Converting a 21 Speed MTB to SS/FG
 

Hey yall,
I've got a 2005 Haro V1 that's seen better days. I'm going to throw on a rigid fork for starters. What rear wheel, cog/chain ring combo's do you use on ss/fg mtb?

what do your dropouts look like?

A good starting point is a 2:1 gear ratio, assuming your primary use is off road. I run a 36x18 off road, and can handle just about any terrain with that ratio (with some suffering). If you're going to be using this on flat land and want to go fast, then you'll want something significantly larger, maybe a 42x16.

There's no need to replace the wheels. You just need to remove the rear cassette (cog cluster), and replace with a single rear cog (I highly recommend the Surly cogs), and add a stack of spacers to adjust your chainline (either beg for used ones at your LBS, or you buy them new). Salvage the smallest cog and lock ring from the old cassette, because you'll need those to clamp the whole assembly together.

If you want to blow a lot of money, then buy a rear wheel with an ENO eccentric hub, this allows you to adjust the chain tension by changing the location of the wheel axle with respect to the frame.

You'll probably want to throw on a single-speed chainring, since mountain bike chainrings tend to be contoured in such a way to make shifting easier, which is undesirable for singlespeed applications. Make sure you check the bolt center diameter.

I'm assuming your bike has vertical dropouts. If you're lucky the chain tension will be perfect, but that's a long shot, even with a half-link. More likely, you will need to add a chain tensioner, or you can just use the old derailleur with the limit screws adjusted to align with the cog location. I use a Rennen Rollenlager because they're frickin' bombproof, but there are cheaper options.

That's it. Go ride.

I just bought an origin8 single speed crankset off ebay. It's not bad for 42 bucks.

Do you have any experience with disc brakes?

I just converted a cheap MTB 18 speed with an SRAM crankset (48t) and a Shimano single cog freewheel (18t). Mine is used for urban riding. I was lucky to have horizontal dropouts so a tensioner was not needed.

There is lots of info here and elsewhere on the web on disc brakes. I have discs on my other bike, but I have not messed with them yet while the bike is still under warranty :-)