also:
http://velosolo.co.uk/faq.html#c2
"Which (drilled cog) hub/set-up is best for my mountain bike ?"
"First be sure you can run a fixed wheel on your frame. See above questions.
Yes? Great - the route to fixed is easy. The rear spacing on virtually all mountain bikes is 135 mm. The best set up is a normal rear cassette hub with 6 bolt disc mount - you probably have one of these on your mountain bike right now.
By mounting a drilled cog on the disc mount and a normal singlespeed cog and spacers on the freehub side you have a flip/flop fixed/free wheel (see pic below). As simple as that, no alterations are necessary and the wheel can be used with gears again simply by removing the singlespeed cog and mounting a cassette.
Using a cassette wheel with a single cog and spacers allows the chainline on the freehub side to be adjusted to match the fixed side perfectly. Thus when flip/flopping the wheel your chainline stays spot on. A cog mounted on the disc side will in most cases give a near perfect chainline with the outer front ring.
Or use a standard mountain-bike singlespeed (ie screw on freewheel) 6 bolt disc hub."
