531colin wrote:does that mean they have kept the centre of the axle on the bike midline and added 2.5mm to each half of the bearing unit/sleeve thing?
I don't know that it is the case for all axle lengths, but certainly from personal experience and measurement of the resulting chainlines it's true for the 122.5mm length, i.e. both ~56.9mm with outer chainring on Spa TD/RD chainsets according to the notes I've just checked.
I think it was the case that the middle ring of Shimano MTB triples would give the required chainline for a Rohloff. That was the route that most Rohloff users took, and Rohloff even supply/supplied 5 x 5mm chainring spacers with the hubs to take the place of the outer ring. I think that it's not quite so straightforward now because the Rohloff chainline increased slightly when they introduced the splined sprocket carrier to replace the threaded sprocket carrier. I think the threaded sprocket chainline was 54mm, and having just checked the manual it's now 57mm, or 55mm with the aftermarket slimmer carrier. I presume the slimmer carrier was developed to offer backwards compatibility for Rohloff owners whose threaded sprocket has worn out and who need to switch to a splined sprocket, i.e. avoiding a need to change the existing chainset or BB presumably because the difference between the 54mm and 55mm chainlines is within tolerance for the hub.
I had a similar concern about fouling the chainstays, but I found that in my case I had enough clearance that if I used chainring spacers I could have possibly even fitted a 113mm axle based on Brucey's measurements and nsew's measurement of the 113mm here. I would have done so to reduce the Q factor, but in the end I never got round to trying it because it looked like my shoes would probably rub against the chainstays.
BTW Brucey, thank you for going to the trouble of taking and posting those original measurements, which saved me wasting money guessing which size UN55 I would need. I was able to measure my chainline with an existing UN55 which I already happened to have spare but which was far too short, and then use that measurement to calculate that I needed a 122.5mm.