I ride a cargo bike (Yuba Spicy Curry) with electric assist that uses an extra-long chain, I believe two spliced together. Not an e-bike specific question, though. 8-speed cassette, one big single ring in front.
I needed a new chain due to an odd circumstance and the bike shop installed one. Then they said I needed a new cassette because the new chain was slipping. This didn't make sense, the old cassette was only 500 miles old and there'd been no slipping prior to the new chain. But I said okay, I needed a working combination.
I got the bike back with new cassette and new chain and the chain rides fine in seven of the gears but slips constantly in fourth gear (of 8). It's unusable in that gear. Also, the chain sits lower than the old chain did, making contact with the bike when at rest in the smallest back cog.
Is there anything to deduce from the fact that it slips like crazy only in that one gear? The shop insists they matched the chain length of the previous chain and suggests if I want them to work on it more, they'll replace the derailleur.