Poor Rear Derailleur shifting
 

I have a Shimano 105 3x10sp group set (I think it's 5700) with a Shimano 105 11-28 rear cassette, and in front 50/39/36.

The shifting has become awful lately, and I can't quite put my finger on the issue.

Symptoms: Toward the smaller-end of the cassette upshifts are becoming unreliable. Toward the middle of the cassette, downshifts are becoming unreliable.

The rear cassette has about 1200 miles on it. The Shimano 105 chain has about 1600 miles on it. The RD shift cable has 200 miles on it. The shifter and RD are original, so about 6000 miles.

I cleaned the shifter pretty thoroughly at the beginning of the season. It still feels crisp.

I keep the RD clean all the time. The RD doesn't appear misaligned but I don't have appropriate tools to verify that. I've never fallen on the derailleur side though.

Here's the fly in the ointment: 1200 miles ago I swapped the rear cassette from 12-25 to 11-28 because I have relocated to significantly more hilly terrain. I didn't add links to the chain. I did adjust the B-screw, tightening it quite a bit to prevent the RD guide wheel from pressing the chain against the cassette when I'm in 30t(front)/28t(back). It's as tight as it has to be, but not even an eight of a turn tighter than it must be to prevent contact.

My inclination is to swap out the rear chain since it has 1600 miles on it anyway, and at that time go a couple links longer, and re-set the B-screw with less tension to promote better shifting.

I could take it into the shop, and probably will end up doing that anyway, but I'm not convinced they'll take the time to get it "just right". If I can manage to improve the situation myself I'd prefer doing it that way. Until recently, it's been working out fine to do my own work. This time I'm stumped though.

...this is probably the commonest cause for poor shifting performance, and you can't eyeball alignment with a 10 cog rear.
I can only tell you while it's not guaranteed that's the cause, it's pretty easy to check with the proper tool. So personally I would do that first, before I started swapping out stuff.

Hanger alignment is critical. If you want to do all your own work, buy one, otherwise take it to the shop. Also cable/housing drag is the most common cause of inconsistent shifting. Have you replaced them? Often suddenly awful shifting comes from the cable actually fraying inside the shifter.

...also, read this thread:

Hanger alignment has become more important as the fudge dimensions have been reduced by the want of more rear cogs in the same old space. ( But I still feel it's mentioned far too much as a prime cause of shifting issues).

The very first thing I thought of is cable condition. Increasing cable friction initially causes problems at the small cog end of the cog set. That's where the der's parallelogram spring is at it's weakest and won't pull back as much cable if the cable friction gets too much. Make sure the cable's first inch is intact and not starting to fray/break. This is out of sight and inside the control lever. Detaching the cable from a frame stop or from the anchor bolt will let you push out that bit of cable from the control lever to inspect. Also the casing condition counts. That last bit of casing looping from the frame to the der is often the most friction prone, being the tightest curve and easily kinked somewhat right at the der adjuster.

The next thing I would look at is the upper/guide pulley. When new and in good condition it will have virtually no rocking slop, no ability to cock this way or the other way on it's bushing, it will have a side to side "end play". When the bushing is worn out it will tilt either way. This vastly increases the pulley's inability to guide the chain as the control lever wants it to. With the down shift over movement of the lever these shifts can be compensated for to a degree (just swing the lever a tad more, most people don't even know they are doing this all the time). But on upshifts the lever only allows the cage to more by the "when in good condition/new" amount. If the pulley is cocking the chain won't be "told" to go directly under that next smaller cog. Almost but sometimes not quite enough.

There are other factors at play but IME these two are biggies. Andy

Thanks for the suggestions. I'll have the guide pully, cable housings (and friction), and hanger alignment scrutinised by the shop. I still may swap out to a slightly longer chain to better accommodate the 28t rear cog without so much b-screw tension.

The cables are practically new because the old one frayed under the shifter hood and snapped a couple weeks ago. The new one is somewhere between 250 and 350 miles, but that's still fresh.

Did the poor shifting start after the cassette replacement?
After the cable replacement? Have you fine tuned the RD since then?

A longer chain does not mean better shifting.
If there's a little slack on the big/big then there's no need to make it longer.
The B screw is about clearance to the cassette, not chain tension.

I'll add to the chain length question. I prefer a longer chain but do know that with a cage/pulley design that has the pulley off set from the cage (as the 105s do) the pulley will raise up (as the cage rotates clockwise to take up the added chain length) and have a better chance to contact the underside of the large cog. So very often a longer chain will want a greater amount of B screw turn in/tension to bring the der body back and unwind the cage a tad. This is the opposite of what the Op is wanting to have happen. Andy

Open the pinch bolt on the derailleur and then run thru the gears up and down on the shifter while keeping tension on the cable with your fingers.
Hold it at the downtube and at the derailleur..
See if it snags. Look where it runs under the bb.
I presume it is external cabling.
You might as well pull out some cable at the shifter and have a look at it when your there.
No harm to squirt some wd or similar in to the shifter while your at it.
A 25 to 28 is not a huge jump.
By the way does it behave the same on all three front rings?
Should be easy solved.

When you replaced the shift cable did you also replace the housing? New cable in old housing doesn't always work well together. Blamester's suggestion to check the cable at the shifter is a good one. Any sudden change in shifting should trigger a fraying check as it if infinitely better to find a fraying cable before it breaks than to try and dig the busted-off head out of the brifter afterwards. .

This is probably a good call. I didn't replace the housings.

I haven't been able to detect much friction, though; I've taken the wheel off and yanked on the exposed portion of the cable to feel it slipping through the rear housing, for example.

The challenge for feeling cable friction with one's hands is that we have a variable strength and a poor feedback system (unless well trained). The der has a very narrow spring tension range and it's feedback is obvious to an experienced wrench.

So feeling for cable friction isn't the black and white that many hope for (including me sometimes).

As I mentioned in post 5 the last loop of casing from the stay to the der is where a lot of friction can happen. dsbrantjr knows this. Andy

I had about the same problem. I replaced the cable housings and everything shifted like new. At a whole $2 in parts it was a cheap fix.

You can make a hanger aligment tool inexpensively and there are a couple of How To videos on Y'tube. I bought the bolt at a walk-in fastener supplier because the hardware and automotive shops didnt have the specific fine thread needed. Less than 5$ for bolt nuts washers,. 25/64 drill bit was 7$, old aluminum shaft hockey stick was free lol.