b-tension: no discernable movement

absinthe said:

Any reason my b-tension screw would be seemingly non-functional throughout its entire range? I installed a new 105 9sp derailleur (11-21; 50/34) and it works fine, but the b screw, which is properly seated on the dropout tab, does not appear to have any effect on the derailleur whatsoever. Thanks!

The b-tension screw should only be adjusted in the lowest gear. The purpose of this is to adjust the distance between the upper derailer pulley wheel to the cog - the nearer it is the better the shifting.

Shift to the lowest gear then loosen it. You should notice that the distance between the pulley and the cog narrows.

Thanks for the response. However, I do already understand how it is supposed to work. In my situation here even in the lowest gear the b screw has no effect that I can see.

absinthe said:

Thanks for the response. However, I do already understand how it is supposed to work. In my situation here even in the lowest gear the b screw has no effect that I can see.

Well you wont really notice any movement while adjusting it. You will only see the difference when you pedal and shift it. Put it on the slackest setting then put the bike on a stand where you can pedal it with your hand and shift it from the 2nd to lowest cog to the lowest, note the distance of the upper derailer pulley to these cogs. Now adjust the b-tension screw to the other extreme limit and then do the same. There should be a bigger distance this time between the upper derailer pulley to the cog versus the prev. setup. This cog-to-pulley movement on the entire range of the b-tension screw is really minute so turning the screw a few turns seems to make no difference at all but its there.