I have a Shimano 3 x 9, XTR RD

When downshifting from the 3rd largest to the next smallest cog, the chain downshifts then rolls back up to the 3rd largest cog.
It will rotate on the 4th cog a time or 2, then roll back up to the 3rd cog.
Eventually, it will jump back down to the 4th cog, sometimes after rotating off chain tooth center, before shifting back down to the 4th cog.

This happens regardless of whether the the chain is on the small, middle, or largest front chainwheel.

Does anyone know the cause & cure for this problem?

Thanks In Advance,
Uni

Odds are it's a simple trim problem. The cable is adjusted too short and the RD trimmed slightly inboard of the center of the sporckets. In theory it should affect all sprockets equally, but in practice it varies sprocket to sprocket based on subtle differences including the distance from the jockey wheel to the sprocket.

The fix is to turn the barrel adjuster inward toward the RD (clockwise if looking from the top) a bit to slacken the cable slightly. Very little is needed, so start with a quarter turn, and see how it goes. Be sure to check shifting in all gears, then fine tune in either direction depending on the changes.

BTW- this kind of problem doesn't develop by magic, so it it used to be OK and now isn't, odds are the hanger got bent in slightly. That's probably OK, but it means the entire RD is now inboard a bit, and you need to confirm that there's no possibility of shifting the chain into the spokes. Do this by having someone hold the wheel off the ground, and while pedaling, shift the RD by drawing the cable away from the frame like a bow string (disregard levers for this test). You want to shift rapidly into low, and repeat shifts from 2nd to low a few times to ensure that all is OK. If it overshifts, or even if the chain rides the tops of the teeth as if getting ready to overshift, you have to adjust the limit to stop this.

You might want to seek out tutoials on how to adjust a derailleur, so you have the full picture.

The other common cause of this problem is increasing cable friction. But this usually has the up shift direction (actually up shift is usually the shift to a smaller rear cog, as in shift to a higher ratio gear) being problematic. At the shop when faced with a shifting issue we almost always check the cable/casing friction being so easy to do and frequently lube the cable anyways. Andy.

Thanks Gentlemen,


Adjusting the cable tension has cured the shifting problems.
' checked for overshifting, & I'm OK there too.


Will you please tell me, if I need to use the adjuster on the Shimano 9spd shifters, turning the adjuster which way lengthens the RD cable?


Thanks Again,
Uni

With the SRAM trigger- and twist-shifters that I use, turning the adjuster out from the shifter (counter-clockwise looking from the cable housing to the shifter) 'lengthens' the housing, which increases cable tension.

It's easy to know which way to turn adjusters to change trim if you think about the housing rather than the inner wire. The adjuster doesn't directly change the inner wire which runs straight through. It effectively lengthens or shortens the housing, or total length of the cable's route.

So lengthening the housing has the same effect as shortening the inner wire, and vice versa. If you want the RD to trim inboard (for normal derailleurs sprung to move out) you turn the adjuster out lengthening the housing. To relax the cable and let the RD move toward the spring (outward) you'd shorten the housing.

Thanks!!!