Official Luthiers Forum!

I've been assuming all along in my little bubble that back reinforcement strips are a default component of acoustic guitars. A thread on another forum pointed out that some Martin models do without. I was surprised by that so a little googling showed that guitars with no back reinforcement strip are not that uncommon.

It was pointed out in the thread that the top plates are joined without any such a strip and do just fine even though they are under a much higher load so the back reinforcement strip is actually completely unnecessary. The cases where I think this argument might not hold are when there is a decorative backstrip inlaid into the back joint, when the grain of the back plates form a chevron at the joint so it's not a true side grain joint, or with highly figured wood in which the grain is going in all directions.

Does anyone here build their guitars without a back reinforcement strip, and if so, under what circumstances?

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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

Yes.

Most Larrivee's don have them, at least when I was there.

If there is to be an inlay, I do use it, but only then.

If you have really high quality back wood, then it shouldn't be necessary. But curved grain, or backs cut so the joining edge comes from near the center of the tree are not so friendly.

Also, softwoods will compress under clamping pressure and squeeze out minor ripples from a power jointer, whereas rosewood will not. So it's harder to get a really good joint with the really hard woods.

Not so fast. Just because Larivee and a few others at times don't do a BJR (back joint reinforcement) does not mean it's a best practice.

I've personally repaired more than a handful of guitars that split the back center seam and did not have a BJR..... We always discuss too when this is the case why a BJR would have either prevented the separation or forced a crack elsewhere on the back.

Mind you my climate is Michigan and we used forced air furnaces in our dry winters. Folks don't humidify and guitars crack or separate.

From my personal experience with separations here if I was still building I would continue to do as I always did and add that BJR. If you live in paradise, the RH never changes, and people are angels floating around on clouds where guitars never get kicked or bumped in the back I might nix the BJR..... Do they have scotch up there? I might never find out.....

I recall one serious separation that I repaired where the edges puckered outward like someone who has had too many Botox injections in their lips..... Getting rid of the pucker required scrapping and finish repair.... On the inside what do you think is done when a back separates and needs to be repaired? A BJR is added at least in the reglued area....

As J said if there is decorative strip I would also always use a BJR. YMMV and if it does it only brings Dave and I more business in time when your guitar splits it's back....

Here's a question for those willing to ponder it:

What about a reinforcement strip for the area of the top that runs from the bridge plate to the tail block? Does anyone do it? Does it hurt the sound? I know it is common to put small cross grain patches in a few spots along that stretch of the top, and repairs can call for larger patches. What about a continuous strip (interrupted by the lower face braces, obviously), like we do for backs? I've not done it; I've only added the small patches. Just curious.

Hey Don!

You mean "diamonds or cleats" I think. My answer would be..... it depends.

I built with very thin tops and that reduced gluing area for the center seam. As such sometimes I installed maybe four very small cleats for added stability for the center seam joint. OTOH though most guitars were made without any reinforcements on the top center seam because braces do the same thing and most guitars never split a center seam. It does happen though and it happens more commonly on the top likely because the back has the BJR.

The additional difference for me at least is the the top is the big kahuna so to speak where excess mass may.... have an impact (for better or worse....) on the sound produced. I'm sure the back does too but I never cared as much about my backs as I did my tops in terms of excess mass.

I doubt that it will hurt anything but if you are thicknessing to traditional thicknesses history kind of tells us that it's not all that necessary.

If I was building a guitar that had to withstand the ravages of say a dorm room, no humidification, drunk kids kicking and puking in it, bong water spills in the guitar, and being repurposed as a place to hide one's stash..... I might cleat the top.

I don't risk it. It's easy to do, easy to glue and helps me to sleep at night. Plus I like the look of it (maybe because I'm used to it).
But I must confess I had one back getting loose at the center seem (prior to gluing the back strip). I could swear that the joint wasn't worse than any other I've made. It was flat dawn, heavy madagascar.

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member of the guild of professional dilettantes

In a classical guitar with traditional fan bracing, that reinforcement is provided by the center fan. Now, to amplify on Hesh's warning, I have seen a lack-of-humidity lower bout top split at the center seam that split the fan brace as well. So humidify those guitars!

