Suddenly ghost shifting into harder cogs?
 

Hi all,

This morning my Ultegra Cervelo S5 began ghost shifting during several points in my ride (I was doing a 2x20 threshold workout). It was shifting into harder cogs, especially under heavy load whether sitting or standing. It was not doing this yesterday or the day before and I store my bike upright so I do not think that I knocked the RD out of alignment. Does anyone have any suggestions as to how I might remedy this?

Thank you!

John

Well, you could try adjusting the RD barrel adjuster a few clicks.

Ah! Thank you! Forgive the stupid question, but I am assuming I should turn the adjuster counter-clockwise?

Yeah, it sounds like the chain is too far off towards the smaller cog.
But "suddenly" sounds unusual. Was the chain kind of noisy from slight gear grinding previously? (an indication of being out of adjustment for a while.)

How old is the cassette? This popping out of a cog can happen with a new chain on a worn cog. On a hard pedaling effort, the chain rides up on the worn teeth, then jumps off to the next cog.

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To check the rear derailleur: (You probably know all this, but it may help new riders.)

In the big chainring, shift all the way to the smallest cog. Then shift 3 more cogs. (Just to make sure it's a clean / normal chain position.)

Get off the bike. From the back, look at the chain position between the cogs on both sides of the current cog.
You want a very slight gap between the chain and the next largest cog. As small a gap as you can make it.

If you turn the adjuster counter clockwise to move the housing outward from the derailleur, it effectively lengthens the cable wire, pulling the derailleur inwards, toward the next larger cog.

Do a small adjustment, maybe 1/8 turn. Then repeat the shifts to the smallest cog and back 3 cogs larger, and check it again. I'll finish with very tiny adjustments to get it "just right"

This process is easier if you can support the rear wheel off the ground on a repair stand. Or have someone hold the saddle while you shift and adjust. But you can always do the 1/8 turn, get on the bike to do the shifts, then get off and look again.

How do you know that some of your cogs are "harder"? Have you done a Rockwell hardness test? Have you sent some (but not all) out to heat treat? Have some of them been hard anodized? Must have answers!!!

Any time you have a sudden change in shifting characteristics you should check inside the brifter/shifter for the shift wire beginning to fray. It is much easier to get it out before it breaks off than after. Also check that the attachment to the anchor bolt is secure.

Maybe that's why the Cervelo guys are so fast...extra workout intensity.

If your cable and housing are not new, dsbrantjr's advice on replacing the cable is sound. If either are new, this might well just be some stretching of the new wire or compression of the new housing. (A lot less of that now with the new cables and housings, but it still happens.) Try turning the barrel adjust 1/8" of a full turn (to tighten the cable, in other words, the hard way). I like making barrel adjusts by easy to remember fractions of turns. Then I ride and see if I need more or less.

Ben

Are you getting "unexpected" shifting or "delayed" shifting.

The latter points to high cable friction, since the RDER spring is what pulls the cable.
Shifting to lower gears is accomplished by "pulling" cable, so friction isn't really an issue vs the force you are using.

'shifting harder' = derailleur tending towards smaller cogs = derailleur not cable-pulled as far towards larger cogs = behavior you would get from cable stretch.

Do check for fraying etc as recommended by dsbrantjr, but if the cable is sound, then a little bit of CCW on the barrel adjuster should restore cable tension lost to stretch. Get the bike in a position where you can sight at the rear derailleur from behind while you reach forward and pedal slowly. You should be able to see the barrel adjuster nudge the derailleur: CCW=left, CW=right. Do this with the chain on any middle cog, and tweak until the chain is as perfectly centered on that cog as possible. For more precision, turn CW until the chain starts to rub on the right side, then count turns as you turn CCW until it starts to rub on the left side. Then halve those turns and CW back to the precise middle.

Wow these replies are super helpful. Cables are only about 3 months old but I ride aggressively and snapped my last RD cable so I imagine that could be a possibility again. Will check for fraying and adjust the adjuster as suggested. Thanks all.

If the cables are 3 months old because the bike is 3 months old, then this is 99.99% certainly regular cable stretch that happens with all new bikes (and new cables on old bikes), which is why bike shops (should) always tell you to bring the bike back in a few months for a quick tune-up (and usually do that service for free, for bikes bought from that shop)

After 3 months , I'd expect to see a pretty dirty cable where it's exposed near the RDER.
I'd suggest cleaning and lubing the cables.

Hmm, Tuesday and Wednesday were gritty "just-after-it-rained" rides so that could definitely be a possibility. FWIW, I got the new cables in early March but have been doing ~250 mi/wk since then inc. hard workouts and races. How often would one expect a cable to last under these conditions?

It's not how long it last, but how long it functions properly.
Clean & lube it. It takes a couple minutes.

Also new cables are expected to stretch a little bit with early use. Coupla months is totally normal.

I've broken rear shift cables completely in under 3 months / 2500 miles and now pro-actively replace at 2000 miles

Under bar-tape routing with a tight 90 degree guide in the shifter body almost cut my cable life in half as one expects from double the fatigue cycles. Before fatigue only came from wrapping around the shift drum, although now each shift also flexes the cable as it runs over the guide.

I break rear shift cable strands within 2500 miles, and replace pro-actively at 2000 miles which is 10 weeks for me and 8 weeks for you. I replace my rear housing every other inner cable swap which is every 4000 miles because friction gets too high for fast shifts to smaller cogs after 4500-5000 miles.

Your mileage will vary based on how particular you are about staying around your preferred cadence. I ride cassettes with one tooth jumps through the 19 cog and pedal like a metronome when I haven't run out of cogs climbing.

It's metal fatigue unrelated to how rainy/gritty the conditions are and how forcefully you work the shift lever. Water and grit just contaminate the rear housing loop sooner.

Front shift and brake cables/housings last almost indefinitely.

Thanks so much for this!!

It’s often caused by a combination of cable relaxation (stretching) with use and frame flexure as you power down. Frame flexure makes minute changes to the chainline. Minor tweaking of the cable adjustment barrel is your first thing to do, then test. Only make one adjustment at a time, to rule out other effects.

That's a first week thing, not something sudden 3000 miles later which sounds like a breaking cable.

Cables don't stretch. The housings sometimes settle a little.