ok, thanks for the explanation

I just got a new bike with a 50/34 compact crank, that came with a 12-25 cassette (105). I had the shop change the cassette out for the 12-27, just to get a lower gear. The only issue is that a short-cage 105 RD will not support the total tooth-range of 16+15=31. So basically I need to stay away from the two extremes of the 50/27 and the 34/12, which don't make sense, anyway.

A long-cage 105 RD will of course handle all of the above gear combinations, but they are normally only used with triple cranks.

For what it's worth, the new 2010 Ultegra RD has a somewhat better total tooth-range (I can't recall the correct term to use for this, but it is the sum of the tooth differences of the chain-rings, plus the cassette.)

The 2010 Ultegra RD will support all gear combinations of a 50/34 compact crank, and the 11-28 Ultegra cassette (and therefore also works with the 12-27 105 cassette).

Thanks for this info. I'm interested to know how you've found this combination. Is hill-climbing appreciably easier?

You can get an older Ultegra mid cage RD 6500 for a fraction of the price of the newest model. Should work great for this application. I picked up a few of them on sale and I'm running them on both of my main road bikes.
https://www.cambriabike.com/shopexd.a...EAR+DERAILLEUR

I think it would qualify as noticeable, although I had the change-out done before I even picked up the bike. From the math it is about an 8% reduction in the lowest gear ratio. Since the replaced cassette was unused, the charge for the swap was $30, which I thought was reasonable. Swapping out both the rear derailleur and cassette for the Ultegra 11-28 cassette would have been a fair amount more money, although I didn't really get a firm price.

The bike I was replacing is (I still have it) a 1980 Raleigh, which has a 52/42 crank, and an "Alpine gearing" 5-speed freewheel with 14-17-23-30-34 cogs. The lowest gear combo on the new bike (34/27) is very close to the lowest gear combo on the old bike (42/34). Actually, since the new bike has longer crank arms, and also weighs a lot less, it's a lot nicer on the hills than my old beast.

As an aside, on the Shimano web site they list the available cassettes for each group, and also the restrictions that apply to the various derailleur choices.

I know all those gear inches charts are easy if you know what you're looking at, but my simplistic method is to divide the cassette into the cranks... as in:
34 divided by 27 = 1.259 and 30 divided by 24 = 1.25. So, as close as damn it... 34/27 = 30/24.

Agree. Both are 34 gear inches.

Ya know, guys, this was a really interesting string. Some thoughtful and useful info. I think that is what I like about the 50+ group the best. Good info, little flame, and thoughtful discussions.....Thanks.

I ride used to ride a triple, but switched to a 50/34 with a 12/27 and found that what has helped my climbing more than anything else....was losing 35 pounds.