Official Luthiers Forum!

First; what Woodie and Hesh have said.

I have seen a couple of examples of side cracks that I thought could be attributed to wood fillets. Both were imported guitars, with rather soft side stock, that came into a local store here in New England in the winter. This is guitar hell at this time of year, with low indoor humidity and wide fluctuations. At any rate, both guitars had wood fillets that had been properly inlet under the liners on the flat part of the side below the waist. The sides cracked in the middle and the cracks ran out for several inches.

I ran some tests several (almost 20!) years ago of some side tapes, comparing different materials and glues, and also including some plain wood coupons to see what the difference was, if any. I compared nylon twill weave tape and cotton-poly bias tape, and used both hot hide glue and Titebond 1. I rigged a lever and a can of marbles to apply a measurable load to a sample on a standard support, and looked at now much force it took to break the samples with the tape on the tension side; as if the load were applied to the outside of a side.

What I found was that any tape raised the necessary load to break the samples by about 40%-50%. I used several different kinds of wood, and, as you'd expect, the benefit was a bit greater on the weaker woods. The nylon tape was strong but didn't glue well: the tapes themselves never broke, but they peeled off the sides. The cotton-poly tapes tended to break along with the sides. The ones glued on with hide glue withstood somewhat greater loads. That's nice because it's easier to use in this application.

People object that they have seen sides with tapes that have been cracked, and the tapes themselves were broken, citing this as evidence that the tapes didn't work. There is, of course, no way to know in retrospect how much force was applied. My data suggests that the tapes probably helped, but. of course, there's nothing you can do that will make them bullet proof. I have seen examples of guitars with side tapes where cracks stopped at the tape, so if nothing else it helps to minimize the repair.

As an extreme example, Mark Blanchard lost some guitars that were being shipped home to him after a show in a 'plane crash: they went into the side of a mountain as about 300MPH. The two that were retrieved from the wreckage had amazing damage. IIRC, one bridge simply sheared it's way cleanly through the top. The heads were broken off and the tuner shafts bent. The sides sheared off cleanly at the end blocks, but were not themselves cracked. Mark had used cotton-poly tapes.

One drawback to tapes is that the glue can deteriorate over time, leaving them flapping. We see this a lot in old instruments. Protecting the glue from air helps, and is a strength of wood fillets. I've been shellacking my side tapes in the hope that it would help prevent that. We'll find out in 75 years or so whether it worked...

I'm interested in the question of whether side braces can cause cracks in sides because of a drop in RH, because I built my last few guitars (almost ready to apply finish) with full width side braces, and installed the kerfed linings in between the braces.

Observation: we glue braces cross-grain all over a guitar. In my limited (but tragic) experience with cracks caused by low RH and the existence of braces glued across the grain, I think the top is far more susceptible to this problem than the sides. Witness the backs. They have cross-grain braces. I have had a top crack due to low RH, but not a back (not yet, anyway). It seems to me that the sides would be less susceptible than the back (smaller span of cross-grain brace), and the back is less susceptible than the top. My point is that, if the RH has dropped so much that the sides have cracked because of it, there are probably already cracks in the back, and most certainly in the top. So, I doubt that RH swings are a good reason to not use side braces.

I'm interested in the thoughts from repair folks about the different parts of the guitar, and how each is more or less susceptible to RH-related cracking.

My experiences with Cracks catalyzed by side braces are these;

Several Rosewood guitars that were built in Southeast Asia at around 70% humidity developed side cracks in the Seattle winter at 35%. The cracks were usually in the middle regions between the side braces. They were not able to be closed because of the tension created by the side braces. I suspect had their not been side braces they would have either not cracked or ben more repairable. The Backs also had cracks.

One of the first ukuleles I made was made with gaboon ebony, probably one of the more crack prone woods. I built it before I had climate control probably around 50-55% humidity. I only used 3 side braces and during the 35% humidity winter I developed a small hairline crack right next to one of the braces.

So these cases were definitely humidity caused but also sort of extreme. With typical woods and typical controlled humidity levels I don't think they would have cracked in either case. Although, I would be worried going from 45% down to 10%.

I have been using side braces just on the two flat parts of the guitar unless I feel the wood needs extra support for some reason.

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Pat