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Jim Kirby
kirby@udel.edu

I've seen too many come apart at the back seam too.,.. One of which was one of mine!!! So I will never not use one again. Also, the top has lots of cross struts, bridge patch and of course the bridge itself to help keep it inline. The majority, by far, of the cracks you see are not on the glue line but all it takes is one brain dead moment to leave a guitar in a hot car or something and the seam can come apart.

Read the thread with AM coffee, stuck a mirror in this factory fresh one. In for a u/p install.



The bevels are a nice touch. Should keep the stress from getting concentrated at the edges............not.

I would rather the time spent installing these Spruce blocks was spent on drilling an endpin hole. Guess thats now an add on.

I have not used a back center strip reinforcement yet. I was just looking at the back of a guitar body hanging up looking sadly at me... (One I may never finish for a variety of reasons)..... The back sucked flat due to 19% humidity - and the center seam itself is holding up fine. The wood in proximity to it isn't so happy because of bug tracks in the wood - but that's not the center seam and I knew about the bug tracks.

In terms of guitars.. I have seen a bunch of guitars WITH center seam reinforcent come apart...

So let's be clear here.... Just because it has a center seam reinforcent strip doesn't mean it won't come apart either.

It seems more to do with whether a high quality joint was made and how squirrely the wood was.... One curly maple back (with a center reinforcement) on a 100 year old guitar required extra veneer purfling strips to be added because the back wouldn't close... The wood had moved or shrunk differently... Or perhaps there was a hollow in the joint right there and the veneer had previously expanded to fill the gap a little better or whatever... I don't know what the case was because it was an open joint when I bought it.... But it had separated both from itself and from the back strip...

Why would you not want to use one?

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It's not what you don't know that hurts you, it's what you do know that's wrong.

The discussion devolved into "oh.. All we ever see is non-reinforced back joint failures." And my point is that the reality is that in pure numbers - you see a WHOLE lot more reinforced back joint failures.... Probably well over 100:1.....

Now... You can quibble that it's simply a numbers game... And how many non-reinforced solid wood (non-laminate) backs are really out there..... But the reality is that for luthiers... If they do see a failed back center seam - it's likely to be a reinforced one.

I haven't inlaid center seams or used purfling strips between the halves... When I do - I will likely use a reinforcing strip of some sort.....

My own quibble is putting a crossgrain patch which expands and contracts at a radically different rate than the long grain wood it's glued to.... I never saw a jointed plate suck inside out till I put a crossgrain brace onto it..... And it's spectacularly so with the old fashioned wall to wall 2" wide spruce bridge graft you see under the bridges of the old ladder braced guitars.

Thanks

We just repaired one of those about a month ago:

Also, fwiw, I've started using cleats on the
tops of my builds after one came back from a client in South Dakota with a big old top seam split. Turns out, he enjoys playing his guitar in the South Dakota moonlight, which apparently dries the crap out of the guitar, LOL.

I've also found that highly figured woods commonly used for backs, especially when there's sapwood at the seam, are more prone to splitting then the generally straight-grained spruce that we use for tops.


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Mountain Song Guitars www.mountainsongguitars.com

The braces running along the seam like that one don't offer a whole lot of protection from the seam opening up. They run along the seam in in the same direction as the easiest way to split them. Still, I would expect the top to split next to the brace because it is much thinner. Shows what I know. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I have used back reinforcement strips on all my guitars. I had one top seam separate on one of my older guitars and have incorporated diamond shaped cleats ever since. I do both of these elements the way Cumpiano/Natelson recommend.

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George :-)

I'll interject something that a violin making friend of mine pointed out once. You often see old fiddles with a row of square cleats across a crack. Sometimes there will be a new crack along the end of the line of cleats, because they're too heavy to move with the wood they're glued to. Other times you'll see cleats that have come unglued at one end, again, because they're too thick to move with the wood. You never see a broken cleat. They're always thicker than they need to be.

