Brad H

Neck Overstand

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2 hours ago, David Burgess said:

On the overstand: More seems to make an instrument easier to play in the higher positions, with less interference from the edge of the top.

Thanks, David.  I was hoping to get input from players or from those speaking for them.  

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1 hour ago, Brad H said:

Jerry's story is an interesting one and one which I will keep in mind, especially when dealing with older, thinner violins.    But, I don't think you have to worry about this.  No competent luthier would set a post so tight that it would endanger the instrument, strung or unstrung.

As I have witnessed many violins where the post has fallen when the strings were loosened, but nary a violin where the post has fallen when the strings were tightened, I would guess that violins configured such that tightening the strings raises the longitudinal arch to be quite unusual.

I also have never experienced a sound post crack.

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2 hours ago, GeorgeH said:

As I have witnessed many violins where the post has fallen when the strings were loosened, but nary a violin where the post has fallen when the strings were tightened, I would guess that violins configured such that tightening the strings raises the longitudinal arch to be quite unusual.

I also have never experienced a sound post crack.

possibly a bad sound post fit (with the curve not flat enough to match the plate) or it was not straight or maybe one of the plates or loose in the middle, or a loose bassbar that it could slide loose from tightening the strings. I also made the mistake of fitting a sound post too quick by not letting the violin regain it's moisture after drying in the UV box after varnishing. the violin wood was shrunken enough that after it settled to room humidity that caused the violin to swell enough that the sound post was to short and fell when the strings where loosed.

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9 hours ago, GeorgeH said:

 

Raising the bridge height increases the distance between the nut and the top of the bridge, so you can't increase the height of the bridge without also increasing the tension on the string and therefore raising the pitch with "all other factors remaining equal."

By "all other factors remaining equal", I meant to include the vibrating string length, with the height of the bridge being the single variable. Experimentally, one could accomplish this by tilting the bridge, repositioning the bridge, or placing a toothpick between the strings and fingerboard at the upper nut end.

When you increase the height of the bridge, you must increase the length of the string (or shorten the neck :)) to keep the pitch constant. When you increase the length of the string to maintain the pitch, you also decrease the tension in the string and the downward force on the bridge.

If you increase the length of the string (the same string), you would need to increase the tension to arrive at the same pitch.

Which brings me back to my question: When tuned to the same pitch, would a shorter string and lower bridge exert the same downward force as a longer string and taller bridge?

No. The downward force would be reduced, partly because of the lower string tension needed to arrive at the same pitch with the shorter string, and partly because of the reduced string angle over the bridge. The lower string tension is easy to verify. Play a C on the A string (which will shorten the vibrating length), and turn the tuning peg with the other hand until it sounds at an A again.

 

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1 hour ago, David Burgess said:

By "all other factors remaining equal", I meant to include the vibrating string length, with the height of the bridge being the single variable. Experimentally, one could accomplish this by tilting the bridge, repositioning the bridge, or placing a toothpick between the strings and fingerboard at the upper nut end.

Ok, except for tilting the bridge changes the experiment completely. :)

1 hour ago, David Burgess said:

If you increase the length of the string (the same string), you would need to increase the tension to arrive at the same pitch.

Yes, we are saying the same thing. If you increase the length of the string without raising the bridge height, the tension, pitch, and downward force will go down. If you increase the height of the bridge without lengthening the string, the tension, pitch, and downward force will go up.

I think that maybe the confusion is that the "length of the string" that I am referring to is the length of the relaxed string, not the distance between the nut and the top of the bridge. By changing the length of the string, I am referring to turning the peg.

1 hour ago, David Burgess said:

No. The downward force would be reduced

Okay. Yes, that makes intuitive sense.

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I don't quite understand it, but a physics site said that 50 pounds of string force makes about 20 pounds of force on the belly at 158 degrees.  The change of force with a plus or minus 1 degree string angle couldn't be much; could it?  Half a pound either way?  Different strings could make more of a difference.

20 pounds isn't very much force.  

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50 pound string tension at 158 degree break angle figures to 19.08 pounds downforce, roughly.

Changing the break angle by 1 degree would change the downforce by 4.5%, or ~.86 pounds .

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1 hour ago, Don Noon said:

50 pound string tension at 158 degree break angle figures to 19.08 pounds downforce, roughly.

Changing the break angle by 1 degree would change the downforce by 4.5%, or ~.86 pounds .

Yes, that means 2° smaller angle will increase downward force by 9% ( almost 10%) , which is not an negligible amount.

If you set the neck for high arched vn (say 17 or 18) and use the same overstand, this thing happens.

 

KYC

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Just now, chungviolins said:

Yes, that means 2° smaller angle will increase downward force by 9%, which is not negligible amount.

 

Or around 1.7 pounds. What would you consider to be negligible, non negligible, and why?

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The other thing is:  what does it take to get that 2 degree difference?

If you start with a 16mm arch and 158 degree break angle as a starting point, and keep the bridge, overstand, and saddle the same, you'd have to have over 3.2 mm difference in arch height to get that 2 degrees... or 12.8mm arch on the low side, or over 19.2mm arch on the high side.  Now THAT would be significant, I'd say.  

So, it looks to me like normal geometry and arching will just end up very close to the same break angle all the time.

For anyone who likes to run the numbers, this is a very handy triangle calculator here to avoid having to remember the formulas and all that stuff.

