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Wood cores

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6K views 35 replies 16 participants last post by  scriv 
#1 ·
I feel like a dying breed and you can confirm it.
Is there anyone else still liking wood cores?

I know if they made limbs with beer cores Bowmania would buy them.
 
#3 ·
I don’t own any expensive limbs .
I have bought an assortment of limbs the last two months that have maple , maple /carbon, bamboo , bamboo/carbon , foam/glass, foam/carbon/glass cores .
Of the different cores I’ve shot I like the wood(Maple)/glass and the Bamboo/glass limbs the best .
They seem to be smoother drawIng and quicker to me with less shock and vibration than the fancier limb cores . My Samick Foam/glass limbs feel heavy to me .
I thought foam was supposed to be lighter and quicker .
As stated in the beginning, these are primarily entry level limbs .
I left out my Bamboo/carbon limbs . I’d put them in the like category .
 
#8 ·
What is the reasoning, feeling? Im pretty ignorant as to what materials bring what qualities to a limb. I can understand weight and speed gains from shedding weight. I imagine though that different materials would draw different, or at the very least react different at the shot.
 
#9 ·
The core serves to separate the carbon and glass layers where the power comes from. Think of it like the connector in an i-beam. The core can be anything, or nothing. A laminated limb must be laminated onto something, thus the core. Speed differences come from difference is weight (density) of different core materials. This is the catch. Low cost foam can have the same density as wood. The other benefit of foam is that it is supposed to be more stable to temperature swings, but few of us experience large enough swings during the day to see the affect. As far as other factors, like smoothness, or quietness are concerned, you have to separate limb design effects from core material effects. That means the same limb design with different core materials. I could never tell the difference. I have never shot the same limbs with different core materials.
 
#11 ·
Delamination is usually a glue failure between layers of carbon/glass material or the carbon/glass-core interface. I wonder how much the core is needed once the limbs are cured. It would be an interesting experiment. Layup a sample limb and then remove the core. Most of the energy is stored in the outer layers so that is where you would expect the failure to come from.
 
#13 ·
Sheetrock doesn't work without the gypsum core as well as the paper on the surfaces. Ditto for limbs - without the core to maintain the cross-sectional geometry of the limb, the back and belly will collapse together (per Larry) rather than load in tension (back) or compression (belly).

For a given limb geometry (cross-sectional as well as shape) and "power layer" material (fiberglass or carbon), the fastest limb will be the one with the lightest core. Beyond that, the core may play some role in dampening vibration, but I'm guessing that the combination of limb design, string material, riser design and material, etc. probably have more influence on that than the core material.

Wood is inherently non-uniform in composition, strength, defect types and numbers, etc. It's therefore not surprising that it occasionally fails in archery applications that push the limits of what can be done with it. I have no personal experience with the kind of foam used in limb cores. However, it can be even harder to achieve consistent results in manufacturing synthetic materials than it is to achieve consistency with wood. I'm thinking about the spine variability of carbon shafts, which Rick McKinney discusses in an article on the Carbon-Tech website.
 
#15 ·
I used to be a research chemist working on graphite epoxy composites. There are parts of air frames made with cores, and parts without cores. This is why I was wondering about removing the core. I believe the original Uukha limbs were all carbon with no cores. I am guessing that without the core, the limb surfaces will not retain their relative position. There is also no cross member in a limb to keep the parallel surfaces from shifting.
 
#17 ·
Regarding air frames, it all depends on what the part is designed to do. I once toured the B2 bomber production line at Boeing with the CEO of Boeing and the Chief of Army Aviation - fascinating to see something that big go together!

Solid limbs of either fiberglass or wood don't need a core to hold the back and belly surfaces in the correct position. But neither performs very well, for different reasons: Fiberglass because the limb ends up being heavier than necessary, and so too much of the stored energy ends up going into movement of the limb. Wood because it can't withstand enough tensile strain - the first step up from a wooden selfbow is therefore either a backing of some sort, or a "flatbow" with very wide but fairly thin limbs. I would guess that a solid carbon limb ends up being sub-optimal for the same reason as a solid fiberglass limb.
 
#16 ·
Uukha has a core.
Here's an all-carbon limb that's more dense than typical laminated limbs. It's designed to be fast and to recover from a lot of shots. This is a good choice for archers who plan to shoot long distances with light draw weights.

• Designed with the Curve profile, which stores more energy and provides a 4% speed gain (about 8 fps, equivalent to a 3# gain at 40#)
• The Curve profile provides limbs that are smooth, low vibration, and faster which are great for shooting long distances with lower poundage
• Uukha technology guarantees limbs will not twist and remain unaffected by weather conditions
• Monolith C25, with 25% carbon fiber content and 75% unidirectional glass fiber is compressed in metallic molds machined by CNC, under high pressure at high temperature
• Aeronautics technology is the key for strength, durability and fatigue resistance which is far superior to laminated techniques
• Visible weaved carbon and monolithic structure offers torsion stability superior to laminated limbs
• Core structure is made of unidirectional carbon and glass which is responsible for flex resistance, high durability and vibration dampening
 
#18 ·
From the Uukha catalog

Since 2009 we have been using Monolith Carbon, we are still the only limb manufacturer that does not make laminated limbs or use wood or foam as a core filler. The
unique manufacturing process and the high carbon content means that our
limbs performance remains constant day after day, in all weathers and are
fully waterproof.
 
