Compact Crank?

snosaw said:

What is a "compact" crank? I assume that things have changed since I last rode in the late 80's.

Compact cranks have smaller chainrings which give you lower gears and a wider spread in gears. IOW instead of a typical 53/39 chainring combo on a road bike(yeah, it's changed a lot from the 52/42 stuff we used to ride) you get something like a 50/34 set of rings. You couple that to a freewheel with an 11 tooth cog and you still get a nice big gear for descending but you end up with a lower low end assuming something like 25 or 27 teeth on your big rear cog. So it gives you a bit more spread in your gears without resorting to a really wide rear cluster and a long cage rear derailleur or a triple crank. Most compact cranksets require you to change your front derailleur to deal with the smaller rings.

-Dave

snosaw said:

What is a "compact" crank? I assume that things have changed since I last rode in the late 80's.

The parameter that permits the smaller chainrings on a compact is the smaller BCD or bolt-circle diameter. Conventional road cranksets use a 130mm (most) or 135mm (campagnolo) whereas "compact" uses 110mm. 110 mm is not a new concept. MTB cranksets in the 80's and early 90s typically used a 110mm BCD for the outer two rings, but now MTB usually use a 94mm on the outer rings.

The smallest chainring that can be used with a 130mm BCD is typically 38T whereas a 110mm can easily accomodate a 34T--therefore allowing a much lower gear than a conventional double. There are numerous threads on this forum discussing the advantages and disadvantages of compacts vs triple for low end gearing and for some the discussion borders on religious fervor.

daveryanwyoming said:

Most compact cranksets require you to change your front derailleur to deal with the smaller rings.

-Dave

I am running FSA SL-K compact (50/34) with a standard Campy Chorus braze-on front derailleur and it works great.

See also: http://www.cyclingforums.com/showthread.php?p=3228386

mwestray said:

I am running FSA SL-K compact (50/34) with a standard Campy Chorus braze-on front derailleur and it works great.

+1 Compacts do not need a special derailleur providing you have sufficient adjustment to lower it far enough (about 6mm). I'm running Shimano compact (50/34) with a standard Ultegra front derailleur. Using Shimano's standard setup clearances, it just needed to be lowered to mate with the smaller 50 tooth "big" chainring.