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Hi. I am installing mahogany strips on the sides of my first guitar. I'm using standard kerfing. Should I make the strips as long as the sides are wide and then piece the kerfing in between? I've read not to just install the kerfing and then glue the strips on in the leftover space, to avoid stress risers. My other option is to cut them part way into the kerfing. Thanks for your help.

If they are thin enough you may be able to feather them into the sides and run the linings over them. Inletting them part way into the linings, as you mentioned, should also work.

What ever method you do don't just abruptly end them and butt them up against the bottom of the linings. If I was going to do it again I'd run them full width from top to bottom and glue a piece of lining on top of it and trim it down so it's the same thickness as the rest of the linings, or just inlet it into the linings.

you mean cut off a little piece of lining and glue it over top of the strip? That sounds like an ok plan to me.

I like full height braces with linings inbetween, but it is kind of a pain trimming all the lining strips to length and installing them individually, plus the braces can cause problems in low humidity. The sides shrink but the braces don't get any shorter, so the braces end up putting pressure on the soundboard and back, potentially causing cracks or popping part of the joint to the linings in extreme cases. Either level the rims in very low humidity, or shave a tiny bit extra off the brace ends before gluing the soundboard and back on.

Why not?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

A side reinforcement strip butted against the lining creates a focus for stress due to the sharp discontinuity in transverse stiffness of the side - this type of construction forces the crack to occur at the edge of the lining, which is a challenging repair to make due to the difficulty in reinforcing the joint. The 'perfect storm' for repair difficulty would be solid, non-tapered (rectangular cross section) linings and butted side supports - a more pronounced stiffness change at the lining/side support joint, and a very difficult job reinforcing a repair.

For side reinforcement and crack stopping, we use cotton/poly bias tape in hot hide glue, and run across the full width of the side. Linings are applied over the tape. The only place we use side supports is on the flat runs of a modern dreadnaught, which are more vulnerable to handling damage than the curved portions of the sides (e.g., 'key' cracks). We use a single support centered on each flat area of the upper and lower bout, made lining-depth and 5/16" wide, with the linings butted against the support.

In terms of repair, we see the worst side cracks on instruments without side supports, but those also tend to be the easiest to repair where traditional triangular linings are used due to the cracks usually being towards the center of the side, with sufficient room for reinforcement where needed. Side crack repairs on instruments with side supports vary - repairs of cracks that run across side supports are complicated by those side tapes or braces, but the chance that the crack will run across those supports is reduced versus no supports. In some cases, we see multiple, parallel cracks from a damage event restricted to a single 'bay' between tapes...the same impact on a side without reinforcement might generate just one longer crack. The worst side cracks we've seen have been on unreinforced sides that are carrying a lot of stress - it becomes a real challenge to align and glue the sides of the crack without mechanical means.

Perhaps the other repair shops might comment on their experience with side damage versus reinforcement schemes, but here - with an emphasis on light, adequately stiff construction - side tapes and the occasional side brace on dreadnaught-style guitars seems to be the best trade of stiffness, damage tolerance, and repairability. Laminated construction - essentially plywood sides - would seem to be the most damage resistant construction, if the extra weight and construction time is acceptable.

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For the times they are a changin'

- Bob Dylan

As what Woodie G said but in pictures:

Using thin veneer or glue soaked bias tape and running the linings over it seems like the best approach to me. For those that do laminated sides 4" fiberglass drywall tape might be an interesting option (I have yet to try it). www.amazon.com/dp/B073X6GJXR/ref=asc_df ... 2620016615

Sometimes I think we create more problems trying to eliminate problems caused by careless owners.

Side supports and side tapes in my mind have different purposes and functions.

Tapes as we see on Martins, G*bsons and many others including most of what I built are intended to assist in arresting a side crack or in other words providing some protection against the crack traveling further along the side if a crack happens.

Side supports OTOH are just that, intended to flatten out perhaps a wimpy side or a side that's too thin or one application that I used them for was with rope figure mahogany. Where ever the rope figure was the wood rippled a bit. Rigid side supports helped flatten out things and kept them flat.

Two weeks ago we installed side supports on a G*bson acoustic, recent vintage that had no supports or tapes. One side had already cracked and the crack had traveled 40% of the length of the side. Another service provider CAed it together, sloppily.... without even leveling or attempting to hide the repair. No backing material and they could not even pull their heads out enough to scrape the glue filet and wet sand it in to help make it disappear.....

Anyway when the other side cracked and this one was over 60% of the length of the side the steward brought it to us. We glued the crack (not with CA....) and did so with it level and matching grain, etc. and then installed side supports because these sides should have never been used on a guitar in the first place. Our side supports in this case were mahogany, quartered, 1/2" wide and butted into the linings but we beveled the butt points and sides of the supports to nothing. Not perfect but we can't reengineer the entire guitar through the stinking sound hole.

