I need help with a compound radius

[GK Breivik]

I originally posted in "Basses", but was advised to post here. If a mod wants to delete the other thread, feel free!

Anyway:

Hi all!

I'm having my 7.25" radius Jazz converted into a compound radius. I think I'd like to keep the 7.25" at the nut. What would be the mathematically correct radius at the 20th fret? I really don't have a head for mathematics ...

Just in case anyone wonders: Standard 34" scale, 20 fret Jazz, 1.5" nut width, 19mm spacing at the bridge.

The reason for the conversion is that I want a flatter radius at the pickups, to help with string to string balance, cleaner string bends in the high register, and the ability to set up the neck a touch flatter. (Less relief) All these points should be addressed with a compound/ conical radius.

Thanks,
GK

 

[Bruce Johnson]

What radius do you want at the bridge? You have to decide that first. Then you can work the math (simple proportions) to figure out what radius you will need at the neck heel.

I can understand that you may want a flatter radius at the bridge and over the pickups, to fit your plucking style, and maybe to help with string-to-string level balance from the pickups. But a compound radius on a bass isn't going to do much for cleaner string bends, unless you are going for super low action and very wide bends. And it won't allow you to set the neck up flatter or with less relief. That's strictly up to the quality of the fret leveling.

 

[Geoff St. Germaine]

Bruce is the man, so his answer is great.

Like he says, if the frets are properly levels (along the string paths) there's going to be effectively no difference between a compound radius board and a standard one. I'd honestly go for a proper fret job before entertaining having the fretboard surface reworked as it is going to be substantially more expensive.

I actually have a conical fretboard calculator that I wrote up that uses an ideal cone for the fretboard geometry and plugging in Jazz bass specs (19 mm bridge string spacing, 29 mm nut string spacing, 34") with a 7.25" nut I get 16.8" at the 20th fret and 21.2" at the bridge. I had went about calculating the difference at the 24th fret between a conical and standard fretboard at one time and recall it being exceedingly small on any sort of realistic fretboard geometry.

 

[GK Breivik]

Thanks guys! This is much appreciated!

I fully realize that we´re splitting hairs here, and that the difference is less than minimal. It´s just that this does´nt cost me any more either way, as the bass needs a refret anyway. My luthier has the new top of the line PLEK machine, which also can level a fingerboard.

21" at the bridge seems a little too flat for me, especially since I have blade style pickups with a 14" radius. If you could be bothered Mr Germaine, I`d be interested to know what I need to end up with 14 at the bridge. Since you have your calulator and all, I mean. I was thinking 7.25-12. Do you think that would feel natural?

Thanks again!

GK