BikeManDan

I'm still a learning bike mechanic (obviously from this question)


SRAM 8spd cassette + Shimano XT short cage derailleur + Shimano Alivio shifters

I can adjust the derailleur to get near perfect downshifts but then the upshifts go to sh^t. The chain stays on the last sprocket and doesnt shift over as if theres not enough movement from the derailleur to froce the chain off to the next. Isn't that an operation of the shfiter itself? (it pulls the cable some specified amount)

What oversight am I making here?

Steev

You may well have a problem with gunked up or corroded shift cable. Less likely but possible is the derailler pivots needing lube or the derailler needing a good cleaning .

AndrewP

For the upshifts, the spring in the derailler pulls it into position, and the cable only stops it moving too far. Therefore it is probably just gunk in the cable housing or in the shifter that is slowing the movement. Cleaning the shifter mechanism with WD-40 is the first thing to do. If that isnt enough remove the cables from their housing and clean them out.

waterrockets 

One quick check is to make sure your der cable is not too tight. If you rotate the der barrel 1/4 turn clockwise, does that make it better?

I agree with the others that it's more likely to be old or guncked-up housing and cables, but the barrel adjuster is so easy to try. If you take the rear wheel out, does it still move the der slowly to the right? If so, go for the cables/housing.

BikeManDan

The bike has had very light use and the cables are in good condition

I just went out and greased the cable and housings. No real improvement

The major problem is the 5->4 shift. This one always gets hung up

Al1943

5 to 4 sounds like an upshift, smaller cog to a larger cog, no?

Al

BikeManDan

Err well I sometimes get confused about upshift versus downshift


I'm talking about bigger cog going to smaller cog
Going from small to large is no problem, it shifts smooth as butter. Going back down is where it gets hung up

waterrockets 

You didn't answer my questions above. Is it slow moving when the wheel is removed? This will tell us if there's something slowing it down, or if it's a mechanical adjustment issue.

Al1943

That's an upshift.

This is often an indication of excess friction due to a dirty and/or corroded shift cable and cable housing. With STI shifters it's best to replace both shift cables and cable housings on a fairly regular basis. The short housing just before the rear derailleur is the most critical.
Also flushing out the shifters with WD40 may help.

Al

Cadfael

Don't shoot me here if I am talking crazy...

But on some deraillers I have seen an adjustment screw that changes the distance of the top wheel on the cage to the cogs on the backwheel... could this make a difference? It just seems to my (often muddy) way of thinking that is the distance is set to far the chain will bend with the deraillier (as they are designed to do I suppose) rather than move with it.