Belly bulge on F-5 top

[NRMiller1958]

I built this F-5 17 years ago, and after a lot of travelling, it's starting to complain. The top (between bridge and tailpiece) seems to be bulging upward, presumably from forward movement of the tail area, though the tail is only slightly out of square (maybe 1 mm) with the plane of the instrument.

I switched from J-75s to J-74s to try and lower the tension on the top, but I'm wondering if there's anything else I should do to stabilize the instrument and prevent any further movement. It's a great sounding mando, and I want to do all I can to retain the tone.

Thanks,

Neil

[MikeEdgerton]

Are the tonebars intact? That would be heart breaking. It looks like a really nice instrument.

[sunburst]

The bulge between bridge and tailpiece is a common occurrence in tops that were carved too thin. Not much to do abut it now, but after all, it's been 17 years. It could stay relatively stable for many more years.
Watch for the beginning of a center seam separation in the bulged area. That follows the bulge in many cases, especially when the cause is a top that is too thin.

FWIW, many of us carve tops with a "spine". That is, a somewhat thicker area in the "string line" area of the top. I usually don't carve the string line area under 4mm anywhere.

[NRMiller1958]

Watch for the beginning of a center seam separation in the bulged area. That follows the bulge in many cases, especially when the cause is a top that is too thin.

Good guess. There is a hairline separation (maybe just the finish at this point?) That obviously concerns me more than the bulge.

This was built using Simminof specs. In more recent builds I've followed Adrian's Loar sketches and increased the top thickness just forward of the tailpiece. I agree they're stronger, though frankly I'm still trying to match the tone of this one.

[fscotte]

That looks like the inevitable... However, as John said, it may hold for some time yet.

[Mark Marino]

Good guess. There is a hairline separation (maybe just the finish at this point?) That obviously concerns me more than the bulge.

This was built using Simminof specs. In more recent builds I've followed Adrian's Loar sketches and increased the top thickness just forward of the tailpiece. I agree they're stronger, though frankly I'm still trying to match the tone of this one.

I had this exact same experience, but mine lasted only 3 years before cracking on the center seam. The crack was about wide enough to get your fingernail into. After one failed attempt to compress the crack together and glue with titebond (it failed again as soon as I restrung it), I used 5 minute epoxy, and did not try to press the crack closed. that has held up for 4 years now, and I gig a few times per month with it. I’d carefully sanded the epoxy almost level and it blended pretty well- doesn’t stand out beneath the strings.

One thing about thinly carved tops is that they sound awesome.

[sunburst]

One thing about thinly carved tops is that they sound awesome.

Not always. A top must be well paired with a back to sound "awesome", and sometimes that means the top can be pretty thick.

Back to the OP, the top grads in the original Siminoff book were pretty thick. It doesn't make sense that a top cared to those grads would be too thin. Perhaps the top is a weak piece of spruce (assuming it is spruce). Do you remember where it came from?

[Jim Hilburn]

While I don't remember any cracks or separations, Grismans crusher has the same issue.

[sunburst]

As did Roland White's Randy Wood mandolin. He had someone reinforce the underside of the top with (If I remember correctly) carbon fiber cloth.

[NRMiller1958]

Back to the OP, the top grads in the original Siminoff book were pretty thick. It doesn't make sense that a top cared to those grads would be too thin. Perhaps the top is a weak piece of spruce (assuming it is spruce). Do you remember where it came from?

The top is sitka bought from Alaska Specialty Woods.

[Ivan Kelsall]

Rather than the bulge being an increase in height behind the bridge,it's the bridge area compressing downwards,creating the impression of a 'bulge'. I'd maybe remove the strings & let the top relax a bit,& then have it reinforced as per John Hamlett's idea,although maybe not with CF cloth if other materials are available.

Here's Frank Ford's take on it :- http://www.frets.com/FretsPages/Luth...ndotopsag.html

Ivan

[NRMiller1958]

I've repaired sagging tops, but this is different. A sagging top results in lowered action, which has not been the case with this instrument. it's really more a bulge than a sag.

My reservation with reinforcing the seam is that if there are structural issues causing the seam to separate, strengthening it with epoxy or carbon fiber, as some have suggested, might well transfer whatever forces are separating the seam to other parts of the top and crack fresh wood. If there is a structural issue, seems like I should address the cause, not the symptom. This F-5 has parallel tone bars, so a transverse brace like in the FRETS post isn't an option.

[MikeEdgerton]

Enlighten me, are the tone bars bulging with the top or separating from it, or is some other force at work here? I would have assumed that the placement of the bars would somewhat keep this from happening. Obviously I'm wrong. Is it bulging between the bars?

[pops1]

Will the separation on the seam close without the strings on?

[fscotte]

A transverse brace is in fact, an option.

Personally, since you built it, I would not hesitate to rip the back off and fix it. This is not a difficult thing to do if you use a sharp japanese saw and slowly cut along the rim joint. You'll have peace of mind knowing that it will hold.

[Jeff Mando]

It seems fairly minor at this point. I would just play it and enjoy the sound until you can no longer play it, raise the bridge, compensate, whatever, then at that point make a new top or repair this one. You might be talking 10-20 years from now.