#22 ·
They still have a core, made up with unidirectional carbon and fiberglass fibers. That is different from the structure of the back and belly with their visible weave construction. I question their claim of the bending resistance being in the core. Tests at Battelle N.W. proved that uni-carbon in the core had zero effect on bending modulus. Think that comes with ample uni carbon fibers in the woven layup.
 
#24 ·
I think you're absolutely right, Larry.

Just from geometry, the stresses and strains in any limb, of any material, are greatest on the outer surfaces of the limbs: As the limb bends, material on the back of the limb is stretched, material on the belly of the limb is compressed, and material in the exact geometric center is neither stretched nor compressed - it just bends. The greater the thickness of the limb, the greater the strain on the outer layers for bending through a given angle, and therefore the greater the bending resistance. Glass and carbon fibers are pretty thin, so a layer of them in the center of the limb will be neither stretched nor compressed as they bend. So... the back and the belly have to be where the majority of bending resistance comes from.

The essential idea of a laminated limb is that you can put a thin layer of high modulus material that withstands tensile (stretching) strain on the back of the bow, a thin layer of high-modulus material that withstands compressive strain on the belly, and maintain the geometry which gives the desired bending resistance by controlling the thickness of a core of lighter-weight material like wood or foam. The result is that by using laminations, you can build a lighter limb for a given draw weight, and thus a more efficient bow.

The core material doesn't need to have a great modulus of elasticity - its job is maintaining the geometry which loads the outer layers as the limb bends. I don't know the relative densities of carbon composites, woods, and syntactic foams: If carbon composites are light enough and are capable of withstanding the simultaneous shear and either tensile or compressive strains inside the limb, then a solid carbon composite limb could be just as fast as a carbon/foam or carbon/wood laminated limb of similar design.
 
#25 ·
Gentlemen, I for one really enjoy these kinds of threads, your shared knowledge is much appreciated.

Of course being a Mfg Eng it gets my geek thing going, I am impressed with the discussion mix of modern eng and the hands on knowledge Larry H has accumulated building bows. In todays engineering I benefit from about every type of software you can imagine in my work and realize when Larry was building bows they didn't have these tools/crutches (ha).

Larry, you sir are a very informed and interesting man, I read about everything you post on and appreciate it very much.

Regards,
Bill
 
#31 ·
I like bamboo (Tonkin Cane) too... low density, relatively high modulus of elasticity, and my impression is that it tends to have less defects than other natural materials. You rarely here of a problem with a bamboo flyrod unless it's been abused in some way, and that's a fairly demanding application too. With bamboo you get a core which adds to the performance in more ways than just being a spacer which maintains the geometry of the back and belly sections of the limb.
 
#29 ·
My favorite hunting bow has a bamboo core and a bamboo riser. A sport by its very definition requires certain limitations, but within those limitations the advancement of personal skills is always the primary objective. In my world, fluid versatility with accuracy is a personal advancement, having equipment that enhances and allows that is a major advantage.
 
#30 ·
bfisherman11, I had the opportunity to spend around 200 hours per year, for three years, with one of the foremost material scientists in the world. That time in his lab, at Battelle N.W. in Richland, Wa., was a valuable education. We had the opportunity also to use software and computer experts in the Computer Lab that would have cost thousands of dollars an hour at the time. I had that advantage because Yakima fell within an economic effect Zone from the Hanford Atomic Site. I hope to never have a day when I do not learn something new!
 
#33 ·
Both of my limb failures have been wood core limbs. I am currently shooting Border HEX6H with a synthetic core. My target bow has Border HEX6S with the lower cost synthetic core. Both are going strong.

I'll updated you when I have a statistically valid sample size of failures.
 
#35 ·
One of the solid glass limbs in my first compound bow (Hoyt ProVantage Laser) failed about a year after I bought it. Sheets of glass started peeling off the back side of the limb at the point of maximum curvature, which would seem to be a sign that the limb was not able to sustain the internal shear stresses. Fortunately, I noticed it before the limb blew up on me.
 
#36 ·
I have bought some foam core limbs for my kids when they were coming up, but I've never been a fan. In 2010 my kids had robbed all my equipment again, so I decided to build another tournament rig. I labored over the foam core, maple core issue for a bit, but decided on maple. I went to target Nationals That summer, and was quite interested as they were all shooting the Hoyt Formula RX gear that I had chosen for my rig. The entire Olympic team was there, and I looked on with great interest. Not a one had foam core limbs. That told me all that I needed to know.
 
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