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In overheard in a post ride discussion this morning about wanting a set of compact cranks.
My search on bicycles.se has only found this question with them in the title, but the answers don't mention exactly what they are.

Are they similar to the old biopace cranks?

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There are 3 size aspects to a crankset (and, of course, lots of variations between product lines as far as materials, color, price, etc.).

  1. The length of the crankarms, usually around 170mm +/- 2.5mm increments--these are the same with both normal and compact cranksets
  2. The diameter of the bolt circle to attach the chainrings--compact are 110mm, regular are 130mm
  3. The number of teeth on the chainrings (and this is the whole reason for a compact crankset, to allow smaller chainrings). Most compact cranksets are sold with 34-tooth and 50-tooth rings (referred to as 50x34), whereas most regular cranksets are currently sold with 53x39 combinations. You can also purchase other chainrings, but those are by far the most common.
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  • You can see how old I am from the fact that I remembered 52x42 as the typical regular chainrings!
    – Duncan McGregor
    Jan 24, 2011 at 8:45
  • 52x42: yep, my first...3? bikes (all '10-speeds') had 52x42 cranksets, with 14x28 5-sprocket freewheels on the back. As I recall, 6-speed rears had just come out, but required a wider frame to accomodate the extra sprocket back there.
    – Jay
    Mar 8, 2011 at 6:03
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They probably weren't talking about the cranks per-se, but the whole crankset, which is the cranks and the chainrings.

Compacts are cranksets with smaller chainrings (typically 50 teeth and 34 teeth) as opposed to standard cranksets which have say 52:42. Combined with wide-range cassette this can give a good range of gears without having to have a triple-ring.

Biopace were oval rings - you can get non-round chainrings for either compact or regular cranksets.

Note that the cranks themselves do come in different lengths to account for different leg-lengths, but this is independent of the chainring diameter.

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    Indeed, compacts are, more-or-less, a triple without the middle ring.
    – Unsliced
    Jan 19, 2011 at 15:23
  • Mmmm...no. A standard is a 53/39, usually. A triple is a 52/42/30. A compact is a 50/34 (usually). Each has its use, but they offer each very different gearing and use cases.
    – zenbike
    Nov 6, 2012 at 3:18
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It's a crankset that allows the use of smaller chainrings e.g 110mm rather than the more common 130 mm.

http://www.sheldonbrown.com/gloss_cn-z.html

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Technically speaking, a compact crank is one with a smaller bolt circle diameter (110mm) than the standard road crankset, which has a 130mm or 135mm BCD. This allows a crankset to have an inside chainring smaller than 39 teeth, which is the smallest that a standard will fit before the teeth hit the chainring bolts.

It allows a better climbing range, and when combined with an 11 tooth rear cog it is actually faster than the 53/12 combination standard on a "standard double" crank.

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