Originally Posted by
leftnose
You know that "Compact Gears" doesn't refer to the fact that there are only two chainrings? It refers to the fact that the two chainrings are smaller. 50/34 is pretty typical for a compact whereas 53/39 is pretty normal for a standard double crank set.
The double is geared lower and is a compromise between top speed and hill climbing ability while maintaining only two chain rings. Really, I think they are aimed at the amateur rider who can't push big gears from a standard up a hill.
Almost there.
Compact chainrings have been around for a long time. TA marketed one in the early 1970s.
What has changed since then is more gears and wider range in the back. Using a 11-25 with a 50/34 keeps a far amount of top end gearing and still gives a nice lower end.
I think leftnose hit the nail on the head though Gary. You may not have been aware that there are two common sizes of double chainrings: a 53 or 52/39 (known as a standard) and the 50/34 (compact) as well as some less common variations.
So to answer your question as I think you meant it:
For a fit rider (not an old fat broken down roadie like myself) on a lightweight bike the standard double up front will work on just about any road you can ride on. Triples are less efficient and can be problematic as to shifting and trim. However they are priceless on loaded touring bikes or for carrying heavy riders. Often they are sold on entry level bikes because the bike frame and wheels are heavy and the assumed buyer is not necessarily very fit. Plus there is the marketing aspect (and it comes with 24 gears!).