Official Luthiers Forum!

Hi all.

My question is simple: why is bellying behind the bridge (and caving in in front of it) is a problem?

The reason I'm asking is because a customer surprised me recently when he told me that (talking about flamencos) he appreciated his old bellied out guitars better than the other ones because they sounded better! And come to think of it, I've seen my share of bellied tops and never was it a problem for me to ajust the strings correctly, neither could I say that the sound was bad because of it. Matter of fact, I've heard many guitars with bellied tops that sounded real good. Only once did I see a top that bellied out so much that the bridge started to come off, although the owner could not tell me which of the bridge coming off or belling came first.

I've heard and read many times about fighting this bellying and caving in (building arched tops being the most popular way to fight this), but never anyone ever explained to me why is it such a big problem?

So your explanations about this are welcomed!

Thanks!

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Alain my friend I am with you and have seen bellied guitars that play and sound great. And a very slight amount of belly tells me that I am not playing a heavily over built guitar.

It becomes a problem if it does not stop at an acceptable point and gets worse and worse and changes the geometry of the string path. The scale gets shorter, intonation goes out the door, and in worse cases bridges can lift and fail as a result of the change in geometry of the string path, saddle, bridge, etc. It tends to be a progressive thing that feeds on itself too with the saddle angle now leaning forward it increases the possibilities of the saddle and bridge failing.

I don't see anything wrong with a trusted old friend that plays and sounds great but has a bit of a belly either. Just keep an eye on it and hope that it does not get worse to the point of impacting the structural integrity, intonation and/or playability of the guitar.

Also just to add my thoughts a bellyed out guitar is obviously lightly braced or has a too small bridge patch, but all that to me says a well loved and played in guitar it'd take years to break a new one in just like some comfy old clothes they fir you just right.

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Formerly JJH

I learn more from my mistakes than my successes

I've seen a few new or near-new handbuilt guitars with bellying and/or caving. I'm much more concerned about these new ones than an older one. I have a 30 yo ply-topped Yamaki that plays like a dream, but has a caving/bellying top that took a good ten years to get where it is. The top is definitely too floppy - it looks like it and sounds like it too. But it hasn't moved in about 20 years.

If it happens soon after being strung up, seems to me they're more likely to just keep on going until something serious happens. If it's an older guitar, I guess I'd be less worried about if it's not too severe and it's not changing. Depends on the value of the guitar too.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

"Never trust a guitar if it doesn't have a belly." - Norman Blake

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Wes McMillian
Santo, TX
http://www.wesmcmillian.com

This effect is the top being pulled into it's max. position .
By string torque.
If it rests there then all is fine and the guitar top will be singing .

If the condition worstens then a new top is in order.

I've seen it 100s of times -on steels also.
It looks scary but as I stated if the top rests there(with no loose braces) and the string height & into. are fine or still adjustable then don't fool with mother nature!

Mike

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Mike Collins

This effect is the top being pulled into it's max. position .
By string torque.
If it rests there then all is fine and the guitar top will be singing .

If the condition worstens then a new top is in order.

I've seen it 100s of times -on steels also.
It looks scary but as I stated if the top rests there(with no loose braces) and the string height & into. are fine or still adjustable then don't fool with mother nature!

Mike

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Mike Collins

I agree about the comments with regard to a newly made guitar that bellies...this indicates a thin top or a lightly braced guitar, both of which are things that contribute to 'better' tone.

I think it also causes stress concentrations, which could lead to additional deformation. I wouldn't buy any new guitar that showed signs of bellying. not that I'd ever actually "buy" a new guitar at this point....it's way more fun to make them!

I'm more of the opinion that a well made instrument should be something that will last more than 5 years and I think substantial bellying/caving in would make me suspect the longevity of the instrument. I certainly can understand if you are a player and you need something a little more cutting edge....as long as you know what you are getting into.

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http://www.PeakeGuitars.com

Ok, so at some point, if the deformation is too extensive, strings will be too high, scale will be too short (thus inducing an out-of-tune instrument) and potentially the bridge will pop off.

This makes sense. But, that said, as anyone seen this situation happen a lot?

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

I've seen this exact thing happen to a guitar built by a well regarded builder. It was built too lightly and the top pulled up making the guitar more or less unplayable.

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http://www.PeakeGuitars.com

It always seems to me that 'belly' is less of a problem than 'dip'. I've seen some gutiars that had lots of bellt behind the bidge and sounded fine. The ones that die are those where the dip in front of the bridge gets past a certain point. Up until that pointis reached, they just seem to get better, and then, wham, they're gone. I wish I knew just what that point was!

