I am new to biking and know very little. Pretty fair mechanic, have been rebuilding/retoring old 70's era two stroke street bikes for a hobby, but I am needing some advice on how to troubleshoot my FD. Shimano 105 FD, Ultegra Shifters and RD.
RD works flawlessly.
The puzzle is this:
With the bike in High Gear on the FD, when I downshift to the middle ring, it is usually a very smooth shift. If I then try to shift to low gear, sometimes it shifts, but most times nothing happens, it stays on the middle ring. Then, sometimes as much as a half mile later, it suddenly drops onto low gear. By that time, I have completely passed the need for low gear, so I am then just freespinning with no resistance. A quick click on the shifter and the shift back to the middle ring is flawless, and then to high is flawless.
The other thing that occasionally happens is that when I try to shift from high to the middle ring, the chain will drop all the way to the low gear ring.
Up shifts from any gear are always flawless, the downshifts, not so much.
I have already read two different tutorials (Sheldon Brown's and another one) about adjusting the FD, have not tried to do it yet, just searching for info before I attempt the repair.

Sorry for being so long winded on my first post, but I did not know how to describe the probloem otherwise.

Thanks,

Wayne

Given your description, I suspect you have either something impeding the free movement of your FD (front derailleur) or a friction issue along the cable.

First take a look at the spring on the FD. If it's badly gummed up or sticky, give it a cleaning with a solvent (wd40, mineral spirits, etc) and a few drops of light lube (tri-flow or similar is good).

Now take a look at the cable guide under the bottom bracket. If it's crudded up enough, the cable may not move freely and reliably under derailleur spring tension. Also, the FD cable makes a sharp upward turn under the bottom bracket. Over time, the cable can cut into this plastic guide and cause the cable to hesitate. Clean or replace as necessary.

I suspect either of the above may be the culprit. If not, you may excess friction in the section of housing running from the shifter to the cable stop on the frame. If your cable stop is not fully enclosed, you can easily slip the housing down to expose the cable for inspection and lubrication. To do so, shift your chain onto the large chainring. Then, without rotating the cranks, release all cable tension via the shifter. The chain will remain on the large ring, but there will now be enough slack to slip the cable through the opening in the cable stop. Once you do that, you can tend to the cable. If the housing is in poor condition or the cable is frayed anywhere, it would be best to to replace both.

fd adjustment or bring it to a shop

It's not the adjustment, it's the cabling. Either the housing is crudded up, or the cable is cruded up, or the cable is attached wrong, or something is bumping the cable. When you downshift, check to see where the tension in your cable is. Somewhere between the place where there is tension in your cable and where the cable is slack and loose is where the problem is.

FDs are pretty basic. They need to be installed correctly #1. For this, refer to Park Tools website. Basically, the outer cage must be parallel with the chainrings and the height of the FD should be 1 to 2 mm above the largest front chainring. Then adjust the hi and low limits as per the Park Tools tips.

From there, explore the cable-related issues outlined above.

Also, are you possibly cross-chaining at any point in this process, like where you go from the middle to the small ring after you have hit flat road? That also might bring into question chain length.

Thanks, guys! I will look at the cable first as I bought the bike used and don't know how old the cable is. It was working correctly when I bought the bike last fall, so I am fairly certain the cable was installed correctly. It may be best to just replace the cables since that should be routine maintainence anyway.

Wayne

Try to figure out what the problem is before you start replacing things. It may be the housing that needs replacement, or it may be that the cable isn't attached to the derailleur properly, or it may be something else that only takes a few seconds to fix.

Yes, Sir, I will try to determine what the problem is before I don ANYTHING else.

Wayne

Go to bike tutor.com and watch the FD adjustment video. You will probably find out it just needs to properly adjusted.