Need some advice - structural problem with my first guitar?

[Mark McLean]

I was pretty proud of myself when I posted the pictures of our first guitar 2 weeks ago - but now I am a bit worried. It seems like the top and neck angle are not holding up so well to the string tension. The main symptom is rising action, and pretty horrible intonation.

When I did the initial neck setup a straight-edge resting on the top of the frets projected just to the top of the bridge (prior to stringing up). We set the saddle height to give an action of 2.5mm to the bottom of the 6th string at the 12th fret. The intonation and tuning were pretty good to start with. After one week at concert pitch the action had risen to 3.5mm and the intonation was off. There is noticable bellying-up of the top below the bridge, and a straight-edge on the fret-tops now lands half way down the front of the bridge. The bridge height is 9mm.

I have never seen a guitar in the first few weeks of its life before - so I don't know how much of this is normal. But this is more physical distortion than I was expecting and I am wondering if it is a sign of critical structural weakness (eg top too lightly braced). I left it strung up and I have been watching it closely each day this week to see if it would get worse. In the second week it has not progressed any more. Perhaps it is stabilizing now?

I can easily make some adjustments to the neck angle. It is a bolt-on, and I haven't glued the fingerboard extension down yet (just double-sided sticky tape at the moment) because I suspected it might need some further adjustment. But if the top keeps buckling I guess it is stuffed.

I am posting a picture of the bracing so you can tell me if you think I carved it way too light (the top is WR cedar, 3mm thick, tapering to 2.4 near the edge; braces are sitka). The X-braces were inlet to the kerfing but the other braces stopped short of the sides. It seems I forgot to take a photo of the finished top but this is close to the end of the carving of the lower braces. Don't worry, I did remember to glue in a bridge plate later.

Any advice would be appreciated.

Mark

[Dave White]

Mark,

Scalloped bracing is not my bag but those X braces look to have started on the low side and look to be seriously "scalloped" in front of the X join and just behind. The soundhole area is a key structural part on a guitar and there looks to be a weakness there to me.

What gauge strings do you have on and do you have any pictures of what the top and neck area look like unders full string tension?

[snidermike]

I think Dave nailed it with the scallops in front of the X. That area of the X definitely needs to be strong, and isn't usually scalloped at all. The rest of the bracing doesn't look too thin to me though, but that's without knowing how stiff or floppy your top is.

[Alfred J]

Ditto to Dave's comment. That scolloping is really extreme. There's no need to scollop the top half of the X braces. There's not enough meat below the X brace joint to support string tension either. I also scollop braces, but not to that extent. Industry standard for sitka tops is 1/8 inch (3.6mm). I have never used cedar so I can only parrot what I've read which is that cedar should be around 10% thicker than sitka to support the same stress. In other words it seems that the top is also too thin. What does the guitar sound like? If it sounds indistinct and unfocussed that will tell you that you that you need more stiffness in the top. I don't know any remedy for that. With a bolt on neck, try fitting a shim under the fretboard extension. That will raise the height of the whole fretboard so that you won't have to reset the neck. The bolt holes will have to be elongated though.

[Dave White]

I think the top thickness is OK - depends on the individual piece of wood as always.

[Allen]

What everyone else has said. I keep my braces in front of the X intersection at full hight until they get very close to the linings. As far as fixing this. I defer to those with more experience.

[Mark McLean]

OK, I live and learn! It is likely that I have made some serious errors in the bracing. Another factor I just thought of is humidity. When I glued the box together, and also when I did the set-up, the humidity was around 40%. Last week when things started to go pear-shaped we had a lot of rain in Sydney and the humidity is still around 80% now. Could be a contributor to how it is behaving.
The strings are 11-52 gague. As far as sound is concerned, I think it is good - bright, loud and good sustain. But I don't really have the experience to pick the sound of an over-braced or under-braced top.

If it is the bracing I guess it won't be fixable. I'll just be a lot smarter when building #2.

[Dominic]

Hey Mark, you might be able do something.

I've thought about using carbon fibre string/rope soaked in epoxy and glued along the braces to stiffen them up. As long as you can attach it securely it should be light and strong. A lot of the lattice bracing/thin top guys seem to do this so it is not new.

Worth trying it out before you try something more drastic.

My local fibre glass shop has CF braded strip stuff that I am eyeing off for the job.