Ok, so far we have two potential issues:

1- A derformation so intense that the string height cannot be set correctly and therefore the intonation can't be set correctly either.

2- The front of the bridge plunging to a point it kills the sound of the guitar.

I agree those are indeed big enough problems to watch out for them.

Now as I asked earlier in this same thread, do you guys see such problems come around often in your shop?

Thanks!

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Since the saddle is fulcrum of this action and not the end node of this action the strings do not get higher. In fact they tend to move slightly or not so slightly lower dependent on how bad the deformation is. The front of the bridge and saddle tend to move inward as the back of the bridge is pulled upward. There by lowering sting position at the saddle. This causes the break point of the stings to move further back on the radius of the saddle break as the bridge rotates. This all leads to the scale length getting shorter as the front of the bridge sinks and the action lowers some what. As other have mentioned intonation goes to pot due to the shortening of the scale length.

What makes it appear as if the string elevates is the sinking of the sound hole. but in relationship to a plane from the top bottom of the stings at the nut and the bottom of the stings at the saddle prior to the belling the stings have gotten lower.

Belling also put a strain on the top’s long grain structure and while it may not be a problem it also increases the load on the top’s glue joint at the tail block area.

Personally I think if a guitar sound noticeably better with bridge rotation, then either the listeners ears prefer a tad flat sound or the saddle was not in best intonation prior to the belling.

Keep in mind I am talking of adverse belling, where the bridge is noticeably rotated

Alain Moisan asked:
"Now as I asked earlier in this same thread, do you guys see such problems come around often in your shop?"

That depends a lot on what sort of guitars you work on. Most of the more 'affordable' guitars tend to be over built, due to the need for production shops to assume that the weakest braces will end up on the weakest top, and design accordingly. Bridges come up, especially if they were screwed or bolted down (when WILL people learn?) without much top deformation in front of the bridge.

Some of the mid- to high-end manufacturers seem to go with a thin top/heavy brace model, and these tops often distort more. You'll see dimples behind the ends of the bridges, for example. Some of these guitars will belly quite a lot behind the bridge without caving noticably in front. These can last a long time, and not sound bad, but they never (IMO) sound as good as one with a more 'balanced' top.

Hand mades can be all over the lot, for all sorts of reasons. some people thickness all of their tops to some 'ideal' number, without taking much account of the considerable variation in wood properties. Others, like the idiot with the pepper shaker, figure that if a little (thinner) is good, then a lot (thinner) should be terrific! We all know where that leads. I've seen a few classicals over the years with no bracing around the bridge at all: just the waist and shoulder bars. These usually sound 'remarkable', in the sense that one would remark about a fog horn in a Beethoven symphony. The vast majority of modern hand made guitars are quite durable in this respect, though.

Ultimately, all* guitars will swallow themselves through their own sound hole. The question is whether the instrument will otherwise meet it's demise before it does so. Those WalMart things will lose the bridge or get stepped on or knocked down or run over or simply be thrown out long before the dish in front of the bridge will get to be an issue. A well made and properly designed instrument assembled with good, non-creeping glue, can last long enough to be superceded by changes in fashion before it actually becomes unplayable, or begins to sound so bad that nobody wants to play it. A little maintainance goes a long way, of course.

* I guess I should note a possible exception: those instruments that have well thought out CF reinforcement. With, say a set of 'flying braces' from the neck end to the waist, and some CF on the soundboard braces, it's hard to say how long it would take for the 'cold creep' of the rest of the structure to take over and destroy them.

I'll have to remember that one....that's great!

I think that some carbon fiber does exhibit creep..it depends on the matrix material (i.e. what kind of epoxy the fibers are soaked in). From what I remember of my little bit of experience with CF, it is probably harder to predict creep in a composite than it is to predict it in wood!

For wood, the only numbers I've ever seen are that the strain in wood will creep up to 1.5 times the initial amounts. The same reference suggested that it takes about 2 or 3 years for most of the creep to be imparted into the wood.

I guess that's the trick - making it musical AND making it strong. If it was easy to do, none of us would probably be interested in trying....

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http://www.PeakeGuitars.com

Thanks Michael and Alan. Very informative.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

With harps the belly is expected to pull up some. This is one of the things that make tuning them a bit of a pain. But if they are so heavily built that they don't pull up then they don't sound as good.
With my guitars I also like to see some "bellying" behind the bridge, but I don't want to see "caving in" in front of it. I build with arched X braces and a flat rim. This seems to give me what I want - a reasonably lightly built top that will pull up some (belly), with no caving in around the soundhole.