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| Top stiffness: latitudinal vs longitudinal http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=17309 |
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| Author: | SniderMike [ Thu May 22, 2008 10:03 am ] |
| Post subject: | Top stiffness: latitudinal vs longitudinal |
The other thread on top plate thickness has me thinking again about this. What do you feel is more important? Stiffness with the grain or accross the grain? Why? And you deflection testers, which way do you measure? I have been going by feel, holding the sides and flexing. So that would be latitudinal stiffness, just so we're on the same page, no? |
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| Author: | Kent Chasson [ Thu May 22, 2008 10:08 am ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Longitudinal stiffness is what's critical from a structural standpoint on a flat top. Both have an effect on sound. |
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| Author: | SniderMike [ Thu May 22, 2008 10:14 am ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Thanks, Kent. Can you tell me more about why, keeping in mind that I'm not an engineer (obviously). |
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| Author: | Kent Chasson [ Thu May 22, 2008 10:35 am ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
I'm not an engineer either so I'll have to keep it simple.... The bridge torque wants to bend the top into an "S" shape. It's the longitudinal strength of the top that does most of the work in resisting that. I assume that, since the X brace goes diagonally across the grain, the lateral stiffness of the top coupled with the X has some effect on resisting that torque. It's an incredibly complex system. But the bulk of the load is resisted by longitudinal stiffness. |
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| Author: | SniderMike [ Thu May 22, 2008 6:38 pm ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Thanks again, Kent. I can easily picture that. Anyone else have thoughts on this? |
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| Author: | SniderMike [ Fri May 23, 2008 10:59 am ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Ain't nobody got nothin' else? Ok, here's some thoughts of mine: When I'm sending a top through the thickness sander, I'm flexing it after each pass, in both directions. Obviously it's a whole lot stiffer longitudinally, and doesn't seem to vary much with each pass. But latitudinally (are these even words?) there are pretty huge changes in stiffness, and a top can get really floppy really quickly, which at some point makes me concerned about its structural integrity. Maybe I'm just not being sensitive enough to the longitudinal change. Or maybe I'm worrying too much about the latitudinal stiffness? Thoughts? ??? |
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| Author: | Alan Carruth [ Fri May 23, 2008 1:52 pm ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
I'm sure the cross grain stiffness has some effect on the stiffness of the completed top, at least in the short term. The problem I see is that over time the cold creep o the wood will probably mean that the cross stiffness has less and less of an effect. From that standpoint it just seems safer to discount it structurally at the beginning. OTOH, the cross grain stiffness certainly does have an effect on the tap tones or Chladni patterns, which ever you like. Changes in the brace angles seem to have onlt a small corrective effect on this: the patterns are more or less dictated by the plate stiffness. And then, of course, we can argue about how much those tap tones or patterns mean in terms of the final sound. |
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| Author: | SniderMike [ Sat May 24, 2008 1:21 am ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Alan, thanks very much for your insight. Can you explain a little more about what you mean by this: Alan Carruth wrote: The problem I see is that over time the cold creep o the wood will probably mean that the cross stiffness has less and less of an effect.
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| Author: | Alan Carruth [ Sat May 24, 2008 4:35 pm ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
The lignin 'glue' that holds the structure of wood togetheris a thermoplastic. As with many such materials, when subjected to a long-term stress, even one well below the nominal yeild pont, it will deform. This is 'cold creep'. It goes faster when you heat the stuff, that's why we can bend sides, but it's also the reason we need to put truss rods in necks. As the top ages under the usual string load it deforms, bellying up behind the bridge and dishing downward in front. Naturally the cross grain stiffness of the wood also communicates some of this force outward to the sides: in order for the wood behind the bridge to belly up in the center it has to pull the wood outboard of it along, and the stiffer the top in the cross grain direction the more width will be effected at first. However, as the deformation goes on I would expect the wood on either side to have less of an effect as it deforms. In other words, if you look at two new tops with the same lengthwise but different crosswise stiffness, the stiffer one should show a smaller and more uniform deformation initially. Over time, however, I would expect both tops to look about the same, assuming the long grain stiffness was the same. It will probably take the stiffer top longer to get there, but I would expect it to eventually. This is theoretical, of course. The experiment might be sort of time consuming. |
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| Author: | SniderMike [ Sat May 24, 2008 8:01 pm ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Alan, thank you for the detailed response. That makes perfect sense. |
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| Author: | Guest [ Sat May 24, 2008 8:16 pm ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Al, what do you think the effect of baking tops might be on long term stability and the strength of the lignin bonds as they may affect cold creep. BTW, I've been having an interesting round of correspondence with Andrew Morrow of Australia's CSIRO, a scientific research arm of the government. He's been testing Aussie timbers for suitability for guitars under sponsorship of Maton Guitars. He agrees that baking tops (or other parts) increases long term stability and lessens the range of expansion and contraction from humidity swings once the wood is back to EMC (Equilibrium Moisture Content). It's a hysteresis effect. Rick Turner |
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| Author: | Alan Carruth [ Sun May 25, 2008 11:02 am ] |
| Post subject: | Re: Top stiffness: latitudinal vs longitudinal |
Rick: I don't know what heat does to lignin over the long term. I suspect that the extra stability of baked wood has to do with changes in the hemicellulose. It breaks down naturally over time with moisture and heat cycling, and baking might speed that up in the way that 'stewing' in hot water does. Alternatively, it might just drive off the 'loose ends' of the molecules, so to speak, and 'cauterize' them so they don't pick up water vapor. It is the hemicellulose that absorbs water, I'm told. It would be all too easy to think up a whole series of material science experiments having to do with wood properties, and the way they change with vibration, heat, moisture cycling, and what have you. Sounds like a whole career, and who's got the time? |
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