Dan The Man

All of the indexing is done at the shifter right? What is the functional difference between double and triple derailleurs. Can a double derailleur be jimmied to work with a triple? I ask because I just replaced my front derailleur, and I may have mistakenly purchased a 105 double. I will check the part number when I get home, but if it isn't a very big difference, or if I can make it work, I would like to use it rather than return it.

bikemore 

I don't know the specifics, but a triple is wider then a double. And a 105 triple will have a much wider (taller?) inside edge to move the chain off the small
ring.

I've heard you can use a double derailleur on a triple crank, but I've never tried it myself.

fiataccompli

FWIW, I have ended up with more 105 triple front derailleurs than I need...I'd trade a triple for your double, depending on clamp diameter (assuming it's not braze-on)

Al1943

Triple FD's are wider than double FD's. There may also be a difference in the depth of the inner rail, this is to accommodate the differences in chanring sizes. Also a 9-speed derailleur is wider than a 10-speed derailleur.
With a triple it's best to have the correct derailleur. You may be able to use what you have but if it is for a double you will have issues with chainrub on the insides of the rails.

Garthr

I friction shift, and use a 9 sp. 105 double FD with 26/44/48 rings. Works perfectly. I don't think triple FD's are any wider, btw. (Up to 9 speed FD's at least, I'm not familiar with 10sp.)

Dan The Man

This is for an 8 speed if that makes any difference

Al1943

My wife's triple Ultegra 6503 9-speed FD measures 12 mm inside at the front and 13.5 mm at the rear.
My double D-A 7700 9-speed FD measures 11 mm inside at the front and 12 mm at the rear.

Chris_W

I think by saying "wider", bikemore actually meant that the inner plate is taller / extends further down / is deeper, he just chose an inappropriate word to describe it.

HillRider

Friction shifting will pretty much tolerate any combination of double/triple crank/front derailleur. Indexing isn't as accepting.

alharris

See:
https://www.ctc.org.uk/DesktopDefault.aspx?TabID=4068
for a comprehensive intro to the details of setting up and selecting front shift components.

djsincla

On the Ultegra, the bodies are sized differently. The triple is larger.

When I purchased a Trek OLCV road bike, the bike store swapped the double chainring for a triple and also swapped the lever. The forgot to swap the derailleur and I suffered poor shifting for a good year before swapping all components for Duraace. Well after the fact I realized there was a difference in derailleur sizing when I was reusing the components on a second bike.

Dwayne

DMF 

Triple FD might also have a smaller arc to the cage to better match the smaller chainrings it's likely to see.

Al1943

I said wider because I meant wider.

JohnDThompson 

Before the days of indexed shifting there was no distinction made between "double" and "triple" front dérailleurs. As long as the adjustment range allowed it, any front dérailleur could be used on a triple setup.

tatfiend 

In the 80s Shimano even made a triple front derailleur for half step + granny gearing. Most such users used a double ring front derailleur for such gearing though. These days if you want to use that gearing arrangement you normally need to use a double derailleur as the triple ring ones have an inner cage that is way too deep to clear the middle ring when shifting to the large ring.

Front derailleur models have proliferated as they are fine tuned for double or triple use and have cage radii which vary from model to model depending on the diameter of the outer chain ring they are intended for. All in an attempt to give the best possible indexed shifting for different front chainring combinations and different outside chain widths.