Official Luthiers Forum!

after the last thread I did a good bit of thinking so I want to pose a few questions and observations , and see how many other do things .
this isn't about my way being the best way , it is about technique and results from observation.

So after really thinking about the bridge and the structure of this here are my observations. What radius is used in not important as the top
has a geometry and we will assume a clean mated glue joint .
My particular method is to glue my braces in a dish and the only radius is on the X braces all others are flat. My rim is prepped so that I have my neck angle from the block to the top of the sound hole and a slight radius behind the bridge. I have 3 distinct areas neck angle , flat at the bridge and a slight radius on the lower end.
Many use 25 foot and radius the full top. I do use a 28 foot and my process is on you tube. As you can see I don't radius the entire top. I use the 28 dish as a way to process my neck angle and to allow a shape in the rim for the radius behind the bridge for rh movement.
so we all know the top is not the same as it comes out of the dish.
What changes do you see in the top? What rh glue up?
On my tops the radius can vary a bit and I assume rh and material differences allow for this movement. But as a whole it looks more like a 35 to 40 foot radius than the original 28 after glue up. This may be from spring back from the flat braces with the rh change. It may even be higher but when set to the rim the bridge area is pretty flat. I would like to hear what your bridge area is doing. It is the same , is it higher or lower and what RH you glue the braces and what rh for gluing the bridge. What is also interesting is how long does it take for the top to settle in?

I look at the top when I glue and prefer to glue at 40%. With tops moving with RH after 20 yrs this method is one I started about 12 yrs ago after getting a chance to see the stressed applied in a lab experiment at work. So just follow me with this and I want to see what your findings on your guitars if they are similar.
What it shows is the the stresses seem higher for a failure in a lower rh than higher. While extremes of any rh shift can be catastrophic the stress patterns show more shearing at the ends of the bridge. On higher RH ( we tested from a rage of 30% to 60%) . As the top sank there was a difference in how the bridge area moved. The stress risers were most noticeable at the front corners and sides of the bridge.
I may be wrong but from what the stress lines showed seemed to the the transition point of the bridge and top structure of where the top stresses changed for compression to Tension . I don't have access to the equipment anymore but it would be interesting to see what other peoples observation is in this area.
The bridge structure while simple in design has a complex application of the stresses on the top. The strings are pulling and we have to start at the ball end to the tuner post. Without getting to engineery the centroid of the force and structure will be different on each guitar so some variance is expected. So assuming the ball ends will pull up , the string tension will come across the top of the bridge hole and the break angle will now transfer some energy from tension to a down force. The overall height I think has more to do with this than the angle when we figure the coupled force that creates the torque. Again this is also influenced by the structure of the bridge and plate along with the braces and how far the holes are from the bridge. So help me see if my observations agree or disagree and we can get a better understanding of what happens in the transfer of energy to the top.
We know that brake angle has little to do and can be considerable lower than once thought. The over all string height , and width of the bridge and bridge plate bracing height are all variables .

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

John,
As always, what you say makes a lot of sense.

I can't offer any comparisons since I take a different approach to tops. Mostly I build in dry, Southern California. But I spent some time along the coast in South Carolina. The RH changes from an air-conditioned house to a screened porch could easily be over 30%. My concerns led to using carbon fiber sandwiched braces on the top. I see far less changes to guitar geometry from RH changes now. We have since sold that house but I still use those practices.

I know you don't radius the bottom of your bridges but you do put a lot of effort into the mating surfaces. That makes sense to me even though I do radius mine. To me the bridge and bridge plate are the second most important braces of the top.

_________________
Joe Beaver
Maker of Sawdust

I live in 'guitar hell'; New England, where the weather changes more often and more radically than just about any place else: something like 70% of the storm systems that hit anywhere in the continental US come through here eventually. I radius tops for structural reasons: I believe it helps to resist cracking, particularly during the heating season. I don't have statistical backup for that; it would be hard data to get in any event. I try to brace tops when the R.H. is below 'average'; around 40% feeling that the problems of a swollen top are less than those of one that is dried out. Acoustically it's really hard to say what difference there would be between a 'flat' top and one that was built with the 25' radius I use. Neither of them stays put for long in use.

I live in Guitar Hell Taiwan, where RH is never below 70% on a clear sunny day.

40% RH is not even achievable. Best I can do is 55% RH with the dehumidifier running nonstop. Perhaps in the summer it is more easily achievable because the heat lowers RH.

I have heard discussions saying to keep guitars in a low humidity box (a dry box but bigger for guitars rather than camera gear). I have seen bridges peel off because of extended time in 70%+ RH. Guys spending big money on Martins and wondering why their top has bellied prematurely and that they are seeing a crack at the bridge/soundboard joint. I told the customer to bring it in for a reglue immediately because it will only get worse...

