Square tapered BB question

[gairym]

Morning people!

I'm thinking of half-inching the lovely retro-chic Middleburn RS2's off my pub bike for a new bike build.

It's a standard 73mm bottom bracket and old fashioned standard 135mm qr rear end and so nothing fancy going on to contend with.

What I'm wondering is what axle length I should go for in order to place the single chainring (mounted in either the middle ring or outer ring position on the spider) to get the best chain line to run it 1*12 (Sram 'Eagle GX').

It's there a website that handily calculates all of the above or a formula I need to muddle my way through or is it just try and see....?

Any help muchly appreciated.

Cheers dudes!

[Lazarus]

i think you are going to have just measure it yourself ans see what length you want
I would be very surprised if there is still anything out there that advises on what square taper for what frame with whatever crank[if there ever was such a site- sheldon?]

[lune ranger]

The Royce BB I’m running my RS7 cranks on was originally specc’d for a set of RS2’s on an Orange P7 from 2001
I can look up the length when I get home. Jealous your RS2’s are still alive. My cracked at the taper interface a few years back.
Will let you know tonight maybe if I remember when I get home. If I forget PM me. I’m doing the B150 itt tomorrow/Saturday so will be out of radio contact till Sunday.

[rando nomad]

Yes, Sheldon has a great article here about chain line
https://www.sheldonbrown.com/chainline.html

sadly there is no calculator to tell you what BB = the correct chain line. In my day job, I have to tell folks it is a try and see sort of thing. Try out X size BB. Determine what the chain line is that results from X size. How does that compare to what it should be? Adjust BB length to suit. For someone who was never good at math, my fear is when someone calls to ask these sorts of questions and wants to check their math!

[lune ranger]

My square taper BB that was spec’d By Clif at Royce to pair with RS2 cranks on an old MTB intended for 8speed is:

123mm

I guess that gives you a reasonable starting point.
Maybe get a cheap BB to test out the theory or treat yourself and give Clif a ring and he will get you the right BB for a bunch of money.
Royce BB’s literally never die.

[wriggles]

Using middleburn Rx7s on 1 x 10 on my 73mm bb shell sherpa with a 118 axle. Any less and the crank fouled the bb.

[javatime]

If you are trying out with a Shimano style BB you can adjust the chain line if needed using spacers on the drive side.

Good luck !