Wedge orientation in takedown limb ??

[Appalachian Hillbilly]

Is there a preference to where the wedge  goes into the stack? Longbow limbs with 2 tapers and a core.  Should the core be on the belly side of the wedge or the back?

[Mad Max]

Wedge is flat side on the back of the limb, Taper side on the riser
2 cores , 1 back 1 belly

1 core on the back, it would help the transition of the fade to glass
 would not look very good on the belly side with no core and clear glass

[Appalachian Hillbilly]

Just got back from the shop. Did back glass,back taper wood, core , wedge, belly taper wood, belly glass.

Similar to my first set of ILF limbs I glued up. I added a longer tapered wedge and a longer straight portion before the curve.  10 thou more stack height total.

Got enough glass left for 3 more sets , then I need to order more. This experimenting is expensive, but I really enjoy this part.

[Crooked Stic]

Hmm I got two curve designs. one has a straight taper wedge the other is a curved wedge. I agree one core should  be on the front side If you got two core one front and one belly.

[Appalachian Hillbilly]

All of the reflex deflex longbows I have from 3 different bowyers all have single cores and back and belly woods and then glass. The traditional D bow I have has 2 cores and 2 show woods. I just looked at my bows when I got home and it looks like each Bowyer had a little different idea.

I will post pics when I tiller it.

I was not at home when I posted at first  and my thoughts were to have the wedge on the belly side of the core so it would be in compression beneath the core.

[Mad Max]

Everything between the glass is core wood except wedges , tapers are core also.

If you have 3 core woods it doesn't mater where they go 

[Arlo]

The only time it would make a difference where your wedge sits in the stack is using short, fast taper, stubby wedges that are hinging at the fades. In those cases you would typically use a power wedge on the belly side to push the fades out further as well as one of your tapered lams under the wedge.   

You are going to like what you see going with longer wedges and pushing the working limb out further.

Something of interest that you may want to consider when milling your lams. Try and avoid using lams over .080 to .090 in thickness in the core. A pair of .040 to .045 will serve you much better. Your limb does better in compression and it will hold its shape better coming out of the form with less spring back....You will also get better limb stability using an extra lam.  If you are using rock hard maple lams you can increase the thickness to.100 on your tapers no problem.

Your mileage may vary depending on core wood selection.    Arlo

[mmattockx]

A pair of .040 to .045 will serve you much better. Your limb does better in compression

Why would that be?


Mark

[Crooked Stic]

You are going to like what you see going with longer wedges and pushing the working limb out further.

Something of interest that you may want to consider when milling your lams. Try and avoid using lams over .080 to .090 in thickness in the core. A pair of .040 to .045 will serve you much better. Your limb does better in compression and it will hold its shape better coming out of the form with less spring back....You will also get better limb stability using an extra lam.  If you are using rock hard maple lams you can increase the thickness to.100 on your tapers no problem.

Your mileage may vary depending on core wood selection.    Arlo

I have to agree with this 100%. My one design has a straight taper wedge about .300 to zero in about ten inches. Kinda a working wedge and moving the working part out all good.

[Kirkll]

A pair of .040 to .045 will serve you much better. Your limb does better in compression

Why would that be?


Mark

More laminations hold the shape better coming out of the form with less spring back. The matrix assembly of wood and epoxy is stronger with more laminations and it provides increased stability in some cases. But… that depends on core materials used too.

If you use a soft core material, most  all your compression strength comes from the glass on the belly side.  Use a bamboo core  on a recurve limb vs a rock hard maple lam on the belly side of the stack…. You will be amazed at the difference.

[Mad Max]

What about Carbonized bamboo verses un Carbonized bamboo?

[Kirkll]

That is a good question Max…. Carbonized bamboo is definitely much harder than natural bamboo and I would imagine it would test better in compression as well.

But the nature of the beast is still there…. Bamboo is basically grass with not good compression ratings at all. But it’s excellent in tensile strength. But there are a lot of bugs in it when it’s harvested. How it is processed and cooked to get rid of bugs, and how it is laminated by different manufacturers gives us a huge difference in finished products. Moso bamboo is the best choice for bows IMO. These engineered products we have these days are pretty amazing, but there are a lot of bamboo products that don’t do real well in a laminated bow.  It broke my heart seeing the hiquerea hardwood flooring get discontinued. That was excellent bamboo for bows I used for many years. I used the carbonized stuff exclusively.

I have started using a totally different product now and the initial test results are looking very promising. I haven’t done any DFC charts and put different lay ups in the shooting machine to get a detailed analysis. But I have got feed back from some customers I’ve sent bows to that are very happy with the product. I’ll be happy to share the name of the product when I’ve accumulated more data. I’ve got another bowyer buddy working with the stuff that is pretty impressed with it too. I see Arlo has started posting here a bit too.  I asked him to keep a lid on this until we’ve got more data.      Kirk

[Appalachian Hillbilly]

With the talk of tensile, compression etc. What do those relate to when looking at the wood databases? We have the Janka hardness which I think is easily understood.

