you are kidding, right?
Chainring tooth count is the difference.
a double has two shots of espresso where a compact is normally only one.
A standard “double” crank has a Bolt Circle Diameter (BCD) of 130 or 135mm, which allows for a minimum chainring size of 38 or 39 teeth. A “compact” crank is a double crankset with a BCD of 110, which allows for a minimum chainring size of 34 teeth.
Typically, a compact crank is equipped with a 34x50 chainring combination and a “standard” with a 39x53 combo, but either crank can be assembled with any chainring configuration that fits within the limits imposed by the BCD minimum, and a maximum constraint imposed by a combination of shifting performance, bicycle frame design, and (maybe…) stifness.
110 bcd chainrings are typically available in 34,36,39,42 (inner,) and 46,50, and 52 teeth (outer.) 130 bcd chainrings are typically available in 38, 39, 42, 44 (inner,) and 46, 47, 48, 50, 52, 54, and 56 (outer.) Combinations that involve a difference of greater than 16 teeth are generally considered to offer inferior shifting performance, and not typically offered as stock configurations.
So, in practical terms:
Compact if you need smaller or lower gears, standard double if you don’t (and yes, most people do…)
.
MM - I think you’re talking cranks and chain rings here. The term “double” refers to two chain rings as opposed to a “triple” which has three. The term “compact” refers to the fact that the spider (the little arms that come off of the crank and bolt onto the chain rings) is different - smaller, more compact than the standard cranks. To specify this: standard rings have a bolt spacing patter that is 130 and compact rings have a bolt pattern of 110. Standard rings are usually 39 teeth on the small one and 53 on the big while compact rings are generally 34 teeth on the small ring and only 50 on the big ring - however you can move around a bit with other rings for each - a compact can carry a 36x52 for example.
For triathlon you gotta love compact, especially if you’re a) not super strong on the bike or b) racing a crazy hilly race - it’s lighter a triple ring and you don’t get caught up in the desire to shift more than necessary with the front derailleur. Also, it’s commonly understood that one can run off the bike better by spinning their way through the bike segment rather than mashing their way through and the compact speaks to spinning a bit more. If you just absolutely love to go fast down hill then you might get disappointed with compact 'cause you’ll spin out sooner in a 50x11 (that’s the chain sitting on a 50 tooth chain ring on a compact crank and the smallest cog you might have - the 11 in the rear) than you would in a 53x11 (standard version of that same story). If you just suck at climbing and need help then the compact is the way to go because you’ll get up the long climbs far better in a 34x25 (34 teeth on the small ring of the compact and a 25 rear cog- could be a 27 or even a 26 if you run SRAM or Campy - or now the new DA has a 28 I believe) as apposed to a 39x25 (the standard version of the same story).
Whew! That was a lot of info. I hope it helps.
Ian
I think there is confusion, usually you compare compact vs. standard chainrings; and compare double vs. triple cranks
Compact or standard refer to the size of the chainrings (compact having less teeth on the rings giving you easier gearing)
Double or triple refers to bikes that have either 2 or 3 chainrings on the front cranks (triple gives you a wide range usually used for touring or MTB)
Hope that helps,
Martin
Edited.
SRAM also has a 28.
I think I read somewhere recently that the new DA 7900 is going to offering a 12x28 - a bit out of range for this OP.
Ian