unexpected shifting
 

New bike: Ultegra 6700 components - rear derailleur (GS) with 11-28 cassette.

On the stand there is no problem, it shifts perfectly. While riding, it's a different story. Although shifting through the rest of the gears is normal, the chain feels and sounds like it wants to shift from 19 to 17 with normal pedal pressure and actually does shift with the added pressure of standing. What is more confusing to me is that, with a 1/2 turn adjustment on the fly, the problem seems to go away or at least get better, BUT the next ride... it's back again. Is there something that could be loose? ?? I can't understand what is going on. Any help to rid me of the horrible crunching sound of unexpected shifts is greatly appreciated.

This problem (auto-shifting) is usually caused by the shift cable stretching. But in your case it may be the fixing bolt that holds the cable to the derailleur is slipping. If you don't know how to fix it take it back to your dealer. Early adjustments on a new bike should be done at no cost to the buyer.

Al

High quality shift cables do not stretch. More than likely the cable system was not pre-stressed after initial assembly. The symptoms you attribute to cable stretch is actually the ferrules seating into their housings fully and into their stops.

1) The OP needs to fully stress the system, one way to do this is to hold the derailleur and force shift it with a reasonable amount of force to seat everything.
2) Once that is eliminated, you can look to a bent derailleur hanger as a cause. It is very common for new bikes with AL hangers to be bent out of the box.
3) Let's make sure the OP isn't skipping, or is using incomplete instructions to adjust the derailleur http://www.parktool.com/repair/readhowto.asp?id=64
4) The final thing would be to check incorrect cable/housing system, whether from bad housing cuts or excess friction due to any number of causes.

Also has a hint of "cable housing might be a bit too short" in the line somewhere.

First of all, thanks to everyone for your input. I've been busy (and sick) lately and haven't had much time to spend riding or working on the bike. One more day until my day off.

I actually did take the bike back to the shop (for a different reason, but also asked him to check the shifting), after my first test ride. He made some small adjustments and apparently didn't see any problems before returning it to me. However, I didn't point out the specific problem I was having because, at the moment, I didn't recognize the specific problem and realize I was going to have a problem that was going to be sporadic. To complicate things, I am an American living in Japan with only limited skills of the language. I am increasing both my skills in Japanese and bicycle mechanics through this process but... that is another thread.


>>operator
1) It is a new bike, but I have close to 300 miles on it now. Would that amount of time on the road fully stress the cable system? If not, how do I "hold the derailleur and force shift it" without bending the hanger? I understand the principle, I'm just unsure how to safely perform the act.

2) The hanger doesn't look bent, but I am not an expert. It's certainly doesn't fit the extreme description found on Sheldon Brown's website. I am still looking for symptoms of a 'slightly' bent hanger. If the hanger is bent, is it better to replace it or attempt to straighten the old one?

3) I'm still trying to think of what "OP" means, sorry. Excuse my 'noobness' to this forum. I'll read the adjustment instructions from Park Tool and get back to you.

4) Still working on 1 through 3.

Thanks. I appreciate your help.

OP = Original post/ poster. Mainly used when responding to a reply someone else has written.

Thanks.

Am I the only one that overlooks the simple solution?
 

Time to report my findings... I am not certain but this may have been the problem...

I did find the derailleur pinch bolt not as tight as if I would have tightened it (I'm gonna buy a torque wrench soon - still hard to believe that my LBS missed it not once but twice), but it was snug and obviously holding the cable well enough to control shifting 98% of the time. The problem was that the other 2% of the time it was very frustrating. I guess eventually, it would have kept getting worse and worse. After I had time to work on the bike, I still had to wait for the cold rain stop falling before I could test my adjustments. One ride with no problems, so I hope that was the solution. If it acts up again, I'll be back. ;) Thanks, Al1943.