Tension/Compression Ratios For Different Woods

[Cameroo]

So I'm sitting at the in-laws today, everyone's lounging around with full bellies, so I figured it's a good time to do some reading from TBB vol 4.  I'm reading about the tests done at the first MOJAM where they are comparing the properties of different woods to settle the "Osage reigns supreme" debate.

Anyway, I haven't had much time to read the 4 volumes I got for Christmas, but after seeing the chart that was made comparing the speed of different bows of different design and different woods, I got wondering if anyone has ever compiled a list of the tension strength/compression strength ratios of common bow woods. 

I think this info would be ideal to know when designing a bow for optimum performance.  For example, when using the mass principle to reduce weight, if one knew that a given wood was twice as strong in tension as compression, then it would make sense to drop mass by trapping the back.  If one knew it was much stronger in compression, you might opt for more of a D cross section belly. If the ratio is 1:1, I'm thinkin a rectangular cross section would be ideal.

Has anyone done any actual tests to determine these ratios?  Keeping in mind that I only have 3 bows under my belt, all of the same design, I'm really interested in what some of you veteran bowyers have to say about this.

[Parnell]

Heya Cam,

It's something that I'm interested in, also.  Tension/Gravity/Compression and most interestingly, to me, how they are used in design.  I'm playing around with some of the tropical hardwoods that grow in zone 10 - south florida- south of the Caloosahatchee River.  Many of the tropical hardwoods here have good tension/compression ratios. 

Good topic, Bro.

Stephen

[Dauntless]

I'd be afraid interspecies variation would reduce the utility of any data put out there.  There is sometimes a big variation in wood density which seems to have a direct effect on compression strength.

I'm certainly not an expert though.

[Cameroo]

I agree.  The SG can vary greatly even among the same species.  But if the info was collected along with the SG, maybe conclusions could be made from that, if enough examples were recorded.

Although you might be able to come to the same conclusions just by bending a sample over your knee   I realize that any bowyer who has made more than a few bows from a given wood will probably get a feel for the best design, and that is likely the best way to learn.  I was more or less just curious if there were any ball-park numbers out there, to help a greenhorn like myself (who has only used maple), when looking to try new woods (without wasting a bunch in the process).  Probably the best thing is to just start with proven designs and go from there.

I just have a scientific brain, and figured some tension/compression numbers would help prove why a certain design suits one wood over another.

[Marc St Louis]

I looked into that about 10 years ago but gave up on it because there is more to bows than tension/compression ratios.  Elasticity is more important

[kiwijim]

I wonder if there may be a direct correlation between elasticity and tension/compression ratio?

[BowEd]

I'm afraid there are things going on in different woods that take a life time of making bows to find out.Nothing earned nothing gained,but TBB4 and this forum are a great start.

[crooketarrow]

  WOOD IS WOOD you can drive yourself crazzy with all the TEX stuff. If your really into this you have to build glass bows where you have some unformity. Like I said WOOD IS WOOD even the same tpye of wood differs for area to area. As you biuld bows you'll find out what you like and the way you like it to be. I don't like to take the primitive out of my bows. The age in which we live in has alreadly done enough of this. But I realize people build bows for different reasons. Just have fun is the way I se it.

[Cameroo]

I wonder if there may be a direct correlation between elasticity and tension/compression ratio?

I would think that they go hand in hand.  Of course there are other factors involved, like moisture content (which also ties into specific gravity), but I think that would be a good place to start.

Crookedarrow - I'm not the least bit interested in building fiberglass bows.  And yes, all wood is different.  But when a guy like Badger, who has made thousands of bows, records so many details about each one, patterns start to emerge that can become useful for other bowyers.  I don't think that there is anything non-primitive about that.  There is no point in re-inventing the wheel.

