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I did some testing of side tapes several years ago. I got samples of nylon twill tape, and also the cotton-poly bias tape that I've been using. I made up cross grain wood strips, cut from failed sides, about 4" long, 2mm thick, and an inch or so wide (all the same, of course), and put tapes on them. I made strips of several types of wood, and had several samples of each type with no tape, with cotton-poly tape, and with nylon tape. I also made samples of each type of tape with Titebond and HHG. The tapes were glued on, and shellaced over, which is what I normally do.

When the samples had dried for a few days I made up a long lever setup to break them. I put the samples across a couple of dowels an inch or so apart at a certain distance from the pivot, with the tape side down. I put another length of dowel across the sample in the center of the span, and then loaded them until they broke. To load them I used a can full of marbles, which I slid slowly along the lever. By recording how far out from the pivot it was when the sample broke, I could calculate the force required.

Basically, any tape about doubled the amount of force it took to break the sample. The worst was the nylon tape, which tended to come unglued without actually breaking, as it did not stretch and the glue didn't stick to it well. The 'weaker' cotton-poly tape took more force, since you actually had to break it, and generally broke where the load was. The samples glued with HHG were stronger with either tape than the Titebond samples.

I think that side tapes must have gotten their bad rep when Martin started to use that awful self-adhesive stuff. The thick latex adhesive allows the side to move and crack relative to the tape, so they don't really do any good structurally. The adhesive also dries out over time, and the tapes start to flap loose, with a lot of dust on the sticky areas. I used to fix side cracks for one local performer who had a Martin with those tapes every time he got back off the road. I finally got sick of it, removed them all, and replaced them with cloth tapes. He stopped having to come back for that particular repair.

As has been said, if you use wood fillets you _must_ inlet them under the liners. If you don't the abrupt end of the fillet at the edge of the liner creates a stress riser, and if the side breaks at all, that's where it will break. That's the hardest place to fix it.

I have seen a number of examples of side cracks that stopped at a tape. One maker points out that, in old guitars with side tapes, if they are cracked the tapes are also broken, and draws from this the conclusion that the tapes didn't work. The missing piece of information is how much force it took to crack the side. My experiment, and experience, suggests that the tapes do help, and I use them on all my guitars.

I have seen guitars with wood fillets that had side cracks because of them. In one case, the crack ran across two fillets, and the fillets themselves were intact. These were deep sides of a relatively soft wood, and I have to wonder if the sides shrunk in low humidity across their depth, while the fillets, being long-grain, did not.

It is pointed out that wood fillets are stronger than side tapes, and that is certainly true. My only rejoinder to that is that anybody buying a hand made guitar should at least try to take decent care of it. Stuff happens, of course, and we makers should take _reasonable_ precautions, but there is no way you can make a guitar bullet proof, and I think it's unreasonable of customers to expect that.