Question: top flexibility after bracing

[HikariGuitars]

Hello guys, I have been working on my guitar and I was wondering : should a top have any degree of flexibility after the braces are trimmed before going into the rim or should it be stiff? My top flexes a bit when I apply some force across and along the grain and it got me worried .

In the direction that the strings will exert pressure on the top ,it flexes when I apply relatively a lot of strength, however , when I try to flex it across the grain it feels a bit weak..

I didn't want to overbuilt the guitar but now I'm starting to think I under built it , and if did so, I'd remake the top ..


Thanks for the help, regards , Hikari.

[Ned Milburn]

Originally Posted by HikariGuitars

Hello guys, I have been working on my guitar and I was wondering : should a top have any degree of flexibility after the braces are trimmed before going into the rim or should it be stiff? My top flexes a bit when I apply some force across and along the grain and it got me worried .

In the direction that the strings will exert pressure on the top ,it flexes when I apply relatively a lot of strength, however , when I try to flex it across the grain it feels a bit weak..

I didn't want to overbuilt the guitar but now I'm starting to think I under built it , and if did so, I'd remake the top ..


Thanks for the help, regards , Hikari.

Yes, it should flex. If it doesn't, it will be an electric guitar. ;-) And cross grain will flex more than lengthwise.

[John Arnold]

What are your bracing heights? Height of the brace is critical for stiffness.
For example, a 10% reduction in height reduces the brace stiffness by 34%.

[HikariGuitars]

Originally Posted by Ned Milburn

Yes, it should flex. If it doesn't, it will be an electric guitar. ;-) And cross grain will flex more than lengthwise.

Thanks mr milburn ,is there anyway to know if it's flexing more than it should ?

Originally Posted by John Arnold

What are your bracing heights? Height of the brace is critical for stiffness.
For example, a 10% reduction in height reduces the brace stiffness by 34%.

Hello Mr. Arnold, if my memory doesn't faIL me the highest part of the x is at around 1.2- 1.4 cm more or less half an inch.

It's parabolic going up to 0 on the edges ....the top itself is at .9 inches or 2.3mm thick .

Thank you very much for taking your time to answer .

[Alan Carruth]

Some builders actually measure the flex of the top in various ways. An easy one is simply to support it somehow and place a weight at the bridge location, measuring the deflection with a dial gauge. Obviously it makes a difference just how you support it, how much weight you use, and so on, but if you do things in a consistent way you can start to develop usable data after a while. Some makers do essentially that with their thumb, and stop carving the top or braces when it feels right. It takes some time to get calibrated on that.

David Hurd wrote a book entitled 'Left Brain Lutherie' that goes into this at some length. He actually makes deflection maps of tops, and uses them to determine where to remove wood from braces. Trevor Gore gives a target deflection in his book: on the completed guitar he likes to see the bridge rotate forward two degrees when the string tension is put on. Less than that and he considers the top stiffer than it needs to be, and much more than that risks collapsing over the long term. I don't know how this relates to instruments by other makers.