The is variance between limbs and especially between different models of limbs. If they all scaled exactly the same at every point (had identical draw force curves) they'd all perform the same and we'd all be shooting $100 SF Axioms.

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Thank you. Do you mean that both should be 38lbs at exactly 28" but that at 31.5" one may be two pounds heavier than the other? Or could it also be age? I can't seem to find a good source for this information.

 

I would expect that you would see no reduction in draw weight solely due to age. Abuse of some sort (overheating, mostly) might cause a reduction, but for the most part, limbs don't deteriorate. Vintage bows as much as 50 years old still often measure exactly what they are supposed to.

Measure both your sets at 28". If one is higher at 31.5", it's likely due to the limb design. Long and involved explanation.

The is variance between limbs and especially between different models of limbs. If they all scaled exactly the same at every point (had identical draw force curves) they'd all perform the same and we'd all be shooting $100 SF Axioms.

There's more to limb design than stored energy (draw force curve). You can get an aluminum bar to give you the same draw-force curve as a top end limb, but it would be useless as a decent bow.

There's the recovery (the transfer of stored energy into kinetic energy), which has mostly to do with the materials and their elasticity and actual structure (look up "static hysteresis"), and the actual quality of the limbs themselves in terms of matching each other. Limbs have to spring back quickly, which means they have to be springy and lightweight, and both limbs have to recover at exactly the same speed and the same bend rate throughout for optimal performance. You want straight nock travel AND both limb tips to reach the end of their travel at exactly the same instant. Simply adjusting the tiller won't do both.

And then there's torsional stability (so the limbs stay in the same plane), and durability and consistency in various environmental conditions to consider as well.

 

They will both be 38# at 28" draw with the tiller bolts at half way - or whatever variation of that that Hoyt uses for their testing parameters - subject to some tolerance. You can imagine that if they produce a set of limbs that happens to read 37# under that test scenario they don't just toss them in the trash, they get labeled as either 36# or 38# and a customer receives a slightly light or slightly heavy set of limbs.

Once you start changing any of the parameters that the limbs were tested under (tiller bolt position, draw length) you will see different scale numbers. Some limbs will handle long draws more smoothly than others. Some limbs are heavier earlier in the draw than others.

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They will both be 38# at 28" draw with the tiller bolts at half way - or whatever variation of that that Hoyt uses for their testing parameters - subject to some tolerance. You can imagine that if they produce a set of limbs that happens to read 37# under that test scenario they don't just toss them in the trash, they get labeled as either 36# or 38# and a customer receives a slightly light or slightly heavy set of limbs.

I will weigh both sets at exactly 28" draw. I am curious if both will measure 38lbs. I do wonder what Hoyt's or other manufacturers tolerance is actually. I can't imagine that if they are off by one pound they would simply sell them. Especially if we are talking about $800 limbs. But maybe I am wrong.

 

I'm not saying that it *can't* be related to the older age of your previous limbs, I'm just saying there's a more simple and straight forward (and IMO more likely) explanation.

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I have a set of limbs I purchased new. On the same bow, they started life as 33.5# at my draw length. They are now at 32.5# about 18 months later.

 

And nothing else on your setup has changed? Mind me asking which kind of limbs those are?

 

Materials and design differences as well as age. Bamboo is one of the hardest woods available (yeah didn't sound right to me either till I had a floor done in it) and resists torsional pressures placed on it more than say Yew or Maple. That along with carbons resistance to load and quick recovery makes for a limb that could by design achieve a greater pound per inch of draw than a limb built with other materials.

My Jeff Massie Longhorn longbows were all made with bamboo cores and one layer of carbon on the face which made them exceptional Reflex Deflex longbows and I had these built more than a decade ago. They built resistance faster per inch but did not stack due to materials and design considerations. I was surprised Hoyt had not gone this route earlier.

 

This is very interesting, Jim. Materials, design difference and age, makes sense to me. I did not know that Bamboo was one of the hardest woods out there!

 

My 2000 Mathews SQ2 (60-70lb) is now maxed out at 64# it once was able to reach 74# though I never shot it at that weight.

 

Thank you for the additional and more in-depth explanation. Much appreciated!

 

Don't believe the markings on the limbs either. They are just an approximation. Stash pointed one thing about limbs that is often ignored. The rate of change in the draw force curve is different from limb to limb at different draw lengths. Some of us like to call that stacking. So numbers can be very different with draw lengths outside of 28 inches. Another thing is the limb geometry is different in the pocket. If the thicknesses of the limb at the limb bolt and the pivot point are different, draw forces will be different, even if you are using the same riser. Then there is the pivot point on the limb. The limb bends about a pivot point of contact with the riser. Different limb designs move that actual contact point around. That changes the draw forces. The brace height changes the draw forces. Hoyt doesn't publish what riser or brace height they used to get that weight at 28 inches. Hoyt doesn't publish the true distance the pivot point it. With a 25 inch riser, exactly where on the riser is that 25 inches? Are all of their risers have that exact same point? No. So what was their test bed that they got that number. Older limbs most likely used a different test bed than the newer limbs because marketing pushes them to publish different numbers to make the limbs look better then the previous ones so that we buy new ones. Limb bolt position affects the draw weight, that affects the limb angle. They don't publish limb bolt positions, or better yet limb angles for their test. With all this ILF equipment, we have to tune the bow to match the number on the limb, so we just think the numbers are consistent from limb to limb. Then there is rounding. Is that a 42.49# limb is it that a 41.51# limb? Almost a pound different, but rounding to integer numbers means they are both a 42# limb. But the test that supposedly generates that number are never published. So legally, they don't get into trouble for misrepresenting their product. So, don't trust the numbers, its just a reference.

 

I learned a bunch of new things. Thanks everybody!