PDA

View Full Version : Compound radius and flat bridge



Flowerpot
Mar-05-2004, 4:28pm
I've been hearing lots of talk about radiused fingerboards and how to adjust string height at the bridge, and it got me thinking. I wondered if theoritically, one could design a compound radius for the fingerboard which would allow for a perfectly flat bridge, and still keep unifor action on every string. The answer is surprisingly, yes. After looking at the problem mathematically, I came up with a solution which keeps the same action across the fingerboard with a completely flat bridge.

The compound radius changes gradually down the fingerboard, with smallest radius at the nut (of course). The formula will change depending on what nut radius you pick, but for this example I used 7.5 inches. The progression then goes (figures rounded off)

nut 7.5"
3rd fret 9"
8th fret 12"
12th fret 15"
19th fret 20"
bridge flat

I used this progression for the fingerboard on my mando #1, and the action is very consistent with a flat bridge, and the neck feels very good to me. My regular mandolin has a constant 7.5" radius with radiused bridge, and the feel is similar except the flat bridge is more desirable. Rather than use a bunch of radiused sanding blocks, I made a few cardboard cross-section templates and hand-sanded until they fit, taking care not to remove material from the center of the fingerboard (so the spot between the 2nd and 3rd strings would be the high point). It was not really any more difficult than a constant radius, and I like the concept.

John Flynn
Mar-05-2004, 6:09pm
Nice work! Makes sense to me. I hope other builders pick up on it.

Michael Lewis
Mar-07-2004, 1:44am
I have been discussing this with a couple of very good pckers. Their preference is to have a flat bridge to make picking easier (more uniform) for the picking hand, and have the more comfortable radiused fingerboard for the fretting hand. I think it's a simple and elegant solution for many folks, but not every one will want their instrument set up like this.