Dom

[Craig]

G'day Mark ,

I agree with what others have stated . There's just not enough support to stop the bridge over rotating. Leave the cross braces in front of the bridge in future , only scollop them towards the perimeter , and they are way too thin on the lower side surrounding the bridge .

I'd try to have a go at strengthening those areas like Dom . suggests, before ripping the top off , as you've got nothing to lose..

Good luck with it Mark , and if it doesn't work out ,you can always ,,,,,,


Cheers Craig

[Craig]

Just had another thought Mark. Your guitar might be a prime candidate for a " Bridge Doctor " . There is a thread running here at the moment about them , if you hadn't already noticed it.

Cheers ,Craig

[Mark McLean]

I am sure you are all correct. I made a big mistake scalloping the X-braces in front of the X. Looking back at the plans I was following there is not supposed to be any scalloping there, as you all identified. I copied it incorrectly onto my brace layout and didn't know enough about what I was doing to realize the mistake.

Craig, the Bridge Doctor idea might just save the whole project from disaster!. I recently played a Breedlove with it installed and was a bit mystified - didn't understand the point of it. But now that I have read the thread and done some Googling on the topic I can see that it is an obvious thing to try. I have detuned the guitar to take the load off and already sent my order off to StewMac. I'll report whether it works.

Dominic, the carbon fible idea is also a good one. If I still have any problems I will use it as plan B. Do you know a particular product or supplier to recommend? I guess I would glue it along the peak of the X-braces for as much of their length as I can reach - or would you put it on the sides of the braces, or the soundboard itself?

Thanks so much for the expert advice. I have learned a lot from this experience - the hard way.

[Craig]

The carbon fibre would be most effective if it were attached to the top of the brace Mark . This , on the principle of ' neutral axis' where the bottom of the brace is in opposite tension to the top , i.e. one stretched , one compacted . The middle area is in a more neutral state . Reinforcement in that area has much less affect.

By attaching Carbon fibre to the top of the brace , you have transformed the brace to a type of " I Beam " . The top of the "I is the carbon fibre capping , and the bottom of the " I " is the soundboard itself which the brace is glued to . The most economical strength to weight arrangement.

At least that's how I understand it to be

Cheers Craig

[Hesh1956]

Mark the top is too thin - 3 mm is .118". I won't take WRC below .130 and I have seen WRC that .130 was too thin.

Also we can't see what you did in the picture as far as the upper transverse brace. Do you have any pictures of that?

And I agree with the others that the scalloping is too radical and especially for a thin WRC top.

Also - just to eliminate this as a possibility is the back of your bridge lifting and separating from the top when the guitar is under string tension?

[Hesh1956]

Craig I just noticed that your knob keeps popping out.......

[Craig]

Craig I just noticed that your knob keeps popping out.......

,,,,,,,, Cheeky .

Hesh is a bit 'cock-eyed'



Mark, I should also have mentioned that a C.F. capped brace is difficult to tune . See my thread " 'Compliance testing "



.

[Dave White]

Mark the top is too thin - 3 mm is .118". I won't take WRC below .130 and I have seen WRC that .130 was too thin.

Hesh,

I don't necessarily agree - it depends on the piece of wood. The two cedar topped guitars I made were around 3mm in the centre bridge area thinned to around 2.5-2.8mm at the lower bout edges. They are fine. One is my cedar/maple "dread" that is nearly two and a half years old now, stable and sounds better by the day:

[Taffy Evans]

Hi
This was posted some time ago you may have missed it, some of the guys said it was useful.....

CONSIDER THIS WHEN BUILDING INSTRUMENTS
What effect size and density have on tone and strength and volume

• Remove 20% off of the height = 50 % less strength
• Taller Braces = Higher response [Treble]
• Stiffer Braces = “ “
• Thicker Timber = “ “
• Smaller Body = “ “
• Larger Soundhole = “ “
• Closer Grain [Stiffer] = “ “
• Flexible Timber = Lower Response [Bass]
• Longer Timber = “ “
• Smaller Soundhole = “ “
• Larger Body = “ “
Also………………
• Longer the Scale [string length] = The more string tension required
• Increase string length = Pitch goes down
• Increase the mass of the string = Pitch goes down
• Increase tension = Pitch goes up
Also……………….
• Timber [tone wood] with the longest ring and highest tone when tapped with a finger is best for use
• The difference [or ratio] of the frequency of the top and the back will have an effect on the response of the finished instrument
• The type and thickness of the finish lacquer will also effect the final sound
• Position of the main crossbraces [closer or further away from soundhole] can affect volume
• Plus a myriad of carving and positioning ideas of all other braces on both back and front plates
• But wait there’s more………………………………..

