Gears jumping. But just one cog. Well, now two.

[Steve vRS]

I had a second hand 34-11 cassette, new chain and new long cage Ultegra rear mech fitted to my TCR. A new cable and outer was also fitted. The gears all shifted crisply and cleanly apart from the 4th cog from the bottom of the cassette. The bike shop who did the job said that there was a chance that the cassette was worn being second hand.

So I bought a single replacement cog off eBay and fitted it. Now the 4th and 3rd bottom cogs skip with all others indexing correctly.

The other potential issue is that in the hinge of the new derailleur what looked like an O ring was sticking out. Broken of course. Without thinking I pulled it out. Could this alter the geometry of the mech? I think not as surely, like a bent hanger, it would throw all the indexing out.

Any ideas?

[wpa1975]

Was the replacement sprocket from the same part number cassette as they do not interchange, the shift pins and ramps are in different places.

Also 11/34 is a large cassette for a road mech, depending on chainrings fitted.

[Steve vRS]

Yes. Same cassette spec for the replacement sprocket. As far as I know the mech and cassette are compatible.

[wpa1975]

What size chainrings due you have fitted?

[Steve vRS]

A controversial 52-34. The gear indexing on the back is the same regardless of which chain ring the bike is in.

[wpa1975]

Steve vRS said:

A controversial 52-34. The gear indexing on the back is the same regardless of which chain ring the bike is in.

Without seeing the bike it is hard but 11/34 on that mech and it will not be happy with a big chainring, you are outside the recommended range.

I am running 11/34 but 50/34, chain length is another issue, with yours if your on small sprocket and chainring, where is your mech cage sitting, ideally it needs to be just off stop and holding tension.

[Steve vRS]

Thanks for the offer. I’ll drop the bike off tomorrow

I’ll be putting the 30-11 cassette back on after the Fred Whitton as I’ve never had a problem with that even on the odd chain rings.

[DE1975]

As the poster above states, your combination of cassette and chainset is beyond the drivetrain capacity of your derailleur.

Ultegra long cage has a maximum capacity of 39 teeth. Your set up is 41. It's the combined difference between the biggest and smallest cogs on your cassette and chainset. I.e. (34-11)+(52-34)=41.

Your derailleur should accommodate a 11-34 cassette in combination with a 50-34 or 52-36 chainset, but not a 52-34.

There may be other causes though, such as the second hand cassette being worn, hanger alignment or incorrect chain length.

Edited by DE1975 on Saturday 2nd April 10:45

[Steve vRS]

Thanks for the detailed reply. But why should that show up on a mid cassette cog rather than one of the extremes such as the very highest or lowest gear?

[upsidedownmark]

IMO unlikely. Also Shimano specs are notoriously conservative, you can usually exceed the spec by 1-2 teeth safely, and yes, I would expect that to give you issues at extremes, or on front shift.

IME bad indexing always starts to show on one particular sprocket. It may be as simple as that. I've also had very worn jockey wheels throw things up..

[outnumbered]

Steve vRS said:

I had a second hand 34-11 cassette, new chain and new long cage Ultegra rear mech fitted to my TCR. A new cable and outer was also fitted. The gears all shifted crisply and cleanly apart from the 4th cog from the bottom of the cassette. The bike shop who did the job said that there was a chance that the cassette was worn being second hand.

So I bought a single replacement cog off eBay and fitted it. Now the 4th and 3rd bottom cogs skip with all others indexing correctly.

Is it an identical replacement from exactly the same model of cassette ? The sprockets are different between different cassettes (thickness, ramps, profiling in different places), so it could mess things up if you've got a mismatched one.

What happens if you put it back to how it was originally, do you end up with just the 4th sprocket fault ?

[OutInTheShed]

Steve vRS said:

I had a second hand 34-11 cassette, new chain and new long cage Ultegra rear mech fitted to my TCR. A new cable and outer was also fitted. The gears all shifted crisply and cleanly apart from the 4th cog from the bottom of the cassette. The bike shop who did the job said that there was a chance that the cassette was worn being second hand.

So I bought a single replacement cog off eBay and fitted it. Now the 4th and 3rd bottom cogs skip with all others indexing correctly.

The other potential issue is that in the hinge of the new derailleur what looked like an O ring was sticking out. Broken of course. Without thinking I pulled it out. Could this alter the geometry of the mech? I think not as surely, like a bent hanger, it would throw all the indexing out.

Any ideas?

Some mixed ideas in this.

Shifting cleanly is one thing.
Skipping usually means the chain failing to mesh properly with a sprocket, once it's in that gear, usually worse under load That's normally all about wear.

The deraiileur failling to put the chain properly on the correct sprocket is all about alignment, slack in the mech or parts incompatibility.
That might result in 'skipping' from one cog to the next.

It can be helpful to put the bike on a turbo trainer on the bench and see what exactly is going on.
If something in the mech is stiff and that results in the chain not being tensioned, that can cause any problem you like.

Cable outers moving or a fault in the shifter are also things to rule out, check the derailleur is actually getting the right increment on every shift?

[Steve vRS]

Thanks everyone. I think I’ll live with it till after the Fred and then revert to the old set up which is good enough for almost every hill apart from Hard Knott.