SHIM_105

I've been beating myself with this thing for a couple of days. I've checked park tools and sheldon brown for info as well as the search here. I have a 105 RD-1056 group. The rear derailer is giving me grief. I've set both limits and set the indexing. When the bike is on the stand it shifts through the cassette no problem from top to bottom. However once under load it will not shift to the smallest cog. I can see the derailer try to make the shift but it just won't kick it down. Sometimes if I pedal backwards it will jump down. Then once back on the stand it shifts fine. I've tried a multiude of adjustments. By the way, the limit screw for the small cog is all the way out and it still wont kick down to it under a load

Capecodder

Cable tension may be too tight.... Release the tension on the cable some and see if it drops down. Also the cable may be binding in the housing so check that as well. Possible derailleur hanger as well.

mechBgon

The RD-1056 also has an adjuster for the parallelogram's return spring, so you could try boosting the return-spring strength to the high setting. To do that, shift the derailleur to the largest cog, then look at it from below. There's a round plastic thing in the rear parallelogram link you can rotate with a flat-blade screwdriver. The end of the return spring rests on it. Turn it until it lifts the end of the spring as much as possible.

Also make sure your cable doesn't have broken strands at the shifter end, particularly if you're using STI shifters where it wouldn't be visible, and make sure the cable moves freely in the cable housings and at the bottom-bracket cable guide.

SHIM_105

I adjusted the plastic piece and slackened some cable. It seems to have helped a bit. Its now shifting to the small cog but with a lot of clattering and quite slowly. If I let off on the pedals it shifts down smoother. By that I mean the bike is moving faster than Im pedaling, or if I stop pedaling, shift, then start to pedal it will shift dowm easier as well but still slow

Capecodder

Have your local bike shop check the derailleur hanger to see if it's bent.

JiveTurkey

This is standard form. It should be done even if the shifting is working as well as possible.