UKH

Just finished a frame swap for my wifes bike, a nice new Willer GTR carbon frame, it was superb to work with compared to the 2nd hand stuff Im used to making myself.

Transferring some of the bits from her old bike which has mostly Sora drivetrain Im having difficulty indexing the gears correctly. I find regardless of how I adjust the cable tension it either skips one gear on the downshift or requires and extra half push on one of the upshifts. Heres what Ive done so far -

New 8sp cassette and chain

New cables and outers

Mech hanger alignment checked (and wheel true)

B screw clearance correct

Limit screws adjusted

Im at a bit of a loss what to check next really.

Cable attached in the right place on the derailleur? Caught me out till I put my glasses on!

You haven't said anything about twiddling the barrel adjuster. Couple of clicks anticlockwise might just do it.

Cable drag in the sheaths?

Yes, the wrong length outer at some point can do this, I just had exactly the same issue.

Good luck.

Toby

Internal routing? If so check for drag under the bottom bracket, and too sharp a bend from chain stay to derailleur. Just done a similar job myself and found the correct length of outer at the back for both gears and brakes is critical.

Thanks for the replies all!

I might not have said the exact words barrel adjuster but I said cable tension = same thing

All the cable outers are good length and theres a nice curve to the cable to the derailleur.

Yep internally routed but it moves freely through the frame and under BB guide

"Cable attached in the right place on the derailleur? Caught me out till I put my glasses on!"

what do you mean by right place?

What about chain alignment? Maybe an extra spacer on the bottom bracket might mover the chain wheel over enough, take some tension of the system. Realign rear derailleur and things may change more smoothly.

Nice idea never needed to do that before though and this frame it's a press fit bb, chainline looks good anyway

Bent hanger? 

How old is the rear derailleur? Is it usually stored in highest gear on the rear sprocket?

Reduced tension in the return spring due to age and or prolonged periods of tension could cause similar symptoms.

If this were the issue the solution is a new rear derailleur.

A good suggestion, derailleur hadn't had much use, just tried another one I had spare no difference, which also reminded me about jockey wheels, but that's ok too.

Oh well looks like a mystery, or it may 'bed in'.

Is there any movement in the cassette?  When I recently did mine the first time on somehow it did not sit properly.   Took off, back on and tight, play removed.

Cheers

Toby

Similarly, check the rear wheel bearings. If there's any movement (side-to-side at the rim) then the shifting is likely to be compromised.

If the cable are new (a frayed cable would be my first suspect) then perhaps the lower limit screw is the culprit.  Slacken it off with the chain on the smallest sprocket and if the DR moves, there is spare slack in the cable which will throw the indexing out.  Use the barrel adjuster, (or start again) to bring the DR back under the smallest sprocket and then move the limit screw back.   If the gods are smiling on you, that should work.

Is the outer cable sheath the "indexing" type, ie: wire strands which form the cable are in-line and not spiralled (as in brake cabling or older, friction based systems)?  If it is, ensure the ends are cut very square and any ferrules used are very firmly seated (using a small amount of grease will help bedding home).

You sound like you know what you are about and all the above answers seem to cover most points. This was the only thing that popped into my mind, as I've had issues with cable outer too, though not newly installed.

Hope it resolves. 

you can clamp the cable both sides of the bolt, I did this once and spent hours fiddling with the indexing to the point of frustration. Looked at it again a day later spotted the error changed over indexed correctly first time!

Days!

Sorry TP I missed your question - GeorgeR has it though.

I struggled with this same problem recently despite many years of working on road bikes. I mean, I know how to adjust gears! In the end I swallowed what was left of my pride and went back to RTFM - the Park Tools rear derailleur instructions reminded me to do things in a structured way, and worked first time.

Thanks all (at least those who read my post fully anyway )

I'll try changing the cable clamping position and update this thread, although I'm sceptical it'll make a difference, I'm happy to be wrong

I'd definitely clamped the cable in the right place.

It seems a bit better after a couple of rides, she's happy with it anyway. I'd like to know exactly what's wrong but I'm satisfied she's happy.

Ta.

GeorgeR, you're a genius!!!  I re-attached the cable to my rear mech on the other side of the bolt and it's made a huge improvement to my shifting!!!

Many, many thanks for that.