Outer-middle difference (again)

I’ve replaced the chainset on my 9-speed winter bike with a triple, Shimano FC-5503 with 52-42-30 rings, then bought an FD-5603 from a friend. Although I knew this to be a 10-speed triple mech, I thought it would work. But as you will know, when the outer plate is set correctly (just clearing the outer ring), the inner plate hits the middle ring.

So my question is: what is the part number of the correct 105 or Ultegra mech, as I cannot find a chart that makes this clear?

Keith Leonard

You’ve provoked the demon of outer-middle difference, that almost secret property of triples and their matching mechs, with which one messes at one’s peril!

Basically, you need the same number of teeth difference between outer and middle ring as the triple chainset that comes in the same groupset as the mech. Previously that was usually 10 teeth, but now it’s usually a bit more. On the road: Sora, Tiagra, 105 are now 11T, whilst Ultegra and Dura-Ace have a heroic 13T difference (don’t go there)! MTBs have mostly gone to 12T, with a consequent reduction in middle ring tooth height and downshift performance.

An older 105 mech, specifically FD-5503 or 5504, will work, but old mechs tend to have worn-out cages. The entry level (8-speed) 2300 group still has a 10-tooth outer-middle difference, so a new FD-2303 will do. Otherwise you’ll have to change one of those chainrings to 41 or 53.

Chris Juden


This was first published in the February / March 2013 edition of Cycle magazine.