KonaRider125

My setup: SRAM 11-32 road cassette, Shimano 105 GS RD, Shimano Tiagra STI levers.

I have been having trouble down shifting from the 3 largest cogs on my cassette. Mostly an issue going from 32 to 28.

Whats the cause of this? Need a new RD? New Shift cables?

Thanks

FBinNY 

Most likely the RD is riding too close to the sprocket. This may be a B-screw adjustment, or a largest sprocket capacity issue.

Before going crazy, try to duplicate the problem on a stand or with someone holding the rear wheel off the ground. Then while pedaling, ease the RD down a bit with your left hand, and see if it makes a difference.

oldbobcat

Also, the derailleur hanger might be bent. Typically, an impact to the rear derailleur bends the hanger inward. This causes increasing overshift as the derailleur moves the chain up to the inner cogs of the cassette. Shifts at the faster end of the cassette work fine, but the cable becomes too tight when the chain is on the big cogs. Then the spring can't knock the chain down on the upshifts.

Have the hanger aligned and re-tune the limit screws and cable tension. This would also be a good time to examine the cables for dirt, corrosion, and damage.

cny-bikeman

I've never seen that effect before, and I'd have to see some more detail before I will buy your analysis. I am aware of no reason that a bent-in hanger will do more than move the shift points inward, produce a bit more noise and possibly degrade shifting to larger cogs. I don't see any reason that it would cause the cable to become too tight.

To the OP: It would be helpful to have a better description of the problem than "trouble shifting." Is the shifting slow/delayed, inconsistent, noisy, etc., as well as the result of trying FB's suggestion is the first step, though.

FBinNY 

The OP hasn't reported back on the test for the most common cause of the issue, namely RD too close to larger sprockets.

It makes little sense to debate other possibilities until that's ruled out. Of course, if RD height is implicated, then it's a question of whether the B-screw can bring it down enough, and/or how to work around that.

Of course the other likely possibility is trim biased inboard a hair, which would manifest as crisp down shifting, slower upshifting especially on larger sprockets where the RD is closer and trim more critical. The trim issue could be the result of the hanger being bent inward very slightly (more would cause overshifting in low). I glossed over that in my earlier post, because the height issue is so easy to eyeball and test for without making any adjustments, so that is the starting point.

KonaRider125

I've had my current road bike a little over a year and modified to the current parts right after I got it, dialed everything in and it shifted perfectly until about 2-3 months ago.

What happens when I try to downshift one gear from 32t to 28t is I just get clicking/grinding noise, I have to shift down twice and shift back up one to use the 28T gear. Shifting in the lower portion of the cassette is smooth and flawless.

Thanks for the help everyone and let me know. The RD hanger does not appear to be bent inwards.

FBinNY 

You're asking for advice while keeping the blindfold tied on securely. It's impossible to diagnose a problem without knowing more. We can't see, hear or touch it (smell and taste probably don't matter) and are 100% reliant on your senses.

The quality and meaningfulness of any answers we might venture are directly related to the quality of the information you provide, which, so far, has been sorely lacking.

So, let's try to get into the nitty gritty.

1- did you try shifting in the stand while pushing the RD down? Did anything change?
2- have you made any adjustments between when it shifted fine and now, especially to the trim?
3- Does the issue manifest any differently if on the small vs. large chainring?
4- how close to the sprocket is the jockey wheel, either radially or tangentially along the chain
5- have you tried shifting directly, ie. pushing in on the lower body and letting go? This is best done by shifting up only one position and back, starting from the various problem positions.



Those are just a few of the items a mechanic would look for or test if he had the bike in his hands, so you're going to have to do that and report back.

That is unless you're hoping that if people throw enough **** against the wall something might stick. Without real input from you, I'm out.

cny-bikeman

Also, we were led astray by your use of "downshifting." Most take that to mean shifting down to a lower gear ratio, which in the rear would be to a larger cog. Apparently you meant shifting to a smaller cog. Did anything significant occur just before the problem arose - accident, equipment change, major maintenance procedure?

FBinNY 

I must be more used to that because I let the "from largest..." guide me rather than up or down.