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Strange thing happened today, 12 miles into a ride, down a small rock garden followed by a short steep climb, as soon as I start pedalling driving train making all sorts of grinding and crunching sounds, get of and see chain is very slack and upper jockey wheel sitting in cassette, nothing looks bent etc, wind in more b tension to a point its rideable to get me back. Can't think how this would happen, threads look fine on b tension screw? Mech has 5 rides on it, however it's the first ride since changing to new wheels where all settings on mech needed to be o be adjusted.
Check the bit on the hanger that the B screw sits on hasn't worn/chipped off.