Rear derailer needs frequent adjustment
 

I have a De Rosa Idol road bike. Having a problem with the rear derailer getting out of adjustment all the time, early into the ride. Seems to me it must be the adjustment wheel backing off on the tension by itself. How to fix this?

I've encountered that a couple of times.

Examine your cable housing ends. What you're looking for is just a couple of stray wires sticking out of the ends of the housing end caps. If you find that it means that the housing end is gradually shortening itself. The fix is to cut the housing off square and install a new end cap.

check the cable in the shifter, could be breaking, catch before it break to save $$$$ and headache for the mechanic that trys to dig it out

Maybe your cable is nearing its end, already broke some strands and about to snap for good somewhere.

Doubt it's a failing inner.

Also, I'm not sure I believe the OP's implication it's effectively slackening; in which case that allows the possibility of dirty or damaged cables, the most likely culprit for dodgy shifting on >8spd.

What's the gear, anyway? Brand and number of cogs?

As others have said, it could be the inner getting ready to let go. They get longer in the last days as the strands fail one at a time.

Of if it's new cables, simply part of an initial settling process.

OTOH id could be that the adjusting barrel spins too freely, and walks down with time and vibration. Years ago makers used to put springs under adjusting screws like the barrel and limit screws. This is a proven design used, for example, in cars for all sorts of adjusting screws that need to stay put. These days the spring has been replaced with a bit of nylock which I don't find as reliable.

If you believe it's the barrel spinning too easily, an easy fix is to bring it all the way up (Shift to low, then with the wheel stationary bring the lever to high to get some cable slack) and paint the threads with something like rubber cement and let it dry completely before threading back in. That will make it harder to turn, but still allow adjustment as needed.

I have also had kinked cable, dodgy barrel adjuster and rupture housing cause shifting issues. However, another possibility is cable is fine but worn chain or worn cassette are both are causing chain to jump around, which would feel similar to out of tune derailleur.

You can check the ends of the housing if the wires are coming through but my suggestion is replace the cable and housing or at least the housing. If the housing is teflon, plastic, lined, it could be hanging the cable up. Housing are relatively cheap, so is the cable.

Step 1: Look at the RD pinch bolt and the der. cable. Does it have fresh pulled sheen near the pinch bolt? Does it look like it's sliding in the pinch bolt? Check it. Usually a 4-5-or-6 mm allen key to loosen tighten. Make sure the cable is in the "groove" of the washer plate that the pinch bolt cinches down onto.

Step 2: Look at the cable tension adjuster knob on the back of the RD. Have you noticed in the last few rides that the knob loosens turns itself slowly and causes the cable tension to decrease slightly cause the RD to go out of alignment? If you can't be sure, use a marker and mark the knob and the same position on the back of the RD body. sometimes, white out works too. If the knob slips and moves during your ride, then you need to secure the knob. Maybe some loc-tite will help, Usually, the tension adjuster barrel, on most Shimano RDs at least, have a bumpy barrel face with spring compressed, notched washer plate. Turning the barrel to adjust tension requires a small extra force to rotate the small bumps out of the washer plate notches and around. But if you release the barrel, the notched washer plate prevents the barrel from slipping because the bumps in the barrel rest inside the notches in the washer plate. If the compression spring is missing, then this can allow the tension adjustment barrel to slip. If it's missing, then you'll need to find a replacement park.

Step 3: If step 1 and 2 were not the issue, check ends of housing where it fits into the stops. Do all cable housing and caps look like they are properly fitted into the cable stops on the frame? If not, loosen as needed (or shift to largest cog or chainring, then stop pedaling and use your brifters to shift back without pedaling - should introduce slack) and position properly.

Step 4: If steps 1 and 2 didn't fix the problem, loosen RD pinch bolt, pull off the crimp cap on the end of the cable, and pull cable out of housing, inspect housing and caps to make sure the housing is bottomed out (I use a 4th hand tool, and old cable, slide it through until one end is bottomed out against the stop fused onto the end of the cable, and then with 4th hand, grip the cut end of the old cable and push up on the housing. This will compress the housing and seat the caps. Regrease cable, and re-install

Step 5. If previous steps didn't work, check out RD hanger and RD for damage, cracked pulleys, etc. (this isn't too common).

Step 6. If previous step still didn't work, check for frame cracks (very rare and I don't want to scare anyone) but it could happen.

The fact that it happens early on a ride points fairly strongly to some sort of seating or friction issue in the cable. The above procedure will work, but frankly if the tension adjuster seems to be properly seated and not movingI would just replace the entire cable and housing, making sure things are properly seated.

Thanks for the hints. I will check out these possibilities one by one.
The hardware is Shimano 7800 series, 10 gears on the cassette.

In case anyone is following any more: The problem was the barrel adjuster un-adjusting itself, which I found by putting a mark on the barrel and keeping an eye on it as I rode. Sure enough, it turned. So I disassembled the apparatus and cleaned gunk out of there with degreaser and reassembled. If the problem continues I'll put a dab of Locktite. Thanks for the suggestions everyone!

Don't use a Loctite threadlocker because you want to be able to adjust it easily. Clean it with alcohol and put a dab of rubber cement on the threads, which will hold it in place, but still allow adjustment. If enough of it sticks out, you have two options next time you replace the cable. You can add a locknut or find a short fairly stiff spring which is what limit screws used to have before folks decided they were too heavy.

Seems like it would be worn out teeth in the adjuster. Usually they are plastic.

If it continues, just try a new barrel adjuster. They're cheap...