Modulus of elasticity which is how much it takes to deform the wood? Modulus of rupture?

Do either of these translate to tension and compression of the wood?

[Mad Max]

Kirk I am down to a couple 38" boards of Carb. boo flooring and will be forced to go to HR maple
x2 on the Carbonized and Moso

[mmattockx]

With the talk of tensile, compression etc. What do those relate to when looking at the wood databases? We have the Janka hardness which I think is easily understood.

Modulus of elasticity which is how much it takes to deform the wood? Modulus of rupture?

Do either of these translate to tension and compression of the wood?

Janka hardness is a measure of how resistant the wood is to denting. It is the force required to drive a 7/16" diameter steel ball halfway into the test sample. It is usually measured on the face grain, but can also be done on the end grain.

Modulus of elasticity is how stiff the wood is. This relates to how much force it takes to deflect it. Twice as stiff = twice as much force for the same amount of deflection.

Modulus of rupture is the stress level where the wood fails in tension. Most bow hardwoods are much weaker in compression than tension and will often show compression failures (in the form of set or chrysals) at stress levels around 50-60% of the rupture stress.


Mark

[Appalachian Hillbilly]

So what are we looking for? The highest values with the lowest weight?

[mmattockx]

So what are we looking for? The highest values with the lowest weight?

For a wood bow you want the ratio of modulus of rupture/modulus of elasticity to be high. If the stiffness is low and the strength high that gives you a wood that will bend a long ways without breaking. In addition to this you want a wood that is compression strong so it won't fail easily on the belly side.

The density is less of an issue for a wood bow, as you need about the same weight of wood to make any given bow, regardless of which wood you use. This is the mass principle from one of The Bowyer's Bible volumes.

For example, yew is a very light wood, but you need quite a bit of it in a bow to get to whatever draw weight you want. Something like osage is much denser than yew, but you need less of it to give the same draw weight as the yew bow. In the end you end up with the same draw weight bow and they are close to the same mass, but the limb dimensions are significantly different.

For a core wood most of this doesn't matter and you want a wood that will carry high shear forces and not take set easily, while not being too dense. Some of the other guys can probably elaborate on more of what you want in a core wood.


Mark

[Kirkll]

Excellent post Mark.

Something I learned years ago about tension and compression strengths in relation to laminated composite bows is that the compression side, or the belly core material up against the belly glass is the first thing to go in extreme stress tests.

With fiberglass backed bows, the glass itself will handle compression without failure much better than carbon. Under extreme loading tests, the carbon typically delaminates from the core on the belly, then the core itself buckles from the belly side first.  If you want to keep a carbon belly backing intact, best material you can use is rock hard maple….. even then…. It’s a tough one to pull off. 

I tried a balsa wood core with bias weave carbon belly and back one time just for fun. All these bowyers were saying “core doesn’t matter” …. After all they were building foam core bows….. but… even on those foam core bows the ones that were holding up had maple on the belly that the carbon attached to.

Btw…. The balsa core failed miserably….   Never got to full draw and she blew up Big time!   Those were fun years building time bombs. Some of those babies were seriously fast too until they blew up.

You want longevity, stick with eastern rock hard maple on the belly side of your stack and your lighter weight material towards the back….. or bamboo.

Kirk

[mmattockx]

Something I learned years ago about tension and compression strengths in relation to laminated composite bows is that the compression side, or the belly core material up against the belly glass is the first thing to go in extreme stress tests.

Yep. This is because the compression strain in the core wood that is up against the FG lam can get high enough that it exceeds the limit of the wood and the bow takes set. The best solution to this is to use a thicker lam on the belly than the back, especially on the HH and other flat bow styles that seem to commonly have set problems, but not very many bowyers seem to go that route.


Mark

[Kirkll]

Something I learned years ago about tension and compression strengths in relation to laminated composite bows is that the compression side, or the belly core material up against the belly glass is the first thing to go in extreme stress tests.

Yep. This is because the compression strain in the core wood that is up against the FG lam can get high enough that it exceeds the limit of the wood and the bow takes set. The best solution to this is to use a thicker lam on the belly than the back, especially on the HH and other flat bow styles that seem to commonly have set problems, but not very many bowyers seem to go that route.


Mark

That makes total sense to me Mark. Even more so Building Tri lams and not having that glass to carry most of the compresion.

Typically glass bows do not take a set too easily unless they are left strung up in an extremely high temp environment. That is the beauty of composite backed bows vs board bows or self bows......   But don't get me wrong bro.... i appreciate the beauty of all wood bows too. They just require more TLC and watching MC levels to keep them alive.   Too much heart break loosing old friends with board bows for me...  I stick mostly to building glass backed bows myself....

I say this as i take my osage stave i'm straightening out of the clamps today...Ha!