Shimano 105 rear derailleur weird specs

"Rick Onanian" wrote in message
news

I'm looking at Shimano's website, considering a triple
crankset for my 2001 Giant TCR2, stock all 105 double.

I'm trying to figure out how little I can get away with
changing to put a triple on here.

I can see that I'll need [obviously] a crankset and a
front derailleur, but I wonder if I can get away with
no new rear derailleur.

How about, instead of a triple, compatibility with a
mountain bike rear derailleur and then I use a really
wide ratio cassette? I'd rather have the triple, but
are Shimano road shifters compatible with Shimano
MTB rear derailleurs?

I think I don't understand a few terms. Here's what the
site says about 105 double and triple rear derailleurs:

Rear derailleur for double cranksets:
Total Capacity: 37t
Front Difference: 22t

Rear derailleur for triple cranksets:
Total Capacity: 29t
Front Difference: 14t

Maybe I don't understand the terminology, but I figured
that the capacity is the amount of chain slack that the
derailleur can take up, and the front difference is the
largest difference in chainring that the rear derailleur
can swallow suddenly (though I'm sure I'm wrong about
that, I think I'm right about capacity).


Rick - read the 'oracle' - Sheldon Brown's site at
http://www.sheldonbrown.com/harris/index.html
The glossary and articles will provide all the information
you're looking for....

You can use a double RDR with a triple crank, but depending
on how you size the chain you could have some slack when
using the smallest ring and some of the smaller rear
sprockets, or a nasty experience when you inadvertently try
to use the largest ring and the largest sprocket if the
chain is too short. Probably better to get a triple RDR.

An alternative to a triple is to use a 'wider' cassette
and/or smaller chainrings. The largest rear sprocket for
most 'road' dérailleurs according to the Shimano spec is
27T, but lots of people report using 30 or even 32 - the
disadvantage is bigger gaps between adjacent sprockets.

The smallest chainring you can use on a 130mm BCD crank like
the Shimano 105 is 38T, so to go smaller you'll need a new
double crank with a smaller BCD.