If I understand your question the maximum range would be limited to the chain wrap capacity which is probably the same as a newer 10-speed short cage Shimano rear derailleur, 29 teeth. Shimano's specs are conservative so that limit could be pushed some. If you are thinking about running a triple crank with your short cage derailleur you can do that if you stay out of the small ring and smaller cogs combinations.
On the other hand, long cage derailleurs are pretty cheap.
What are you thinking about doing?
One thing to know is that when people speak of a "8-spd. derailleur," it doesn't mean the derailleur is limited to 8-spd cassettes. The 105's will be equally at home on a 10-spd. cassette or a 5-spd. freewheel.
Originally Posted by Cateye
Only panthers007 is stupid enough to believe that this is a good idea.
Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
Originally Posted by Dural
You can do at least: 32t with a 42/53,
Maybe, maybe not.
In addition to the chain wrap issue there is also a largest cog capability. Shimano rates their road derailleurs, regardless of long or short arms, for a 27 tooth maximum rear cog. Like Al1943 said, you can generally cheat a little on that but 32 teeth is quite a bit of cheating.
The bottom line is that, if I already owned the derailleur, I'd try it to see if I could make it work to my satisfaction. If I were buying a derailleur, I'd be sure to get one that was rated for the gear combination that I was using.
06 KHS Urban-x, 07 KHS Flite 300, 08 Cannondale Capo
Thanks, guys. What I have is a single chainring with 48t, an 8-speed cassette, and a short-cage 105 derailleur. I'm guess I'm gonna go up to 28t for sure, then, for a 12-28, but maybe I'll wait and see if I need something bigger.