Why does the cog slip forward?
 

last month I took my bike to my LBS to get the cog/lockring tightened because I felt some slippage when backpedaling. Now just recently I felt a big slip pedaling FORWARD, few minutes later it slipped again but this time backpedaling.

I rotafixed it (upon doing so I noticed the cog was really loose) and then tightened the lockring.

My question, why did it slip forward? I understand that when you pedal forward the cog gets tighter, then this must have been the lockring that slipped and gotten loose? but why was the cog so loose when I rotafixed it (I was able to tighten it quite a bit). if pedaling forward would tighten the cog then why did it slip forward? i'm confused.

and sorry if what I'm describing isn't making any sense. I'm new to all this.

I'm riding a raleigh rush hour.

[edit- i should mention i rode it hard today and didn't have any more slipping problems, could this happen again? I don't want this $@%# happening in traffic.

thx

Rotafixing puts way more force on it than pedaling. It's possible you've stripped the threads on your hub if you overtightened the cog, which could cause slippage in either direction without the lock ring being lose. I wouldn't worry, just tighten everything firmly but not with death torque and ride, if it's stripped it will let you know.

Sounds like it was loose at first then when you pedal the slip you felt was actually the cog re-tightening. Hope that makes some sense. As for why it's loosening up, someone else may be able to answer that for you.

it might be that your cog is eating into the non-threaded part of your hub creating space between your lockring and your cog, allowing it to slip back when you backpedaled.
edit: uppon further review, im not quite sure how this could happen
when you rotafixed it on you were(maybe) just tightening it onto the "threads" you had created, and in the process maybe wrecking your hub. hopefully im wrong though, i didnt totally understand your problem.

Sounds to me like your threads are screwed up. FWIW, my Rush Hour had a really loose cog/lockring when I got it too. I wouldn't trust an LBS to tighten it, unless they know fixed gears. Spend the bucks on a chainwhip and lockring tool, and then you won't have to worry again.

I doubt this, you wouldn't just be threading it, you'd be butting up against the flange over a pretty large surface. That metal would have to go somewhere, and I've never heard of anything like this happening.

how tight its to tight, on a lockring?

i have, though only once and recently so it came to mind. but yeah the hub that i saw had had the cog bite into the hub and "create" more threads for the cog. the metal just shaved off. of course, it was impossible to tighten anything anymore, so the hub was bunk.

i think it as a combo of a cheap cog and a cheap hub that did it, though i would agree that it would be unusual,

hmm, should I just bring it back to my LBS and have them inspect everything? or should I take everything off and inspect it myself? I'm not too good with bikes though.

when I rode it today for the first time after it slipped I was paranoid as hell that it would happen again especially going up a hill, but like I mentioned before I rode it hard and everything was fine. I even tried tightening the lockring again after the ride to see if it may have loosened again but it was still tight.

Can it be overtightened by using a chainwhip? The LBS guy used a chainwhip and lockring tool to tighten everything up. It was only after it started to slip forward that I rotafixed it myself which so far hasn't slipped anymore.

sounds like you stripped your hub dude.

damn this sucks and I only had the bike for less than a month. do I need a new wheel then?

for now I'm flipping it to the singlespeed side.

Yeah stripped it.
get a new hub off the interweb and have somebody rebuild your wheel. it should cost like 80 bucks total.

then buy a chainwhip and lockring tool and do it yourself from now on

also, no-name cogs and lockrings are a no-no

I have a rush hour and have had the exact same problem. It's happened a couple of times during skidding, but the slippage is only about 1/8 of a full crank turn (hope that makes sense). Then when I pedal hard it tightens back up. I've taken it to the lbs and got it tightened and then it's good for a while. The cog and lockring were quite loose when I got it, so I'm not sure if the hub is stripped or just loosens with time.

The lbs guy swears it's not stripped but I don't think he deals with fixed much. If you feel it slip again just don't skid or back pedal and then tighten it until you're ready to upgrade the wheelset.

Same thing happened on my friend's Pista.
Only a month old and he was having the same problem...the LBS he bought it from probably didn't tighten it down hard enough or installed it incorrectly.
So, he took it back to them to have a look at it...they couldn't remedy the problem.
After a phone call to Bianchi, the LBS managed to have Bianchi send a new rear wheel since the bike was still under warranty.
Try to do the same...your bike should still be covered against these problems.