X braceing "top sinkage"

[108 Mile]

Hi all,
I own a top tier x-braced mandolin from the 90's. Upon close examination I have realized why these have the reputation. The top isn't so much sunk, as pinched because the bridge is a loar with two feet suspended over the abyss. If it had a solid bridge the stress would have been borne by the x-brace instead of the top itself. Does that make any sense? Your comments are appreciated
Charlie

[sblock]

Huh? I am not aware that X-braced mandolins have any special reputation for top sinkage, per se. In fact, given that the vast majority of modern mandolins are tone-bar braced, then the vast majority of those exhibiting top sinkage are probably also tone-bar braced. Do X-braced mandolins have a greater propensity for sinkage, on a per-mandolin basis? Not so far as I know. In fact, most x-braced mandolins are made by custom makers and small shops (factory-made, budget mandolins are mostly tone bar braced), and I would have thought these were put together a little better, on average.

Anyway, UNLIKE guitars, where the top bracing serves a key structural role, the "braces" (tone bars, really) in carved mandolin tops serve mainly to distribute the vibrations, and not to support (i.e., brace) the top. So your proposed explanation makes little sense to me. Top sinkage can certainly happen, especially with too-thin tops, but it seems unlikely to be directly related to the bracing pattern.

[108 Mile]

Tone bar bracing puts the tone bar directly under the foot of the bridge (give or take) I think bracing has a great deal to do with structural strength in a top.

[rcc56]

Yes, bracing is a significant structural component, even on a carved top instrument. I've repaired enough carved top instruments of various sorts with loose braces to be sure of that.

The single transverse brace behind the soundhole of the old oval hole Gibsons has a significant effect on the stability of the whole top. I've seen sinking and distortion of the tops of these instruments be minimized, sometimes even completely eliminated when the brace was re-glued. Tone bar and X-braced tops will also sink when a brace is loose.

If Charlie's mandolin is built right, the legs of the X should intersect the feet of the bridge. A bridge base with a solid foot would be helpful if they do not. If the top is distorting, the X should be inspected to make sure that the glue joints are still good and that the brace has not split.

[pops1]

There are carved top mandolins with no bracing on the top, the arch is the strength. The bracing is the voicing not the strength. An A model mandolin has but one transverse brace and is still a solid instrument because of the carved, arched top. I find it hard to believe that X bracing contributed to a sunken top.

[rcc56]

It might if the X is loose, split, or mis-located. The problem is not X-bracing itself, but it may be the integrity, profile, or location of the brace. Agreed, some tops are strong enough to take all the stress, but some are not. In any case, on Charlie's mandolin a solid bridge foot would spread the stress out more evenly. And the brace should be inspected.

We have to look at the structure of any top as a whole, and treat each top individually. On a carved top instrument, the thicknesses of the top, the inherent stiffness of the wood, the height of the arching, the location and size of the braces, and the quality of the glue joints can all come into play.

One of the trickiest things about repair work is correctly diagnosing the root cause of a problem. When I've assumed too much, I've sometimes missed something important. I do not mean to make a direct comparison to a carved top mandolin here, but I once had a Santa Cruz guitar come back 3 or 4 times in two years with the bridge coming loose, always at the corners. When it finally occurred to me to darken the room and put a strong light inside the guitar so I could see the position of the braces, it turned out that the X brace was mis-located and was not intersecting the back corners of the bridge.

[sblock]

We seem to have some genuine disagreement about whether the strength in a carved mandolin top comes (1) mainly from the arch, or (2) from a bracing effect coming from tone bars or x-braces that are glued to the underside of the top.

There is no doubt that some arched mandolin tops occasionally sink. The question is whether this sinkage happens mainly in those cases where the wood is carved just a bit too thin, or if it is mainly caused by some kind of failure in bracing. Pops1 is quite correct to point out that some carved-top mandolins have no bracing at all, and these mandolins do not all experience top sinkage. Also, in many mandolins where some sinkage has occurred, the braces can get bent or distorted, but are neither cracked nor separated from the top. These sorts of observations argue against the tone bars being key structural elements.

