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There are 2 ways to make any bow faster, store more energy or deliver more of the energy you already have to the arrow with a higher efficiency. Efficiency is expressed as a percentage of the energy imparted to the arrow vs the stored energy (KE). While efficiency can be important when the ACS-RC came out I asked John Harvard about the lower than average efficiency and he said stored energy trumps efficiency. He's right of course, but by how much? Based on Blacky's tests (@ 9 gpp) the ACS longbow shoots 200 fps, and the ACS-RC shoots 198, so basically identical. While the RC stores more energy the LB is at 85.9% efficiency, the RC is 81.1%. The best I found in Blacky's tests was a Morrison longbow at 87%. This means that a sizable portion of the energy you put into the limbs is lost to vibration, limb/string oscillation, and noise (it has to go somewhere). Less wasted energy would mean a faster arrow, and perhaps a quieter bow too. But the quest for speed has been mainly through limb shape changes more than anything else leading some to question if conventional RC limbs are dead.
When I posted about lateral stability (TS) and draw weight I was looking to see if this affected efficiency. Turns out it does as adding TS adds weight and possibly width, which reduces efficiency. Then I noticed my EBF limbs shot the same speed as my fiberglass Palmer in Blacky's test, and my Palmer was actually more efficient (82.5% vs 84.5%). This has to be the limb design as fiberglass weighs more than carbon fiber (CF). I wondered how fast my Palmer could it be if it was made out of modern materials, keeping everything else the same (very little torsional stability as is the case with all glass bows). It would be faster than the EBFs and just as accurate for sure!
Not everyone likes the feel of a super RC, especially those who don't use draw checks (like hunters). I had trouble consistently finding my anchor because the flat DFC (low increase in draw weight) of a super RC gives little physical indication of drawing at the end. Plus the high initial higher draw weight might be an issue for some. This begs the question, why no super high tech conventional limbs designed for hunting?
All else equal recurves (RC) can never match the efficiency of a longbow ( LB) as the limb length is longer (it's curved) so it's heavier and has more air resistance, it needs more torsional stability (TS) which also adds weight, and the string is longer which adds weight too. A super RC will always be less efficient than a conventional RC, the limbs are even longer, they need a lot more TS and the string is also longer (more weight and air resistance). This is also true because whatever materials and methods used make a super RC can be used in a conventional RC, so the conventional limb will always be more efficient. There is no free lunch, more stored energy (KE) comes at a price. But (so far) KE trumps efficiency, so they shoot faster. JP clocked the EBFs at 194 fps, Hex 6 at 201 and Hex 7 at 209. The fastest RC limbs on the planet are 15 fps faster at 10 gpp than my EBFs, which sounds like a lot but that's only a 7% increase in speed (14% inc in KE though).
But is this really a fair comparison? My EBFs are based on a target limb, but is all the TS too much robbing efficiency? Or can the limb design be tweaked (like the fiberglass Palmer) for all out speed? I've owned Hex 5 and 6 limbs, they are very light, noticeably lighter than my EBFs so they had efficiency in mind too, but nowhere near what could be achieve in a conventional RC limb. Going by Borders data for the Covert Hunter (204 @ 9 gpp at 50#, 1.17 se/pdf) the Hex 7 efficiency is only 71.1%. That's the price paid for those big energy storing hooks.
Border uses very expensive exotic materials in their Hex 7 limbs, but what if they specifically designed a conventional RC hunting limb using the same materials? I know they have a CX limb, but that is a target design and likely a compromise between TS and speed. You could give up a bit on some of the target priorities like the extra TS and go for max efficiency, AND lower the brace height (another reason why super RCs are faster). The EBF limbs are 82.5% efficient, if you got that to 90% you would be at 203 fps, so only 6 fps slower than a Hex 7 and 2 fps faster than a Hex 6!. Drop the BH to 6.5" and slim up the tips (less air resistance) and you are almost in the same ballpark.
I think there is a market for conventional RC limbs for those who don't like the feel of a super RC, but want an all-out hunting limb. This would be cheaper too and gives us more options. If you like the let-off feel go for the super RC. Actually that's perhaps the biggest benefit of the design. All the current top end conventional RC limbs are designed for target shooters. So how about a conventional ACS-RC, or a CX designed for hunting that is more focused on performance (speed) than ultimate accuracy? It might not be as fast as a super RC but that's not the point. It might be the best balance of all the attributes needed for a top of the line hunting limb.
