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  #1  
Old 06-16-2019, 04:02 PM
rainier38 rainier38 is offline
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Default Martin Bridge Question

Hey all,

Got a really good deal on a used Martin 000-18 that was basically brand new. It's a 2019. Never had a super nice guitar and been inspecting it, as most do.

Only issue I could notice is the bridge dips with the top behind the sound hole a smidge. Some suggest Martin builds this in. Others say it could be a dry top.

Any thoughts?

Last edited by rainier38; 06-19-2019 at 04:25 PM.
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Old 06-16-2019, 05:00 PM
keith.rogers keith.rogers is offline
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They're built flat. But, over time, the tension of the strings will make the top belly up slightly behind the bridge and dip in front. How much and how soon can be different with every guitar. If it didn't do that, it would be a very stiff bunch of top-wood and braces that got put together.

I can't tell from the picture, which could be distorting things, so I'd want to see a straight edge against it (hard to do, I know). Or, get an opinion from an authorized Martin repair person. It looks like a lot of movement for a guitar that is "new", but I haven't stared at new guitars lately.

My 40 year old guitar, built from a Martin bracing pattern, has a tiny bit of dip, I'd estimate it at less than .5mm (1/64). My 7 year old Martin is still essentially flat. I might convince myself there's a dip of maybe the thickness of a piece of paper. It's one of the A-braced models, though, and not lightly braced, either!
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Old 06-17-2019, 04:54 AM
B. Howard B. Howard is offline
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Quote:
Originally Posted by keith.rogers View Post
They're built flat.
No! They are not made flat. They are made with a slight curvature from the factory. Approximately a 53 foot radius on the top of any Martin guitar.

If they were made flat the stress of the strings and their 180 lbs of pull would cause the top to break in short order. In engineering terms the curve of the top makes it pre-stressed so as to not further deform under the pressure.
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Old 06-17-2019, 05:47 AM
keith.rogers keith.rogers is offline
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Quote:
Originally Posted by B. Howard View Post
No! They are not made flat. They are made with a slight curvature from the factory. Approximately a 53 foot radius on the top of any Martin guitar.

If they were made flat the stress of the strings and their 180 lbs of pull would cause the top to break in short order. In engineering terms the curve of the top makes it pre-stressed so as to not further deform under the pressure.
53 feet? Down? I.e., in the opposite direction from the fingerboard radius?

I'm not disputing your number, but I am pretty sure you wouldn't see that radius on a piece of wood only a few inches long. The picture posted shows a much more obvious dip than a radius of that size.

Edit: I've been reading about the top/back radii over at UMGF and remembering all the stuff I forgot about this. Of course, the top is radiused during construction - the amount doesn't matter. And, it is done to counteract the force of the strings pulling the bridge forward and down. A properly made bridge would be radiused to match the top curvature at the factory. Clearly, they distort to a flat or even downward curve (without breaking) over time. The picture seems a bit much for a new guitar, but as a subsequent post says, if the action is correct with that much bridge and saddle it's likely Ok. It is certainly not built in, though.
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Last edited by keith.rogers; 06-17-2019 at 06:25 AM.
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Old 06-17-2019, 06:05 AM
jonfields45 jonfields45 is offline
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If with that high saddle the action is fine, then ALL IS WELL. Try to enjoy the guitar with your hands and ears!
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Old 06-17-2019, 07:14 AM
redir redir is offline
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I've built at least 20 guitars now with true flat tops. I'll have to disagree that the tops will break in short time. My understanding of arching tops was more for changes in humidity then for structural purposes though it probably acts as both. But there are many small shop luthiers that build true flat tops.

---

As was mentioned it's a bit hard to tell in the pic,a shot with a straight edge would help. I do see what you are saying and it could be cause for concern but probably not. If you live in the desert then get a humidifier and use it regularly with the guitar in the case and see if it changes anything. You might want to look around inside at the bracing too with a good light source and a mirror. Do that with string tension on and off.
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Old 06-17-2019, 09:05 AM
rainier38 rainier38 is offline
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Thanks all for the words. I’ve been playing 15 years on an old Korean made Ibanez. This is my first nice guitar purchase. I bought it off a guy who bought it new online (unseen) and was upset to find it had a two piece neck, which obviously doesn’t bother me and is found on many of these newer Martins. I had to jump because the price really was right.

