Side Reinforcement Stress Riser

[redir]

I stopped using side reinforcement some time ago and imho here's a good reason why! Well, I guess as many have mentioned here if you extend the reinforcement to the very top and back or at least inlet into the linings a bit then it will help. Here's a flamenco guitar I built about 10 years ago that went through some winter drying stress. That little ripple that you see is the very tip end of the reinforcement, mahogany brace I believe, and the lining.

So take warning and DON'T do this

[TEK]

No picture on this end???

[rogthefrog]

Originally Posted by redir

I stopped using side reinforcement some time ago and imho here's a good reason why! Well, I guess as many have mentioned here if you extend the reinforcement to the very top and back or at least inlet into the linings a bit then it will help. Here's a flamenco guitar I built about 10 years ago that went through some winter drying stress. That little ripple that you see is the very tip end of the reinforcement, mahogany brace I believe, and the lining.

So take warning and DON'T do this

The picture isn't working for me It seems to be set to private in your Google Photos account. I'm very curious about this.

[redir]

Sorry, perhaps now you can see it?

[dekutree64]

Picture is working now.

I do think the lazy style of side braces do more harm than good, and are still a pain to install. Better to use tapes that extend under the linings, or nothing at all.

I usually use full height side braces with linings glued between, but it's a lot of work, and they do have the drawback that their ends can push against the top/back in dry weather. I recommend gluing them in extra low humidity to reduce that. If you want to use very thin sides for whatever reason, this style gives great stiffness and strength.

[fazool]

Is that the edge of the kerfing?

[redir]

Yeah that crease is right at the edge of the linings.

[Alan Carruth]

Yes, side fillets MUST extend under the liners or they're worse than nothing. Any pressure on the side is concentrated right at the juncture of the fillet and the liner, and that's where you'll get a crack eventually: just where it's hardest to fix.

I have seen a couple of guitars with properly inlet side fillets that developed cracks in the center of the side. Apparently the sides wanted to shrink a bit in dry weather but the fillets didn't get any shorter, so the sides cracked right across the fillet. Up until I saw those I thought of that as only a theoretical issue.

[printer2]

No picture on my end.

Sort of on the topic. I installed some reversed kerfed linings on a jatoba back and sides. Had a couple of cracks form due to low humidity (my bad) and they formed at the kerf. I am thinking if the non kerfed side was next to the back there might have been a little less cause for the back to crack.

[redir]

I'm wondering if feathering the ends of the side brace to zero would help relieve some stress. Maybe I should just remove them. But anyway, I'll learn one the hard way. It's my own guitar so I am not too worried about it. This guitar incidentally was sort of an experiment in that I built almost the entire guitar using CA. I think the top and back were the only thing I didn't use CA on.

When I started building I used tape, then I thought that was useless so I started making braces and then at some point I read Cumpiano and Natelsons book and they didn't use anything! So I thought heck if it's good enough for them then why bother. But with this flamenco the sides were made very thin so I thought I should sure it up a bit.

I've never had any issues with the guitars I side braced in the past, my guess is because the sides were the normal thickness of around .9in. The flamenco sides were closer to .7in IIRC. Course back then I didn't know anything about letting the braces into the linings, I do now

[dekutree64]

Originally Posted by redir

I've never had any issues with the guitars I side braced in the past, my guess is because the sides were the normal thickness of around .9in. The flamenco sides were closer to .7in IIRC. Course back then I didn't know anything about letting the braces into the linings, I do now

That makes sense. .09" definitely shouldn't need side braces. .07" probably would be fine too. I've done as thin as .04" sides with Honduran rosewood (which has very high Young's modulus, so similar stiffness to .05" Indian rosewood) on a small classical/flamenco (3.75" tail depth) using the full height side braces. 5 years old now and no sign of trouble.

If you feel like taking the back off, you could shave the side braces down to .02-.03" tall, so they function more like tapes, but still have a slight stiffening effect, but not enough to concentrate a lot of stress. Or if you have an idea of how to remove them while working through the soundhole, go for it. Feathering ends may help a little bit, but probably not enough to bother doing it.

[John Arnold]

I only use wood side braces when the sides are too flexible, or when I want to reduce rippling of the sides that happened during bending. To eliminate the stress riser, I like to inlet the braces into the kerfing, but not extend them all the way to the top or back. Typically, the braces I use are hardwood that is about 5/8" wide by 1/16" thick. This matches the visual appearance of the ribbon that I use most of the time.
Older Gibsons have flat spruce braces (1/2" wide by 3/32" thick). These were scarfed thin on the ends, and the kerfing was just glued on top of them....no notching. While this leaves a gap in the glue joint, it seems to work OK.

The same rule applies to this as when gluing braces on a top or back....make sure the wood is as dry as possible before gluing.

[yellowesty]

Originally Posted by Alan Carruth

I have seen a couple of guitars with properly inlet side fillets that developed cracks in the center of the side. Apparently the sides wanted to shrink a bit in dry weather but the fillets didn't get any shorter, so the sides cracked right across the fillet. Up until I saw those I thought of that as only a theoretical issue.

This makes good sense. But neck blocks and heel blocks are usually made with the grain running top-to-back. Why don't they cause side-splitting with humidity changes?

I use sturdy hardwood inserts to affix weights to the sides (ala Gore/Gilet) and, though I haven't had problems (yet), I thought I might orient the grain of those blocks along the sides rather than top-to-back. But if the neck and heel blocks don't cause a problem, my side blocks shouldn't either.

???

[murrmac123]

Originally Posted by yellowesty

But neck blocks and heel blocks are usually made with the grain running top-to-back

And what evidence do you have for that assertion ?

[Alan Carruth]

yellowesty:

Cussed if I know! FWIW, the sides and backs on those guitars were of a very soft wood that looked like an acacia, and the sides were quite deep. Maybe the wood had higher shrinkage than the usual rosewoods or mahogany, both of which are pretty stable.