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Belly vs Back Arching Asymmetry


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I would like to make sure I properly understand a subtlety.

The long arch has a high point in the middle of the plate and slopes down to the top and bottom edge of the plate.

For backs it seems like the arching is typically symmetrical - the top-side arch and bottom-side arch are mirror images.

For bellies it seems like the bottom-side arch is very similar to the back.  But the top-down arch is rather different.

The belly top-side arch seems to stay flatter as you move from the center, then fall more steeply once you cross the upper bout wide point.

Do I understand this properly?

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2 hours ago, Shunyata said:

I would like to make sure I properly understand a subtlety.

The long arch has a high point in the middle of the plate and slopes down to the top and bottom edge of the plate.

For backs it seems like the arching is typically symmetrical - the top-side arch and bottom-side arch are mirror images.

For bellies it seems like the bottom-side arch is very similar to the back.  But the top-down arch is rather different.

The belly top-side arch seems to stay flatter as you move from the center, then fall more steeply once you cross the upper bout wide point.

Do I understand this properly?

Do you mean in modern making, or old German making, or maybe in Old Italian making?

There was recently a long and rather combative thread about this very thing.

Instead of dragging us back to rehash the same ground, could you perhaps look up that thread. It's only a few weeks ago.

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Looking at the other discussion, I can see this topic is a real can of worms.

The short answer seems to be that what I am seeing actually exists in old instruments.  Whether I should duplicate it is another matter.

I will simply note that the instruments I make with a flatter top profile sound good... perhaps this is answer enough. 

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19 minutes ago, Shunyata said:

Looking at the other discussion, I can see this topic is a real can of worms.

The short answer seems to be that what I am seeing actually exists in old instruments.  Whether I should duplicate it is another matter.

I will simply note that the instruments I make with a flatter top profile sound good... perhaps this is answer enough. 

You can also say that the most player favored instruments historically all show this feature.

 

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1 hour ago, David Beard said:

You can also say that the most player favored instruments historically all show this feature.

 

One could also speculate that copying 200+ years of distortion will consign an instrument to a quick death, unless it is re-arched regularly.

I know of one maker who copied the age-related distortion, who ended up making a lot of new tops for their instruments.

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8 minutes ago, David Burgess said:

One could also speculate that copying 200+ years of distortion will consign an instrument to a quick death, unless it is re-arched regularly.

This raises an important issue, where those involved in restoration at a high level will have a greater understanding of arching, and how arches will both creep and distort.

Those who can only use second hand information, such as pictures, plans, posters etc, will just never understand it in the same way.

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Some of the difference in arching that I am seeing likely would not arise from string load on the plate. 

I make the assumption that load would tend to flatten arching.  The profiles I am seeing actually tend to concentrate increased curvature at the upper bout widest-point.  As a mechanical engineer, I don't see how this would occur solely through load.

If course that does not mean the historical shapes are undistorted.  But there is more than just distortion going on.

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16 minutes ago, David Burgess said:

One could also speculate that copying 200+ years of distortion will consign an instrument to a quick death, unless it is re-arched regularly.

I know of one maker who copied the age-related distortion, who ended up making a lot of new tops for their instruments.

If you copy them well, there is a slight crown to the flattish stretch.

That helps.

And, of course, we do have a good number of old examples that do show collapse and distortion around the bridge and require repair.

But that doesn't mean it isn't still the way the best violins were built.

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6 minutes ago, Shunyata said:

Some of the difference in arching that I am seeing likely would not arise from string load on the plate.

I make the assumption that load would tend to flatten arching.  The profiles I am seeing actually tend to concentrate increased curvature at the upper bout widest-point.  As a mechanical engineer, I don't see how this would occur solely through load.

Don't forget that the compressive force, end-to-end, is much higher than the bridge downforce. While the downforce on the bridge tends to hold down the center of the top, there is no such opposing force constraining the top in the upper and lower bouts. So as you have noticed, they bulge.

