I took the gears off the Salsa Mariachi and replaced them with one Surly cog and a spacer kit I made at work. The spacer kit was pretty simple, 35mm ID and a 40mm OD, cut from 6061 aluminum stock. The tricky part was figuring out how long to make the spacers, because I only wanted one on each side. So I took apart a bunch of old cassettes and stacked the plastic pieces up so I could set the chain line and measure the lengths.
It turned out pretty nice, similar to anything you'd buy from a company only this was free instead of $40. The Surly cog and a surly chain ring are both steel should last a long time.
I didn't really know what to expect when I took the bike out for the first time... Did I pick the right number of teeth? Will my spacer kit work? Everything ended up perfect.
I rode 30 miles of single track at kettle moraine, and only had to get off for a couple hills. The steep hills I was actually able to still climb just fine, the technical ones with tight turns in the middle are what got me. I never got the feeling that I was "spinning out" while on the trail. On the road the 32 front 16 rear gives me about 17mph tops.
The Macho Man is still running the same 2x10 setup, with a 46-36 front and a 12-28 rear, and that makes for a do anything gear range. I have ridden tight single track with it, and done fast Tuesday night group road rides as well. That is what I will be running for Trans Iowa, low enough for the climbs and high enough for the decents.
I had a blast on the single speed though, in fact I think that's what the Mariachi was made for. Salsa just added gears so they could sell some more, but its really meant for single speeding. I have never ridden a single speed before, and I came away really amazed at how much fun it was. Just a simple rigid fork and single speed makes for a bone jarring and knee bursting good time!