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Permalink Reply by Uncle John on December 17, 2014 at 3:45am

I've had this problem too.  Wrecked a nice old solid wood Corina box - and I thought I had strengthened it adequately.  

One method that  has worked for me is to have the neck not touch the box right under the bridge - but have it touch and strengthen half an inch or so away - in both directions.

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Permalink Reply by Bluesheart on December 17, 2014 at 4:42am

Wayfinder,thank you. My main issue was the box crush not the bridge so maybe that was not the best picture choice. I do appreciate all the input about bridge design and this one is not typical of what I do. I made a new neck for the box this one was on and did not want to see this one go to waste since it was my first one with frets (1/16"brass rod superglued into saw cuts, way more trouble that it was worth!)  (no separate fret board) and an angled headstock (end mitre cut and wood dowels instead of a scarf joint, I learned my lesson there since I have repaired the joint several times)

The neck has sentimental value so I put it on this box. Now I'm thinking I will take the guitar apart and at least angle the neck back so that I can then use a taller and better designed bridge and while I'm in there I'll brace up the top so it does not crush anymore. The box is one of those nice deep JFR plywood boxes and does give good acoustic volume. Who knows, while  I'm at it maybe I'll pop off the brass, add a fretboard with proper frets too and some binding to cover the original fret slots!        

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Permalink Reply by Tom T on December 17, 2014 at 4:49am

Wayfinder has nailed the problem. If your current bridge is about 1/4" x 2" then the string pressure is being concentrated into 1/2 square inch of your box top. If instead, you used a a bridge foot that was about 1" x 3" then the string pressure is spread over 3 square inches. That is a reduction in pressure per square inch of 84%. That should be enough to stop the top from collapsing. The foot doesn't need to be very thick. a piece of hardwood only about 1/8 inch thick should be rigid enough to get the job done.Your bridge height can be cut down a like amount and you're back to the same total bridge height you started with.

 

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Permalink Reply by Bluesheart on December 17, 2014 at 5:15am

Yep, that makes perfect sense. Though in this case the box has already been pushed in so I doubt it will pop back out and stay without some help from underneath.

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Permalink Reply by David McQueen on December 17, 2014 at 5:21am

I do two things for this:

1. My bridge is about 1/4" to 3/8" but it is 5 " long.  This gives a lesser weight distribution per square inch.

2. When I cut out the neck so that the top can vibrate, I leave three pads. Two pads for the bolt fasteners I use and a third pad for the bridge.

So the final affect is the bridge is long enough to straddle both the neck and the open face of the box and should be giving some energy to both, but it is also directly pressing on the neck for 1.5" of the 5" bridge width, enough to stop dimpling or crushing.

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Permalink Reply by JL on December 17, 2014 at 6:12am

I apologize now, I am so sorry to bring out the physics soap box to stand on...

A bigger footprint for the bridge/saddle stops it from gouging into the surface of the box (SHEAR FORCE) but does not change the total loading of the box surface (COMPRESSION FORCE).  distributed across 1/4"square or 2" square the strings still put a total load of 20-30 lbs on the soundboard. 

As an example: you try to pick up two grocery bags, each has 4 bottles of soda in it.  In the plastic bags the handles bunch up and cut into your hand painfully, but in reusable cloth bags the woven strap is easier to grip. But your back still groans at the 35lbs of weight you are trying to carry, no matter which kind of bag it is in.

Ok, I'll climb down off my soapbox now and go make a stompbox or something out of it....

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Permalink Reply by Tom T on December 17, 2014 at 10:10am

Correct, the total loading of the surface is unchanged. The load, however is distributed over a greater area of the surface. Old office buildings often specified the maximum load that could sit on an area of the floor (they may still do it). The building designers knew that focusing a high load on a small area generated high local stress. I sort of equate it to laying on the floor, taking a spear and sitting it on my chest, then sitting a sand bag on the top of the spear. Total weight isn't much, but the local stress is going to make me a shush-k-bob. Put a one foot square of plywood on my chest, followed by the sand bag and I might just fall asleep (I'm an old guy).

Anyway, your point is correct, total load is unchanged. However, distribution of the load will help. Don't know if it will be enough to prevent the top from depressing, but the load from three strings shouldn't be that high in the first place. Someone, somewhere actually calculated the stress and posted it. (too much time......)

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Permalink Reply by Bluesheart on December 17, 2014 at 8:48am

I just love all you guys and the spirited discussions that spring up around a seemingly simple question! I 'm no scientist but I think both JL and Wayfinder make good points. I agree with JL that string pressure will be "what it is" but it also makes sense that a larger footprint of the bridge will spread that "what it is" out over a larger area and would therefore be less concentrated on the top and would/could result in less deformation of the top. It reminds me of my cat Mojo, she weighs about 10lbs, and it is very easy and comfortable to have her lay on my belly but, when she steps on my belly with one or two paws only and I have to pee cause it is first thing in the morning, that 10lbs cat feels like a lion since her weight is concentrated in only the square inches of her paw/s instead of the whole length of her body. How is that for physics?

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Permalink Reply by john mackay on December 17, 2014 at 9:27am

No kids, do not try this at home.  You are not a cat. ;D

oh thats me out then.

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Permalink Reply by Ron "Oily" Sprague on December 17, 2014 at 10:33am

Typical string tension for a 3 banger with a 25.5" scale is approximately 60-75 lbs, depending upon which string diameters you use. That'll definitely depress a box top, especially if concentrated over a relatively small area. You increase the area (distributed load), you lower the concentration of forces at any given point.. Wide bridge does exactly that. It's the guitaristic equivalent of a flying buttress, especially if you use a triangular saddle.

Another easy fix for this is to install a sound post, directly below the bridge, attached to both top and bottom. Violins do this, for increased bass response, and to get the whole body resonating. Try it. Jam a piece of squared wood under there. Simple experiment.

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Permalink Reply by Bluesheart on December 17, 2014 at 12:01pm

Sound post and wide bridge coming up!

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Permalink Reply by Tom T on December 17, 2014 at 12:56pm

The sound post may be a really good idea. I looked inside an old violin I had and sure enough there was a round dowel connecting the top to the bottom of the box. It was right under where the bridge was located. Never considered that vibrations could be mechanically transferred from the bridge to the bottom surface of the box. Should help overall volume.

Nice idea.........

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