What kind of adjustment needed in this case?
 

My trainer bike has an ultegra 9 speed cassette. I like to workout at a certain cadense in the 53-17t combo. Problem is, the chain often (but not always) drops down to the 16t cog without me shifting. What kind of adjustment is called for here? Thanks!

Have you tried a simple cable adjustment? As one pulls on the cable to shift to the 16th cog you would need to release tension to keep it on the 17. That would mean moving the adjustment barrel on the derailleur down (clockwise, toward the derailleur) - about 1/4 turn at a time until it no longer jumps but still gives good shifting.

I would go the other way. If the bike is dropping from 17 to 16 there is too much slack in the cable, add tension by turning the adjustment barrel counter-clockwise. In general, you can determine if an adjustment is necessary when the bike is on the trainer. Remove all resistance so that the rear wheel spins freely. Starting at the smallest cog, with the chain on the large front ring, make a single shift while "pedaling" with your hand. Does the shift happen immediately, does it hesitate then shift, or does it not shift at all? If the second or third option, turn the adjustment barrel counter-clockwise a half turn, shift back to the smallest cog and repeat the single shift. Repeat as necessary until the shift happens smoothly and immediately. Continue to shift one gear at a time, evaluating each shift and adjusting as necessary, until you have covered the span to the largest rear cog. Now reverse the process and shift down. If the shift hesitates, release tension by turning the adjustment barrel clockwise. (At this stage you are fine tuning the shifts so you'll probably want to adjust in 1/4 turn increments.) Repeat until you are back at the smallest cog.

BTW, the bike can also shift on its own if you are out of the saddle and twisting the frame aggressively. But this isn't something that happens randomly, it is something you should recognize as a pattern.

Always.. a good thing to check is theRt Rear Dropout hanger alignment,
with the shop tool for the purpose.


so everything is in synch.. parallel planes ..

+1. And getting too loose is the more common issue when cables break in.

Thanks for the reply mate, I'll give it a try. This is what I suspected I needed to do but just wanted to check with someone first. I have a way of fiddling with things and making them worse. :D

Keep track of where you start.
Rotate the adjuster 1-2 "detents" at a time, write down how much and what direction.
Test.

As long as you know you can go back to the original start point, you shouldn't get in trouble.