Medium cage vs short cage

the cage is where the two pulleys are, and a longer cage derailleur means there is more distance between those two pulleys, meaning it will take up more chain slack over a higher range of gears, like on a mountain bike. On a road bike built for racing you will have a shorter range of gears, so you will want the short cage derailleur, because it doesn't have to take up as much chain slack so the chain will be shorter and your shifting will be more responsive.

The newest Shimano 105 RD's will work with sprockets up to 27 teeth in size. The difference between the short cage and long cage is in their total capacity. Capacity is, in # of teeth, (big cog-small cog) + (big chainring-small chainring). The short cage has a 29 tooth capacity whereas the long cage has a 37 tooth capacity. Just figure out what cogs and chainrings you'll be using, and from that you'll immediately know which derailleur to get.

Shifting speeds won't be so different as to cause any real difference in a race.

(big cog-small cog) + (big chainring-small chainring)
then ... if I have (52 big chainring and 40 small chainring) + (26 big cog and 14 small cog). I could use either the short?

then ... if I have (52 and 40) + (26 and 14). I could use the short?

Souto Adrian said:

then ... if I have (52 and 40) + (26 and 14). I could use the short?

Yup.

Quote: Originally Posted by Souto Adrian [IMG]/img/forum/go_quote.gif[/IMG]
then ... if I have (52 and 40) + (26 and 14). I could use the short?

You CAN use a short cage rear derailleur ...