Should be an easy one: 120 mm hub to 135 mm?
 

I bought this nice Surly Flip Flop hub, 120 mm spaced. As I look at it, I wonder if, by adding an equal number of identical spacers on both side of the axle, would I be able to fit it in 135 mm dropouts?

Yes you can.

Yup, you've got the right idea. Just make sure you add the same distance of spacers to each side, so you keep chainline. But it sounds like you're already aware of this. I'm pretty sure that Surly makes their axles long enough to change up spacing as you're talking about.

Right, but you might have chainline problems. Surly "track" hubs, like most track hubs, are designed for a chainline of about 42 mm.

135 mm frames are most often mountain bikes, and the chainstays of typical mountain bikes won't allow any but a quite small chainring to be installed that far in. Typical singlespeed mountain bikes/hubs are designed for a chainline of about 52 mm.

See also: http://sheldonbrown.com/chainline

Sheldon "Oops!" Brown

Absolutely true: I have experienced this with a project where I used a 135 mm wheel with hub for separate freewheel (oldstyle thingy). On the MTB frame I used, the largest chainring I could use was 36 teeth. Still, it works quite well, have a perfect chainline (thanks to some spacers on the crankset bolts).

Which SS MTBs will allow for such a large chainring (52 T)? I was looking at the following frames:

Surly Karate Monkey (doesn't have sliding dropouts unfortunately, and they're really handy).
On-One Inbred
VooDoo Dambala
Kona Explosif/Unit (only 26" though).

Do all of these allow for a large chainring?