Of course when it comes to building bows, the whole point is to have fun.  This is just something I have been pondering in between scraping sessions.  But if a guy like Mark says this is futile, I'm sure that it probably is

[Badger]

   I don't think their is a correlation between tension, compression strengths and elasticity. Yew is proably the most elastic, elm and osage are also up their. Strength of tension or compression is simply a measure of how much something resists bending. The elasticity is obviously how much it can stretch before it deforms. Usually the main things we are dealing with is avoiding a bow taking set or developing chysals or simply breaking. As a rule of thumb most woods are slightly over or under 1% in ability to stretch and compress. The majority of them being slightly under 1%. The most elastic woods are probably about 20% more elastic than the least elastic bow woods. (don't quote me on that). But to make it simple we can treat them pretty much alike as far as thickness, the width needs to be adjusted to develop the draw weight we need. As we get to know a specific wood better we can hone in closer on how it likes to be treated. When I built the 24 ft bow for the giant catapult I just figured it based on taking a flat piece of wood and bending it into a circle, then figuring both the inside and out side circumferences of the circle using trial and error for the thickness until I got a difference of about one percent, then I gave myself a 20% safety marging and went from there.

[Cameroo]

Thanks for you input Steve.  You and Mark are the two people I most wanted to hear from. 

I didn't realize that the compression/tension was so similar amongst the thousands of different types of woods that will produce a bow, or that the elasticity could vary that much.  That would make sence then that elasticity would be a more important factor, as Mark stated.

I still think that in theory, knowing the compression/tension of a specific wood could help reduce mass from the correct portion of the bow.  But I guess it's also true that it would take for ever to collect enough data to be useful, and would probably be easier just to learn by trial and error.  I guess that's one symptom of growing up in my generation - we're always looking for shortcuts.  Some would call it lazy, I call it smart and efficient   But in my defence, if you consider the bowyers of thousands of years ago, they would have the same benefits of the knowledge passed down from their ancestors.  We are just playing catch up to them, after much of that knowledge was lost or disappeared.  But they knew what worked.  They might not have understood exactly WHY it worked, just that it did.  As someone new to bow-building, it is the "why" of it that facinates me.  After reading more of the "Design and Performance Revisited", I'm slowly learning.  Thank god for the TBB's!!

[Marc St Louis]

A wood may be strong in compression but low in elasticity, take Black Cherry as an example.  Because it is low in elasticity it will chrysal quite easily.  Bubinga is another wood, very strong in compression but a bit low in elasticity.  Consequently it will chrysal if stressed too much

Knowing if a wood is tension strong is good to know if you want to use it as a backing

[George Tsoukalas]

Cameroo, Tim Baker talks about specific gravity, elasticity, compression and tension in his bow design chapter in TBB 1 (P. 96). I'm not ready to dismiss compression and tension but he says compression and tension have to be coupled with elasticity in wood choice. I agree. I prefer not to use a wood that is weak in compression (belly), weak in tension (back) and low in elasticity. Osage is high in compression, tension and elasticity. Petty good stuff. Cameroo, I like the way you are thinking on this..just find a way to couple that compression/tension ratio with elasticity which is also very important. I also don't want to use a wood that has  a low specific gravity (similar to density). Low to me is less than 0.60 because that means I have to make the bow excessively wide which can be an issue in the deer woods. Longer can also be an issue which is why compression and tension are important. Sorry for the rambling thoughts but it is getting to be nap time. Keep up the good work. Jawge

[nyarrow]

Cameroo,

  We must think alike. My plan was to slowly build the data over time by detailing the layout, dimensions, design, wood and force draw curve of each bow I make. There is great info out there on the types of woods and many guidelines that have been developed over the years but I see no real data that defines tension, compression, elasticity and etc. I know many say the difference between trees of the same species pretty well makes this effort futile; however for years many of the traditionalists have also been incorrect on the usefulness of kiln dried lumber.

[ScottSing]

Guys i found it

http://www.woodworkweb.com/woodwork-topics/wood/146-wood-strengths.html