One thing you could do is roughly measure the existing crown on the board and do tests by stringing a single (or a few) plain wire strings to see where it's at.

See what it sounds like by sliding the action in and listening to the tone in that region and checking with your downbearing gauge. Poor tone results from lots of things, not just poor crown, so listen to see if you can isolate how the soundboard crown is affecting the tone.

If you want to experiment, you can also remove the board from the inner rim, then add compression when reinstalling by drying the board, reinstalling, and shimming/clamping around the edge before raising the humidity carefully to see if you can increase the crown. Measure and test like before.

Other options are experimenting with reinstalling new radiused ribs, but this, like many other piano design factors, is hard to test and change once you do it. Many rebuilders just guess what will produce good tone and go with it - based on past success and their experience.

There is no number for good crown, but there is a number for a specific piano (optimum region). What you want to do is balance the potential energy stored in the soundboard with the potential energy stored in the strings (based on the mass, length, elasticity and tension of the scale).

I hope these thoughts are helpful to you and spur freedom and experimentation.

Probably, but minus spring, jaws and screws.

OK, I need some clarification on this. What units would the crown be measured in and what units would the down bearing be measured in. I could see someone measuring 1/8 inch of crown along a 5 foot rib and then setting 1/8 inch downbearing over a 1 inch length of string!

Jeff,
What note are you asking about?

No particular note. Perhaps if you gave example it would be good.

There are very few 5 foot ribs.
Few notes have a back scale of one inch or less.

It should be plain to see the vertical measurements of crown vs bearing.

It would be helpful to start there if that seems vague.

Great! Then could you give an example of vertical measurement of crown vs bearing?

That one is too easy Jeff ... I will insist on leaving that one to you.

Well, its just one more thing that has not been adequately explained. I have not done it, so I don't want to lead anyone astray.

Pete,

It is not my intention to be elusive.

Down bearing is only one of several considerations when deciding on the voice of a piano.

One could write an entire paper on that subject here on PW.

Also, to only look at bearing, without an understanding of the entire relationship, risks failure. I don't want to oversimplify nor do I want to "write the entire paper" here.

Many technicians work a lifetime for the understanding of this relationship. They can be hired. It is not wise to give the entire thing away here.

Try things, gain experience. Practice on pianos of no consequence. We all did. Attend the PTG conventions. Over time, one will acquire the understanding.

Larry:

Then it is not as simple as your earlier posts suggested, and who is Pete?

Not exactly what I am saying.

"Pete", I meant Jeff

Then for "Pete's" sake, what exactly ARE you saying???

Jeff, I think I may have called you Pete before ... I am a recovering Blond.

In the OP's situation, I would venture the ONLY consideration is preventing too much bearing and risking reverse crowning the board. Other than that, he should take all the bearing the crown of that board will let him have reasonably.

In a perfect world where one has control over ALL the parameters ?? Del has described the considerations used in this process as well as anyone can.

To write at length on my specific choices here is simply not wise.

From time to time, I have a piano here I have put a lot of work into. It is good to play those. They reflect the summation of my choices and I can speak about those choices in context.

Roy123 recently had the opportunity to play a Baldwin L and write about it on the Piano Forum. That piano is still here for anyone to play and examine. Playing that piano puts into context my choices. That Baldwin L will here for maybe another week.

Larry:

OK...

Would it be wise, on your part, to just explain how someone might: "take all the bearing the crown of that board will let him have reasonably"?

Otherwise, why did you even post? Just to say you really won't explain anything?

Measure the crown.

In this piano, that vertical distance is ALL you have in down bearing.

I said this right at the beginning.

I might not go all the way to zero BTW if I thought I could get away with it.

A Soft light hammer properly voiced will make the most of that situation.

The rest of the answer must be in the trial and answer process.

There is a lot of discussion about "proper execution". The Trial and error process is what sorts all that out.

As good an answer as can be given right here IS the simplest and least wordy.

Next step .. do it and listen.

Here's an instance of to much down bearing


<http://leacocks.com/FeaturesBlog/wp-content/upLoads/2011/11/crushedpiano.jpg>

For this old board inside of this old piano, if it does have - for the sake of numbers - 1mm of crown and the plate height is set so the strings are deflected by the bridges by 1mm, when the strings are brought up to pitch, the board may not be deflected by 1mm - possibly less. Not likely on an old board but possible.
The good news is that it could not invert.