it has a sort of rubble
Did you keep your old chain when swapping the cog? I've had this before when I put on a new MX30. The old one was well eaten and the chain had probably slightly stretched along with it. Stuck a new chain on - problem solved.
@rab started
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Did you keep your old chain when swapping the cog? I've had this before when I put on a new MX30. The old one was well eaten and the chain had probably slightly stretched along with it. Stuck a new chain on - problem solved.