had the new mark v for 3 or 4 days now, and on one of my first rides the cog spun unscrewed just a little bit while skipping. so i cranked hard up a hill and then stopped by dragging my feet, then tightened the lockring real tight, per tony's instructions.

then today i was riding down a slight hill, skipped pretty good to slow for a stop sign, and it happened again. it only spun a little bit--maybe 1/8th or 1/16th of a turn or something-but was quite noticeable. then when i get back on the gas of course it screws back on and i can feel that too.

this has significantly affected my confidence in emergent stopping ability. so looking at the hub, the threads are greased and the cog is a soma, 3/32" width. there appears to be plenty of lockring threads that extend well into the shadow of the cog's shoulder, so i don't think the cog is too thin. and the lockring does appear to seat firmly against the cog when tightened.

so i guess i want to clean off the grease and use some licktite. do i locktite the cog threads, lockring threads, or all of the above? or should i use jb weld?

No jb weld....red loctite on the cog

is this a common problem? i have talked to a couple of the local guys here and they have greased threads and apparently don't have any issues with the cog unscrewing. why me? and is the locktite a sure fix?

Try this- Ride around, pushing really hard on the pedals, trying to get that cog as tight on there as you can, but DO not resist the pedals, i.e no skids/skips. Use your front brake to stop, if you have one.
After a little while (30 mins maybe?), yor cog will be really tight, then tighten the lockring again, so its really tight. Make sure that you do this on a side street or something so you don't kill your self.
The proper tools, a lockring wrench, and possibly even a chain whip will help here.

Loctite should be a last resort, but use blue strength, so it is not permanent. No JB weld/Red loctite.

Maybe the base of your cog is just a tiny wee bit too narrow, and the lockring isn't actually locking down on the cog enough. I had this problem with a Surly cog. I put a 1mm steel spacer between the cog and the hub. Otherwise my lockring would bottom out on its hub threading before putting enough force on the cog itself. Hopefully this is clear... Please try this before using any kind of loctite.

Do yourself a favor: figure out what's actually causing your problem.

What you're describing is a setup that should work just fine. As you discovered, it's not. So take the time to figure out why. As I've said before, this setup has been used successfully for a century without needing loctite. It would behoove you to divine what's busted up in your particular instance.

that's just it. the system is built up perfectly. the problem is that when applying EXTREME back pressure (such as that applied by a heavy rider stopping fast) is a predictable cause for failure in this way. i really can't see how any heavy rider can stop quickly on greased threads without some slippage occuring. maybe i answered my own question here. i don't know. i mean, do all you guys skip really hard to stop your fg's (with greased threads) whithout ever having any failure of this sort? i just don't see how that's possible without some sort of more direct opposition to the rearward force being applied to the cog. know what i mean?

Yes, it's called a lock ring.

BostonFixed's method will fix your problem, Andy. 30 minutes is most likely more time than you'll need to spin that cog on as tight as it will go, I'd say 10 would be a lot of time to spend spinning it on. Your problem is caused by the cog not being spun on as tight as possible, then the lockring being threaded on only as tight as the "loose" cog will allow it to go. So there's some play in the cog, it spins forwards and spins off a little bit, but is stopped by the hub or the lockring. Spin the cog on, then retighten the lockring and you'll be fine.

The grease isn't the problem, the loose cog is. This is a very common problem, and is asked at least once a month, it seems. It's always been solved via BostonFixed's advice. If he's wrong this time, I'll pay for the plane ticket so you can fly out here and kick me in the junk, while pushing me into a busy intersection on my fixed gear, while I yell "Get out of my way you fools, I have no brakes!!". Sound fair?

ok,fair enough. i give. i will try again tomorrow and this time push REALLY hard up a big hill, walk it back down and then tighten the lockring again. thanks guys. will report back. by the way riding fixed is THE funnest thing i've ever done on 2 wheels... like riding my single speed but 10 times better. super psyched. much respect

ok, cog is good. i pedaled up the biggest hill and walked back down, twice. then tightened the lock ring REALLY tight. turns out the lockring was never being tightened down enough by the guy at the bike shop. he was tightening it without removing the chain, and therefore could not put a lot of pressure on the spanner since it wasn't on straight (chain in the way). i borrowed his spanner and did it myself, with the chain off, and was able to get it like a zillion times tighter. thanks everybody.

hmmm. in my experience, some bikestorefolks don't like trackhubs and fixies and **** and stay away from them... and make mistakes like that.

also, you gotta have a good locknut spanner. some bike toolkits sell real cheap hinged ones that don't give the solid torque you need to firm up a fixie hub.

I had the same problem, and on my way to get the lockring tightened, the rear hub completely stripped out. major bummer.

anyways, like queerpunk said, make sure you have a good locknut spanner. I know Paul makes one, but it's super expensive.