One thing you know for sure, if you mess with it, it will sound different. But, not necessarily a bad thing, IMHO.

Good luck with it!

[HoGo]

I agree with what John said. The top is likely too thin in the bulged area. Even if the numbers don't seem too thin, the older Siminoff plans had apex of the arch well behind the bridge which helps start the problem (the further the arch is from straight line between bridge and tailpiece the weaker it is for given thickness).
The seam will start top open and the only thing I found to help is restoring shape of arch with hot sandbags against plaster cast followed by a large chest patch to make the area stronger. I did exactly this to Schofield with same symptoms. (I posted some pics here a year or so ago). It may take some time, but once the joint gives, things will go south pretty fast.
Some players love the type of tone these mandolins produce but, IMHO, they are just woofy and hollow without the "core" of tone and often lack real volume - they can apear to be loud and responsive when played alone but they get lost among other instruments.

[Ivan Kelsall]

A question from the un-enlightened - with so much down pressure,how does the upward bulge come into being ?. What force is acting on that specific area causing it to move upwards ??. I'm finding it hard to understand - but that's just my lack of knowledge. I know that it doesn't happen to banjos !!!,
Ivan

[HoGo]

A question from the un-enlightened - with so much down pressure,how does the upward bulge come into being ?. What force is acting on that specific area causing it to move upwards ??. I'm finding it hard to understand - but that's just my lack of knowledge. I know that it doesn't happen to banjos !!!,
Ivan

There is around 80kg (160 pounds) of string tension that pulls on the tailpiece which tends to rotate the tailblock and compress the top along the centerline. The bridge holds the center down so the bulge is concentrate in the wide open area behind the bridge. Imagine pressing ends of stick against each other, the stick will bend, same principle. Some mandolins with substantial tailblock and back thickness can withstand this to some extent but over time it will show.
On violins, thin top results into "camel back" arching with bulge at both lower and upper bout and bridge area lower.
On mandolin the area between the neck and bridge is much stronger due to lower-smoother arch, less width and tonebars being closer to centerline.

[MontanaMatt]

My tone gard keeps my belly bulge off the back, but if your belly bulge is affecting the top, diet and exercise are in order!
I couldn't resist, sorry!

[Willie Poole]

I have a mandolin made from the early Siminoff booklet and it too has a slight bulge behind the bridge , I never really noticed it or thought about it until this post came up, anyway mine was built in 1981 has no apparent cracks but has lost some of the original tone which I figured was due to lack of playing time on it since I now have other mandolins to play...If it fell apart tomorrow It wouldn`t worry me because it more than paid for its self over the years when it was my #1 mandolin at all of our shows...

Willie

[rcc56]

Try the simple approach-- glue the crack with hot hide glue, splinting it if necessary, and cleat it. And check and re-glue the tone bars if they are loose. No need to rebuild the mando unless/until it caves.

Maybe the crack was caused by the graduation, maybe it is only a humidity crack. I see an awful lot of those.

[HoGo]

Try the simple approach-- glue the crack with hot hide glue, splinting it if necessary, and cleat it. And check and re-glue the tone bars if they are loose. No need to rebuild the mando unless/until it caves.

Maybe the crack was caused by the graduation, maybe it is only a humidity crack. I see an awful lot of those.

In the OP you can see the arch nearly touching the tailpiece, that is not just small amount of deformation (unless it was carved that way). Cleats won't help in those cases as more often than not the joint is still closed (or almost) on the inside edge and wide open at the outside.

[rcc56]

I stick by my guns on this one. Simple repairs first, disassembly only as a last resort. This is not a violin that was built to be taken apart.

Start with a complete inspection.
1. Check tone bar glue joints.
2. Check tailblock joint, which no one has mentioned.
3. Inspect crack thoroughly, both inside and out.

If the distortion is indeed very bad and the glue joints are good, I would still opt for crack reinforcement first. If you like and you're crafty with magnets, you might be able to install a reinforcing strip through the f-holes. Or drill tiny holes through the crack and install the reinforcing strip with a flexible caul, using Don Teeter's "cross patching gadgets," [Stew mac now sells a version of it, called "Crack Clamp, item #6271], to align the patch and pull it into place with wire. Reference Don Teeter's "The Acoustic Guitar" for this technique. Let the instrument relax for several weeks before attempting any such repair.

If any repairs executed from the outside do not hold, they can be reversed upon disassembly. Better to try the simple repairs first rather than risking the mandolin.

A lot of things can go wrong disassembling a mandolin, especially if it was not built with hide glue. Plates can crack or shatter, stubborn binding joints can cause wood tear out, linings can be broken, etc, etc, etc. Think long and hard before pulling a mandolin back.

Another approach to this repair which is not as invasive as removing the whole back, but is still difficult, is to reference violin repair techniques and "slip the tail block."

J-75's are a heavy set, heavier than I would use on anything but a heavily built factory mandolin. Lighten the load and the instrument might stabilize.

[pops1]

I agree with rcc56. If a simple repair doesn't work then decide an new route. I have glued many crack with hide glue with and without splints or even shavings of wood for a lasting stable repair.