To do a conversion, you'll need
♦ A frame with horzontal dropouts.
♦ A new rear wheel with a fixed gear hub (+cog and lockring)
♦ And possibly a new crankset/BB to get a good straight chainline.
I'm not trying to insult your intelligence by posting this but I find a lot of people don't know what a "horizontal dropout" is.
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While conversions are great, your budget is pretty close to affording a new bike. If you can sell some blood plasma or find a little more loose change in your couch cushions for about $300 you can buy a brand new factory built FG @
Track Bikes, Fixie, Fixed Gear, Singlespeed and Track Bicycles from bikesdirect.com . You may be thinking- "A $300 bike must be a cheap piece of junk" but the reality is that since FG/SS bikes do not have the expensive and complex components of other bikes like shifters, derailleurs, and cassettes they are remarkably affordable.
I weigh about 250lbs and I've been rocking the "Motobecane Messenger" for a couple years problem free [i did add 28mm tires to replace the 23's it came with].
Good luck whatever you decide. Riding FG is a lot of fun and it is sure to make you a more efficient rider on your freewheeling multi-speed bikes as well. Just be ready to grow thighs like treetrunks.