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I have an engelmann top that is at .107" thick, has a deflection of roughly .150" at 18" span and 5 lbs weight, and a density of approximately 6.71 g/cubic inch. Is this OK for an SJ or should I save it for a smaller bodied guitar? Will still have to do finish sanding and such...

Thanks

should be ok

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I concur with John.
Tom

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A person who has never made a mistake has never made anything!!!

I just did a run where the tops deflected .200 at your weight and span and they turned out great. I don't think species means much in itself as far as strength goes.

I should add the deflection test was longitudinal, I think (end grain at supports)

At one point I would have agreed with the yes people but lately I would say set it aside. I would look to a much smaller guitar for that thickness in that g/cu" range. You didn't mention the cross grain stiffness either, that also makes a difference. Do you have that ratio? It could still make an excellent guitar, of course. I would want it to be thicker for an SJ though.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

I can check the cross grain, how wide apart for the rails, 12"? Same weight?

I have more of these tops so its no problem to use another, but thought maybe I want this thickness anyway.

Edward, it doesn't matter what the span is as long as you do it for both the long and cross the same. I usually use 13 1/2 or so.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

.800" at 13.5" and 5lbs for cross-grain and about .035" for longitudinal.

I get about 23:1 for those measurements. I would consider that pretty far off what I look for. What is the end grain like? Is it quartered in the center and then splays off as it moves out?

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Burton
http://www.legeytinstruments.com
Brookline, MA.

Perfectly quartered, fine grains.

I am with Burton. I don't do deflection testing, but every Englemann top I have used ended up about 10% thicker than any red or Euro. .107"would be in the range of what I'd use on a 11 1/4" parlour. On a 000 type I'd never use that stuff thinner than .125". There are, of course, exceptions, but they're very rare.
The other important issue is that Englemann is usually much softer than other spruces and tends to sand faster (and thinner) during finish prep.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Depends if SJ means small-jumbo or super-jumbo!

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https://www.facebook.com/profile.php?id=100008907949110

Whatever happened to;
It all depends on how you brace your top?
Thinner top- taller braces
Thicker top - shorter braces


Granted, I'm in the school that would probably save it for a 00 or parlor, but if I was feeling gutzy it might go on an SJ, or better yet (this is Engelman right?) a classical. Keep thinning for that tho.


dave

I agree you are near the threshold . If you match your braces to achieve a better deflection you would be fine . This is one of those things you need to decide . After all you have the top not us . Best advice , err in the side of safety .. Using it on a smaller body would be fine , if you have a top available , use that .
It is easier to make thinner than it is to make thicker . I have built with other spruces to .095 and seen some Martins with sitka and red as low as .090 . Thinner you will loose some headroom so think about that . Engleman is more for a finger style guitar in my mind .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Granted it is like comparing apples to potatoes, but with my deflection setup and a 5# weight I take my 6 string SJ's to 250 to 280. I take my 12 string SJ's to around 210-230. This usually ends up with a much thinner top than many of you are used to.

By your description, your deflection setup is very similar to mine.

With this in mind, my last 2 SJ 6 strings had tops thicknessed to 0.094 and 0.093 with Shane's Lutz spruce. 6.8-6.9 g/cu density

Obviously bracing as well as so many other factors come in to play. As a Somogyi grad, I do prefer thinner tops and well thought out lighter bracing.

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Andy Z.
http://www.lazydogguitars.com

How do you guys answer a question like this? It seems to me that there's some missing information such as how wide was the top. is it rectangular or guitar shaped, was it load applied across the the whole width, or just in the middle? So, I personally don't think you have a real good answer, but .107 isn't too thin imo for an average top depending on your bracing, as Dave points out.

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Jim Watts
http://jameswattsguitars.com

Sorry Jim, it is rectangular (15.5x22), loads were applied nearly across the widths as part of the weight used was a smooth plane.

Guess I should not of thought asking here would make it any easier, given the nature of the subject.
Do appreciate all the input. Like someone mentioned engelmann does seem to sand easily so I think I will join another top up and shoot for around .110 after finish sanding. Maybe save this one for another SJ but when I have more experience.

Thanks

Edward, there's no reason to be sorry, as an online community we tend to have lots of opinions about things we don't have our hands on, including my statement BTW. That said, there is some value in hearing and processing every ones opinion. I didn't mean to sound like a crouch, which I think maybe I did. Sorry bout that.

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Jim Watts
http://jameswattsguitars.com

Edward: At the figures you gave in your first post I was tempeted to tell you to go thinner but was trying to be conservitive. If it was mine I would make it thinner.If you do another top and make it thicker let us know what deflection you get with the same set up.
Tom

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A person who has never made a mistake has never made anything!!!

Both my training and my intuition tell me that increasing bracing size is not the answer for a too thin top.
Unless you reduce the spacing between braces so that it has less span between braces then the weakness in the top remains.
Not saying that this one is too thin, just that in principle, the top and the bracing both need to be optimized rather than trying to fix one with the other.

Good post Jeff! This top is not too thin, or might be too thin. It depends on how it is braced. My tops will have deflection three times that of the OP.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.