Tap tuning as a rough guide when shaving the braces

Construction and repair of Classical Guitar and related instruments
Alan Carruth
Luthier
Posts: 2986
Joined: Mon Dec 24, 2007 6:56 pm

Re: Tap tuning as a rough guide when shaving the braces

Post by Alan Carruth » Mon Jan 29, 2018 7:15 pm

The string tension on a steel string guitar can be as high as 250#, but it's usually closer to 180#. The torque is that force acting through the lever arm of the height of the strings off the top, which is usually about 1/2" So it's somewhere between 90-125 in/pounds. If the bridge is 1.3" wide, then the maximum force on the back edge, assuming all of the load was being dissipated there and the saddle was all the way at the front of the bridge, would be between 70#-95#, roughly. In fact the load is distributed all along the glue line between the bridge and the top, and that's the total static load. It doesn't produce any sound, as you can easily observe since it's there even when you're not playing the guitar.

The actual vertical force pushing the bridge in and out to make the top move is no more than about 20% of the sting tension at most, and usually much less: I'd think an amplitude of about 10% of string tension would be more like it for a hard strum. For simplicities sake any one string would be about 1/6 of that, so, say, 1.5#-2#.

This really encapsulates the problem of making a good guitar top: the top has to be stiff enough to resist the torque load over a long period of time, and at the same time light enough to respond easily to a single string.

amezcua
Posts: 593
Joined: Tue Oct 04, 2016 9:50 pm

Re: Tap tuning as a rough guide when shaving the braces

Post by amezcua » Tue Jan 30, 2018 11:52 pm

One tip about bridges and cracks forming at the ends of the bridge was to taper the wings slightly near the tips .About a centimetre I think . But that was the same maker telling us to have a stiff bridge . Does that sound like a contradiction ?
The Jansen site say the best guitars have the lightest bridges . That`s a short version .
Bert Eendeback also says to tune the area below the harmonic bar but to leave the area above that a bit thicker as it`s not officially involved in making sound . About 0.5 mm thicker above the harmonic bar . He has some interesting ways to brace the backs too .

SteveL123
Posts: 1025
Joined: Thu Feb 09, 2017 5:05 pm

Re: Tap tuning as a rough guide when shaving the braces

Post by SteveL123 » Wed May 09, 2018 4:55 pm

Subscribed! Lot's of valuable info.

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