Yeah, pretty much any frame can be run with just one gear. The simplest or perhaps cheapest way to try it would seem to be using a chain tensioner. I only point to that "kit" as it's the first one I found, I'm sure that the helpful folks here can chime in on which tensioners to use/avoid.
But the general idea is to get a single cog out back, and lacking track/horizontal dropouts, a device to manage chain tension.
Yes, you can get lucky and find the "magic gear" for your frame and components, but that can take a little bit more time and/or patience to figure out.
The other stuff that p_nut refers to, such as the eno hub or the Forward Components eccentric bb start to get a little bit more spendy -- and perhaps a little bit more on the commitment end of the spectrum.
NOPE... Just pull your cassette, install a single cog and spacer set, replace your deraileur with a tensioner ,
If your bike is a 3X9 MTB, remove you big ring and your granny and run your chain on the middle ring and single cog out back.
A suggestion.... get the spacer kit with a stack of rings rather than one with a single outer and inner spacer... you have to get the chain aligned between the ring at the front and the cog at the back
If you'd like to see if your frames chainstay length is JUST RIGHT to run a "magic gear" try this sight.... My first conversion worked with this. http://eehouse.org/fixin/fixmeup.php
I wish I would have taken pics today of what I did. I bought a a Raleigh road bike from a rummage sale. It wasn't on sale. I kept looking at the bike hanging in their garage, and it was very dusty. Caked on dust. I told the lady I'd give her $40 for it. I knew it was my size, and the bike is in MINT condition. She's in her 50's and told me she's not ridden it in years.
So I brought it home. In 10 minutes, it was all taken apart. My new rainy day SS bike. Pics to come.
Any bike can be single speed. ENO, fake tensioner (using a derailleur as a tensioner), horizontal drops, etc. Any bike.
Jessica Alba, if you are reading this, you got a nice smile.
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