Two days after I bought my Langster, I managed to unscrew the sprocket while resisting the pedals and this popped off the lock ring and stripped the thread off the hub. My LBS replaced the entire rear wheel as a warranty job, but tonight after I was doing a bit of skidding the sprocket has come loose again (second time in 2 weeks).

I'm going to take it back in to the shop tomorrow, but I just though I'd ask if this was a common problem. Is the hub or sprocket poor quality, or should I have paid a bit more for a Bianchi Pista? Or am I just really unlucky or ham-fisted? I'm not all that heavy (about 70kg/150lb) or powerful so I don't think it's me.

i know i had to tighten mine when i first got it

Even if they tighten the cog and lockring by hand, you will certainly need to tighten the cog and lockring again after some riding.

I usually ride up a large hill, or rotafix the cog on, then tighten the lockring down as hard as possible...Even after it was tightened down by hand.

Thanks guys, I just hope it's a matter of tightening it up. I'm missing all the nice weather with the bike ending up in the shop all the time, and we don't exactly get too much nice weather here

i had the same issue with mine now i just use red loctite and a super tight lockring