Question For Acoustic Experts - How Much "Belly" At The Bridge Is Normal?

[frankie5fingers]

I'm told that on most higher end acoustics some bowing of the top behind the bridge is normal and in fact, is likely to be built into the design of the guitar; making a flat top actually flat on the bottom but with a slight convex curve in the bridge area. Is this so; that a good acoustic's top should have a slight hump behind the bridge? If so, what's considered normal? Assuming there isn't a playability issue, do acoustic experts recommend doing something about it only when it exceeds a specific amount? What would that amount be?

 

[zombywoof]

Bellying is perfectly normal on all flattops.

If you want to check it, lay a straight edge across the top of the guitar just under the bridge. Measure the gap between the top of both edges of the guitar and the bottom of the straightedge. If the combined total of the gap is less than 1/2" you are OK. If it is more you may have a problem.

 

[Seorie]

when you design an ‘acoustic guitar’ ( a round hole steel string western), to achieve a ‘good playing action’ on the fingerboard things that have to be considered are, broadly speaking - neck angle, fingerboard thickness, bridge thickness (height) + saddle height, soundboard radios (doming which is done to add strength to the top).
If the bridge / saddle are to high this causes undue torque (of the bridge) on the soundboard which results in the bulge behind and dip in front - or the bridge parting company with the s/b.
All these considerations are taken to put the ’string plane’ at it’s optimum in relationship to the ‘layout’ of the fingerboard / neck and bridge.

here are Radius Dishes back ’n front for Bracing Acoustic Guitars
http://www.stewmac.com/Luthier_Tool...dius_Dishes_for_Bracing_Acoustic_Guitars.html

with all this geometry going on with natural materials that ‘move’ the ‘pinpoint in the universe’ point of balance is when there is just the right height ‘in the saddle’ for minor adjustments to achieve a good playing action (which is ultimately down to the player !).
You can read up on all this stuff, I went to luthier collage which was a great experience.
hope this wasn’t to long winded, bit early in the morning for me not enough coffee yet.

 

[frankie5fingers]

Bellying is perfectly normal on all flattops.

If you want to check it, lay a straight edge across the top of the guitar just under the bridge. Measure the gap between the top of both edges of the guitar and the bottom of the straightedge. If the combined total of the gap is less than 1/2" you are OK. If it is more you may have a problem.

I'm having no problem(s) but I'm curious as to whether or not there's a fixed "ratio" for building the bow/dish/(whatever it's called) into the soundboard top. I know a bit f bellying is normal was wondering how to discern between normal and something to be concerned about. Thanks!!

here are Radius Dishes back ’n front for Bracing Acoustic Guitars
http://www.stewmac.com/Luthier_Tool...dius_Dishes_for_Bracing_Acoustic_Guitars.html

with all this geometry going on with natural materials that ‘move’ the ‘pinpoint in the universe’ point of balance is when there is just the right height ‘in the saddle’ for minor adjustments to achieve a good playing action (which is ultimately down to the player !).
You can read up on all this stuff, I went to luthier collage which was a great experience.
hope this wasn’t to long winded, bit early in the morning for me not enough coffee yet.

That link was quite helpful actually. I wasn't familiar with those and am trying to find how an essentially "flat" top could get the proper radius built into (only) that area on a large scale vs. hand preparation on a case by case basis. That helped me find some other info as well - Very enlightening...thanks for that.

 

[Seorie]

your welcome Frankle.
here is a picture of a guitar I built @ collage, look closely round the edges and you'll notice they're 'curdled' up from the board their lying on.
You place the (joined) soundboard in the radius dish and shape the underside of the braces to that radius. When you glue then into position they
'push' the soundboard into the curvature of the radius thus adding strength - think of buildings with a dome or try crushing an egg - length wise between thumb and fore finger.
If your interested further re construction check out Robert O'Brien's videos
Luthier Tips du Jour Mailbag

though my personal favorite of the luthier videos is Nigel Forster, he doesn't say much - just gets on with it.

http://s264.photobucket.com/user/uilleann49/media/soundboardbraces-1.jpg.html