I've always used center seam reinforcements, in part because I normally incorporate an inlay line in the back, and it seems appropriate, Since that little chat with my friend I've alwys made them as thin as I can, and tapered out at the edges. The width of the reinforcement is proportional to that of the center inlay; often the width of the inlay plus 6-7 mm on either side. Keeping the reinforcement no wider or thicker than needed helps eliminate cupping over the seam.

I have a 120 year old Bay State. It is BRW. The back center seam is reinforced with BRW running parallel to the back. The whole guitar is badly cracked, including the back center seam. Undoubtedly this guitar spent many years, if not all 120 years, in dry winters and no humidifier. The sides look like they are reinforced with cross grain BRW. Although badly cracked, the guitar held together. It had been strung with metal strings. The original bridge was gone, so it had a tail piece and floating bridge.

What about inlaying a wood strip just for the purpose of increasing the gluing area? Maybe one on each side 30% into the plate?

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Pat

I like that idea. It almost serves as the back graft, especially if cross-grained. Hmmm, like radial purfling....


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Mountain Song Guitars www.mountainsongguitars.com

End-grain-to-end-grain joints sacrifice about 75% tensile strength versus long-grain-to-long-grain joints, while long-grain-to-end-grain joints suffer from the instability of the relative movement of the cross-grained construction and some reduction in tensile strength. When adding a decorative back strip, we assume at least a 50% reduction in cross-grain tensile strength, and very little if any tensile strength contribution from the back strip - particularly where the composition of the strip is largely cross-grain or end grain construction. We also assume creation of a big stress riser at each edge of the back strip channel...so even if the wood retains enough tensile strength to exceed the minimum needed, the rapid change in stress across the joint will ensure failure at that point. We address these strength and stability issues with an applied cross-grain graft that eliminates the stress concentrations and adds tensile strength.

For back joints without a decorative strip that are well made (planed joints with fresh surfaces) and glued with adhesives that do not creep or heat-release at moderate temperatures, we view that back graft as purely decorative and unnecessary. Likewise, on tops that are joined with care and glued with suitable glues that resist creep, we see no reason to add mass with diamonds or other devices, particularly where they create a stress riser in an area where none existed before..

On the repair side of things, when we see failed center seams on guitars, it's often a case of poor factory joinery (jointing done with dulling blades on machines) and thick glue lines or heat-related creep damage which softens or releases the thermoplastic glue (aka, Titebond). Taylor top seams on 199-2005 guitars seem to be the worst of the lot - we saw so many of these over the last couple years that it became a bit of a joke when a customer would call with a top crack..."Let me guess...an early 2000's Taylor?"

So to recap, we reinforce for recovery of tensile strength and elimination of stress risers when using a back strip, we cap the back joint for cosmetics (our customers expect to see it) even when we do not use a back strip, and we always joint with a plane for maximum strength and glue with hot hide to avoid the poor creep performance of Titebond.

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A constellation only takes shape when one maps the whole.
- Beth Brower

Do they have scotch up there? I might never find out.....

Hesh, I wouldn't worry, I hear the other place serves a good single malt as well.........

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"I need to hurry up, the years are going by far too fast"

I have come to no defiinitive answer on this topic . I will use a x grain fir , wrc or spruce strip to reinforce the back, I use titebond HHG, and sometimes epoxy depending on the wood .Sometimes I will glue a thin piece of 1/4 sawn sycamore , on ukes long grain to long grain to reinforce the back seam . I rarely sand a joint unless it/s a real PITA. I usually use my L/N low angle jack with microbevel for hdwds and ece 24in jointer planes or ECE bladed 18in DIY try plane the ece /s have a HSS blade which is nice as they tend to last a while before sharpening. A sharp blade and a freshly jointed edge are indispensibile in creating a lasting joint. In his last book . Jose romanillos mentioned that he had switched to gluing a long grain reinforcement strip on the back .As others have mentioned long grain to x grain creates poor adhesion. problems. Vlnmakers will typically use a washcoat of HHG /sizing of the joint before the final gluing . Food for thought,. My 2 cents worth cheers, and practice making joints and testing them to see if they come apart . Practice on scrap!!