Shimano ultegra - shifting problem from largest (23) to 21
 

I have a CAAD10 that has ultegra components. I've tried turning the barrel adjuster on the rear derailleur, but to no avail it keeps acting strange when I shift from my 23 to 21. Here are the details:

1) It doesn't *always* do it. But when it does shift poorly, it can do so in many different ways. One way it feels like the chain completely jumps off of the 23 onto the 21, spooking me out a little bit because of the momentary second or so that I don't have the chain running with the teeth on the cog. Sometimes, mainly on the trainer, it will "act out", like it's slipping, and not shift. Sounds horrible when it does this. Sometimes it shifts just fine, and actually most of the time it does. Maybe 8 out of 10 shifts are fine. It's that 2 out of 10, and in a situation where I'm hammering it with a group or racing, that worries me that it won't shift properly one of these days.

2) all of the other gears work just fine. If I turn the barrel adjuster any more to help persuade the chain to go from the 23 to 21 a little easier, it will start to be difficult to go back up the gears (from 11->12, 12->13, etc...)

What's going on with my cassette? Is the cassette nearing replacement? I just put a new chain on about a month and a half ago. Put roughly 150 to 220 miles per week on this bike during the season. I do tend to use my large chainring nearly 100% of the time unless I'm climbing something over 10ish percent grade for a while. Otherwise I just use the big ring and the 23 if I'm riding hard. I'm guessing that cross chain action puts a lot of stress on the parts. But I do take good care of the drive train, cleaning the chain regularly, replace when needed, clean cassette when it starts looking bad, cleaning chain rings, etc...

I'm sorry, scratch that happens on the trainer part. I put a different wheel on when I'm using the trainer. But, that makes me even more confused. Ugh.

How does one check wear on the cassette/each cog/sprocket/whatever you call them?

I would suggest that you check for excess friction in your shifting system; inability to find a cable length adjustment which suits both up- and down-shifting is often indicative of a friction problem. A common culprit is the loop of cable going into the derailleur; it tends to collect water and road crud. The other cables can likewise collect sweat and other contaminants.

Never would have thought to check that! Makes sense though. Wonder if the housing needs replaced anyways? I've had the same housing on since I bought the bike back in 2012.

Cable and housings are so inexpensive that I tend to just replace them together whenever I am uncertain of their condition. One problem with replacing just the inner wire is that the wire gets worn into the outer housing and the new wire may not play well with the worn housing.

Try lubing that loop going into the rear derailleur with some light oil. Also, don't forget to lube the cable guide under the bottom bracket. If it's dry the cable can actually cut into the housing causing a lot of friction. But you do put a lot of miles on so maybe a cable/housing replacement would be a good idea.

Stupid question... does the cable tension change depending on which gear you are in? Or should it be the same?

I've never measured, but it feels uniform throughout the RD's lateral movement, but can be slightly looser when the RD is resting on the H travel limiting screw. The thing to remember is that the cable's tension is from the RD's spring. With a high normal RD the spring is what moves the RD to the smaller cog. Any unusual cable friction can cause upshift problems.

Brad

The cable tension is provided by the spring in the derailleur; it will vary depending upon which gear it has selected; it has more tension the farther you pull it, just like any other spring.

The so-called "tension" adjustment provided by barrel adjusters and the like is actually a cable length adjustment or more accurately a cable housing length adjustment, which sets the relative position of the derailleur vs the shifter index "clicks".

Great feedback! So, say the chain is on the 23 cog, and I turn the adjuster 1/8th of a turn. That 1/8th of a turn should be the same adjustment for the other cogs right? Or does that 1/8th of a turn turn into more/less of a turn further down the cassette due to the spring tension in the RD? Or... am I just overthinking all of this, and I just need to replace my housing? :-p

Many (most?) random shifting issues can be fixed by aligning the RD.

Park Tool Co. » DAG-2 : Derailleur Hanger Alignment Gauge : Frame & Fork Tools

You are overthinking this. Forget about the spring tension, it will take care of itself.

Just make sure that there is free motion in the entire system, then adjust the system using this procedure, starting at the beginning and not skipping any steps: Park Tool Co. » ParkTool Blog » Rear Derailler Adjustments (derailleur)

You may find that as you approach the optimum settings you need to make smaller adjustments so that you do not overshoot the sweet spot. Sometimes less than 1/8 turn makes all of the difference between perfect and pretty good.