I completely agree with Woodie that abrupt termination of side supports creates a stress riser. That's why we beveled to nothing but I would have preferred if it was possible (meaning when the thing was built) to use side tapes full height and under the linings AND side supports or better yet throw these sides away or cook fish on them or what ever and not use them for Lutherie.

Anyway my pedantic point here is that comparing side tapes and side supports in my view is like comparing a spoon and a fork. You can eat with either but one is better for some things than the other. If your sides are more likely to crack use tapes. If they are more likely to distort use supports. Insecure about Lutherie and unaware that guitars are not JUST.... a woodworking project use both and sleep better at night.

My sides are a bit thin and are a bit cupped in one spot so I figure the solid strips make sense. I'm my only customer though, so i'll deal with any problems in the future. The idea of shortening the strips a bit makes sense too, although it is COLD and dry here right now.

Cool, be sure to have some fun too!

I'm trying to understand this so bear with me. Doesn't the edge of the lining itself create a sharp discontinuity with respect to transverse stiffness of the side? How does butting a brace end against the edge of the lining increase the discontinuity at that point? It seems that if the brace end was the same thickness as the edge of the lining that, if anything, it would reduce the transverse discontinuity where they meet. I'm not an engineer so I would like to improve my understanding of how this works.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

If the end of the brace and the top of the lining wear the same height, the energy from an impact from the outside would go right to the middle. The side would want to Pivot on that intersecting line. It would not be true for an impact that originated from the inside if they were touching. If it was a tight fit and they were glued, there would be no stress riser from an outside hit. However, usually what you see is lining that tapers and side braces that taper leaving the point in the middle the weak link.

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Pat

The brace and side combination are much stiffer than the lining and side combination (for a kerfed lining), but both are stiffer than the side alone (the lining and side about 70% stiffer than side alone, and the side brace/side combo about 3-1/2 times stiffer), so the crack will develop there at the butt joint, versus clear of the linings. It's usually abrupt changes in stiffness that are a problem, versus more gradual changes; this is an example of a design flaw concentrating stresses in a small area.

I'm not an engineer, either, but there is a surfeit of them in the shop...this discussion would be worth a couple pages of the big sketchbook used to explain seemingly everything except why engineers are so fond of over-explaining.

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For the times they are a changin'

- Bob Dylan

Are you able to see the pic I posted? Sometimes Google doesn't post images for some reason... But anyway if you can that crack is right at the point where the kerfed lining meets the side reinforcement. It's not easy at all to fix and would not have happened if the strip went from back to top.

jfmckenna - No, I'm not able to see the photo, but I can imagine what it shows.

Ok, the weak link argument makes sense to me. The side is essentially reinforced across its width by the lining and the brace, but not right at the butt joints themselves making them the weak points.

So based on the above posts, if one is going to have side reinforcement, the brace or tape should be let in to the lining or you're better off having no brace or tape due to the difficulty of repairing a crack along the edge of the lining.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

This topic makes me think about cost/benefit as it applies to what lengths builders go to to accommodate low probability events or careless or negligent guitar owners. I certainly could invest the extra time and effort for letting in side braces into the lining, but for how much actual real world benefit? Is it time well spent over many guitars? Is the risk of a crack at that butt joint really so high that it justifies the extra time and effort? I’m concerned about cracks that might result from expansion/contraction with humidity because that is something that a lot of guitars do get exposed to, while I’m not terribly sympathetic to people who bash and drop and abuse their guitars and cause cracks that way. I don’t feel obliged to build beefed up guitars as a matter of course on the off chance one of my guitars might end up in the hands of one of those folks. On the other hand, if there’s something simple to be done that can minimize or eliminate a risk, like not using the type of end pin that can split a tail block or using unslotted bridge pins, that’s fine.

Since my main interest in side reinforcement is its function as a crack stopper for cracks caused by expansion/contraction, I’m wondering, if a crack is going to happen in a side because of humidity changes, does a butt-jointed side brace substantially increase the chances of it happening at the butt joint instead of somewhere else on the side and if so by how much? Is it slightly more likely or is it pretty much guaranteed to happen there?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

"On the other hand, if there’s something simple to be done that can minimize or eliminate a risk, like not using the type of end pin that can split a tail block or using unslotted bridge pins, that’s fine."

Glue saturated bias tapes or thin veneer strips are relatively simple and don't interfere with linings, and offer some "crack stop " qualities. More substantial side reinforcements are probably not needed on properly thicknessed sides. Many of the old guitars have neither (and some of them are split from end to end ).

I think the best defense against side cracks from the humidity change is no side braces at all because they don't let the side move. I'm not sure if tape would have the same effect or if it would be more flexible.

