Pointless to do a Soundpost patch on this back?

[HongDa]

Seems like we're just going in circles now,

unless someone has another unusual or innovative suggestion on a way to do this repair.

With the goal of keeping costs down, could anything be done with really rigid carbon-fiber cleats, placed as close to the soundpost location as possible without interfering, to spread the load, and reduce the flexing of that area to near zero?

Perhaps one of these cleats could even be placed directly beneath the soundpost, if it was wide enough to allow for moving the post around a little?

I'd think that the back would need to be dehydrated well before gluing them in, so they wouldn't pull the crack apart if moisture levels get low and the back contracts in the future. And that the ends of the cleats would need to be staggered, and cut at an angle, so a new stress concentration isn't created.

We have quite a bit of experience now with both cross-grain, and along-the-grain reinforcement of necks with carbon fiber, and that seems be working out quite well. Also with reinforcing peg holes.

It does seem to have gone full circle now.

I'd just like to say that I've mentioned several times on this thread that on cheap school instruments I have done quite a few soundpost crack repairs to backs and tops by just gluing the crack and putting a few cleats above and below the soundpost area to allow for post movements..... and I've seen them years later still holding.........longer than I or the customer ever expected.

As terrifying as it may sound I've even glued the cleats (on backs) in through the f-hole using gentle pressure to glue them in place with a soundpost (On shorter cracks). Again....it's definately not my usual practice but if it's a matter of life or death of a cheap school instrument I've done it.

I wonder if any others here have tried it and what their observations were.

I'm happy to see this post getting into alternative possibilities.

One thing I'm curious about is how much pressure the soundpost puts on the back.

On a package of Dominant violin strings on the back of each envelope  it gives numbers in kg and lbs. I assume this means what the string pressure will be on the bridge? The total for one set = kg 21.8.........48.3 lbs.

If this means that much pressure is going onto the bridge......then how much of this pressure is absorbed by the top and how much by the back? And therefore how much rigidity is needed if one decides to cleat?

Again I've had success with maple cleats and from my experience they were to my surprise.....enough. So I'm very curious how much pressure the post puts on the back.....is it alot less than we would expect? I've actually come across a few instruments people played for years with no distortion to the top AND no soundpost at all   nor did the players know that such a thing excisted. Does that suggest the top takes the bulk of the string pressure?

Do any of you have the capability to measure the pressure? I thought someone here mentioned some type of thin pressure measuring device.

 

 

FiddleDoug wrote:

David, I know that you're talking about carbon fiber, but that's pretty much where I was going with the 2mm thick overlay maple patch. Quite rigid, large enough to spread pressure, rounded edges to avoid edge stresses, and because it's the same type of wood, and only grain rotated a little, it should avoid shrinkage problems.

And again I 'll mention I've just cleated areas like this with much success before but felt some extra wood between closely fitted cleats ,as near the post position as possible, would help because the back is much thinner than what I have done on other instruments. Then you suggested your larger patch idea which I like. If one were really concerned that such a patch would still flex enough to open the crack.....would it make any sense to also put cleats on the patch as near the post position as possible? Or even leave the upper and lower sections of such a patch thicker?. It wouldn't be a pretty sight but in any case this isn't going to win a beauty contest

As for Jacobs advice to just glue it....interesting and daring but the climate I'm in makes me nervous. I think I have an instrument of my own though that I may try it on to see what happens.

Just looking up at my rack now...there is another customer's violin with a full length repaired back crack that had a post patch put in it many years ago as well as cleats.....it's developed into a hairline crack and it seems he played it like that for quite some time not even realizing it ever had a crack. It's been hanging here months as he's too depressed over the fact he was suckered into buying it......and can't afford to have the repair done properly which would be a new patch, cleats and removing the dark stripe of varnish over the original crack. Since he played it in such condition for a period, perhaps he'd be willing to just have me glue it and see if it works. Worst case scenario is the hairline crack reopens and he then takes the strings off and waits until he decides to do a complete repair in the future.

 

[Violadamore]

One thing I'm curious about is how much pressure the soundpost puts on the back.

On a package of Dominant violin strings on the back of each envelope  it gives numbers in kg and lbs. I assume this means what the string pressure will be on the bridge? The total for one set = kg 21.8.........48.3 lbs.

