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Hey all
I'm working on a side by side build. Both OM size. One adi top, one redwood.
I decided to do a deflection test to see how the stiffness compares. I was thinking it would be good to get them both in the same ballpark because I wasn't sure how thick to leave the redwood.

So I thinned the adi down to where I usually go with it between .110 and .115
Deflection result was .220" with 6lbs on it.

Then I started in on the redwood. Most info I found said to leave it thicker than spruce but not as thick as cedar. I figured I'd shoot for .125"

Ran the deflection test at .145" and it was already deflecting more than the adi.

After much deliberation I brought it down to .130". Which deflected .35" with 6lbs on it.
I also thinned the adi closer .110 and it deflected about .275"

I figure I can brace the redwood stiffer and I do plan to do free plate tuning on both tops so I figure I'll get them both to sound fine.

My question is, does this data seem right?

I've not built with redwood much and can't say as I understand whether or not I did the right thing with the final thickness.

Thanks in advance for the advice

Dave

Deflection tracks thickness but it doesn't show the quality of the actual wood until you factor in the weight as well. If the redwood is sufficiently lighter than the Adi then they will perform the same, resulting in similar resonance points when doing the free plate tuning.

And then there is the actual quality of the wood which can vary a lot. As an example, in my brace-blank deflection tests at the same weight, size and density the best euro moves about 2mm, most of it is in the average camp say 2.3-2.6 while the bad ones can move even 3.3mm. In percents that is quite a range. With such less impressive tops my approach is to refine the bracing as much as I can, going for the best stiffness to weight wood for braces, taller and narrower profiles, extra light bridges and so on.

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Build log

Dave: In general terms you are on track. You are dealing with the two extremes of wood stiffness with the Red Spruce and the Redwood.The Red being the stiffest and Redwood near the least stiff. Both can vary a great deal but on average I'd say things are as one would expect them to be.
Tom

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

When I started using deflection tests I used what I call the bare top to help me plan the braces. The more deflection the higher the braces. The key is learning to know what you are looking for. I have a weight that I use on a bare top , then when the top is braces I place the weight at the saddle area and the rim area and manipulate the braces to get the top to move or not move where I want.
In this case I may do scallop on the spruce and non scallop on the red wood then , if you are more comfortable on the spruce top , then work manipulate the braces to get a similar result on the redwood top. That will get you closer.
For the record I do the Red spruce on a forward shifted top at .110 and rear at .100 , redwood I only did a few all forward and at .145 thick.
you should be safe. I also use hot hide glue or fish for the bracing. Keep us posted. It can be a very good learning thread.

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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