J-San,

Search "Composite Beam" and you will find all kinds of video explaining terms like compression, tension, shear, core, and neutral axis.

In short, the job of the core material is to define the thickness of the limb. (thick at the limb butts and thin at the tips.) The thicker the limb the stiffer. The core material needs to bond well to the outer layers and be strong enough to not tear in use. The core material must also be flexible and light. The lighter the core the lighter and faster the limbs.

All the strength and springy qualities of the limb come from the outer layers of fiberglass, carbon, and wood held together by resin. At brace or when drawn the outer layer of the limbs on the back of the bow (facing archer) is under compression while at the same time the outer layer on the belly side is under tension. Explained another way, imagine each outer layer of a bow limb working like a million little coil springs. On the back side the spring ends are squeezed together (compression), while on the belly side the spring ends are stretched apart (tension). When the arrow is loosed each spring is released to "spring" back to its original shape, on the back side the springs fly apart, and on the belly side the springs snap together.

While all the compression and tension is happening at the outer layers the job of the core is to keep the outer layers working together.

Hope this helps,

Rasyad

 

Thanks, Rasyad. I will certainly do some reading about the subject.

 

Rasyad provides a good description. My understanding is that the core acts as a separator between the power providing glass/carbon laminates on the two surfaces. The thicker the core, the further apart the power surfaces are. Separating the surfaces provides stability to the structure, and makes it harder to bend. You can test this with Popsicle sticks. Separate the sticks by different distances and see how the rigidity changes. (I will have to try hanging a weigh and measuring strain under a constant stress for different separations) The important thing is that the surfaces cannot slip or deform. You must be able to maintain the proper geometry while under strain.

 

That does make sense. Thanks, Hank. Don't have any popsicle sticks on hand, but I did glue together a cut up expired credit card. Allowing the two "laminations" to slide past each other does little to really increase rigidity. When I glued them together with CA, they became much stiffer. That and if I wrapped them around a dowel and glued them, they maintained a curved shape and would exhibit even more rigidity if pulled against the curve like a recurve.

I suppose it would make sense to have the core material as light as possible, hence the foam core? I would imagine the more mass a limb has, the slower it would accelerate and the slower the arrow would fly. So the foam works to separate the working laminations further apart to increase draw weight, while minimizing mass? I wonder how Uukha does it with their solid limb designs.

 

I suppose it would make sense to have the core material as light as possible, hence the foam core? I would imagine the more mass a limb has, the slower it would accelerate and the slower the arrow would fly. So the foam works to separate the working laminations further apart to increase draw weight, while minimizing mass? I wonder how Uukha does it with their solid limb designs.

That is correct on lighter being easier to accelerate. Something I picked up from Sid is the two scale test where you put the tip on one scale and the butt on the other. It gives you an idea how the weight is distributed. You want the end that moves to be as light as possible without losing needed mechanical properties. The test is a bit sensitive to how you place the limbs on the scales. It is now part of my standard test set when I test peoples bows.

 

since almost all limb s are glued with epoxy, and epoxy exhibits glue creep, the layers do move a bit through a draw length. you can control how and when the recurve portion opens by using tapered lams. also determine preload in the strung position.
lot's of variables. all limbs are not created equally.

 

A core needs to be as light as possible, but still hold together under the sheer stress of the nutral axis, and be strong enough to withstand the compression of the draw cycle.
It also has to be able to be bonded.
Anything heavier than that doesn't make any positive impact on the shooting of the bow.

