question about surly hubs
 

i'm pretty sure surly only makes fixed and/or free hubs, but if they don't that's what i'm referring to. my question is that do the various sizes (120, 130, 135) all have the same flange width and hub body or are they different for each size? for instance, with a change of axle and spacers, could you effectively change the hub from one size to another? i've dealt with both the 120 and 135, but never had them together to compare. thanks!

No. I was at a shop last week and I held a Surly track hub and a 135mm Surly singlespeed hub next to each other and both the flange width and chainline were not the same.

You could take a 120 mm hub and either change the axel or just cut off the ends and use a quick release. But then you would have a really messed up chainline unless you used really narrow cranks.

As above, I'm sure you can make a 120 hub into whatever you want, but you may have to dish it.

That is true. But then your flip flop hub will neither flip nor flop.

The 120 mm hubs are made for 42 mm chainline, typical on track type equipment.

The 135 mm hubs are made for 52 mm chainline, so the chainwheel can clear the wide chainstays of a typical mountain bike frame.

See: http://sheldonbrown.com/chainline for details on this.

Sheldon "Actually, Surly Doesn't 'Make' Hubs At All, They're Made By Formula" Brown

Yeah, wish I'd bought Formula and saved myself the $100.

Now here's another question. On my Surly Crosscheck, A 118mm bottom bracket with SR Signature cranks gave me about 2 or 3 mm of clearance between my chainstays and the cranks.

However, that same combo on my fixed gear worked.

My question is this. Can two cranks have the same chainline, but different clearance between the crankarms and chainstay?

Most definitely......and even more critical when using track specific cranks.