As of 2013, Shimano and SRAM have introduced cranks where the chainring bolts lie on a circle but are unevenly spaced.

Here is an 2013 SRAM XX1 Crank and a chainring with an uneven bolt circle [SR13] :

     

Here are the Shimano 2013 FC-9000 "Dura-Ace" and 2014 FC-6800 Ultegra [Sh13] cranks:

     

Why

There's at least two possible engineering reasons for doing this:

• The chainring bolts more easily clear the crank. This is possibly an issue for the SRAM crank, but not an issue for the Shimano cranks.

• The chain and sprocket loads are highest at 3 o'clock and 9 o'clock positions, so putting the bolts closer to those positions offers better chainring support. "Blurbs" on Shimano cranks often list this as a reason.

If bolt access is a problem, there's alternatives. A spider with threaded holes is used commonly and widely for the smallest "granny" ring on triple cranks. Note the SRAM XX1 is designed for one-ring-only operation, so there is no problem using designs that allow mounting only one chainring.

So it seems unlikely that bolt access is important for either crank.

The Shimano crank is about 25 grams lighter than the predecessor. The predecessor had 5 arms rather than 4, so most of the drop is probably due to simply removing one of the arms. Still, it is possible a 4-arm design with even spacing would add 5 grams.

And, after all, why carry around 5 grams if you don't need to?

Why Not

Durability

A conventional chainring can be rotated, which gives about a 50% improvement in chainring life. Exactly because crank loads are highest at 3 o'clock and 9 o'clock, wear is fastest there, and rotating the chainring 90 degrees lets you switch the worn and unworn parts. You don't get 2x the life because (say) 1 o'clock and 2 o'clock get worn some in the first position, so are already worn some when you switch to the second position.

Of course, with an uneven bolt pattern, you cannot rotate the ring. Your choice: 5 g or 50%. I buy my own parts, and few of my friends are sponsored riders. Folks often pay lots of "extra" dollars and grams to get more-durable parts.

50% longer chainring life for no extra dollars and 5 extra grams seems like a big win for most riders!

Availability

There's also the issue "can you get chainrings, and how much do they cost?

Standard 4-bolt chainring patterns are "everywhere". These new uneven patterns are "nowhere". Bash a chainring, or somebody bashes it for you? Now you need to wait a week without riding your bike, while you wait to get a replacement! Wear one out? You may be able to make do with the old one for a week until the new one shows up... but you may also be paying 5 times as much as a standard ring [PP13].

Standards

Sometimes you have to break with standards. If we had stayed with standards always and forever, we would today be riding iron-tired boneshakers. Or, maybe, we would live in the ocean and breathe through gills.

But when considering a change, you want to consider cost and benefit before you switch. If these "drunken" bolt patterns saved 5 grams (or maybe 10!) and had no downsides, I would say sure: let's all switch. It will cost some moeny and take a few years, but we will all get there eventually and all get to save 5 grams.

But the "no downsides" bit seems to be missing. They want me to pay more for the next decade and lose 50% chainring durability? If sponsored professional racers want to go for it, great. But for ordinary riders, paying more to lose 5 grams and 50% durability is a bad deal.

Of course, there's a long history in every industry of "let's rip off our best customers to increase our short-term profits!". But that doesn't mean we have to play along every time.

Specific Cranks

SRAM XX1

Derailleur bikes have ramps and pins at specific locations to improve front shifting, and if you rotate the ring you give that up.

For the SRAM XX1, there is only one chainring, so there's no front shifting, and there's no ramps and pins anyway. Further, there's only one ring and it is pretty small, and it is for an "off road" group to be ridden in gritty dust and mud — so it will wear faster than if you have three chairnings.

5 grams or 50%?

SRAM puts a lot of "majik" in the chainring, so it is not entirely fair to compare it against a standard ring... but it is still educational. As of 2013/07, the discount price of SRAM XX1 chainrings is around US$100 (US$90-130). A brand-name (not generic) conventional-pattern 34T ring from the same discount retailer is... US$15. The hugely-more-expensive 42T version is a whopping US$18! [PP13].

And, being an "elite" part, good luck finding a same-day replacement if you smash it. Which you might do, if you were using it. Hey, if you never use it, it won't wear out!