This is still the factory setup. Action is a touch high but string height and neck angle check out in my minimal experience. Using a flat edge or a piece of string shows a slight dip in front of the bridge, favoring the bass side a little. Behind the bridge seems normal with gentle bellying. I’m a bit far from a luthier but will get it in for a checkup. Everything else seems normal.

I’m thinking taking the saddle height down a little might reduce torque and I will humidify until I can get to a luthier. Just curious about others experiences on the matter as I’ve never dealt with a Martin before. Just wondering if it’s recoverable or has the potential to get much worse.

Last edited by rainier38; 06-17-2019 at 09:12 AM.
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Old 06-17-2019, 09:56 AM
Alan Carruth Alan Carruth is offline
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FWIW, Trevor Gore, in his widely appreciated and well researched books, suggests that the bridge 'should' rotate toward the neck by about 2 degrees under string tension. You can measure this fairly easily using a laser pointer taped to the bridge. Clamp the neck to hold the guitar still, and see where the laser spot hits the wall. Slack off the strings and see how much the spot has moved. The math from there is fairly simple.

Note that even with the strings slacked off the top probably will show some evidence of distortion. Wood 'cold creeps' over time: takes a permanent 'set' in the direction of even a fairly small load. This creep is faster at first and slows down over time. It may never really stop, but seems to reach a level of about three times the initial displacement where it becomes relatively stable.

As the bridge rotates forward the stress on the glue line at the back edge increases. That 2 degree short-term displacement implies about a 6 degree long term deformation. Although I've never measured it, my 'eyeball' judgement suggests that a bridge that has tipped forward more than six degrees is well on it's way to flying off and killing the cat. Too little rotation indicates a top that is too stiff to produce sound properly, so that 2 degree number makes sense.
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Old 06-17-2019, 11:38 AM
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rick-slo rick-slo is offline
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Quote:
Originally Posted by rainier38 View Post
Hey all,

Got a really good deal on a used Martin 000-18 that was basically brand new. It's a 2019. Never had a super nice guitar and been inspecting it, as most do.

Only issue I could notice is the bridge dips with the top behind the sound hole a smidge. Some suggest Martin builds this in. Others say it could be a dry top.

Any thoughts?

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That is totally normal. No worries.
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Old 06-17-2019, 01:24 PM
redir redir is offline
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What I *think I see in that image is that the bridge itself is actually arched along with the dip in the top just in front of it. That bothers me but that's why a shot with a straight edge would be good to see too because it might just be the pic.
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Old 06-17-2019, 02:34 PM
rainier38 rainier38 is offline
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this is maybe not a better photo, but i just moved and my tools are still in boxes. this is a chopstick laid across the interior of the bridge. a definite dip, more pronounced on the bass side (maybe makes sense because more tension here?). bridge appears to flex slightly with the top.

here's the gap...

Last edited by rainier38; 06-19-2019 at 04:25 PM.
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  #12  
Old 06-17-2019, 06:59 PM
Howard Klepper Howard Klepper is offline
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Your guitar is okay. Do nothing but play it, love it, and keep the humidity where it lives above 35%--preferably in the 40-55% range.
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Old 06-18-2019, 04:49 PM
nikpearson nikpearson is offline
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Default A little worrying in a new guitar.

There’s clearly a concavity in the top between the bridge and soundhole - that’s not ideal - especially in a new guitar. Before doing anything else I’d want to ensure the guitar is properly humidified for a few weeks at close to 50% Relative Humidity.

None of the guitars I own display this issue and only have slight bellying behind the bridge as well. Once you’ve humidified properly if the issue persists I’d take the guitar back to the shop it was purchased from or to a luthier for an expert diagnosis. Form the original post it’s not clear where the guitar was purchased.
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Old 06-18-2019, 10:51 PM
lar lar is offline
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If you have a mirror, you could look inside the guitar to see the condition of the braces. If a brace came loose, maybe it could lead to something like this?

I shove my phone in the soundhole and take photos - works pretty well.
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Old 06-19-2019, 07:02 AM
redir redir is offline
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As Howard said, it's probably fine. It can't hurt to have a look inside though. Truth is the best sounding guitars are ones that you see top distortion on, telegraphing and so on. So chances are you have a more responsive guitar there then not.
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