Originally Posted by
cpper
With I've had this before I was speaking about what was following, the cassette wear namely. Sorry if you got it wrong, english is not my native language.
You're saying the problem is wear. The 10th cog worked perfectly until today, even when pedaling as hard as possible. In a matter of a second it stared slipping even when pedaling slowly. I knew wear causes problems gradually. Is this not true ?
Anything that wears goes from working to not working at some particular point in time - especially if you only have 11t. You appear to have reached that point. It certainly doesn't appear to be related to the lateral movement of the derailleur and you said you've checked your chain (I'm assuming with a chain checker and not just a ruler). There isn't anything left.
Put a new cassette on. If it goes away, then it was wear. If it doesn't go away, post about it again.
You might also look at a gear chart and see if there is a better gear combination that doesn't have you riding in your highest rear cog as much. Most people only use that cog downhill or when sprinting.