Carbon in bow limbs

[SELFBOW19953]

Will someone please give me the "facts" on using carbon in limbs.  At least 2 well known bowyers state "Carbon does nothing in the center EXCEPT run the cost up" (taken directly from an e-mail). Several renowned bowyers offer carbon on the back AND belly, some on the back only, some as center laminations. Who's right?

Thanks.

Phil

[Apex Predator]

From what I understand it's a waste of money inside the lam stack.  Back and/or belly only.

[vermonster13]

Back is the best place for it, it stretches well. It doesn't do as well under compression which is why more materials are being explored for belly use. As a core material, there are better choices.

[Pat B]

Does it help with limb stability?    Pat

[msamn]

I notice that in the recurve limbs made by Hoyt and Win&Win for FITA recurves, they use carbon on both back and belly. I personally have no experience with either in bow making. But I have made flyrods with both fiberglass and carbon. I know a little bit about bamboo in flyrods too. The bamboo has a slower action and is more smooth. But being less stiff, bamboo does not cast as far. The carbon can be made more stiff and has the ability to cast further. I would guess the same is true for bow making.
 Just my two cents.

[Cupcake]

Phil,
I am an engineer so I approach these subjects with application of some of my book learning.

Carbon in the core of the limb is a waste of money.  The core of the limb is simply just the spacer for the outer layer of fiberglass or carbon in the composite sandwich we are building.  Some may claim that the limb is more "stable" with carbon in the core but the math refutes that claim.

I have recently calculated the contributions of the core and outer layers of the limb and the core does not contribute anything but weight.  That is a bit extreme, but valid.  If you research sandwich panels you will see that the outer fiberglass or carbon provide at least 95% of the stiffness.  The core exists merely to get the outer layers in the proper spacing for the desired spring rate.

The ideal limb core has no mass to it.  That is why foam cores are used.  The density of the foam can be as small as one-tenth that of wood.  If the limb has lower mass the energy is used to move the arrow and not the limb.  Red elm is also a favorite in limbs and that is because it has low density compared to most any hardwood.

The thing about carbon is that it is almost four (4) times as stiff as fiberglass.  That means that one can use thinner carbon lams and not have to space them as far apart - the limb is thinner so there is less mass in the core.  My calculations tell me that I can reduce the mass of a limb by 35-40% using .035" carbon as opposed to .050" glass.

I don't agree with the notion that carbon is not as good in compression as tension.  The material properties show that it is stiffer in compression than tension.  The problem may be that the core material and epoxy does not have enough shear strength to handle the shear strain at the boundary with the carbon.

I am going to build a bow on the same form as my glass bows using my calculations and carbon and we will see what happens.

If you were to build a bow using carbon with the same limb thickness as a 50# glass bow it would end up about 100#.

Kevin

[bjansen]

Great analysis Kevin.  I think everyone will appreciate those facts and would be interested to see how your bow turns out. I surely would!  

I always wanted to try a carbon limb bow as well...but does anyone know where to get it.  Bingham sells it but only sanded on both sides.  I understand that you can still finish sand that rough sanded side after it is glued up but I was just wondering if anyone else had a better source to obtain it.  

Thanks!!

[SELFBOW19953]

Kevin,

Thanks a lot.  Your analysis makes since.  I doubt I'll ever make a bow with carbon, but I may have one made.  Based on your data, and the fact that it explains and supports the statements made by other longtime bowyers, I feel I will be a more informed consumer-always a good thing.

Phil

[7eyes]

When doing the math you need to calculate the human factor. Engineered buildings, bridges, and roads still fall down. Empirical  knowledge is the best, if you practice.

[tradwannabe]

Kevin,
I am a mechanical engineering designer and one of my hobbies is studying race car design, all aspects.
The aerospace and race car people (F1) design carbon fiber in tension use only.These are the pioneers and experts in carbon fiber, Period! that is the actual fibers strength, tension.
Carbon fiber on its own works this way.
as you say the binder comes into effect in any compression use since carbon fiber on its own doesn't work this way.
aerospace and highest levels of racing use pre-pregnated carbon fiber that is heat activated (autoclave) at high temperatures. this minimizes the binder weight and volume.(maximum performance)
however, i can see that less carbon fiber is required than glass and replacement will add weight benefits but probably not much compression benefits in itself, although roll protection in race cars is done with carbon and kelvlar matrix combinations with multiple angles.
Tortional benefits would require different strand orientations to be beneficial.

Primarily, back of bow is where it is/should be used, continuous strand from tip to tip, do not use half pieces from grip to tip.
 Belly might have some benefits but the binder is critical.same tip to tip requirement but not as critical.
Rob