I just bought a Specialized Langster off Ebay. I haven't ridden it much yet, but the gearing seems a bit too high. I had knee problems when I was young and don't want to aggravate them. Also, the bike is currently set up as a single speed and I would like to change it to a fixed gear.

I have been searching and reading and it looks like there are 2 different thread systems and cogs are spaced differently. I can't stand bike noise or grinding. What cog would work well on the Langster? Also, what tools should I get to remove the current cog/freewheel and attach the new one?

Thanks.

No one has any advice?

I am just anxious to get out and comfortably ride this bike.

Everything you're asking is pretty simple.

First off, tools:
You need a BMX 4-prong freewheel puller (looks like this) to get the freewheel off. If you're not going to be using it a lot I'd just take the wheel down to your LBS and have them pop it off; it takes about 5 seconds.

To do things up right you should have a chainwhip to tighten the cog and a BB lockring tool to get your lockring nice and snug.

As far as cogs and lockrings go, don't skimp and go no-name; your hub threads will thank you. I like EAI cogs but Dura-ace and the new Phil Wood cogs are options as well. Your drivetrain is probably 3/32" so make sure to pick up the proper cog. Finally, I'd go with a Dura-ace lockring to keep everything in place.

Now, to get things ready to ride. Pop off that freewheel. Flip the wheel around and toss on your cog. Tighten it up as best you can with the chainwhip. Spin on the lockring (remember it's reverse thread) and tighten the hell out of it. You shouldn't have to add or drop any links in your chain if you're not altering your cog size too much but that's up to you. Stick the wheel back on the bike and get everything hooked up. Off you go. After your inaugural ride make sure you go back and check the cog and lockring to see if they're still snug. Tighten as necessary.

That should do ya.