So, as Larry pointed out. The way a limb works in DB^3. (breadth x depth, but the depth value is cubed) so the depth is 3x more important than the width.
the draw cycle of the bow is a geometry thing. as all composits used are linear in their strength.
for every unit of compression, you get the same increase in strength.
so the geometry means that you can play with the DFC.

the core as long as it has the same dimensions, will give approx. the same draw weight and DFC if the laminate is the same...

an example of this, is that a core in a recurve would equate to about 1lbs of bow weight if it didn't have any laminates either side.
so even if there is about 50% difference in materials, that would only equal 50% of 1 lbs.
sandwich this in composit and it will jump to say a 40lbs bow. so your bow would be 40lbs or 40.5lbs.
Not really worth considereing.

what we have discovered is that the mass of the core is more important.
we have 3 cores. to which the biggest difference is about 20% difference in mass. this only comes out at 2-3fps. on a recurve.

so what is in a core?

we don't think that much!

we have a core that 20% lighter than maple.
say 0.1" of maple = 0.027" carbon for mass.
that's why we don't have a sold carbon core!
carbon is about 3x heavier than maple.

granted a carbon core would contribute more to the bows draw weight, but remember every 0.001" inboard, = a cubed reduction in the work it does. so this leads to a quick reduction in benefits. till its mass out weighs the work its doing!

this is the summary of a lot of R&D!

 

Wow. That was a very detailed explanation, Sid. Thanks!

How about a curved limb profile vs a flat one? My W&W Synerzy limbs have a dished profile to them with what seems to be a hollow on the belly side. I have never encountered such a limb design before and I would imagine such a design would effectively stiffen a limb, much like how a measuring tape can remain rigid when held out horizontally.

 

i beleave harry drake made flight bows back in the mid 60s with that idea.

we are not convinced. a tape measure is desinged not to bend when out. a limb is designed to bend.
what happens to the tape measure when you bend it?

 

A&H ACS used the curve section in their limbs to improve limb rigidity so it does work. Not sure if the concave side is facing back or belly. If it works like a tape measure, then I would speculate that its going to provide some let off if over flexed?

 

Or collapse

 

If the wood cores have no bearing on the limb performance, why do bowyers put different types of different specialized woods in their limbs and not just pine (assuming it would flex enough)?

I can understand the use of a good looking veneer under the glass for aesthetics.

 

If the wood cores have no bearing on the limb performance, why do bowyers put different types of different specialized woods in their limbs

Weight would be one consideration and possibly degree of heat sensitivity as that's the raison d'etre for the synthetic cores.

On the positive side, wood cores adhere well to the laminates and the limbs should be less likely to delaminate.

 

A tape measure collapses sideways. So does the dish carry a potential risk of twist? I wonder if anyone has had one of these limbs fail, and what did it do when it failed?

I've had a limb twist and fail, no joy at all...

 

this is a quite interesting thread. particularly as i am eyeing a pair of "dished" limbs for my dorado. please keep it going.

 

The core material does matter. If doesn't, then bowyers will just make a solid glass limb. Cheaper and faster that way. Like what others say the lightest material that will withstand the shear forces will make the best limbs. Besides foam, actionboo should make the fastest limb core as it is much lighter than maple or red elm. Second best will be maple.

 

Well, my W&W Synerzy limbs with the dished profile are holding up well and have easily over 10,000 shots through them by my doing alone and I got them used from another FITA competitor.

Speaking of different woods, what exactly is "actionwood" and "futurewood"? I have seen these terms being used in some product descriptions of limb core materials and riser materials. Are they just resin-impregnated wood?

Anyone broke a set of Uukha limbs yet? I would be curious to see what is inside them.

 

I remember an old story about Archery division of Yamaha having problems because Piano division gets the best wood, so they began experimenting with other materials.
About different woods do not forget the “magical/romantic” thinking.
Thanks
Martin

 

By separating the power generating glass/carbon surfaces, you also get more resistance to twist. Take the Popsicle stick example again. The more you separate the sticks, the harder it is to twist. The key is that the core must be light, and it has to maintain the correct positioning of the power generating layers.

 

By separating the power generating glass/carbon surfaces, you also get more resistance to twist. Take the Popsicle stick example again. The more you separate the sticks, the harder it is to twist. The key is that the core must be light, and it has to maintain the correct positioning of the power generating layers.

Interesting. I would have thought that most foams would be more compressible than wood. BUT, I'm certainly no plastics engineer!