I have a guitar I built in the 80's and a more recent one, both using the same timber, Indian Rosewood/spruce/mahogany. the wieght difference is really noticable. I used to over build my guitars in the early days, gradually getting lighter and lighter over the years, as these two guitars show.

[Hesh1956]

Dave mon that is a very cool guitar and it's clear to me that it's no rasta impasta......

Excellent post Taffy!!!

[Mark McLean]

I thought I'd give you an update on my collapsing soundboard. Basically, it seems that Craig's advice about the Bridge Doctor has been a lifesaver.

I first played around with Dominic's idea of carbon fibre reinforcement. I got some CF tape from a local supplier. It only cost a few bucks and I tried soaking it in epoxy and layering it along some sitka bracing. It probably would work but it was hard to keep it in place while it dried even on a brace on the benchtop - let alone inside the guitar. I ended up keeping that as plan B.

Then the Bridge Doctor arrived from Stew-Mac. Installing it was very simple (less than an hour; could probably do it in 30 mins next time). As soon as I strung it up the bellying in the top and the rotation in the bridge were gone. I have left it at concert pitch for a month and it has remained stable (whereas the top started to deform within days without it). Last week, feeling confident that it was now solid, I went back and redid the setup of the neck angle and saddle. The end result is really pleasing. It sounds good, action and intonation are fine. Thanks so much for the advice Craig!

This device is mainly marketed to remedy older guitars that have developed a bellied top. However, it is also being installed in all new Breedlove guitars. I presume that Breedlove think this allows them to get away with a more lightly braced top, with some presumed advantages for the sound. JLD also claim that their invention improves volume and tone - but some of it sounds a bit like snake oil advertising (good for halitosis and restores sexual potency too).

Don't get me wrong - I don't think it is a good idea to deliberately under-brace a guitar and rely on this to fix it – and I have doubts that my guitar is going to last to be a family heirloom unless I eventually rebuild the top. But the Bridge Doctor is a simple and cheap ($US22.50) solution to a bellying soundboard and seems to have retrieved even this severely weak top (for now). And I won't be making the same mistakes in the bracing of my next build! I took some detailed measurements and photos of it and it would be easy to make one yourself. I'll post them if anyone is interested

[Hesh1956]

I wish they made a bridge doctor for 52 year old men that would stop bellying.....

Glad to hear that it is helping your guitar Mark.

[DarwinStrings]

I was curious to see the device that had helped with your belly troubles and was surprised to see what it was when I went to the Stewy Mac site.... I'm not 100% sure but I think this idea was developed to try to create a soundboard with less phase troubles and therefore increase volume by way of stopping the bridge from rocking and getting it to drive up and down better. Looks like it turned out to be a good way to fix a problem as well.

I am interested to know if you noticed any increase in volume of the guitar after you installed the "bridge Dr" Mark.

Hesh, maybe the only way to help with that sort of belly trouble is to cover it up with a Ferrari.

Jim

[Allen]

Hey Mark, yes, please post pictures and measurements. You just never know when something like that will come in handy. It'll go into my library of potentially useful stuff.

[ozziebluesman]

Thanks for the update Mark! I'm interested in the device details too!!

I have a project planned to build two concert size guitars and would like to set them up as either fingerstyle or use a steal nut raiser and use them for lap slide! I was thinking along the lines of using ladder bracing and install a bridge doctor type device to allow the use of medium strings. I don't know what I'm doing really so it's just a prototype. All good fun!!!

Cheers

Alan

[Taffy Evans]

Hey Mark
Glad you got it sorted, thanks for advising of the outcome. I still have two of my early guitars, and I remember I overbaced and overbuilt every thing for a good few years. My most recent Rosewood body guitar compaired to the 1980 Rosewood model feels like half the weight when both are held one in each hand. Shows how more confident I've become with the chisel, and other areas of the build. The sound of the heavier guitar is full, well balanced but of course naturally a brighter treble responce. I'm told it has good tone, "like a big Gibson".....I can live with that, especially as the person making the comment is known to us all.

The big luthiery secret is knowing when to stop, when is enough, enough?
experience helps. Tone wood being the monster it is, makes the exercise a challenge each time one picks up the chisel, or selects the brace material. I try not to have lightness at all costs as my main criteria.

You never know that guitar could turn out to be a family airloom.