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 .
