Why is wood stronger in compression than in tension?  Because it has to support the weight of the tree.

Related: The underside of conifer branches are stronger in compression for the same reason.

This is not the case for broadleaf branches, which are stronger in tension. Good to know when making branch bows.

Interesting.

Sounds logical. Conifers have needles all year and have to support snow and wind loads. while the broadleaf's loose the leaves and have an easier time with wind and snow...I think God knew what he was doing!

Thanks for sharing this.

You have too much time on your hands. LOL

But it's not actually.

Not clear to me why the underside of broad leaf branches would be stronger in tension, since they spend their life in compression.

The top side is.

As far as conifers go you would need to test the top and bottom to see if the the bottom is actually stronger than the top or just stronger than normal trunk wood.

Conifer branches for bow wood? Where do we sign up? Lol

In Northern Eurasia if you ever end up there.

From Google: Reaction Wood In leaning stems and branches, trees form a special type of wood known as reaction wood. In softwoods, this is known as compression wood and in hardwoods, it is known as tension wood. There are important differences between these two types of wood. In softwoods, compression wood forms on the lower side of leaning stems or on the lower side of branches. This wood is more highly lignified, contains a special hemicellulose called galactan, and has a high microfibril angle. Compression wood generates compression strain, which pushes the leaning stem back to the upright position. Tension wood in hardwoods occurs on the upper side of the stem, is less lignified, and has a small microfibril angle, generating tensile strain, which pulls the leaning stem back to the upright position. Both types of reaction wood are associated with eccentric growth so that the stem grows more on the reaction wood side as characterized by eccentric pith. Both types of reaction wood have increased longitudinal shrinkage as well as high residual growth strain and are associated with distortion of sawn timber.

Here's a link to the full above piece: https://www.academia.edu/15159430/Arctic_Bowyery_the_Use_of_Compres sion_Wood_in_Bows_in_the_Subarctic_and_Arctic_Regions_of_Eurasia_an d_America

little bit of wood trivia

... Balsa one of the worlds softest woods ... is a hardwood

Jack, not to much time on his hands. Tim Baker can carve a red oak bow out in a couple of hours that will rival most glass bows. A pioneer in wood bow making along with guys like John struck,Jim Hamm, Dean Torgeus, Paul Compstock, and others. Super informative, and willing to share. Highly respected in the world of wood bow making. Quiet the craftsman.

Thanks for link, Tim.

Ok so a simple composite with the bottom of the limb for the belly and top of the limb for the back. Super Bow. ?????? Tim you do know your stuff!!!

This thread began with:    "Why is wood stronger in compression than in tension?"  This could mislead newcomers. It is stronger in compression in some cases, in shear across the grain for example, but for bow-making purposes wood is two to three times stronger in tension than compression.

That’s why I trap the back of even Osage these days Tim . Less mass , less set ,and faster bows.

So, a bow made from the middle of the limb would be very good in tension on the decrowned back, while also good in compression on the belly?

That would be the case if somehow the properties of hardwoods and conifers could be blended together--which has artificially been done with conifer branch bows of various cultures: 

With hardwoods reaction wood is stronger in tension but no stronger or weaker in compression, so a hardwood bow of reaction wood, with the pith at the neutral plane, would be a bit stronger than normal in tension, no stronger in compression. But hardwoods are already two or three times stronger in tension than in compression, so not much point.

With conifers reaction wood is stronger in compression but no stronger or weaker in tension, so a conifer bow of reaction wood, with the pith at the neutral plane, would be stronger than normal in compression, no stronger in tension, a recipe for tension failure, and the reason why those traditional conifer limb-wood bows are backed, often with tension-stronger wood, or sinew. We have clever, clever ancestors.

Tim, if you didn't see the limb or tree cut how can you tell? Would the tighter growth rings be on the down/compression side? Thanks, >>>----> Ken

If you can view a branch crossection it will be somewhat oval, the pith above center, the compression wood below center. Lumber generally comes from straight trees, so no compression wood.

Every so often you see an older hardwood tree, especially maple or oak, with massive scaffold branches growing horizontally outward from the trunk. Seeing one always makes me marvel that the wood in the branch can support such a load--imagine holding your own arm horizontal with a gallon jug of milk in hand, and doing it 24/7/365.

I wonder what the tension/compression balance would be in such a branch, and whether it would make a good bow...any thoughts, Tim?

If a hardwood branch, the top of the limb could be up to 60% cellulose, so extra strong in tension, but low in lignin, therefor weak in compression. A bow made of that top wood would take unusually large set. The bottom-branch wood would have fairly normal properties for its species.

On the other hand, branch bows of vine maple, top of the limb as bow back, do well. Those I've made and seen were from fairly small diameter branches. The pith of reaction-wood limbs is above center, so possibly such bows have normal-properties below-pith wood bellies, no excessive set taken.

And surely there is variety between wood species as to tension and compression strength ratios in their reaction wood.  Way complicated. But that's why we make bow of wood instead of characterless plastic.

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