Thicker or thinner gauge for frequently snapping harp strings?

Question

I have a kora (West-African harp) on which the highest two strings only seem to last about a week before snapping.

The two strings are tuned to A (880Hz) and G (784Hz) respectively and are stretched 220mm and 250mm between bridge and machine head.

I'm currently using 30lb nylon fishing line, as recommended by the instrument maker.

In order to prolong the life of the strings and avoid damaging the harp, would it be sensible to replace them with thicker or thinner strings?

Answer 1

The tensile stress on the material is independent of the thickness that you use: it depends only on the length of the string and the pitch you are tuning to.

For a string of a given material, of a given length, it takes so many Pascals (or PSI) of tension (pulling force divided by cross-sectional area) to tune to a given note, regardless of whether the diameter is .008" or .011" or whatever.

The short life you're getting is because the material you are using as string does not stand up to the tension due to tuning and the stress of vibration, or some other aspect of the instrument, such as the string pressing against a sharp edge. (Perhaps the instrument can be modified to relieve some source of stress.)

Once the string is damaged (develops a crack), extra thickness doesn't help; the crack spreads almost instantaneously and the string snaps. It's like a balloon succumbing to a pin prick. So a thicker string might not help.

Thicker strings will put more stress on your instrument because of all the extra tension needed to get them up to pitch.

The way to go here is to fix any sources of unusual stress in the instrument, and to search for a better quality string material.

Look for product that quotes higher tension values, for the same thickness. Don't just look at the absolute pounds that a given filament can handle, but divide that by the square of the diameter. A .018" line that can take 30 pounds is stronger material than a .022" line that takes 30 pounds, obviously. How much stronger? We divide 30 by the square inches: 30 / (.018 x .018) =~ 96000 PSI and 30 / (.022 x .022) =~ 62000 PSI. The .018" line can withstand 54% more tensile stress. This stronger material is better choice unless it is also 54% (or more) heavier than the weaker one. A heavier material has to be put under more tension to get to the same pitch, which can offset (or more than offset) any strength advantage it has.

By the way, I pulled some of these numbers from a table on this web page. As you can see, there significant differences among the monofilament lines, and some of the microfilament (spun thread) lines are way stronger. The best monofilament listed ("High Seas Black Widow") requires a .016" diameter to achieve a 30 pound strength, but the Berkeley Superline does it with a .014" thick braid, and something called "Sufix HercuLine Dyneema" gets it down to only 0.009".

Answer 2

30lb line should fine on those two top strings. We've used it on 100s of koras at that point. You could of course always try line a couple of pound lighter as every kora is slightly different, and worth a go, but it is more likely to be something else.

Just to add to the information above - if the line always breaks at the same point, as suggested above, check for faults:

• if at the bridge, run fine sandpaper in the two relevant notches.
• if at the eyelet, check inside the neck where the string comes through the wood - check no possibility of snagging/wear.
• although very unlikely as it is happening on two strings, check the spindle of the machine head, perhaps swap one over with one a little higher up the neck - if that then starts to break, there must be a small burr on the spindle hole.

I'll get in touch with you direct to see if you still have the problem - simply can't have one of our koras causing problems, even if from a kit :-)

Answer 3

You may want to check the breaking points on the string (for a traditional stringed instrument, the bridge/saddle and nut), and tuners, and see if they are comfortable to run your finger over.

If you notice any tearing or pull against your skin that may be what's causing the string to rub up against unnecessarily, burr into, and break. If you carefully smooth all contact points with the string, that alone may solve your problem.

In my experience, nylon strings are much more susceptible to burrs and digging into which compromises them.

Also, how good is the quality of nylon fishing line you're using? Does it feel smooth to the touch as you run your fingers along it in both directions? It may have a natural grain it likes to follow that you may want to tweak with as well.

Answer 4

Check the length of the string, you may need to move the bridge (and wooden base) a little higher to shorten the string a little, a small amount will make a big difference to your string tensions & the balance across the range.