Rear Derailleur Misery

[lonegroover]

The rear derailleur cable broke on my hybrid. I've replaced it, and spent an hour this afternoon trying to adjust the gears.

I can get the cable tight enough so that it changes into the biggest cog without a problem. I can get it loose enough so that it changes into the smallest cog. But I can't get it to do both.

The shifter looks OK. The cable isn't jamming or sticking.

It's not the limit screws - I've looked carefully. I'd be tempted to think the spring was too weak, but it was working fine before the cable went.

Any thoughts? I know it sounds a bit vague but thought maybe someone had had the same experience.

[orienteer]

Did you replace both cable inner and outer? In my experience just replacing the inner is often insufficient to restore perfect shifting. Also need to ensure the derailleur mechanism is clean and lubricated.

[chrisbainbridge]

That sounds like inadequate pull length if you cannot go all the way across the cassette. I wonder if you have made a mistake in the cable run or even something as small as the wrong side of the cable clamp will do this.

[Torslanda]

Start from first principles. Assuming you have fitted like for like. Undo the derailleur cable.

Adjust the outer (H) limit screw until the chain just drops from the second smallest cog to the smallest.

Adjust the inner (L) limit screw until the chain climbs from the 2nd largest cog to the largest.

Bottom the cable adjuster on the derailleur and shifter. All the way in and out again 2 turns.

Manually tension the cable and ensure the shifter has its full range of movement in both pull and release. Ensure the cable does not snag, is not frayed or kinked. Pay careful attention to the section under the BB if that is the routing.

Manually tension the cable and fit it correctly to the pinch bolt. There is a groove in the derailleur body and the clamp. Just in case the clamp is 180^o out, the groove in the body is the correct orientation. Don't worry at this stage about cable tension. Turn the pedals and change down until you run out of clicks. Ignore the indexing, what you're doing is seating the outer cable as it will have relaxed slightly when you took the tension off.

Change up all the way to top, ensuring the chain ends up on the smallest cog under its own steam, adjust limit if necessary. Undo the clamp and pull any slack from the cable manually. Retighten the clamp.

Turn the pedals and click the shifter once. If it changes to the next cog smoothly continue across the cassette. If not add a half turn to the adjuster until it does.

CAVEAT. If your mech is new and your chain/cassette worn it may only be possible to adjust downshifting (pulling the cable) reliably. A worn chain can 'hang' on the cog needing two clicks to get to the next.

HTH

[hatler]

CAVEAT. If your mech is new and your chain/cassette worn it may only be possible to adjust downshifting (pulling the cable) reliably. A worn chain can 'hang' on the cog needing two clicks to get to the next.

Oooo. I didn't know that. That could well explain a few frustrations I've had over the years.

[lonegroover]

Appreciate the replies, thanks very much. I will start from scratch again tomorrow.

[Brucey]

FWIW you don't say how many gears you have but in most shimano index systems a minimum requirement is

- a polished stainless inner cable
- correct new SIS cable housing, well prepped
- good ferrules
- a BB cable guide that is not binding
- a good lubricant on the cable (I use finish line teflon grease with a little oil added, but it is not the only way)
- a free-moving shifter that is well lubricated.

if the cable housing was old then the shifting will probably be compromised even with a new cable.

There are lots of other things (wear on parts, derailleur alignment etc) which can affect shifting too.

cheers

[Kim]

My money's on something cabley, but bent hanger is another favourite.

[nikki]

Start from first principles.
[...]
HTH

Unsure how lonegroover got on, but this approach got my rear derailleur working smoothly - thanks Tors 

[Torslanda]

You're welcome, Nikki. Woohoo! I can fix bikes by remote control. 

Having just re-read the OP I mistook the problem. I'd read 'broken' and assumed a replacement mech, not the cable.

Broadly, Kim & Brucey have it and I would stake money on corrosion in the outer cable housing OR a bent hanger.