Bicycle Mechanics - Front derailer adjustment issue

Hi folks,
I’ve been working on swapping parts from my carbon bike to a Centurion Ironman frame. The swap is complete but I’ve been dealing with an adjustment problem that I have not been able to resolve. When the bike is in the stand, the front derailer will shift from the small chainring to the large chainring in a satisfactory way (i.e the shift is quick and crisp). When under load, however, the shift can no longer be completed (the front derailer reaches the same position as it did when not under load, but the chain does not make it to the large chainring—it won’t catch). The only way to adjust the front derailer to complete a shift is to further increase cable tension (already pretty high) and loosen the outboard adjustment screw. When in the stand, this will throw the chain, but when under load the upshift can be completed.

The problem is that this “fix” has the front derailer too far outboard and gives me far fewer usable gear combinations. One of my co-workers (I work at a shop) took a look and was stumped too. Is there a reason why the chain might not be “catching” onto the large chainring while under load? Anyone think this is compatibility issue with the parts I’m using?

Here’s the component spec:
FD: 105 (FD-5600)
RD: Ultegra short cage
Cassette: 105 11-23
Chain 105 (CN-5600)
Shifters: 105 (ST-5600, the earlier double/triple model)
Crankset/BB: Truvativ Elita 50-36 w/ GXP; for 68mm BB shell (not a 50-34 as previously stated)

Check to ensure your crank arms are torqued to the proper spec and then check your chainline again. Also you could try angling the front derailleur in/out to see if that helps the situation. From the specs you list it seems like you are running a compact crankset...double check that your front derailleur is compatible with a compact crank.

-j

Any chance the pedals are turning slower when you're riding? ;)

Check to ensure your crank arms are torqued to the proper spec and then check your chainline again. Also you could try angling the front derailleur in/out to see if that helps the situation. From the specs you list it seems like you are running a compact crankset...double check that your front derailleur is compatible with a compact crank.

-j

Left side crankarm torque looks to be okay; I measured the chainline at 44.5mm which is within spec according to SRAM. Fiddling with the derailer angle didn't improve the situation when I tried it. I was thinking about the capacity of the derailer. Shimano's tech docs say that the FD-5600's capacity is 15 teeth or fewer. Obviously I am working with a 16 tooth difference (this is how the donor bike was originally spec'ed). I have seen cases where the 105 FD worked successfully with a compact crank--think it might be the problem here?

Correction: helps for me to count correctly. This is a 50-36 compact crank, not a 50-34. So the FD's capacity should be okay.

Hard to say from this side of the monitor, but one thing I'd look at is DR height. There's a possibility that the DR mount is different between frames. I've had and seen the problem you describe, and increasing the cage to big ring clearance took care of it.

Hard to say from this side of the monitor, but one thing I'd look at is DR height.

+1. The most common cause of inability to shift up is the derailleur being far above the big chainring. It should be only a few millimeters above the chainring.

Other causes of poor FD shifting:

1. FD cage not parallel with chainrings.
2. FD cage bent.

++The very first thing to look at with front derailleur shifting problems is rotation and height. Adjustment is useless if those are not proper. There is of course more stress on the derailleur on a shift under load, so it's important that the derailleur be in good shape. Did it shift with no problems at all on the other bike?

throw limit screws heght (from Big Ring) and rotation around seat post , are all you have to work with..

other than that it's the cables , the lever or something else. not the FD itself.

Beg to disagree, but as we know nothing about the history of the front derailleur it cannot yet be discarded entirely.

So...to resurrect this thread, after further examination of the shifters, FD, and crank, I determined that the crank spider was bent. Deciding that it was time for a gearing change anyway, 1 new crank/BB later everything's perfect. Didn't even have to change the FD height/rotation after installing the new crank! Appreciate the help.