This is what I mean... the guy who did the work said it started as a small crack and turned into this:



I told him the bridge plate is likely gone and needs reinforcement but he felt it was not necessary. I hope he's right. That guitar is plywood.

_________________
Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

Note the 'tan line' around the spot where the bridge was glued to the top. If they routed off the finish where the bridge went they stayed about 2 mm inside the line of the edge all the way around to be sure that no wood would show. This meant that there was a gap between the bottom of the bridge and the wood of the top at about the thickness of the finish coat, which is not all that thin. You can't fill gaps with glue; a good bond depends on having the thinnest possible glue line.

It's also possible that they just glued it down over the finish. Either way that bridge was never really glued down properly. The proof is in the fact that there was no wood shear whatsoever.

Add to that the fact that the plywood top and the bridge plate were not stiff enough to keep the top from folding up along the pin line. I wouldn't touch that repair unless I could reinforce, or replace, the bridge plate.

And the cost to do a bridge plate reinforcement would be about the cost of that plywood guitar unfortunately.

about the only thing they are good for is smashing them after a concert. They are not built properly to begin with. Weird thing is, I've seen better guitars built this way! 1000 dollar Cort all solid acoustic was built this way too.

_________________
Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

This is the reason I shy away from bridge reglues and replacements on plywood tops. Once the outer layer of spruce is compromised, the top has little resistance to folding across the grain, since the center lamination is a cross ply. On a lifted bridge, this fold is usually at the pin holes, as illustrated. But I have seen this fold on intact bridges, at the bottom edge of the bridge. Even a good stiff solid top guitar can break at the pin holes, once the bridge lift has gone this far.
The classic resultant failure involves a rectangular hole in the top, bounded by the bridge pins and the front edge of the bridge.
You need to add structure in the direction of the string pull, and a conventional bridgeplate with grain oriented across the top is not ideal. I prefer installing a triangular spruce patch under the bridgeplate, with the grain parallel with the strings. Once this is installed, the integrity of the top is restored, and a normal bridgeplate (approximately the same width as the bridge) can be used. Spruce (particularly red spruce) is the ideal material for the patch, since it has superior stiffness along the grain, and is much lighter than the hardwoods typically used for bridgeplates.

Sent from my SM-G950U1 using Tapatalk

_________________
John

As I have stated before I see it as an accumulation of cycles that does the real damage. A few extreme swings, as long as not so severe as to cause imediate damage are not good but they are not what causes a bridge to lift gradually over time as all the ones that don't fly off immediately seem to do.

Every movement jarrs the structure and stresses not only joinery but the actual wood under tension from the strings. It's kinda like bending a piece of metal. Bend it in half and open it and it doesn't break. You can fold it in half maybe a dozen times and it will only begin to crack. But bend it much more slightly back and forth a few hundred times and all of a sudden it tears in two...... Stress is like that it accumulates. Why metal work is always designed with rounds and fillets, to keep stresses from accumulating.

Now, yes wood is a very different material than metal, but this engineering concept is universal and I could have used plastic or even cardboard in my first example. And this is what I see lifting bridges. My relocation to the Delmarva peninsula and the exceptionally high and variable RH has only seemed to confirm my observations. It is more the stability of the RH where the instrument lives than how low it gets in the winter or high in the summer alone that pulls bridges off..

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

A customer told me he sees a crack, and I told him to bring it in immediately for repair because the crack is sign that the stress has begun to cause damage. Kinda like the fine crack you see in metal and all of a sudden it breaks.

However I have seen many Martin guitars in Taiwan that has developed rather severe (for new guitars) belly right out of the box. But the shops that sell them won't provide any services for it.

_________________
Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

I agree with Brian - damage accumulates as the structure is made to deal with expansion and contraction. I don't have any data on this other than observation but I've noticed that wood tends to accept humidity very, very quickly but take much longer to dry out. So for folks taking their guitars out in high humidity, even for a few minutes - I think this causes damage.


In terms of bracing, every brace is radiused to 25' - including the bridge plate and upper transverse brace. The bridge also has a 25' radius. I use laminated sides and also use a center seam brace on the back (no cross grain and it is not cut out for each ladder brace). I glue the top and back on with the sides in a mold. Doing all this ensures that the top and back geometry end up very close to the radius I chose for the plates. After the strings go on, the top geometry obviously deforms somewhat but the overall geometry stays in the general ballpark.

I'm sure there are lots of different approaches but this is the way I've always done it and it works well for me.