Front Derailleur Rub
I have a 52-42-32 front crank and a 12-26 cassette. I had a 28 chainring laying around and I was thinking about lower gears so I put it on. The chain now rubs on the bottom of the front derailleur with the three smallest cogs on the cassette because the derailleur lacks the needed capacity. Will this cause any problems?
It's not primarily a capacity issue (that's your secondary problem), it's that the height between the bottom of your FD and the top of the 28 chainring is too great. Other than the problems already noted 1) chain rub and 2) excess chain capacity you should be OK if you stay away from the small gears in the cassette (ie; avoid crosschaining).
Originally Posted by rmac
It will wear parts more quickly, but in practice why would you need to use those combinations anyway?
Thanks. It actually shifts great. I'll try it for awhile. Just have to remember not to use those gears.
Mad bike riding scientist
There are some general fixes to try:
The "Doctor! Doctor! It hurts when I do this!" aproach. Don't do it! As noted elsewhere, those gears really aren't needed since you have duplicates of them anyway.
You could snuggle the front derailer up to the outer chainwheel a little. Measure the distance between the outer plate of your derailer and the top of the teeth on the largest chainwheel. If it's more than 3 or 4mm, you can move it down.
You can change the front derailer. You don't say which one you have but I've found that the Shimano Tiagra is a bit more forgiving then the front derailers higher up in the Shimano line. It's a little wider and longer.
Change your chainwheels and your cassette. A 50/39/28 with an 11-27 cassette would give you a higher high gear (122" vs 117"). In my opinion, the smaller chainwheels shift a bit better too.