Rohloff Chainline

[westofsouth]

Recently bought a Sterling frame and Thorn chainset.  Thorn apparently recommend a 73-118 bottom bracket with this setup. This bottom bracket gives a chainline of just over 58mm and a chainstay clearance of about 6mm.     
Rohloff specify a chainline of 54mm.
This 4mm increase over the Rohloff spec seems excessive.
It seems that Thorn specify this BB in order to guarantee chainstay clearance. Is there anything that can be done?

[expr]

Looks like you have a couple of choices, one of which you can do right now if you choose.

Option one is change the bottom bracket for a shorter one, or slightly move it off center bias towards the non drive side.

Option two, take the chainwheel off the spider and mount it on the back, this fits ok i have done this before, this will give you about 4mm difference back.

Dave.

[stutho]

westofSouth

I don't have all the figurers in front of me but I would be a little surprised if  Thorn were recommending a set up of the chainline that was less than perfect... Unless they expect the Rohloff when fitted to the Sterling to be set up with a 13 tooth sprocket.  The 13 tooth sprocket has a different chain line to the others of 58mm.  It  also might make sense on the sterling to use this small sprocket - as it would mean you end up using a smaller than normal chain ringing  giving increased ground clearance.


Stutho

     

[expr]

Well spotted Stuart, I had forgotten about that you could well be right.

[westofsouth]

Thank you for your comments.

According to the Sterling brochure, the standard sprocket is a 16t.
If you do the arithmetic,  a 110mm bb will give the correct 54mm chainline, but the pedal arm is in danger of scraping the chainstay. A 113mm bb will give an extra 1.5mm of clearance.

I have now checked the Sheldon Brown website, and have discovered something called 'Q-factor'....... apparently not all cranksets are the same.

[westofsouth]

Now had the opportunity to examine a Thorn built Sterling, and can confirm that it does indeed have a 118mm  BB  and  a chainline of ~58mm.

Have now fitted a s/h Deore M510 crankset with a 73-110mm bottom bracket. The chainline is ~54mm and there is  plenty of chainstay clearance.

Can offer no opinion why Thorn choose to fit their crankset to the Sterling.