We will never know how much the shape of a limb affects speed until someone builds matching limbs of different shapes. Bigger hooks are faster, but not by as much as the stored KE would suggest. And there is a limit to how far you can go with a super RC design as efficiency loss and KE gained will converge to a point where bigger hooks will mean a loss in performance even if they store more KE. Eventually someone will figure out the perfect shape for a hunting limb, or perhaps the choice will be based on draw feel (smoothness). It seems like all the top target limbs are very similar in shape and construction, that's no coincidence.
When I posted about lateral stability (TS) and draw weight I was looking to see if this affected efficiency. Turns out it does as adding TS adds weight and possibly width, which reduces efficiency. Then I noticed my EBF limbs shot the same speed as my fiberglass Palmer in Blacky's test, and my Palmer was actually more efficient (82.5% vs 84.5%). This has to be the limb design as fiberglass weighs more than carbon fiber (CF). I wondered how fast my Palmer could it be if it was made out of modern materials, keeping everything else the same (very little torsional stability as is the case with all glass bows). It would be faster than the EBFs and just as accurate for sure!
Not everyone likes the feel of a super RC, especially those who don't use draw checks (like hunters). I had trouble consistently finding my anchor because the flat DFC (low increase in draw weight) of a super RC gives little physical indication of drawing at the end. Plus the high initial higher draw weight might be an issue for some. This begs the question, why no super high tech conventional limbs designed for hunting?
All else equal recurves (RC) can never match the efficiency of a longbow ( LB) as the limb length is longer (it's curved) so it's heavier and has more air resistance, it needs more torsional stability (TS) which also adds weight, and the string is longer which adds weight too. A super RC will always be less efficient than a conventional RC, the limbs are even longer, they need a lot more TS and the string is also longer (more weight and air resistance). This is also true because whatever materials and methods used make a super RC can be used in a conventional RC, so the conventional limb will always be more efficient. There is no free lunch, more stored energy (KE) comes at a price. But (so far) KE trumps efficiency, so they shoot faster. JP clocked the EBFs at 194 fps, Hex 6 at 201 and Hex 7 at 209. The fastest RC limbs on the planet are 15 fps faster at 10 gpp than my EBFs, which sounds like a lot but that's only a 7% increase in speed (14% inc in KE though).
But is this really a fair comparison? My EBFs are based on a target limb, but is all the TS too much robbing efficiency? Or can the limb design be tweaked (like the fiberglass Palmer) for all out speed? I've owned Hex 5 and 6 limbs, they are very light, noticeably lighter than my EBFs so they had efficiency in mind too, but nowhere near what could be achieve in a conventional RC limb. Going by Borders data for the Covert Hunter (204 @ 9 gpp at 50#, 1.17 se/pdf) the Hex 7 efficiency is only 71.1%. That's the price paid for those big energy storing hooks.
Border uses very expensive exotic materials in their Hex 7 limbs, but what if they specifically designed a conventional RC hunting limb using the same materials? I know they have a CX limb, but that is a target design and likely a compromise between TS and speed. You could give up a bit on some of the target priorities like the extra TS and go for max efficiency, AND lower the brace height (another reason why super RCs are faster). The EBF limbs are 82.5% efficient, if you got that to 90% you would be at 203 fps, so only 6 fps slower than a Hex 7 and 2 fps faster than a Hex 6!. Drop the BH to 6.5" and slim up the tips (less air resistance) and you are almost in the same ballpark.
I think there is a market for conventional RC limbs for those who don't like the feel of a super RC, but want an all-out hunting limb. This would be cheaper too and gives us more options. If you like the let-off feel go for the super RC. Actually that's perhaps the biggest benefit of the design. All the current top end conventional RC limbs are designed for target shooters. So how about a conventional ACS-RC, or a CX designed for hunting that is more focused on performance (speed) than ultimate accuracy? It might not be as fast as a super RC but that's not the point. It might be the best balance of all the attributes needed for a top of the line hunting limb.
We will never know how much the shape of a limb affects speed until someone builds matching limbs of different shapes. Bigger hooks are faster, but not by as much as the stored KE would suggest. And there is a limit to how far you can go with a super RC design as efficiency loss and KE gained will converge to a point where bigger hooks will mean a loss in performance even if they store more KE. Eventually someone will figure out the perfect shape for a hunting limb, or perhaps the choice will be based on draw feel (smoothness). It seems like all the top target limbs are very similar in shape and construction, that's no coincidence.
