Compound Radius Question

DrBob

Right !
I’m  having a bit of a back & forth with a bloke on a Jazzmaster/Jaguar page and I wanted to check my logic with the Fretboard hive mind.

There’s a guy who wants to stick a neck with a 7.25”-9.5” compound rad board on his guitar and he’s asking what radius bridge he should buy.

I’m saying that logically it should continue the “cone” radius and be flatter (although probably not much) than 9.5”.

There’s another guy that’s saying that as the radius only matters over the fingerboard section then 9.5” is fine.

To my mind that’s wrong because if the bridge is 9.5” then at the 21st fret the string radius will be tighter than 9.5” and so the middle strings will be further off the fingerboard ?

What say you ? 

Maynehead

You are correct, although in reality it makes little practical difference, and most compound radius guitars with a fixed radius bridge will have a bridge radius that matches the radius of the last fret.

Regardless, your theory is sound for achieving a perfect string height profile across the board.

ICBM

You are correct, and in fact the bridge radius should always be slightly flatter than the fingerboard even on a fixed-radius board. This is because the individual string action should increase from treble to bass - which is achieved with a slightly larger radius curve, raised at the bass side... if you do that with the bridge radius the same as the fingerboard, the middle strings end up too high.

To be exact, a compound radius which increases from 7.25" to 9.5" from nut to approximately the 24th fret (ie by 2.25") would increase by another 0.75" (a third again) if the fingerboard went all the way to the bridge, so the *minimum* radius of bridge that would be correct is 10.25", but because of the above I would probably use 12" radius.

poopot

I have 7.25-9.5 on my tele... roughly 12 at the bridge.

Danielsguitars

/\ this as poopot says around 12 at the bridge

I have 7.25 to 9.5 fretboards and love how they feel personally

ICBM

Not only do they feel better, there is a very good reason for that... a *non*-compound radius is actually geometrically wrong on a tapering neck - it needs to be a section of a cone, not a cylinder. Trying to squeeze a fixed radius onto a tapering neck effectively produces a hump in the middle of the neck at the high end. So all fingerboards should really be compound radius.

Danielsguitars

ICBM said:

Not only do they feel better, there is a very good reason for that... a *non*-compound radius is actually geometrically wrong on a tapering neck - it needs to be a section of a cone, not a cylinder. Trying to squeeze a fixed radius onto a tapering neck effectively produces a hump in the middle of the neck at the high end. So all fingerboards should really be compound radius.

I don't get the hump on my 12 inch radius fretboards, probably because i make it and sand flat again after it's been glued on, i think it actually ends up a tad flatter up the high end naturally, anyway gets me really nice low actions to start with which is the important bit