Replaced chain and cassette, now new chain slips in a specific gear

Question

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.

Answer 1

You have a mid-drive (Bosch) motor according to search, in my experience these can be very hard on drivetrains, for two reasons. One, the motor puts the extra torque through the chain and cassette, as if you were riding full gas the whole time when you think you are cruising. Two, the rider can be a bit lazy and leave the bike in their favourite gear, greatly increasing the speed the favoured sprocket will wear out (see point one).

Sadly that means that 500 miles is not unthinkable before wearing a chain out (I personally think this will lead to innovation away from derailleur gears in the near future). It then becomes a reasonable routine to forego the old 'replace the chain before its worn too far' mentality and budget to replace both the chain and cassette together, slightly later on. Not a mandatory rule, but a reasonable adjustment to consider. Especially if you are paying the cost of two chains every time.

Now as for the remaining problems, it is strange that the cassette is slipping in the 4th sprocket, if the cassette was replaced with new. I would double check this, the behaviour described is consistent with a new chain on a worn sprocket.

You have recourse with the shop who have done the work, they should be able to service it in a way that you are happy with, or be able to explain why not. Show them the behaviour and also show them the chain length. Ask them to try another new cassette if they are confident it was replaced; new chain should not slip on new cassette.

The derailleur is unlikely to affect a single gear, perhaps if it is a little misaligned, this can affect the indexing halfway through the cassette. This can be measured with the hanger alignment gauge.

Someone will be along soon to slag off the bike shop who did the work, I would ignore that kind of confrontational anger as it doesn't achieve much. Take them the bike back in and look at it in person, explain it as you have written above and ask them to help solve it. They are the best people to sort it out, rather than internet guesswork.

Answer 2

The chain should not touch the frame of the bike at all. It will destroy the frame of the bicycle. The shop should never have returned the bike to you in that condition. Request to speak with the mechanic who worked on your bike; go through the issues you found, and watch what they do with your bike in the repair stand.

In my own experience, many shops and mechanics dislike servicing cargo bikes because they are more complex and their owners are more sensitive about their continued reliability; don't take no for an answer.