I am currently running a Surly Mr. Whirly crankset as a single speed. The frame is a Soma Double Cross DC, a cyclocross frame. I need to check my math, or the install, because something seems off.
I have a 68mm bottom bracket shell that is NOT faced (not a problem from what I can tell). I am using the 47.5mm chain line spindle designed solely for 68mm bottom bracket shells, Chris King bottom bracket, and one 2.5mm spacer on the drive side. I achieve 50mm chain line with this configuration. My cranks are symmetrical in relation to the frame's center line and their distance away from the chain stays (keep in mind this is with the spacer).
I have ridden this install ~100 miles with no problems.
This is what I don't understand. Currently there is a ~2mm (could be ~2.5mm) gap between the end of the spindle on the non-drive side and the edge of the machining on the left crank. I want to believe this is lost space due to the spacer on the drive side. But, maybe this is how it is supposed to be. Without the spacer my chain line would be 47.5mm (as stated on the spindle) but the Q-factor would become lopsided in relation to the frame's center line.
Should that ~2mm gap mentioned above be there? A faced bottom bracket shell might drop the gap down to ~1.5mm, but no more; there would still be a gap. Why is that spacer needed to maintain a symmetrical Q-factor? Is the frame lopsided?
Did you ever get this figured out? I'm considering a Mr. Whirly with the 47.5mm chainline spindle too.
It doesn't make sense that you had to go to a 50mm chainline to get a symmetrical crank. That would imply with a 47.5 mm chainline it would be 2.5mm further to the left.... which does not sound right at all.
Try measuring the crank arms from the inside of the arms to the inside of the seat tube. This will tell you if you chainstays are out of wack.
Yes, I learned that some frames are imperfect. Unfortunately I got one of the imperfect ones. My frame was assembled by humans, not robots, so I guess imperfections are to be expected. The slight asymmetry of the chain stays is what threw me off. The cranks are fine.
To answer your question... According to Surly, if you want to use the 47.5 spindle you shouldn't use any spacers. I believe you could get away with using one 1mm spacer if you needed to, but no more.