Ultegra RD - Short cage vs Mid Cage
I pooched my rear derailleur and have to replace it.
What is the trade off in going with a mid cage Ultegra rather than a short cage? At my age, the prospect of throwing a 30+ tooth cog on the back for hill climbing is an attractive option. Is it simply a bit heavier, or are there going to be other performance differences?
Also, what is the difference between "Largest Cog: 30 teeth" and "Chain Wrap Capacity: 40 teeth"? How big a rear cog can I use with a 6700A GS Mid Cage Rear Derailleur?
I doubt you would discern any difference in performance between the two.
"Also, what is the difference between "Largest Cog: 30 teeth" and "Chain Wrap Capacity: 40 teeth"?"
The largest cog rating is for the largest rear cog the derailleur will accommodate. Many folks routinely exceed this number in actual use, how far you can push it depends upon the individual situation.
The chain wrap capacity refers to the number of links the derailleur will take up without slack; this is found by taking the difference between the largest and smallest cogs and largest and smallest chainrings and adding them up. So a 13-28 cassette is 15 teeth and 48 and 28 chainwheels is 20 teeth; you would need a minimum chain wrap of 15+20=35 teeth for this example. This can also be exceeded somewhat since you would not normally use the small cog and small chainring together and a slack chain will do no damage. You MUST however make sure that the chain is long enough to engage the large chainwheel and large cog without strain, lest you accidentally shift to this combination.