Indexing problem

[Kestevan]

I've just replaced the rear mech cable on the Mountain Bike, and I'm having indexing problems I just can't seem to fix.

The shifter moves perfectly through all the gears except the move on/off the smallest sprocket; that needs 2 "clicks" of the shifter to force the move. The shifter can then be clicked back without affecting the selected gear.

I Can't seem to adjust it out without affecting the remaining changes. It's not the end of the world, but still irritating. What am I doing wrong?

 

[subaqua]

sticky cable or mech. lube the inner cable and lube the pivot points on the mech .


a good squirt of GT85 down the outer cable sheaths will help

 

[Kestevan]

The cable and the outer are both brand new, and have been fully lubed immediately prior to installation. The rear mech is clean, well greased and moves by hand with no stickiness at all.

 

[2Loose]

Stupid question, but is the wheel completely central and backed up to the end of the drop outs? I once had similiar when the wheel was slightly out. Removing and re-seating the wheel sorted it..somehow.

 

[fossyant]

Check the B screw - i.e. the one that controls how close the jockey's sit to the sprockets, although I'd suggest what has been said before - some cable drag.

 

[HLaB]

I had a similar problem a while back, the thing would either hang on the 2nd upper cog and then shift down the block correctly, I played with the B screw like Bicycletutor suggested and found at first I could transfer this problem to doing the eaxact same but at the other end of the cassette (hanging on the 2nd smallest then shifting up correctly) but with a wee bit of patience it sorted out.

 

[RecordAceFromNew]

Unfortunately there are quite a lot of possible explanations. Diagnostic is also somewhat dependent on whether you have a top normal or low normal rear mech.

Most mechs are top normal where the cable is slackest at the smallest sprocket. It is worth propping the back wheel off the ground and see whether the first click lines the pulley up with the small sprocket. If not it could e.g. be the H limit screw setting, or because the cable is not slack enough. Usually the cable is pretty slack for this gear and another push on the lever should have no impact.

On the other hand you might have a low normal/rapid rise mech, where the cable tension is highest at the small sprocket. Again you could observe whether the top pulley does line up with the sprocket by the first click. If it does not engage until the second click then it could e.g. be because the new cable housing is a little too large and its engagement into the bottom of any of the ferrules only happens with maximum tension - i.e. the cable housing is behaving as if it was compressible, or that the cable can stretch, with tension.

ps oops sorry only just noticed it is an Altus which I believe only comes in top normal...

 

[Zoiders]

The coil springs that power the mech lose their power after so many years so they stop shifting down to a smaller sprocket cleanly.

New one.

 

[fossyant]

The coil springs that power the mech lose their power after so many years so they stop shifting down to a smaller sprocket cleanly.

New one.

Really...... not that likely. I have a 20 year old DA Mech that's perfect and a similar Ultegra age mech that shifts perfect.

 

[Kestevan]

The bikes only a couple of years old, and the mech looks to move fine.

If it was a problem shifting onto the smallest sprocket itself I could see the issue being the stop limit or the B adjuster (both of which I've tried to adjust anyway).

I think I've actually described the fault incorrectly. It changes on/off the smallest cog with no problems, but then wont shift off the second one until the lever is pressed twice -

i.e. 8-7 1 click.
7-6 2 clicks
7-8 2 clicks

I'd normally guess it was down to a sticking cable, but with new cables etc I doubt it.

 

[RecordAceFromNew]

The bikes only a couple of years old, and the mech looks to move fine.

If it was a problem shifting onto the smallest sprocket itself I could see the issue being the stop limit or the B adjuster (both of which I've tried to adjust anyway).

I think I've actually described the fault incorrectly. It changes on/off the smallest cog with no problems, but then wont shift off the second one until the lever is pressed twice -

i.e. 8-7 1 click.
7-6 2 clicks
7-8 2 clicks

I'd normally guess it was down to a sticking cable, but with new cables etc I doubt it.

If that is the case I think there is a chance that the cable tension is not set correctly. If you look at section 5 of the Altus (actually pretty much the same in any Shimano rear mech) manual, you will see that cable tension is set at the second smallest sprocket (7th gear in your case), so that some pressure on the shift lever will cause the chain to touch the 3rd sprocket, meaning it is inconceivable that it will then take 2 clicks to shift into 6. If not already perhaps you could try following that part of the instructions and see what happens?

 

[Kestevan]

Sounds like a polite way of saying "Read the Fckiung Manual"

I'm a bloke - I dont do manuals.....If you cant fix it with a hammer and a lot of swearing it's obviously zarked


I'll have a crack at that - thanks Record

 

[RecordAceFromNew]

Nah. There is method in the trial and error madness - after spending a few days adjusting all 3 screws on the mech and the cable tension knob through all the possible permutations and combinations to get it to work, one will definitely be able to do it in the middle of a moonless night!

I spent a couple of days in Holmfirth last year, definitely wouldn't want to ride a SS around there!

 

[ColinJ]

I spent a couple of days in Holmfirth last year, definitely wouldn't want to ride a SS around there!

I rode round there with Kestevan last year and was struggling on my 27S!

 

[RecordAceFromNew]

I rode round there with Kestevan last year and was struggling on my 27S!

That is actually quite impressive Colin. I was staying next to the church where Compo was buried. The only way I could get up that hill was with my 24" gear!