Shimano FC-6800 and FC-9000

These cranks are often used with a front derailleur, and derailleur bikes have ramps and pins at specific locations to improve front shifting. If you rotate the ring you give that up.

Maybe. Lots of riders I know are happy to "pay" for longer part life with sluggish shifts. Worn cassette and sluggish shifts? Keep riding! Worn derailleur and sluggish shifts? Keep riding!

But, okay, a bunch of folks who buy these are amateur racers who care about fast and predictable shifting and will not "rotate for wear". Arguably, Shimano should have two high-end lines: weight, and durability. If you care most about racing you will get the Dura-Ace. If you care about durability you will get the Ulteg... oops.

It also seems weight cannot truly be the driving factor for the Ultegra FC-6800 crank. It has a stress-riser "vanity ridge" reaching from nearly the pedal eye to center. Or, perhaps, from nearly the pedal eye to hospital. I have not done structural analysis or durability testing on it, so I may be wrong, but it appears either the vanity ridge makes the cranks failure-prone, or the vanity ridge means the cranks pay a weight penalty compared to a smooth-faced crank.

(For more, see some "vanity groove" failures such as HERE, HERE, HERE, HERE, HERE, HERE, HERE, HERE, HERE, HERE, HERE, and HERE. Note that anything other than a flat outer face acts a stress riser for loads near the bottom of the pedal stroke, and especially where the outer face is narrow. As examples, consider failures such as HERE, HERE, HERE, HERE, HERE, and HERE. The FC-6800 crank is especially notable because the ridge is very narrow, thus requiring substantial over-build compared to a flat face to resist bottom-of-stroke loads.)

In a little more detail, when you bend something, the "skin" on one side has increased tension, on the other side has increased compression. In a crank near the bottom of the stroke, the "outboard" face is under more tension, the "inboard" face is under compression. In the following (from HERE as of 2013/07), red is "tension", blue is "compression":

  

The problem with a "vanity groove" is the groove carries very little load. Here is an over-simplification: imagine milling all the way through the crank to make a slot instead of just a groove. If the groove is 1/3 the width of the crank, then the 2/3 which is left gets 3/2 the stress compared to "no groove". Make the groove 1/2 the width, the parts left get 2/1 or twice the stress.

Well a "vanity ridge" is the same: mill away 1/3 the material and leave a ridge 2/3 the width, width, it gets 3/2 the stress. Mill away 9/10 the material leaving a ridge 1/10 the width, it gets 10/1 the stress!

For an actual groove or ridge (not a slot all the way through the crank), the above is a drastic over-simplification, but a groove or ridge is often just asking for trouble unless extra material — that is, weight! — is added to compensate.

In other words, it is hard to take seriously a claim "we switched to a customer-hostile bolt pattern in order to save weight!" when they also use a heavier-than-needed arm, especially one that presumably costs more to make than a more-durable and lighter arm. The FC-9000 has a similar problem (narrowed load-carrying section) but it appears to be much less severe and is probably an issue only for very long-distance riders.

Oh, and the FC-6800 and FC-9000 asymmetrical bolt pattern is a 110 mm circle (BCD or "bolt circle diameter"). Which violates an unwritten but often-followed rule, that there should be only one bolt pattern in any given BCD. "I'd like a 110 mm chainring" used to work pretty well. Not perfectly — is that 5s, 6s, 7s, 8s, 9s, 10s, or 11s? But pretty well. Now you can get Shimano 110 mm chainrings that won't even fit on other Shimano 110 mm cranks. And, since Shimano is a big maker, unless consumers see the new crank as entirely laughable, it is likely that at least for a few years there will also be aftermarket incompatible chainrings offered by other makers.

At higher prices and worse availability.

Related

Related is putting the holes at equal angles, but each arm is a different length.

References

[PP13] Price Point "Where price is the point". Standard-pattern Race Face single-speed chainring 34T US$14.98, 42T US$17.98 (as of 2013/07); SRAM XX1 X-Sync chainring 28T US$91.98, 38T US$128.98 (as of 2013/07).

[Sh13] Shimano FC-6800 crank. http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/road/ultegra/product.-code-FC-6800.-type-.fc_road.html as of 2013/07.

[SR13] "SRAM XX1 Crankset" http://www.sram.com/sram/mountain/products/sram-xx1-crankset as of 2013/07.