smgmk2

Hey guys, my rear deraileur is having some issues. When i am in the big ring in the front, and i am trying to shift up from the lower gears (lower as in easier, shift up as in moge to harder/smaller cogs) my gear doesnt shift. Then when i shift down again my gear shifts down twice. Therfore right now when I am in the big ring, the only way to shift one gear up is to shift up twice then down. However, I dont have issues shifting down, and I have no issues shifting whatsoever when I am in the small ring up front. Any help would be apreciated, as in which screws on the deraileur and cable tension things I should try fiddling.

I have Shimano 6800 on my road bike fyi.

Thanks guys.

Wingsprint

Check that nothing is bent on the drive train and that your cables are in good condition, then follow this guide:

Derailer Adjustment

Andrew R Stewart 

AS the chain is played out (as when in the big ring) and the rear der cage rotates the guide pulley gets more distant from the underside of the cogs. This means that more links of chain are between the pulley and the cog's teeth. This means more points of play/slop (each link's pin/bushing) to go through their play/slop before a sideways motion is transferred to the next link.

Back in the day chains had far less side to side flexibility. Todays chains are designed to have a lot of this slop so the chain can ride on two different cogs at the same time. This side flex gets worse as the chain wears (and this wear isn't what we measure when we claim a chain is worn out. But this wear most definitely effects shifting performance.)

Chain length effects the amount of chain between the pulley and the cogs. I try to use as long a chain as possible w/out having pulley knock (on the underside of the large cog) to reduce the amount of open chain when on the large ring.

To the OP's situation- Adjust the der's "B" screw so that there is ALMOST pulley knock when in the small ring and largest cog. Check the cable friction. That is a very common reason for up shifting issues when down shifting is spot on. As the up shift is dependent on the der parallelogram's return spring pulling the cable through the housing and perhaps the frame as well. Andy.

FBinNY 

Shifting differently depending on which chainring is in use is evidence of a bent hanger.

The upper pulley swings an arc as the cage rotates to take up slack. If the hanger is bent, that arc isn't on the plane of the bike and the upper pulley swings in or out slightly affecting trim and shifting. The bent hanger theory would b confirmed if this is a new problem on a bike that shifted fine before.

So, step one is to check your hanger. If it's OK, then it might call for a slight trim adjustment toward the inside. The upper pulley is a bit farther from the sprockets when you use the large ring so downshifts can be a bit more sluggish, and call for more of an inboard trim. It's difficult to get perfectly uniform shifting in both directions with both chainrings, so you try for the best balance.

reshp1

Back out the barrel adjuster 1/2 a turn or until you have problems shifting to smaller cogs (harder). Big ring in front, small in rear is the worst case scenario and your problem may be as simple as your cables stretched slightly and the problem is only just starting to be noticed in this one combination.

xlbs

You might also wish to check your chain for wear. If your shifting performance is fine each time you move down the cassette (to smaller cogs) your chain may well be worn just enough that it "likes" the cogs as you shift to a smaller cog. If it is worn or stretched then the teeth on the cog won't engage properly as you attempt to move to larger cogs.

Likewise, your cogs may be worn too.

In each case there are some simple diagnostics you can try.

For your chain, place it on your big chainring and small cog. Take and grasp a couple of links on the front of the chainring and pull on them towards the front of your bike. If you get a lot of play you should then find someone with a chain-stretch guage to ascertain just how much stretch you have.

For your cogs, clean them and then examine them closely. If you see wear marks on the leading edge of each tooth you may also need to have someone check those for you.

All of the other suggestions are on the mark as well, but if you haven't changed any of your derailleur settings (you didn't indicate that you had done so) your problems may be from wear and stretch and not from alignment or cable tension...

In each case, above, cleaning your chain and cogs will make it that much easier to diagnose any problem because you will be able to see the mechanics of the shifting while your bike is on a workstand.