Okay I recently bought a fixed gear from a guy off of my local craigslist. After buying I a realized that the rear cog is screwed directly to the rear hub with no lockring.

So my question is can I simply add a lockring to my original rear hub? or do I have to buy a new hub or wheel. The tool at my LBS told me I had to buy a new wheel, but then again he was a total toolbag.

I am no expert but it sounds like maybe a suicide hub? I've heard you can ride it but is unsafe. But like i said i'm no expert.

I know people who ride a suicide hub and they're fine I guess, even according to Sheldon Brown it's okay as long as you don't skid or heavily backpedal.

And woody you wouldn't need a new wheel just new hub with track threads and spokes, it's not that hard to build your own wheel, but it'll take time.

you -could- ride a suicide hub, but it's called a suicide hub for a reason. it would be insane to ride a suicide hub with no handbrake because if you backpedal hard enough, the cog -will- unscrew and you will not be able to stop.

if it is a fixed-gear hub you will easily be able to add a lockring. if it's just a hub threaded for a freewheel (for a road bike) you had better get a new hub, or a new wheel, or a brake.

I'm on board with almost everyone, but a photo would help.

I run a suicide hub on my 80's conversion. I run full brakes on it and I have never spun the cog loose. I'm not sure I would trust one on a brake-less setup or a bike with just a front brake.

Post some pics of the hub.

uhmm, just buy a lockring, bro. but i wouldn't do anything realllll crazy like skiddin' and $h!t. of course i'm a mtb'er.

Can't you supposedly use a BB lockring on a suicide hub?

Look at your hub. If it's got a smaller-diameter section toward the outside, then just go buy a lockring and remember that the lockring bit is reverse threaded. If it's a continuous diameter, go invest in some red Loctite, make sure you're running a brake, and possibly a BB lockring (more info on the above from someone with experience would be helpful) and start saving for a new rear wheel. Otherwise, stop riding your bike right now.

can you skid with your suicide hub without unscrewing the cog? i've never tried but i find it hard to believe that it wouldn't spin off.

A suicide hub is a hub without the ability to add a lockring as it only has one thread the diameter of a fixed/freewheel cog. Hence the possibility of a fixed cog spinning off if you skid, hence the name, suicide.
A track hub has, as you say, "a smaller-diameter section toward the outside". This is where the lockring threads to the hub. Not to run a lockring may be termed suicide by some, badass by others & stupid by the rest.
I'd say ride SS, with brake(s), or get a new hub.

you don't need to skid if you have brakes you see.

of course. but you can still put a lot of backpedaling pressure on a cog without skidding. also, the OP never said if he has brakes or not ..

so the guy at the bike shop is a tool because he gave you the best advice? huh
get yourself a proper wheel man...you will have peace of mind and well, a new wheel