Originally Posted by spud
you can go the super cheap route, pick the gear you like, put it in that gear and clip the derailuer cables and remove the shifters. a very time efficient SS too, and you probably wont even have to wash grease off your hands.
Originally Posted by poopncow
Careful, if your chain line is not relatively straight (with the gear you pick), the chain will try to "shift" when you stump on the crank...first hand experience. btw, there is no rear deraileur, right?
the cable holds the derailleur against its natural tendency to spring to either the highest or lowest gear, depending on make/model. cutting the cable in one of the middle gears won't result in the derailleur staying in that gear like some kind of singleator. it will just free it to go to its default position at the top or the bottom of the gearing.
op: easiest way to go singlespeed on a derailleur bike is to select the cog you wish to use from the cassette, and replace the other cogs with cassette spacers. chainline is important with an s/s bike, which means that the chainring and the cog should be in line, looking from front to back or vice versa, so that the chain runs straight between them. you place the cog at the perfect position along the cassette by adjusting how many spacers are on either side of it.
any other method requires a new hub and a rebuilt wheel.
chain tension's important to keep the chain from falling off the chainring. horizontal dropouts and trackends allow for front-to-back repositioning of the rear wheel to assist this. it's harder with vert dropouts, but there are other options, including a chain half-link, an eccentric hub or a surly singleator (google any term you don't know).
use a bmx chain.