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1 hour ago, David Burgess said:

Don't forget that the compressive force, end-to-end, is much higher than the bridge downforce. While the downforce on the bridge tends to hold down the center of the top, there is no such opposing force constraining the top in the upper and lower bouts. So as you have noticed, they bulge.

This talk about bulging has made me think about how an unsupported/glued violin top might bend when compressed end to end in a vice. It would probably bend upward at the centre bout.

Now if it was part of a completed box and the longitudinal force exerted by the string tension was the dominant force why would it not do the same when compressed?

 

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11 minutes ago, Dennis J said:

This talk about bulging has made me think about how an unsupported/glued violin top might bend when compressed end to end in a vice. It would probably bend upward at the centre bout.

Now if it was part of a completed box and the longitudinal force exerted by the string tension was the dominant force why would it not do the same when compressed?

 

The amounts of movement needed at the ends and bridge to produce the shapes seen would be more than anything justifable by observation.  The directions are as you would like, but the amounts are not.  Not by a long shot.

Remember, 1) the compression hypothesis is a guess, 2) it solves a problem that doesn't need solving - why not believe the arching were simply carved basically as we see they?  3) the proponents of this theory are biased. They made tops with a modern through curve, and they want to beliveve that's what the old masters did.

*****

Do you really believe that uncontrolled distortions turned all the old instruments from modern through curved tops into well formed old style tops, every single one if them.  Until we get to modernized French makers in the 1800s.  And then, only the very explicit bench copies of Old Italian stuff deformed, but the modern style instrument didn't?

Yeah. And the tooth fairy is real for those that truly believe.

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Yes, I agree. My guess is that the top doesn't bend or bulge much anywhere under string tension.

It's a very compelling hypothesis visually but I can't see the box structure distorting enough to determine the shape of the top, even over time.

I can see the downforce from the bridge sinking the area immediately ahead of it on a overly thin top. And I can also see the possibility of a bad neck set causing the fingerboard to rotate closer to the top arching instead of the top bulging upward. Early makers thought the best way to avoid that was to drive nails into the neck.

 

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1 hour ago, David Beard said:

The amounts of movement needed at the ends and bridge to produce the shapes seen would be more than anything justifable by observation.  The directions are as you would like, but the amounts are not.  Not by a long shot.

Remember, 1) the compression hypothesis is a guess, 2) it solves a problem that doesn't need solving - why not believe the arching were simply carved basically as we see they?  3) the proponents of this theory are biased. They made tops with a modern through curve, and they want to beliveve that's what the old masters did.

*****

Do you really believe that uncontrolled distortions turned all the old instruments from modern through curved tops into well formed old style tops, every single one if them.  Until we get to modernized French makers in the 1800s.  And then, only the very explicit bench copies of Old Italian stuff deformed, but the modern style instrument didn't?

Yeah. And the tooth fairy is real for those that truly believe.

So why did the ancient archers destring their bows when not in use?

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29 minutes ago, Marty Kasprzyk said:

So why did the ancient archers destring their bows when not in use?

Maybe they were like careful gun owners today who don't leave their weapons loaded.

 

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There are two issues being discussed here. The first is the initial shape with the long arch of the top having a flatter area approximately between the widest points of the of the bouts. The second is the deformation which is often seen where the bouts bulge upwards where they are not pushed down by the bridge. The large majority of good instruments have the former while the latter is something which most makers try to minimize with proper shaping and graduation.

As David B. said there were makers a few years ago who tried to carve in the deformation thinking the instruments would sound (and sell) better with fairly unsatisfactory results.

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21 minutes ago, Urban Luthier said:

Wondering if anyone has tried the spyderco ceramic stones I posted above 

They are made from an ultra-hard alumina ceramic material and require minimal water for sharpening. I've been experiment with a couple of slip stones to hone at the bench in-between sharpening. Since these have different properties than normal water stones, it is taking a bit of time to get use to them, but they look promising. 

