Can Shift To The Top, but not back down...

parkinpants

I'm having trouble getting the shifting right on my rear derailler (Deore LX)

Shifting up from smallest sprocket to largest works fine, but the first click down from the biggest cog goes nowhere, it's then okay for a few clicks, struggles and then skips a gear before landing to the smallest cog.

It was doing this at the weekend. Since then I've:
- put new gear cables in (though old ones looked fine)
- aligned the derailler hanger
- shortened the chain (it was a few links too long before)

Still not right though.

Bike is about 8 years old with original deraillers/shifters. First problem I ran into was chain snapping. Lost a few links. Was riding with a short chain a few timesand couldn't reach the furthest gears, no big deal. Then I replaced cassette/chain/front chain rings.. but shifting hasn't been right since.

nicklouse

Did you change the outer? No change it.

Does the mech move fine with out the cable attached? If yes then it points again to the cables.

parkinpants

No I didn't change the outer.

I didn't check the mech movements before cabling it up - but I can unhook it all and check that out if there are no other ideas.

graemeferguson

You could try a slight reduction on your cable tension. This will help with going from larger rings down. Might need to press by the click to get it up the rings and don't reduce too much or you wont get it on the big ring.

cooldad

99.998789899999999% of shifting problems are cruddy cables. Always worth replacing outers at the same time. Minimal cost, maximum benefit.

maddog 2

sticky cable

1. the force needed to go to a bigger sprocket comes from your thumb, which is strong.

2. the force needed to go to a smaller sprocket comes from the mech spring, which is weak.

Hence if your cables are sticky, then 1 still works as your thumb is strong enough to overcome the stiction, but 2 doesn't because the spring isn't.

Cable can be sticky due to grubby inner/outer or kinked inner/outer cable somewhere, or tight routing.

m_cozzy

Similar issue on mine was the b screw was wound in too far.

Ouija

parkinpants wrote:

I'm having trouble getting the shifting right on my rear derailler (Deore LX)

Shifting up from smallest sprocket to largest works fine, but the first click down from the biggest cog goes nowhere, it's then okay for a few clicks, struggles and then skips a gear before landing to the smallest cog.

It was doing this at the weekend. Since then I've:
- put new gear cables in (though old ones looked fine)
- aligned the derailler hanger
- shortened the chain (it was a few links too long before)

Still not right though.

Bike is about 8 years old with original deraillers/shifters. First problem I ran into was chain snapping. Lost a few links. Was riding with a short chain a few timesand couldn't reach the furthest gears, no big deal. Then I replaced cassette/chain/front chain rings.. but shifting hasn't been right since.

That sounds like a perfect description of something that can happen with a warn and wobbly upper jockey wheel. Unlike sticky cables, where you sometimes can't get it to jump up into or down into a specific gear, forcing you to double click and watch the chain jump two rings at once a worn and wobbly upper jockey wheel shifts fine in one direction (up, if you've set the cable tight enough) but doesn't shift down on the very largest cassette ring. And when you shift down another gear the chain just jumps down to the next ring so that it's always one gear off where it should be until it gets near the middle of the cassette and suddenly does a double jump until chain and gear indicator are back in agreement with each other.

And if you've got it set up so that it sort of works in one front chainring, as soon as you switch to another front chainring it all goes to pot as the chainline shifts.

The main reason is that the bearings, or the greased depression if it's a non-bearing jockey wheel wear down over time so that the jocky wheel can lean a little too much to the left and the right. This means that if you tighten the cable so that the chain is sitting under the right cassette ring, the jockey wheel is leaning slightly outwards (away from the bike). This doesn't stop the derailleur shifting up the cassette smoothly until you get to the biggest ring but as soon as you switch to trying to shift down, the derailleur moves outward by the appropriate amount but the jockey wheel simply straightens up, or even leans to the left (inwards) preventing the chain from jumping down. This leaning then causes the shifts to be one behind where it should be (so the chain is in 8th gear when the indicator says "7th", and then 7th gear when the indicator says "6th" etc) until you get to the middle of the cassette and the straighter chainline causes the wheel to staighten up, at which point it jumps two rings until everything is back to how it should be.