In those cases where the tone bars (or braces) partially separate from a sunken top, it is not possible to conclude from this observation alone that they were "supporting" the top in some fashion against downbearing string pressure transmitted by the bridge. Instead, the sinking top itself may have born most of the downbearing pressure, but the arch flattened as it sank, causing the tone bars that were glued to it to split away. Both scenarios are equally consistent with an observation of loose tone bars or loose braces.

[JeffD]

Hi all,
I own a top tier x-braced mandolin from the 90's. Upon close examination I have realized why these have the reputation.

I was not aware of any reputation.

[rcc56]

I'm not aware of such a reputation either.

And I'm not really disagreeing with anyone, I'm just taking the view that each instrument has to be judged individually. I've seen a late F-4 with no top brace that still had a perfect arch. I've seen distorted tops restored to their original shape simply by re-gluing a loose brace. I've also seen sunken tops show little improvement after braces were re-glued.

Our job as repairmen is to find the best solution for each individual instrument. Sometimes that means thinking outside of the box. Theory is great, but no two pieces of wood respond the same way, even if they are from adjoining billets out of the same tree. What is holding one instrument together may not be the same as what is holding its littermate together.

The builder's dilemma is learning to build an instrument lightly enough so that it will sound its best, but not so lightly that it will fly apart. And the repairman's dilemma is to determine how to execute structural repairs in a manner that will not compromise the tone of the instrument, but again, will keep it from falling apart.

At any rate, somebody needs to look at Charlie's instrument and make sure that nothing is loose or broken. If everything is intact and the distortion doesn't get any worse, there isn't a problem. If it keeps getting worse, then he'll need to make sure that the problem is not being caused by heat exposure or too heavy a string. If the instrument is indeed getting proper care and it keeps getting worse, then a determination will have to made of what is causing the structural problem and what is the best way to stabilize it.

[fscotte]

Hi all,
I own a top tier x-braced mandolin from the 90's. Upon close examination I have realized why these have the reputation. The top isn't so much sunk, as pinched because the bridge is a loar with two feet suspended over the abyss. If it had a solid bridge the stress would have been borne by the x-brace instead of the top itself. Does that make any sense? Your comments are appreciated
Charlie

I wouldn't worry about how it looks. Is the top sinking, most evidenced by the need to retune the strings to bring them up to pitch. Is it starting to buzz when it didn't before?

Even my 98 Gibson has a bit of deformation at the bass side of the bridge where time and pressure takes its toll. But properly built arched topped mandos are incredibly strong, despite the horror stories you hear. Most can withstand 75-90 lbs of downward pressure, which attests to the strength of their arching. As long as its playable and no cracks appear, I'd not worry.

[Tom Haywood]

The top isn't so much sunk, as pinched because the bridge is a loar with two feet suspended over the abyss. If it had a solid bridge the stress would have been borne by the x-brace instead of the top itself. Does that make any sense?

No. Sounds like the top has not sunk, so what's the problem? Top is "pinched" by the bridge? Not remotely possible IMO. Photos of course would help. Is there an actual problem going on with playing this mandolin, or is it all speculative? What reputation are you referring to regarding X-bracing? Maybe the original bridge was replaced and the new one isn't long enough for the feet to rest above the braces? Have you looked inside with a mirror? That's the only way to tell for sure what, if anything, is going on.

[pops1]

A full contact bridge would place more of the downward force on the X and there by reduce top deflection under the outer edge of the bridge. Whether it is necessary is hard to tell without looking at the instrument. If it is severe change the bridge, if not play it.

[108 Mile]

Thanks for the replies, just to clarify....the top is still perfectly arched. But the top has been distorted by the two feet of the bridge. The inner sides of my F holes are lower (1/8") than the outsides (particularly the bass F hole) So the top has distorted by being pinched. This just came back from a very competent luthier who spent 2 hours trying to get a solid fit. He succeeded but the top looking bent is disconcerting. It has probably not moved in years.

