Bicycle Mechanics - question about 'sloppy' rear derailler

I'm having problems w/ my RD. while i'm riding, it has a tendency to move to the smaller cogs (away from the center of the wheel). it seems there is some sort of mechanism inside the RD which pulls in this direction, and it is too strong. i have downtube friction shifters. i have the friction set as tight as possible on the shifters (meaning: i've cleaned the 'washers' inside the shifter and tightened the little D ring as much as possible. but still, when i try to put the RD in biggest cog (closest to the wheel), it slides out to the 2nd biggest cog, and stays there for a while, and then finally settles into the 3rd biggest cog. it's pretty stable there. but if i get up out of the saddle and really lay into the bike, the RD will slip one more gear out. it's very annoying not to be able to use the big cogs (easy gears). for what it's worth, got the bounds set correctly (via those 2 screws on the RD), and i've got the shifting cable tight; no slack when the RD is in the outermost (smallest) cog.

is there some way to reduce the force that pulls the RD away from the wheel?

i hope this makes sense. if not, please someone let me know and i'll try to explain it better. thank you in advance.

dan

The cable is probably too tight.

There is spring inside the parallelogram that pulls the derailer outward. It is not adjustable. If your shifter is not moving and assuming your derailer is not real old with worn pivot points, then I'd look at the cables and housings. Make sure that the cables move freely and are not dirty. If your shifter is being pulled back, then perhaps you have assembled the unit incorrectly. I use the old Suntour design powershifters which have a ratchet mechanism to allow easy pulling on the cable but friction to hold the shifter in place. If the internal parts are assembled the wrong way I belive that the shifter can slip.

well, since this was happening before i tightened the cable (there was slack in it, so i tightened it), i'm guessing it's not b/c the cable is too tight.

the shifter is moving when the RD moves outwards. the shifter slides back to it's position parallel w/ the downtube. so maybe it just needs more friction somehow. i'll play around w/ that, but if anyone else has any suggestions, i'd be grateful to hear them. thanks.

dan

It sounds to me like your fraction shifter has finally given up the ghost. My bet is that if you replace your shifter your derailleur problem will go away.