If this means that much pressure is going onto the bridge......then how much of this pressure is absorbed by the top and how much by the back?

No, the numbers for the strings are string tension, that is, the pull on the pegs and the tailpiece.  The pressure on the bridge is the vector resultant of the two pulls taken at the bridge, and if memory serves, since the two pulls are balanced at the bridge, the force will be twice the average tension of the four strings times the sine of the angle between the strings and the plane of the neck.  Easiest way to get the angle is to measure the angle between the bridge face and the strings, add that to the 90 degrees between  the bridge and the belly, then subtract the total from the 180 degrees in a triangle.  Just for illustration, lets assume that the string angle with the bridge is 80 degrees, so 80+90= 170; 180-170=10, sine 10 degrees is 0.1736, so the answer is 2(11)(0.1736)= 3.8 pounds (rounded to one decimal, of course). 

 

Edit- this has been edited to fix an egregious error. The 11 pounds in the equation given now was figured from the strings in a set of Dominants. Mea maxima culpa. 

[Carl Stross]

No, the numbers for the strings are string tension, that is, the pull on the pegs and the tailpiece.  The pressure on the bridge is the vector resultant of the two pulls taken at the bridge, and if memory serves, since the two pulls are balanced at the bridge, the force will be twice the the total tension times the sine of the angle between the strings and the plane of the neck.  

 

Is that so ?

[uncle duke]

What about poundage breaking strength per string.  Then the totals of 4 strings are totalled for a poundage breakage set number.  I'm still half asleep but you know what I'm saying.

[FiddleDoug]

"If one were really concerned that such a patch would still flex enough to open the crack.....would it make any sense to also put cleats on the patch as near the post position as possible? Or even leave the upper and lower sections of such a patch thicker?"

 

I don't think so. Just make the whole patch a little thicker. If you were to make the patch 2.5 mm thick, and rotate it a couple of grains, it would be stronger, and stiffer than the original back. No need to complicate things with extra cleats or thicker areas.

[Stephen Faulk]

No, the numbers for the strings are string tension, that is, the pull on the pegs and the tailpiece.  The pressure on the bridge is the vector resultant of the two pulls taken at the bridge, and if memory serves, since the two pulls are balanced at the bridge, the force will be twice the the total tension times the sine of the angle between the strings and the plane of the neck.  Easiest way to get the angle is to measure the angle between the bridge face and the strings, add that to the 90 degrees between  the bridge and the belly, then subtract the total from the 180 degrees in a triangle.  Just for illustration, lets assume that the string angle with the bridge is 80 degrees, so 80+90= 170; 180-170=10, sine 10 degrees is 0.1736, so the answer is 2(48.3)(0.1736)= 16.8 pounds (rounded to one decimal, of course). 

What about baroque gut strings that are equal tension sets? Does the same equation apply?  

[Stephen Faulk]

What about poundage breaking strength per string.  Then the totals of 4 strings are totalled for a poundage breakage set number.  I'm still half asleep but you know what I'm saying.

 This is violin making not bass fishing, doofus. 

[uncle duke]

Yeah, I found out 4# line is not strong enough. But I really do believe that the poundage numbers for violin strings are thier breakstrength. Where did I go wrong in trying to understand that- what did I miss?

[David Burgess]

Yeah, I found out 4# line is not strong enough. But I really do believe that the poundage numbers for violin strings are thier breakstrength. Where did I go wrong in trying to understand that- what did I miss?

String tension numbers are indications of the tension at normal playing pitch, with a normal string length. Breaking tension is usually much higher.

[Brad Dorsey]

...One thing I'm curious about is how much pressure the soundpost puts on the back...

 

 

According to "The Violin Explained" by James Beament, page 50, typical cello strings exert a static downwards force on the bridge of about 23 kilograms.  I assume that almost all of this force is transmitted to the back by the soundpost.

[FiddleDoug]

"I assume that almost all of this force is transmitted to the back by the soundpost."

 

Actually, about half would transfer to the bass bar through the bass foot of the bridge. The arching of the front would also take up some of the force.

[Jeffrey Holmes]

 

As for Jacobs advice to just glue it....interesting and daring but the climate I'm in makes me nervous. I think I have an instrument of my own though that I may try it on to see what happens.

