Along and Across Grain Stiffness Ratio

[Mitch Lees]

This is my first post and so I thought I had better introduce myself. I am an amateur guitar maker living in Findhorn in northern Scotland. I have 3 classical and 3 steel string (plus a number of electrics) to my name, built mainly using the Bogdanovich, Cumpiano methods. I am quite pleased with these instruments but I would like to improve the design and hopefully the sound. I was introduced to this Forum by Trevor Gore as I have just got his books and was going through what seems to be an ‘initiation rite’ of trying to put the Target Plate Thickness equations into Excel – I was struggling with this, and Trevor though I would benefit from the information on this forum, and he was right – I now have an accurate spreadsheet and quite a lot more knowledge of Excel! I am looking forward to contributing to the forum and I was pleased that you accepted me – being from the ‘wrong’ hemisphere, although my father was a New Zealander and so I feel some affinity. There are a number of disadvantages of living in the UK at the moment, but probably the most serious is that I am too far away to attend one of Trevor’s courses!

Now to my question which is to do with the ratio of along and across grain stiffness (EL/Ec). I understand how the ratio was used in the sensitivity evaluation (section 4.5.4), but looking at the Elong/Ecross column in Table 4.5-3, I wonder what (if any) conclusions can be drawn from the value of the ratio with respect to the quality of the plate? What is the utility of the list in the Table - how should we use it? I cannot see any real correlation between a high/low ratio and the predicted plate target thickness. Is there any?

I realise that I am coming to the party late, and that most of you have probably put this sort of thing behind you, but I have searched the Trevor Gore section of the forum and can find no one else who has asked this question – which makes me quite nervous (stupid question?).

Grateful for any advice
Mitch

[Trevor Gore]

I cannot see any real correlation between a high/low ratio and the predicted plate target thickness. Is there any?

They're correlated via the thickness equation, but not an easy one to see, of course.

I wonder what (if any) conclusions can be drawn from the value of the ratio with respect to the quality of the plate?

None that I can see. Most of the wood listed in Table 4.5-3 was made into guitars and they were all perfectly good guitars. So it's very hard to hear much correlation between the EL/Ec ratio if you build using the thickness equation. I'd still avoid wood that is ridiculously floppy across the grain, though.

A (genuine) question I ask at my modal tuning courses is why some people have a strong preference for high cross grain stiffness, as it seems not to have much impact on anything. I have yet to get a satisfying answer. As far as I can tell, the preference comes mostly from those building classicals. Steel string guitars are traditionally factory built and wood isn't "tested" for this. Classical guitars come from much more of a hand making tradition, so cross grain stiffness would get (sort of) tested. The stiffer cross-grain wood produces less "print through" for the predominantly longitudinal bracing, so I'm guessing that might be a possible reason. A slightly larger fan angle would compensate for low cross grain stiffness whilst Martin X-bracing avoids this problem.

Western red cedar is typically lower damping with a lower Elong/Ecross than spruce. Treated right, cedar tops produce more sparkly trebles (contrary to popular opinion that WRC gives a darker sound). So is that down to the lower damping, the Elong/Ecross ratio, or some of both?

I haven't been able to isolate the cause, but I would be inclined to think it's more to do with the lower damping.

[Mitch Lees]

Trevor,

Thank you again for your time. Glad I have been through the 'spreadsheet process' it helped a lot with gaining a more detailed understanding of the relationship of the various equations, and being able to do more with Excel will be useful in the future.

Regards

Mitch

[mqbernardo]

Hi Mitch, Tevor has already given (at least one) courses in the northern hemisphere. Maybe one day he could come to europe? i´d be in.