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How should I know when my rear derailleur is getting worn? I can measure/inspect chains, tyres and brakes; gears tend to show problems on a new chain if they're worn. But when changing my chain at the weekend I noticed the that there seemed to be quite a bit of play in the pivots - more than I'd expect from mechanical bits in general. Also the jockey wheels were quite worn (and didn't run very smoothly even after a good clean and dry PTFE).

In my specific case it's only Altus, with about 20 000 km on it, and the bike doesn't get cleaned as often as it should.

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    20.000km for an Altus is quite decent. You could put in new jockey wheels. But they might cost nearly as much as a new unit. I'd decide to get a new derailleur. – Carel May 18 '16 at 7:35
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    @Carel I'm sure he can find cheap jockey wheels, maybe not genuine. C'mon, it's just kind of helper, there is no need to go hi-tech genuine. I would say derailleur is worn out when the spring isn't strong enough to keep chain tensioned at every gear and/or spring responsible for going to higher gears seem to fail somewhere beetwen. How to measure it -> compare torque of new unit, don't ask how to obtain this data. ;) – krzyski May 18 '16 at 7:42
  • @Carel, I'm inclined to the view that new jockey wheels are a false economy in the case of cheap hardware, but didn't want to bias the question in that direction. – Chris H May 18 '16 at 7:57
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    @krzyski I doubt a lack of spring tension is what's casuing my new chain to skip even after changing the cassette (despite the chain rings looking OK). Shifting is spot-on. I reckon it would be possible to measure spring tension by measuring the deflection using a known load and a pulley. – Chris H May 18 '16 at 8:00
  • My limited experienced is that you can go for at least 10K miles on a standard rear derailer. – Daniel R Hicks May 18 '16 at 12:38
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You seem to have the answer embedded in your question:

  • sideways play in the pivots
  • worn jockey wheels

The pivots seem to get worn mainly by contamination - during mountain biking. The rough terrain doesn't help the situation either.

The result of worn pivots seems to be worse chain retention over rough terrain. So I would say, if you ride down the street, hit a pothole while pedaling, and the rear switches gear - then the RD is to be replaced.

On a side note, RD wear is one of the easiest and quite important steps in checking a used bike. Put a finger on the bottom end of the cage and rock it gently sideways.

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    Comparing old altus to new acera demonstrates that 20 000km (and some knocks that left deep scratches) was indeed enough to wear the pivots. It never shifted going over potholes of even kerbs (though I try to avoid the latter especially). Although I don't do much off tarmac, our roads can get quite dirty, and there was certainly muck in places that can't be washed with the RD on the bike. – Chris H May 28 '16 at 9:41
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I cannot comment on derailleurs in general, just my own experience.

My retro ride is 35 years old, and in that time I've ridden it about 150,000 km. It has Campagnolo Record derailleurs, and both are fine. Shifting is still good with the downtube shifters. The parallelogram is rock solid.

I cannot claim to have maintained it especially well. It's only had about 10 chains and clusters in that time.

Observing the milage of others, I think the main end of life event is a crash. Some people do seem to wear the jockey wheels, perhaps due to running lower quality chains.

My conclusion is that it all depends on the initial quality. We get what we pay for.

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    Is that bike indexed or non-indexed? I would guess that an indexed setup is slightly more prone to malfunction due to wear. – Daniel R Hicks May 18 '16 at 11:45
  • @Daniel Non indexed (I don't think indexed gears existed in 1981). In my understanding of how indexed gears work, I think the opposite regarding wear. – andy256 May 18 '16 at 12:28
  • But Campagnolo wears in, while others wear out... – Rider_X May 18 '16 at 15:52
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    @andy256 - with friction, your shifting procedure does a lot of compensation over the life of everything, making it more robust in that regard. – Batman Nov 10 '16 at 15:44
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I have the same problem on a mid 90s rigid steel MTB, which simply won't drop to top gear reliably.

The RD mech no longer moves out far enough to engage the last cog, which implies the spring is lacking in sproing. I can get the chain there if I suspend the bike and pull the RD with my hand while pedalling with the other hand.

A replacement rear mech is the only option.

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A late addition, but from decades of owning Campy and Shimano, each should last forever with basic cleaning and lubing. Seventeen year old Dura-ace rear has zero play. By contrast, I've worn out SRAM MTB X7 and X9 rears in one to two years, despite frequent TLC. Their weakness is mostly in the soft metal of the parallelogram bushing pivots, with tiny plastic inserts which get loose no matter what lube you can get into them, and soft alloy main pivot bolt which screws into the hanger. Pulley wheels get all the attention, but you can run them forever if clean, and they are cheap to replace, whereas the main derailleur parts are not at all.

  • I think this might apply more to the more expensive ranges, but it's a good point. I say it because the cheap shimano ranges accumulate muck in hard to reach places and aren't really built to be stripped down. – Chris H Jul 17 '17 at 6:32
  • Are there any derailleurs where the main parallelogram can be disassembled? In early 2000s Campagnolo had almost every small part available as spare, the exception being rear derailleur body. – ojs Jul 17 '17 at 11:01

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