 

 

Yes..  climate can complicate things...  I see nothing wrong with cleating the crack if you can get into the area effectively without disassembly... though I think it may not add all that much to the integrity.  Again, I would consider that this sort of repair has a limited expected "life".

 

Just as an aside; I just finished repairing a flank crack in a 18th century fiddle (Tomaso Eberly; Naples) that appears to have previously restored a very well known shop in the mid-20th century.  It has a repaired soundpost crack in the top (nicely glued and touched in) that extends from the lower bout to above the ff hole.  It's just barely "off" the post (less than a grain from the outside edge).  No patch.  No cleats.  Still perfectly closed.  Been that way for more than 50 years, living in Michigan (wet in the summer, dry in the winter).  Can't see why I'd patch it at this point.  I informed the owner and placed one small cleat as a safety, but that was really just to make myself feel better.  Of course the pressures are very different for a top than a back crack in this area...  but still... this is one of those times when the repair performed had no business lasting this well by conventional thinking.

[Violadamore]

What about baroque gut strings that are equal tension sets? Does the same equation apply?  

It's the reverse of the pulling your car out of the mud problem that can be found in most freshman physics books.  I made an error last night, for the way I lumped the tensions to simplify the problem, one should take either highest (for the extreme case) or average (for a better overall model), rather than additive tension.   For equal tension, you'd take the value for the single string, because they are equal.  

 

I gave a rough approximation, mainly to show that it's a vector problem as well as that the force on the bridge is a fraction of the string tension.  The only way to really get good numbers here would be to set it up in 3-D and work a network of tensors, but for the question asked, that's overkill. 

[Brad Dorsey]

"I assume that almost all of this force is transmitted to the back by the soundpost."

 

Actually, about half would transfer to the bass bar through the bass foot of the bridge. The arching of the front would also take up some of the force.

 

Of course you are right.  About half the force is carried by each bridge foot.

[Violadamore]

"I assume that almost all of this force is transmitted to the back by the soundpost."

 

Actually, about half would transfer to the bass bar through the bass foot of the bridge. The arching of the front would also take up some of the force.

What supports the bass bar?  And where does the arching go?  IMHO, part of the force on the bridge will be distributed to the edges of the plate, and thence to the blocks and ribs, part will go to the soundpost.  How much goes where is an interesting question.  When I look at this, it seems that the back is also resisting the force pulling the neck and endpin together.

[uncle duke]

What supports the bass bar?  And where does the arching go?  IMHO, part of the force on the bridge will be distributed to the edges of the plate, and thence to the blocks and ribs, part will go to the soundpost.  How much goes where is an interesting question.  When I look at this, it seems that the back is also resisting the force pulling the neck and endpin together.

And where does the arching go?--  what hasn't been answered yet is an area of the arch where it changes it's direction, called inflextion nowadays, - does inflection move out towards edges or does it move in towards center?  Is it a waste of time deciding?

[Stephen Faulk]

What supports the bass bar?  And where does the arching go?  IMHO, part of the force on the bridge will be distributed to the edges of the plate, and thence to the blocks and ribs, part will go to the soundpost.  How much goes where is an interesting question.  When I look at this, it seems that the back is also resisting the force pulling the neck and endpin together.

  I have a better idea, make a sound post that can measure pressure. Then get rid of the all the silly maths and speculation. 

 

A sound post created out of nesting tubes with a tough spring inside which is then calibrated to measure the pressure. Or some digitally sensitive pad must exist that you can put pressure on and it measures the weight, put that sensor on the end of a sound post. 

[Bruce Carlson]

String tension numbers are indications of the tension at normal playing pitch, with a normal string length. Breaking tension is usually much higher.

To perform the Mosé variations (Sonata a Preghiera) by Paganini, the g string is tuned up to b flat. It doesn't break.

[JacksonMaberry]

On 10/21/2015 at 6:06 AM, Brad Dorsey said:

 

According to "The Violin Explained" by James Beament, page 50, typical cello strings exert a static downwards force on the bridge of about 23 kilograms.  I assume that almost all of this force is transmitted to the back by the soundpost.

Beament, page 58 - chapter 4, section 6. Shockingly little is transmitted to the back.