Parallel Vs. Pre-Loaded Limbs
Equipment
I have no intentions of buying a new bow anytime soon, but my curiosity has been sparked by the new 2010 line-up. I'm seeing more and more of the pre-loaded, sharply curved limbs out there. The newer PSE and Bear bows are a good example. I fully understand the concept of Parallel limbs. I shoot a 2005 Allegiance, which was one of the early parallel limb designs, and I love it.
In the advertisements for these "pre-loaded" limbs, I've seen some focus on the fact that the limb pockets are vertical, but the limbs are parallel, or beyond parallel. My question is this:
What is the benefit of a vertical limb pocket compared to a limb pocket that is angled more towards being a horizontal limb pocket? Can anybody explain the significance of a limb pocket's orientation on the riser and how that translates to a bow's performance? Thanks.
I have wonder the same thing. Why no responses?
Well...I guess it's just a marketing ploy.
I have been told that the Pre-Loaded Limb allows the shorter (compact)bow to generate more speed and power, i you notice the limbs are also much shorter than any parallel limb design; i have a 2010 Bear, Assalt and it is rated at 328 fps at max draw weight with a 30.75" from axel to axel.
If you look at the 2010 linep from Bear you will notice the upward rise in speed with a much shorter bow.
Shorter limbs result in a shorter brace height which translates into a faster arrow. That I understand. I'm still unclear as to how the orientation of the limb pocket to the riser affects performance.
We might need an unbiased physicist to explain. The problem with many of these bow designs is that the marketers make all their claims and then hire physicists to back them. It would be nice if the physicists did their work and then the marketers worked off of that. Anybody drive a Prius? HAHA Just kidding on that one.
Sorry to keep adding to my own post, but one more question:
What about the curved limb makes it "pre-loaded"? Is there a difference bewteen a normal curved limb and a "pre-loaded" curved limb?
Preloaded just means that the limbs have been flexed under a load prior to the draw. Claiming that a bow's limbs are "preloaded" is virtually meaningless in archery because if any bow is strung, the limbs are "preloaded".
All limbs are basically flat springs. Their spring rate (force per unit of deflection) is determined by their length, width, thickness, shape and modulus of elasticity (varies with material). Having the limb pockets mounted in a more vertical position require that the limbs be of a certain configuration to achieve the desired attributes (spring rate, brace ht, A-A, etc) that the bow designer needed. That is not to say that a more horizontal pocket with a differently configured limb could not have achieved the very same attributes.
Thanks Purdue, that was about the way I was thinking about it. Glad to know I'm not the only one. In essence, using the orientation of the limb pocket as a performance advantage is really just a marketing point.
Are the limb pockets on these new PSE bows "warfable", especially now that a new "adapter" plate may possibly be modified to use ILF limbs? Angle deg.anyone?
Thanks
The 'verticle' attribute of the pockets just allows for more stored energy in the limbs - in simple mechanics think of the limb pocket as a lever. If you start the lever in a more horizontal orientation, the 'power stroke' is shorter - verticle alignment allows a stronger fulcrum to the lever. Additionally, it in theory should help cancel out some vibe - transferring Y axis vibe to X axis vibe from the limbs.
But it's really all marketing - everything I said above can be cancelled out by limb construction.
Only thing I could think of is for a given ATA and brace height the "pre-loaded" limbs can be designed to be somewhat longer than if it were a parallel limb.
What advantage that would be I have no idea. A shorter limb would have to be a stiffer limb to store the same energy?
My understanding of the preloaded limbs is this.
They are laminated with the layers each being slightly different length. But they are laminated so the ends are flush. This puts a curve in the limb at rest.
Ordinary limbs are straight when you take them out of the bow. PSE XForce limbs I believe are curved when you take them out.
That is my understanding anyway, I could be wrong. Nobody sells PSE around here, so I have not had one in my shop to work on yet. I've had a couple Monsters in, but I haven't had them apart far enough to take the limbs out.
Sometimes structural members that, for example, are more sensitive to compression (like carbon fibers) are stretched while being molded into the compression side of a member so that when a load is applied it first relieves the built in tension before the fibers see any compression. In this way the fibers are exposed to less compression and therefore it extends their life.
In the case of a "pre-stressed" limb (if that is how they mean the term) the fibers on the string side of the limb were probably stretched while being molded into the limb. However, if the limbs curved after being released from the mold due to their "pre-stressed" condition and the previously stretched fibers returned to their neutral position, the benefit of pre-stressing would be lost. In fact, additional stress would be put on the tension side of the limb.
I have no idea how they make their limbs or which way they want us to interpret the term. Time will tell if it is an advancement to bow design or a solution to a problem that doesn't exist.
A recurve is curved the opposite direction when unstrung. I would suppose that is the ultimate "pre-loaded" limb design? Do any traditional guys have insight as to if and/or how a recurve outperfoms a longbow?
I can't tell you why but I can tell you the fastest trad bows today are longbows and that they out perform recurves.
Can anybody elaborate on how longbows outperform recurves? I'm assuming Tilzbow is saying that if two bows are designed at the same weight and draw length that the long bow shoots an identical arrow faster? Just curious, because this is fascinating to me. Thanks again guys.
arctichill,
You'll probably have a better chance of getting an answer to your question over at Stickbow. Search for a thread titled "Walk the Talk". It's all about trad bow speeds and one of those guys who posted can probably explain why the modern longbows are so much faster than longbows of the past.
Great suggestio. Thanks Tilzbow.
