

Hi Chuck,

I used to test every bow at 26" AMO, 28" AMO, and 30" AMO.  I always found that the fastest limbs at 30" were also the fastest at 26".

I think I understand the notion of fitting a bow to a specific draw length.  The theory is that by tailoring the limb/riser combo so that at the archer's draw length (say 27") the limb just begins to stack (draw weight increases incrementally more per inch of draw).  

The problem with that in practice is that I've never found a combination of shorter risers/limbs that increases SE/PDF at short(er) draws.  The limbs that pull smoothly to 32" (longer limbs) are obviously very far from their stacking point at 27".  Yet they will have stored more energy per pound of draw at 27" than similarly-designed shorter limbs.  At least that's true for each and every bow design I've ever tested.  Not saying that it might not be possible, and I've certainly tried every design option I could think of to maximize SE/PDF.  But if a solution is out there for how to make a shorter limb store as much energy as a longer version of the same limb I haven't found it.

The difference in performance FOR SIMILAR-DESIGNED LIMBS (as measured with AMO speeds) isn't huge - perhaps 1-2 fps.  But the longer limbs store more energy at any draw length (certainly true beyond 24" AMO) than the shorter limbs.  I used to think that longer limbs would have measureably lower DE than shorter limbs but that has proven to not be the case.  The difference in DE is definitely on the edge of not being large enough to even measure.  Given that the DE of longer limbs and shorter limbs are VERY similar for both recurves & longbows, the difference in energy storage makes the longer limbs produce slightly more speed.

Gotta go to a seminar!

[khardrunner]

While it seems the OP is (mistakenly to a point) concerned with the KE of a bow as it pertains to efficiently killing game, his points still work with the more important conservation of momentum. Both KE and P are products of velocity. Hence the discussion concerning speed for a particular draw length. In reality, as soon as the arrow leaves the bow, the bow efficiency doesn't matter. In that I am saying the arrow is a projectile ideally working agains friction horizontally and gravity and friction vertically. While momentum provides a truer value for calculating penetration potential (all other values remaining the same), both KE and P are velocity based.

[floodman]

Ya...What He Said...I knew I should not Had dropped out of Skool in the 3rd Grade..All I remember in Physics is,, Pie R Round and Cornbread R square..

[SELFBOW19953]

I give up, what are OP and P?

[reddogge]

OP=Original poster. I don't know what P is though.

[floodman]

Penetration..is the P.....?

[khardrunner]

P is the symbol for momentum. It really is a rho and a greek letter that looks like a P.

OP is original poster

[MAT]

John, thanks for the response.  Do you have any draw force curves of shorter limbs (or just the data, I can plot the D/F).  I'm trying to visualize what you are saying. The longer limb D/F curve since it's flatter would store more energy at shorter draw lengths (actually all draw lengths) than a limb that starts to stack sooner.  Can you confirm that this applys when you compare short limb/long riser vs. long limbs/short riser with equal bow lengths (both at 58" AMO at 26")?

That is my dilemma, I like longer risers in a 58" bow but might be giving up some KE (or P) at my 26" draw.