[Text and index last updated December 2012.]
I do a lot of long rides on Alpine roads (I live in Switzerland), frequently doing multiple passes in one day. The only way to achieve this comfortably (given that I'm not a pro' rider) is to have a decent low gear - grinding up the hill in an unnecessarily high gear seems more tiring / less efficient and can hurt your knees. The 33 tooth minimum chainring size on road compact cranksets (110 mm BCD) is not low enough to do my kind of riding (multiple bouts of several kms at >7% gradient) at a decent cadence (>80 rpm). I could use a compact crankset with a mountain-bike cassette, but I don't like that option due to the large difference in tooth counts between adjacent cogs. Hopefully this has negated all of the "tiples are stupid / unnecessary / for wimps" comments.
First, I'll present an index of all two-piece road triple cranks that I'm aware of, then make some notes about some aspects of this list and compare the available cranksets and give my opinion on which are the best in which situations. The purpose of this is to make comparisons and choices between the options a lot simpler, and it may result in people telling me about options that I'm not currently aware of.
The index below contains all cranksets that I am aware of that have the following three properties:
(1) An inner BCD (bolt circle diameter) of no more than 94mm (so that the minimum inner ring size is no larger than 30 teeth),
(2) A chainline appropriate for a road bike, and
(3) A 2-piece / integrated axle design.
Click the image below to download the attached pdf. I've tried to make all of the information as accurate as possible, but there are still a few missing data points. I'm happy to receive any input regarding additions or corrections.
Links to manufacturers' websites:
Campagnolo, Driveline, FSA, Lightning, Samox (Chuan Wei), Shimano, Specialized, Stronglight, Sugino, Truvativ, Zinn.
Nearly all of these cranks come stock with a 30 tooth inner ring. With the 74 mm inner BCD that most of them have, chainrings down to 24 teeth are possible. These are quite easily and cheaply obtainable for about US$20, and changing the inner chainring is not difficult. Using a ring smaller than 30 teeth may remove the ability to use the small-small gear combinations even more than usual, due to the chain rubbing on the middle ring and the rear derailleur not have enough capacity to take up the slack in the chain, but it works fine if you avoid those gear combinations, which you should be doing regardless of the size of the inner ring. My favorite size of inner ring is 26 teeth - if you're going to have some low gears, then they may as well be low enough for all situations. When installing such a small ring, it is a good idea to also add a chain retention device so that the chain doesn't drop off the inside of the inner ring when you shift down to it, such options include a Third Eye Chain Watcher, a Deda Dog Fang, an N-Gear JumpStop, or a Jtek DropStop (the N-Gear is my favorite because it is the most solid).
A tripleizer middle chainring can be used in place of a normal inner ring on many double cranksets (both 130 mm and 110 mm BCD versions are available), which then allows a third, inner ring to be mounted (normally 74 mm BCD). However, with a two-piece crankset, the chainline would again be very wrong, and the inner ring may not even clear a lot of frames. Tripleizers are therefore only really options for 3-piece cranksets where a bottom bracket of the appropriate width can be used to achieve the appropriate chainline.
The Ultegra 6703, Dura Ace 7803 (plus the older, 3-piece 7703), and the Zinntegrated cranks use a standard 130 mm spider and have a tripleizer middle ring with a special BCD (92 mm) to mount the inner chainring. To obtain the correct chainline, the axle length is longer than for the equivalent double crank, and there are a couple of extra spacers. Unfortunately, the only available inner ring for these cranks is Shimano's 30-tooth model; if you want to go smaller then you'll also have to change the middle ring to a tripleizer that has a 74 mm BCD for the inner.
Nearly all of the cranksets currently available that fit my three criteria are triples. There are now two exceptions to this. The Sugino "Compact Plus" or "Compact+", which has a BCD for the outer chainring of 110mm, but the inner chainring is either a 110mm or 74mm BCD ring; one of the available versions is a 46/30 setup which is very useful. See this thread for more details. Also, the Lightning crank can take a 94mm BCD spider, allowing you to use down to a 29-toth inner ring (and the same spider can also be mounted on a Speciliazed crank). This type of double chainring combination is often called a super compact, and has similarities to the "half-step-plus-granny" setup that was common when cassettes/freewheels only had 5 to 7 cogs (the half-step gearing is no longer needed when using a cassette with 9 to 11 cogs). Super compact cranksets can be shifted with road front derailleurs designed for standard or compact doubles (it tends to work fine even though the chainring sizes exceed the recommended guidelines) and integrated shifters designed for doubles (thereby allowing the use of SRAM Double-Tap levers or Shimano Di2 equipment with very low gearing). Having a frame with a braze-on mount for a front derailleur can cause problems in getting the front derailleur low enough on this kind of setup, but aside from that there shouldn't be any compatibility issues. Such setups can also be created by using only the inner and middle positions of a crankset designed to have three rings.
I always recommend Shimano cranksets: they have the best-shifting chainrings and are a competitive weight for a decent price. The Shimano 105, 5703 is my current favorite because it is the only current Shimano crankset that has the proper mounting posts for the inner chainring (which the Ultegra 6703 does not have) and has hollow crank-arms (to make it reasonably light).
The best lightweight triple crankset that has the proper mounting posts for the inner chainring is probably the FSA SL-K. The Lightning and the Stronglight X-Wing cranksets are even lighter, but are more expensive and are not as readily available as the FSA. However, I've found that FSA chainrings don't shift as well as Shimano's.
If you want extra-long cranks, then the Stronglight Z-Light is the only stock crankset that is listed as going up to 180mm, but someone responding below reports that the FSA Gossamer may be available in a 180mm version. The custom Lightning and Zinn-tegrated cranks go longer but are probably outside of most people's budgets. The larger manufacturers list several stock models as being available in 165 mm sizes, but often those lengths are difficult to find.
Campagnolo makes the only triple crank that is officially compatible with an 11-speed chain. However, most of the other options that are listed as 10-speed compatible will probably work fine with an 11-speed chain (or with a 9-speed chain) - the differences in the spacing between the rings is minute.
The Truvativ cranks (made by SRAM) are sometimes re-branded as Bontrager. These have the worst-shifting chainrings that I've tried.
It's very hard to find much information about the Samox and Driveline cranksets. There are websites showing the models and I've seen one Samox crank at their booth at Eurobike, but they don't appear to be for sale anywhere and I cannot find the complete spec's for them. Samox and Driveline may be the same cranks, just re-branded, but again I'm not sure about this.
I've been told that Stronglight are discontinuing their 2-piece cranksets, but they are still listed on the company's website, so I'm not sure what is happening there.
There are still lots of 3-piece cranksets available with road triple chainrings. My criteria of having a two-piece / integrated axle crankset makes this list much shorter. If you are happy with a 3-piece crankset, in addition to the road triples then even on a road bike you can also use most cranksets that are designed for mountain bikes by using a bottom bracket with the correct width, but you may still end up with a wider than necessary Q-factor / tread. It's not ideal to use a two-piece crankset designed for a mountain bike on a road bike because the chainline will be too large.