And no, if there were side braces and the humidity caused a crack it would most likely be in the middle somewhere which I have seen a few times. Probably just depends on the most susceptible place in the wood. Not to say that it couldn't happen along that seem though.

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Pat

I do mine like this, for the reasons stated above.

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Joe Beaver
Maker of Sawdust

As Mr. Breakstone mentioned earlier in the thread, side tapes appear to me to be much more about limited protection against impact damage and a spread of a crack, versus a way to straighten or stiffen a side against all loads. Tapes function more like a bending strap than a brace - just as a bending strap carries much of the tensile loads when working a side or binding on a bending iron pipe, side tapes allow more of the side to stay in compression during a damage event, which avoids cross-grain tensile or 'spreading' loads and limits the length and severity of cracks which do occur.

If a few side tapes or braces are good, many would be better, and that is the idea behind using a cross-grain lamination in laminated side construction - the sides get much stiffer across the grain and much better at handling loads that would crack or buckle a conventional guitar side.

The problem with this approach is that making one component significantly stiffer can shift stresses to another part of the guitar. We have had several guitars in with crushing damage - a SCGC instrument lent by an indulgent uncle to a careless college-bound nephew, and a luthier-built 'double-sided' custom guitar that suffered from the owner's forgetfulness (and the rear wheels of his F-350). Both guitars were partially crushed, but it was interesting to note that there was far more top and back damage on the instrument with laminated sides than the conventionally built 000. While the SCGC instrument required a portion of the top to be replaced, along with repair of well over 60" of cracks on the treble side (and lots of loose braces), the double-sided guitar was not economically repairable, and would have required a new top and at least half of the back replaced, as well as at least one of the sides (hidden damage was suspected in one area where the finish on the side of the instrument showed extreme checking, but the side still appeared to be in one piece). The loads the sides transferred to the top and back were such that the linings sheared off the sides...but only after the top collapsed and the sides drove the back into the rigid foam lining of the case.

One of the gentlemen that is frequently here suggested that this was another case of a stiffness-driven structure where making one component of the instrument much, much stiffer than the others (e.g., top, back) resulted in a big change in how the guitar responded to a damage event. While the purpose of the builder's decision on side construction was to avoid energy losses in movement of the sides, the consequences of that decision affected a number of other characteristics unrelated to tone.

On the topic of humidity-related side cracks, none of the other repair people that frequent the shop could recall seeing low-RH events that caused side cracks, although it is difficult to tell in some cases whether a crack is due to the nature of the wood (e.g., stump wood) or some other cause. If the goal is resistance to cracks originating from RH-related wood movement, it seems like Mr. Macaluso's suggestion of adopting SCGC's and Taylor's approach - no cross-grain reinforcement to impede movement and ensuring that grain orientation in the side, neck, and tail block is uniform - might be a more viable strategy than side tapes or braces.

_________________
For the times they are a changin'

- Bob Dylan

I think I fixed the image now but anyway that happened to this guitar due to low humidity. I like to build guitars as light as possible, for the most part, and at the same time take into consideration at least a minimal abuse. But I tend to agree, I'm not going to build a guitar that is able to withstand baking in the oven in the back seat of someone's car in Arizona. But at the same time I want it to be able to handle the desert climate.

" If the goal is resistance to cracks originating from RH-related wood movement, it seems like Mr. Macaluso's suggestion of adopting SCGC's and Taylor's approach - no cross-grain reinforcement to impede movement and ensuring that grain orientation in the side, neck, and tail block is uniform - might be a more viable strategy than side tapes or braces."

The down side to that is that even moderate "bumps" can split the sides from end block to end block.

Just want to amplify another poster's comment about RH related side cracks.

I can't recall after seeing and repairing likely hundreds of side cracks a RH caused, for sure... side crack.

It's always a blow, a hit, being dropped, someone sitting on it, the mic stand fell over, etc. It's also at times poor materials as in the case of the example I posted prior.

Think about it. Our sides are framed by binding. Most guitars, f*ctory guitars are built with binding material specifically engineered to absorb a bit of a hit because the corners are where a lot of hits happen. Sides are in the same region and very subject to being kicked, what ever.

With this said I'm not sure that anyone understands with complete certainty why tapes, supports, none at all are all choices in terms of for what. For me they are insurance against additional damage from an accident not insurance against RH swings. Sides are also not expansive to the degree that tops and backs are and as such dimensional instability from RH swings has less impact simply because sides are less wide than tops and backs.

I'm not sure that every maker is attempting to address the same issues. Clearly SCGC and Taylor are addressing RH and they say so but others may not see RH as something to address when it comes to sides.

Don't get me wrong I am sure that I could dry out a guitar enough to crack a side. I'm just not seeing it though in the day job.