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No idea, but he kept getting pinch flats. He bent his wheel when the bike fell over, loaded! He got a new wheel, then the rim tape was awful and kept slipping then getting more pinch flats. Not sure how he got it so wrong tbh. I'd run tubeless on that ride anyway.
I think it was a cassette spacer, therefore the cassette was sliding about on the freehub body. So shifting was all over the place.
What tyres was he running? How did the wheel break? If you forget the spacer don't you just use the barrel adjuster to compensate?