how did my cable tension increase all of a sudden?
 

When I tuned up the RD two weeks ago, it was 60 degrees F. Today, it was 40. I noticed the up shifts were instantaneous, but the down shifts were noticeably more cumbersome. Sometimes it was smooth, but other times, the chain would just rub the sprocket, and not budge unless I shift it twice. I thought that this could only mean that the cable tension increased. But then again, 20 degrees F is nothing... Is adjusting cable tension a common practice when it comes to index shifters?

Hm... could be the 20°F.

Shimano 10spd?

40, not 20; I meant to say 20 degrees F difference between 40 and 60. and yes. ultegra 6700

why, does the manufacture matter?

Easier up shifts means the cable is looser.
Maybe your "tune up" was lacking and things simply "settled in"?

I meant Δ20°F.

Cable pull per shift varies by manufacturer and number of speeds; Shimano's 10spd uses the shortest cable pull yet, an average of 2.3mm. This minimises signal-to-noise, obviously.

Some people call going to larger rear cogs upshifts, some call going to higher gears ratios upshifts. It's helpful to state which you mean. If by upshift you mean a higher gear ratio (smaller cog) then it is likely that there is excess friction in the cable/housing or less likely in the lever mechanism, which could be caused by the colder weather or by contamination from the poorer weather if you ride on wet streets. Put the bike in a stand and shift to the largest cog. Stop the wheel and shift the lever all the way to the small cog (most relaxed cable) position. Move the cable in each section of housing back and forth to find the source of friction. If one lubricates cables (not necessary for lined housing and stainless cables) one should never use grease unless working with old-fashioned unlined housing, as it will often thicken with time, temp, etc. Use Phil or other good quality oil or "wet" chain lube.

up shift = going to larger sprocket.

up shift = going to a higher gear

In that case, your cable housing ends have probably just seated in the cable stops. tighten the cable tension a tad and you'll be good-to-go for a long time.

Exact same thing happened recently to me, and I fixed it by tightening the cable. Same deal a week later. Then the cable broke right at the shifter. So make sure the cable isn't getting longer because it is fraying, not just stretching, in a place you can't see well, like in a shifter housing.

I suspect the issue isn't with the cable tension or adjustment, but with the colder temperature creating greater friction inside the cable housing. If the cable is greased a lower temperature may have made the grease much thicker making the cable move more sluggishly or even binding.

Well, as you were speaking of the rear derailleur we will assume you mean larger cogs - convention is generally to call the rear parts of a freewheel/cassette cogs and the front chainrings/chainwheels/sprockets.

Not correct - that would not make for noise, partial shifts but it would be consistent. The OP describes sometimes shifting, sometimes not.

Correct, as I explained earlier.

so this whole time I've been working with a bent hanger. This is a commonly sited problem for BD bikes, and I had my suspicions when putting it together. The RD just didn't look right from the beginning, but I didn't think much of it because everything was working nicely. Good thing they had a spare hanger in the box.

Now it's all good. There's some really small clicking noise on certain gearing combinations, but I'm assuming that's just part of the drive chain - nothing out of the ordinary.