(above numbers are edited... determining the parts of the triangle are not so simple; I ended up using a computer graphics model of the violin geometry rather than trying to figure the appropriate lengths for the triangle sides)

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My violas are wide on the C bouts, so I use a high bridge with a 10 mm of neck projection, with a 9 mm high saddle. It was an advice Christopher Landon gave me many years ago. 

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1 hour ago, David Burgess said:

Or around 1.7 pounds. What would you consider to be negligible, non negligible, and why?

9% is like sales tax in most states, which ppl don't want to pay.

If you have to think about tax, then it is not negligible.

Of course there is no clear boundary, but there is no reason to subject your top under constant 9% extra force.

I would avoid 9% tax if I can.

 

KYC

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28 minutes ago, MANFIO said:

My violas are wide on the C bouts, so I use a high bridge with a 10 mm of neck projection, with a 9 mm high saddle. It was an advice Christopher Landon gave me many years ago. 

Goody - that would make varnishing under the fingerboard a piece of cake. - cheers edi

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1 hour ago, Don Noon said:

For anyone who likes to run the numbers, this is a very handy triangle calculator here to avoid having to remember the formulas and all that stuff.

That does look handy, but it will probably take more than a knock on the head to dislodge "soh cah toa"...but, I am rusty on the application.   Using your triangle calculator, plugging in values of 325 mm, 90 deg. and height of 49.5 (arch of 16 + 33.5 for bridge), you get a string angle of 81.4 deg from nut -bridge-base line.  What do you do for the string angle from bridge to saddle?

 

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1 hour ago, Brad H said:

That does look handy, but it will probably take more than a knock on the head to dislodge "soh cah toa"...but, I am rusty on the application.   Using your triangle calculator, plugging in values of 325 mm, 90 deg. and height of 49.5 (arch of 16 + 33.5 for bridge), you get a string angle of 81.4 deg from nut -bridge-base line.  What do you do for the string angle from bridge to saddle?

Your dimensions assume that the base of the triangle is the top/ribs interface... but the string end at the nut doesn't necessarily like on that plane.  Close, yes, but not exactly.

Hmmm... actually, I made a similar mistake in my previous calculations for the arch height required to change string angle by 2 degrees.  I fixed it.  Keeping the overstand fixed introduces a bunch more problems, as the neck angle changes and the endpoint of the string at the nut moves all over the place.

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I go about it backwards.  Surprise! I figure out the angle to the saddle first, because it isn't as variable.  Pick a number for the saddle, 5.5 mm?  and subtract it from the arch height (A) and a bridge height guess 32? (27 projection + 5 for string clearance.  That gives some angle in the teens, maybe 14 degrees.  If you are looking for 158 you want 8 more degrees.  Go backward, and take the inverse tan of your horizontal plane stringlength. 325?  That will give you a number for the height.  Subtract that, and you will find the location of the top of the nut at your perfect angle.  

Now you have to check to see if it will work.  

The top of the nut doesn't help you any.  Neither does 8 degrees. You need the neck angle. Take 8 degrees and subtract .85 degrees for the string clearance over the fingerboard, and .85 degrees for the taper of the fingerboard.  Those might not be exact numbers, but they are close. Do the math if you want them closer. If you are figuring out a viola, the numbers will be higher.  We end up with 6.3 degrees for the neck angle.  Now subtract 7 mm or so, (the nut) from the number you had for the top of the nut. Let's say the number was 5.5 mm, now the number is -1.5 mm.   This is the top of the neck at the nut, 1.5 mm below the plane of the belly.

We are now ready to see if the overstand is in the ballpark.  

Multiply tan 6.3 by the length of the neck.   I use about 5 more,  or 135 on a standard neck, which happily divides the fingerboard in half too.  That gives us, in this example 13.4 mm.  That is the total to the block.  But not yet!  The end of the neck is at -1.5 so the number is 11.9 This number is far more useful to builders than the measurement to the top of the plate, but that is what people SEE, so  subtract 4.5 and we get an overstand of 7.4.  In this example we'd have to adjust a few things if we feel it is too high.

Tapered ribs complicate things.  If you have them, you have more figuring to do at the start to define the points better.  Keep the plane at the top block at zero, and figure out where the saddle would be, and how much to add or subtract to the arch height. It's not hard, just tricky. Drawing it out helps.

If you did the same models all the time, you don't have to do it very often, only if starting something new.  Or you forget to write it down, or where you wrote it down.  I've done that.

 

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I don't know; It seems like a lot of agonizing over minutiae unless really radical changes in overstand depart from the accepted normal average.

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8 minutes ago, Bill Yacey said:

I don't know; It seems like a lot of agonizing over minutiae unless really radical changes in overstand depart from the accepted normal average.

Yes, if you make new instruments with standard arch height it is hardly an issue,but if you do the neck setting very high arched vn, then this becomes very important issue.

I've seen many  resetted necks from reputable shops ,which repair wasn't done properly mainly bc they dodn't understand the things we discuss here.

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Rather than going through all the calculations, I found it simpler and faster to glue two pieces of wood together at a 158 degree angle. One end rests on the saddle, and the other rests on the fingerboard, with shims attached to the fingerboard portion to approximate the height of the strings above the fingerboard. Spend ten minutes making this, and be done. You can use it forever.

Simply place it on the fingerboard when setting a neck (or on any fiddle for that matter) and you know right where you're at, or which direction you need to go.

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