[fscotte]

Wood isnt rigid so its perfectly normal for the top to deform a bit under pressure from the bridge feet. Many great sounding guitars have a top bulge behind the bridge and sunken a bit in front of it.

[HoGo]

There are carved top mandolins with no bracing on the top, the arch is the strength. The bracing is the voicing not the strength.

That is certainly not true. The mandolins that have no bracing have extra thick tops to compensate. Mnadolin top made to typical graduations without tonebars would fail in short period of time. The wood is needed. With tonebars thoughtfully placed you can leave less wood in the top than would be required if top was just left thicker.

To the OP. What you see is commn on tone bar mandolins as well and has little to do with bracing pattern, though the x right under center of strings may make it a bit easier for bridge to dig into arch. I've noticed exactly this on my Krishot (tone bars and quite high "gothic" arch) few months after I got it in '96 and I immediately filled the base with new wood and fitted it as full foot bridge (that was years before that became commonly used) and that bridge is there still today without any change since then. I've reworked failed F-9 a year or so ago that had pretty thick top but showed the same thing... looks like the inside edges of bridge feed are digging into arch or center of top bulging out to fill the empty space.... From my experience this cn have several reasons. FIrst is the bridge base that slowly bends under pressure and ends of feet going up - often seen on bridges that are quite thin at the center of base. Second is the string tension puts top under strain that makes the arch want to bulge up in the apex of the arch which is most often right under bridge or close behind and since the pressure of bridge feet doesn't allow deformation, it will concentrate in the most vulnerable place between the feet. Third is the weakness of top but that typically shows as sinking of the whole top together with this bridge feet deformation (and often without it).
The tops on best mandolins are typically close to the very edge of structural strength to vbe sensitive so bridge deformation can be sometimes seen.
These reasons are why I use full single foot bridge on all my mandolins, It sound teh same as two footer but without any of these issues. I tend to pre stress the bridge during fitting to top so the ands of feet seat first and center after application of pressure, this helps even on single foot bridges to keep the fit of bridge and top shape healthy.

[peter.coombe]

I was not aware of any reputation.

Neither was I. The usual practice with X bracing is to locate the arms of the X so they are directly underneath the bridge feet, and the intersection of the X is in front of the bridge. That is how I have been doing it for nearly 25 years, and Gilchrist for close to 40 years. If the intersection of the X is directly underneath the bridge, then what you have observed may be more likely.

[Buck]

I can't say for sure how it's built, since I haven't had it apart, but my 1999 X-braced Big Sky has presented no problems at all. It's been strung to full tension with medium strings since I bought it new.

[HoGo]

Neither was I. The usual practice with X bracing is to locate the arms of the X so they are directly underneath the bridge feet, and the intersection of the X is in front of the bridge. That is how I have been doing it for nearly 25 years, and Gilchrist for close to 40 years. If the intersection of the X is directly underneath the bridge, then what you have observed may be more likely.

Well I was aware that usual pratice was crossing right under the bridge for Gilchrist, Flatiron and most other builders.
Here: http://www.gilchristmandolins.com/bracing/
The top is arched so the arch supports the bridge feet but may be a bit more elastic than feet supported directly by bars. (perhaps therefore more bass sound from such x-braced)
I've never understood it so the only one I made wit x was crossing 2" north of bridge and bars went right under the inside edges of bridge feet (if the feet were there - I used full foot bridge anyway) The mandolin had some of the X tone character but also some of the tonebar (midrange was still there), though it is impossible to generalize as the wood was extremely floppy engelmann that required really thick top as well.

[fscotte]

I haven't built an x-braced top, but I have always thought that X bracing allowed the center of the top to be a bit thinner while keeping the outer areas a bit thicker. Whereas traditional tonebar allow you to keep the outer areas thinner and the center a tad thicker.

Maybe its just the lighting, but Gilchrist's page on bracing that Adrian linked seems to support that idea no? If you look at the profiles of the tops held up to the lightbulb, you can see how the x braced tops seem to allow more light through the center (thinner) while the tonebar tops allow more light in the outer area.