Why is the middle chainring skipped?
 

I know that a common reason that the middle chainring is skipped when changing from a large ring to the middle one is that there are too much cable tension.

But why does this happen with too much tension? Is the tension so high that the shifter can not reach it's correct position and "believes" it is in the middle chainring even when it is the large?

I have a Shimano 105 triple front derailleur that has trouble shifting to larger chainrings if the tension is too weak. But I think it might be defect and has ordered a new one. And yes I have adjusted it both according to parktools and shimano instructions :-)

Are you shifting from large chainring to middle chainring while in the largest rear cog? If so, the angle of the chain causes everything to sort-of catapult through the stop.

I've never had the issue that you're describing with a triple crankset, but I do know that a compact double can drop the chain if shifting occurs as I described above. So if you're doing that, stop it! Shift before you run out of rear cogs.

If you are skipping chainrings regardless of rear cog position, then you've set something up wrong. A front derailleur only has 4 adjustments: mounting height, high stop, low stop, cable tension. You could always borrow a derailleur from a buddy and try to set it up on your bike. If his works and yours doesn't, then your derailleur is messed up as you mentioned. If you can't set his up, then your methodology is flawed and you should ask somebody to check your work. You could also take it in to your LBS and pay them a couple of bucks to set it up for you.

This happens no matter of what cog is set at the rear. But there is a distinct difference in the shifter when this happens as well. When it works OK, the shifter lever can only be pushed one click, but in the cases where it skips the middle ring, I can feel that the shifter performs two clicks - I sort of "click through". I think this is the normal behaviour when shifting from middle to small ring.

But thats why I would like to know why cable tension makes middle ring skip - I suspect that too much cable tension makes it "impossible" to move the shifter further enough to take it to the "last index", so that it believes I'm in the middle ring instead of the large ring and thus performs this "middle to small ring" shift even on the large ring.

No ideas anyone?

I had the same problem...
 

I changed a 18 speed twist shift, to a 21 speed with rapid fire shifters, but did not buy a new front dérailleur. It was to narrow, and would jump from smallest front chain ring to largest, had to change the dérailleur to wider one, if yours is the right one , it could be bent in a little.....
Richard, Hope this helps..

Here's my understanding of cable operation for front derailleurs:

High stop and Low stop set the range within which the derailleur moves. If you were to remove those constraints, cable tension would be the only control over the derailleur location. As such, too little cable tension has the cage too close to your seat tube (being drawn there as a result of spring tension). Too much cable tension pulls the cage too far away from the seat tube. The indexed shifters aren't smart enough to know where the chainrings are, they only measure how much cable is pulled. So it possible that too much cable tension will shift past your middle ring while your shifter still has room to go further. At that point, the high stop won't allow the shifter to draw that much cable, and that's the only thing keeping you from sending your chain sailing off into the night.

There is, however, one more variable than has been mentioned so far. If your front derailleur is mounted by a clamp, rather than bolting onto a braze on, it is possible to adjust shifting by rotating the shifter. Sometimes that's just the tweak that's needed. Good luck in any case.

If you push the shift lever too far it may go to a trim position instead of the middle shift position, depending on how you do the shift, and with the inertia of the chain that may be enough carry it to the large chainwheel.

The issue is the other way around really - it skips the middle on the way down...

If it skips the middle ring when shifting down, the inner cable is way too loose. It needs more tension, assuming the high and low stops are correctly set. Turn the barrel adjuster counter clockwise. If there were too much tension, it would tend to skip the middle gear on the way up. bk

Shaimano road shifters have four main positions plus a trim position for the smallest chainring. When downshifting from the big chainring to the middle it should take one click. The second click will put the derailleur on the second middle position. A third click will shift to the smallest chainring. I think you have too much tension and the low limit screw my also be set too close to the seat tube.
I suggest that you follow instructions for setting up a front derailleur found at http://www.parktool.com. Perform each step in proper sequence leaving nothing out. When set up correctly there should be very little tension on the cable.

bkaapcke: I think it is the other way around? Shimano service manuals says that it is due to too much tension if it skips the middle downwards?

I have followed the park tools instructions and also the Shimano ones. The low limit is set so close to the chain as it can get (it almost touches when having the middle chainring and the largest cog). I can not loosen tension more. It won't skip upwards if I do.

I guess that there is something wrong with the front der. It might be bent or have to much slack in the joints. I have ordered a new one, and will try that. Thanks for all advice.

I doubt that the derailleur is the problem.
Are you sure that all of the cable was out of the shifter at the time the cable was attached to the derailleur? If not this could be the reason for the problem shifting up to the big ring.

Yes it was. When I shift to the small chainring - the cable is quite loose. When I attached it I tensioned it with my hand, not pliers or anything.

I have done quite a lot derailleur adjustment, but this one seems to cause me a lot of trouble...

It may be a long shot but have you tried shooting WD40 into the shifter?

Set the FD cage so it is parallel to the chain when on big ring/small cog

I've done all the adjustments in the book...

When you get the new FD, follow Shimanos installation instructions very carefully. Their step by step procedure works very well. Don't peel that red tape of the der until you have set the height with it. bk

I fitted the new front derailleur today, and everything works superbly now! Even the trim-functions work.

I couldn't spot any huge differences between the old and the new one, but I guess there must have been some issues somewhere. I saw one thing that might have had a difference. Where the cable connects to the derailleur, there is a "lump". I saw in the shimano instructions that the cable should rest on top of this lump and not go behind it. But I guess this shouldn't make a difference? Or might it affect the tension at different angels of the derailleur?