Bike Shifting On It's Own Again

[Dansky]

I have a relatively new road bike (Giant TCR Advanced 2) that I bought just before this Spring. After about 250 miles, the shift cables had stretched/loosened and I took it in to my LBS for an adjustment.

This was about 2 months ago, and I ride 100+ miles per week, sometimes more. The bike is beginning to suddenly shift to an easier gear---on it's own---again, and this isn't happening when I'm standing up to accelerate, just during normal riding. I have indexed shifters, and was wondering if this is just a simple matter of using my adjustment barrel near the rear dérailleur to tighten the cable; going half a turn counterclockwise, as per Sheldon Brown's website advice.

Opinions are most welcome.

 

[rparedes]

I have a relatively new road bike (Giant TCR Advanced 2) that I bought just before this Spring. After about 250 miles, the shift cables had stretched/loosened and I took it in to my LBS for an adjustment.

This was about 2 months ago, and I ride 100+ miles per week, sometimes more. The bike is beginning to suddenly shift to an easier gear---on it's own---again, and this isn't happening when I'm standing up to accelerate, just during normal riding. I have indexed shifters, and was wondering if this is just a simple matter of using my adjustment barrel near the rear dérailleur to tighten the cable; going half a turn counterclockwise, as per Sheldon Brown's website advice.

Opinions are most welcome.

If it's going to a larger cog: it could be that tension is too tight (barrel adjustment) OR the guide pulley is tracking the cogs too close (B screw adjustment). Check from behind to see that the pulley aligns exactly under the cog and is as close as possible (vertically to the cog) but not rubbing ; check at each cog. Also, make sure your low and high limits are set correctly.
For any RD there is an "indexing range". If you are at the extreme, any change in tension (like slight flex in frame) could cause the pulley to "jump" to the next cog. You can find the sweet spot in the range by rapidly turning the cranks while you listen to the cogs as you turn the barrel adjuster in either direction. When the chain begins to rub a cog, that's one "edge"; as you turn in the opposite direction and you hear the noise, that's the other edge. You want to be "in the middle" of those two "edges". This, of course, assumes the H and L limits are set correctly.

I hope it helps