If any of this sounds vaguely familiar to what your experiencing, replacing the jockey wheels, or even the whole derailleur may solve your problem. Also be aware that jockey wheels are meant to have a little bit of lateral play in them, but too much and you experience the problem described.

parkinpants

Good analysis. The jockey wheels are brand new though, but maybe I didn't fit them right? The chain goes through the GUIDE jockey wheel first, then the tension jock wheel. Only one of the jockey wheels had a 'rotation' arrow and I set that up so that the arrow follows the way the jockey wheel spins.

m_cozzy - I'm not sure what the b screw is but I'll look it up and check it out.

I'm not convinced that it's the cable just yet because it was doing it before I replaced the cable, and the outers felt pretty slick.

Think I'm going to borrow someone's rear mech to see if it starts working, and then fails again when I replace mine. Presumably if that happens I can say it's the spring that's gone in the mech?

Ouija

Only mentioned it because i've had the exact same problem twice (one a few years back, the other last month). The first time it happened i replaced the jockey wheels and when it happened again last month i just replaced the whole derailleur. In both cases the problem instantly disappeared. So i'd ignore the "it's your cables" brigade as sticking cables manifest a very different problem to the one you described (if i read your original post right). Focus on the derailleur as i'd bet a £100 that's where the problem lies. You don't have any spare derailleurs you can try out or temporarily swipe one from another bike to try out?

parkinpants

so you think spring has gone in the derailler? I was thinking that might be the case, having done a few rides with a short chain might have damaged it, so yeah I'm going to check the B screw first and then try and borrow a spare derailler.

cooldad

So ignore the obvious outer possibility? (always worth changing both at the same time, costs very little, solves the problem most of the time), and assume wrecked mech?

parkinpants

No, I'll get some new outers first before I start with the mech.

Adjusing the B screw seemed to help a bit, but I think I'm just 'moving' the problem around to different sprockets.

Ouija

cooldad wrote:

So ignore the obvious outer possibility? (always worth changing both at the same time, costs very little, solves the problem most of the time), and assume wrecked mech?

Depends on my intepretation of his initial post. If the chain doesn't jump down to the next biggest ring when he shifts gears and then jumps down TWO gears when he presses the lever a second time then it's probably cable outers (usually that curved bit that goes into the derailleur). If the chain only jumps down ONE gear when he presses the lever a second time and continues to lag one gear behind most of the way back down the cassette it's a derailleur/jocky wheel problem. Simples.

cooldad

Or censored indexing.

parkinpants

The shifting pattern at the moment is requiring two clicks to drop from largest sprocket to next one down, then skipping out the 3rd sprocket. Here's the pattern:

1 click - stays on 9 (largest)
2 clicks - 8
3 clicks - 7
4 clicks - 6
5 clicks - 5
6 clicks - 4
7 clicks - 2
8 clicks - 1

Climbing the gears works fine.

So, on ouija's analysis, it's not the outers, it's either the jockey wheels or the derailler. Jockey wheels are brand new, derailler is about 7 years old.

cooldad

cooldad wrote:

Or censored indexing.

Ouija

cooldad wrote:

cooldad wrote:

Or censored indexing.

Nope. If he loosens off the cable tension so that it goes back down the cassette perfectly it then won't go UP the cassette (essentially flipping the problem around so that it requires TWO shifts to make the chain go up ONE gear).

Like i say, i've had this problem in the past. I could easily drag my old Alivio derailleur out of the cupboard and refit it to my bike (which currently works perfectly and suffers from no cable binding whatsoever) and the problem would instantly reappear. No amount of tensioning and loosening of the cable in quarter turn increments would ever get the problem to go away so that you could shift all the way up and all the way down without any "lag" (always being one gear out of where it was supposed to be for most of the cassette).

parkinpants

That sounds about right. No matter how I adjust the b screw and barrel adjustment it will not shift properly in both directions. If I loosen the barrel adjustment I can get it to descend okay, but I need to tighten it to get to climb properly.