I had some free time today to putter, so I went to work converting the drive-train on the Cross bike.
Removing the old ATB cranks was necessary, both because they take 4-bolt rings (ick), AND mostly because at 175 mm they're too long for my legs. I swapped in some budget cranks that should hold up to my style of riding, with the beautiful Surly ring:
Once the new cranks were installed, I dialed in the chainline and installed the Surly cog, and the Convert (the component that allows me to run a singlespeed setup on a frame with vertical dropouts):
The Convert, from Soulcraft, comes with helpful --and funny -- instructions, which really should be read all the way through before you install it. Important note to the D-I-Y'er if you use the Convert: do NOT use cogs or ring with ramped teeth, and if possible go with a chain that's dedicated to single-speed use (it will be tougher and last longer). If you're paying a shop to set this up for you they [should] already know this stuff. I chose the KMC 1/2 x 3/32 "BMX" chain (for use on bikes that take 3/32" chains rather than the wider 1/8"). As you can see, it's made of pretty stern stuff. When I hefted it in my hand I felt like one of the Jets or Sharks in West Side Story...
For now I'm using a 39t ring and a 21t cog. I chose this based on my cruising combination when I commute (I prefer to spin low gears, rather than push high gears). Of course, there's room for experimentation, but I thought I'd start on familiar ground. I also have 19t and 17t cogs available if I need to go a little bigger.
I was hoping to install a pair of Crank Brothers 5050 pedals that I'd splurged on -- I intend to ride 'cross with flat pedals -- but out of the box there is a problem with the bearings in the left side pedal, so I will have to run something else for now. Grr.
Meanwhile, it's coming along nicely. All that's really left is to set up the brakes and dial in the handlebar position. Next time.