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hello,

Im wondering about how to set downbearing when installing a new soundboard?
As the original won't give you much useful information in this regard due to the loss of crown, how do you then determine the correct ammount on the new bridge before stringing?-Also, do you need to compensate for the downward pressure of pulling it up to pitch?-Would adding more downbearing before stringing add more pressure overall thus counteracting the effect?

How is net bearing worked out?-Is there a formaula to use?-And how does it relate to front back bearing??

Can anybody help?

Thanks!

Lewis.

(deleted)

First of all, even though there is some loss of crown, (unless the soundboard is inverted or has no crown) measurements should be made on the old one, both with tension and without to get an idea how much to compensate for; yes you must account for the down bearing force, ~ 4-6 lbs per string wich is close to half a ton over all. If the soundboard is being replaced measure downbearing at each end of all bridges along with a few measurements in between; you need to take additional measurements of the height of plate over soundboard. There are other measurements needed also such as pressure bar height, height of coils on pins ect... Most well made pianos have the downbearing angle set fairly equal on both sides of the bridge. Its still important to measure on both sides of the bridge, not just the combined single angle...this indicatess if there are structural problems with the soundboard such as a colapse in one area. The hitch pin side has a bit of leeway if your new height comes in a little high since you can put a felt washer under the loop if need be.

If there is some downbearing, you can slightly compensate on the rebuild from the original amount by listening to the sound of the piano. If the strings and hammers arn't totally shot, you will have fairly decent sustain but the tone will be somewhat frail and weak when the downbearing is too little. If too much downbearing is applied the tone will be decently loud but with a very short duration. There is some leeway to allow for seasonal rise and fall of the soundboard and to find a nice balance of amplitude/sustain. Typical change from dry to wet seasons can be about a 1/4 of the overall downbearing angle. 1 degree of downbearing is often a good start for the central to treble region.

There is an free online vector force calculator you can use here if you want to determine how much force change there is for certain tension @ a specific angle.

http://www.engineeringtoolbox.com/vector-addition-d_320.html

For a string that is say 166 lbs of tension...fill in 166 for F1 and F2 and then for an overall 1 degree downbearing, fill in 179for the angle field...results indicate a downbearing force of 2.9 lbs.

Ideally you should be looking at about 1/32" lift off of a thread from the agraffe or capo bar for the ends of the treble bridge and most of the bass bridge. Depending on the rigidity, ribbing ect.. the more central part of the soundboard can take usually between 1mm-2.5 mm of downbearing. You really must assess the original condition or the piano to know where you are going with the rebuild.

The answers to questions like these for me were hard learned over time by taking many classes at PTG conferences with people like Del Fandrich and working in a local rebuilding shop with people that know what they are doing and studying piano design.
You set downbearing on a new board by planing the new bridge cap to the correct height in relation to the plane of the strings.
Original information is very useful and should be recorded.
Yes you do need to compensate for the strings being up to pitch. You do not want to flatten or invert a new soundboard.
A good place to start thinking about this is to first know how much crown the board has unloaded, know about the capability of the system to support string downbearing and not design the bearing to exceed these parameters.
Bearing formula is simple geometry based on string length, string height above the lower termination and the desired bridge height as well as the string tension.

Gene is correct, nobody can give you a concise answer online to this question. There are simply general guidlines you follow and most of the rest is determined by digging in there, hands on. I can not overstate that the more information you record about the original condition measurements, the better off you will be in the end.... without this, it is simply hit or miss in the results.

Thanks so much for the replies. Very interesting.
I see now that i was being far too dismissive of the original soundboard!
Very useful information.

A couple of other questions... Emmery you mentioned 1 degree as a good start in the treble region,.Is that 1 degree each side, totalling 2 degrees?-or 1 degree in total?

Also, is it unheard of to put the original bridge (no new cap) back on a new soundboard?-Or would that be impossible to get right?

I would so love to attend those seminars, but Im in the UK which makes it a lot more difficult!...They don't seem to have as much here.

Thanks again!

There is a known formula to compute the amount of give of the ribs, so to stay in a accepteable level of constrain.

The slanted Bridges are supposed to level somehow when pressure is installed. Some boards will accept more pressure, some also need it , as there are differentways to install crown the load may vary a little.

That said, the crown is primarly structural, for whatI understand. On old boards , evaluating the level of downbearing implies evaluating thecrown. Old boards are too soft to accept a lot.