I really need to chime in here . . . .
Ed, I am sorry, but Kawai's testing does not bear out what you are saying regarding capo bar hardness. I do agree with most of your points – a smaller V contact area and not too steep of a string angle up to the front duplex definitely decreases the stress on the wire. But you are getting people to accept an incomplete statement that hardness on it's own will contribute to string breakage – this is not accurate according to Kawai's testing.
About 10 years ago Kawai launched a pretty exhaustive research project into treble string breakage. A lot of different duplex / capo configurations were tested, including various hardness specs and profiles for the V under the capo. Here are the main points that our R&D department found, many of which are in agreement with Ed McMorrow's findings:
- Soft capo bar combined with a broad cross section at the string contact (3mm radius or more) leads to very accelerated string breakage after 5 to 10 years of steady use and tuning.
- Harder capo bar combined with the same 3mm cross section made the piano easier to tune (easier string rendering) and reduced string breakage. This is partially why almost all piano makers have a hardened capo bar these days.
- High string angle between the string segments of the duplex and speaking length greatly increases string breakage. It was found that 15° gave very good string reliability, 17°-18° gave equally good string reliability and lowered string duplex noise. Angles in excess of 20° can be common in many grand pianos, and this definitely contributes to accelerated string breakage.
- Longer and shorter duplex string lengths had little effect on the string stress / reliability, but affects the tone of course.
- Worn hammers (and therefore longer string grooves) cause the hammers to drive the strings with a long, flat contact area which greatly accelerates string breakage.
If a piano is breaking treble strings, the technician must look at all of the variables that contribute, and not look at one cause. This reminds me of technicians who contact me to ask if they should add a touchweight reducing spring rail to the keys to make an older Kawai feel lighter, yet the technician hasn't done anything to the action yet to get it working correctly! Similarly, if a piano is breaking treble strings you don't want to jump on a single cause like the hardness of the capo bar.
So here is what you look at:
- How old are the strings?
- How many hours a day does the piano get played?
- How aggressive is the pianist in practicing? Some pianists have a positive gift for breaking treble strings, others can play an equal amount of time but not break any!
- How worn are the hammers?
- How close is the letoff? (if it is very close, the string stress really increases! 2mm is usually plenty safe, though)
- What is the string angle at the capo bar?
- What is the condition of the V-bar? Are the strings buried into it with long 'saddle' grooves? This is very bad.
I would like to mention here that many music schools simply become accustomed to changing treble strings every 5 to 10 years in their practice room grand pianos. This can be true of all brands and ages of pianos – and mainly depends on the pianists, hours used every day, and condition of the hammers.
As Ed had confirmed in his testing and Steinway originally pointed out over 100 years ago, the ability of the string to pivot at the capo is very important for lowering string stress. A 'sharper' V contact area helps this. If the iron is reasonably soft this also allows the strings to settle in just slightly and create very clean terminations, and the sharpness of the V helps prevent long, deep saddle grooves.
A 3mm radius V bar works well if the iron is hard enough to keep the strings from digging into the plate. I agree that the sound is not as good as with a smaller radius, but as far as string reliability, some hardening of the capo does decrease string breakage by keeping the strings from settling into the iron as much. Keep in mind that the capo bar should not ever be as hard as the wire, or the wire becomes flattened and distorted. But as long as the iron is softer than the string and the angle is not too steep, it is beneficial for the V to be hardened some.
In the newest Kawai grands, the capo bar radius has been reduced to about 2mm, and the iron is being tempered slightly. In addition, the string angle has been reduced to about 17° - 18° so that the string is not being forced to dig into the capo bar V as deeply. We have had absolutely no complaints of string breakage since these changes were implemented! We have only heard about some breakage in some very extreme use locations where the string breakage would happen in any piano, but even with those the number of broken strings has been greatly reduced.
Based on the list above of causes, here is what I recommend when strings are beginning to break in a heavily played piano:
- If the strings are more than 5 years old and are breaking, all of the strings in the weak area should be replaced. It is foolish to just replace the strings which broke and service the hammers – the rest of the wire has been weakened and will still break!
- While the strings are out, file down on the front and back of the V under the capo to sharpen it. Then smooth the tip of the V to eliminate all remaining string grooves. Sand with fine emery cloth, then polish with 600 grit paper. I also like to paint the resulting shiny contact area with McLube 444 before installing new strings.
- If the duplex string rests are high, they can be filed down while the strings are out. It is tedious, but a short length of thin file can be used to reduce the string angle to a reasonable level. I have done this on quite a few pianos with excellent results, and it takes 2 to 3 hours (in addition to all the other work, of course). I also polish the contact area and add McLube to the duplex bars as well.
- For very heavy players, the Paulello XM wire also seems to hold up a little better, so you can string with this wire (it's easy to buy from them directly from France using PayPal, just do an internet search for it).
- Once restrung, then reshape the hammers to eliminate string grooves and restore the original pointed treble hammer shape.
- Regulate the action!
When all of this is done, the piano will be at least as reliable as when it is new, and if you have made the V sharper and lowered the string angle the breakage will most likely completely stop until the hammers become flattened again.
Don Mannino
Kawai America
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