I also have a Poprad with an ENO but I've not had any problems. I usually tension the chain while the bike is on the ground and make sure that you tension the chain with the eccentric pointing toward the rear of the bike. I think that's confusing, let me put it another way. Facing the drive side my 19mm wrench is in the 6-9 o'clock position and I'm moving the wrench clock wise to add tension. This way if it does come loose the it will only get tighter.
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I'm not one for fawning over bicycles, but I do believe that our bikes communicate with us, and what this bike is saying is, "You're an idiot." BikeSnobNYC