One of us is on the wrong topic.

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20 hours ago, Dennis J said:

Yes, I agree. My guess is that the top doesn't bend or bulge much anywhere under string tension.

It's a very compelling hypothesis visually but I can't see the box structure distorting enough to determine the shape of the top, even over time.

I can tell you it does. Not only from seeing very, very old instruments, but also from forcing distortion on my own instruments over intense intervals, and from seeing violins of mine which are fifteen years of Evah Pirazzi use old.

If you start with more of a catenary arch in spruce, the top will come to resemble a "top arch" with a plateau, and the ends of the sausage will bulge out rounder. How extremely you take the initial shape will change this, of course...

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21 hours ago, David Beard said:

The amounts of movement needed at the ends and bridge to produce the shapes seen would be more than anything justifable by observation.  The directions are as you would like, but the amounts are not.  Not by a long shot.

Here is simple sketch of few arches... the reference arch  (third from bottom) is derived from Sacconi map. The first is bulged roughly 1mm in the bouts, the second has bulge with 0.5mm sinking under bridge, third has more saddle and fourth is the reference (Sacconi, shown as dotted line in the rest). Fifth is less flat version roughly the typical back arch and the bottom pic is comparison of the first and fifth as two extremes - difference in the bouts is 2.5mm.

Who dares to guess difference of the LENGTHs of the arches from edge crest to edge crest? (the straight distance with no arch is 350mm)

I've got exact numbers from CAD to post later....

Long arch.png

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22 hours ago, Dennis J said:

This talk about bulging has made me think about how an unsupported/glued violin top might bend when compressed end to end in a vice. It would probably bend upward at the centre bout.

Now if it was part of a completed box and the longitudinal force exerted by the string tension was the dominant force why would it not do the same when compressed?

 

You've left out the bridge downforce in your thought model. :P

22 hours ago, David Beard said:

The amounts of movement needed at the ends and bridge to produce the shapes seen would be more than anything justifable by observation.  The directions are as you would like, but the amounts are not.  Not by a long shot.

Remember, 1) the compression hypothesis is a guess,

No, it is not a guess by any means. Both longitudinal compression and downforce at the bridge are easily measurable, and the consequent distortion is also easily observed or measured, via long-term observation. You're "tilting at windmills".

Bear in mind, as I mentioned in another thread, that the area where you live is one of the most friendly climates on the planet for violins, so you probably haven't had a chance to observe the changes that many others of us have seen. That is no excuse to hide your head in the sand, ignoring them, or pretending that they don't exist.

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Agreed... there is no way that "folding" distortion - bringing neck and tail together - can cause flattening of the top.  In fact it would cause buckling of the top and an INCREASE in arching!

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13 minutes ago, Shunyata said:

Agreed... there is no way that "folding" distortion - bringing neck and tail together - can cause flattening of the top.  In fact it would cause buckling of the top and an INCREASE in arching!

Were it not for the downforce at the bridge. ;)

Here's a simple experiment for you: Lay a piece paper flat on a table. Push the ends together. You will get an arch. Then push down at the center. The center will come down, and regions closer to the ends will bulge.

I'm find it amazing that some people don't seem to grasp even some of the most simple mechanical principles.

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But the top is binded together with back at soundpost via bridge pressure from top and stretching of back from bottom side. Also the perceived folding or distortion of the violin will optically differ based upon which two points you set as reference points for measurements.

Years ago when I drew F-5 mandolin drawings I had also CT scan and made sure to trace cross arches and long arches at many places only to find that the difference of height of long arch and cross arch at center was well over 1mm but the ribs were of uniform height all around and considering factory build on full height outside form they started pretty much square and flat. After analyzing all the curves I think the whole thing just bent and the arch of top and back deformed similarly to what we are talking here. There is no soundpost in mandolins and ribs are pretty thick (2-2.5mm) with strong kerfing but still it bent in 80 years...

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