It's just been reported SRAM is coming out with a 1×11 road system. This is hardly unexpected: they've already done this for mountain bikes and cyclocross.
Single chainring road? Are you crazy?
Hardly. First, I have several friends who like riding around the San Francisco Bay area on not just single front, but single rear as well. Fine-tuned gear ratios are somewhat over-rated.
But in a race situation, this wouldn't do. Only 11 cogs? That compares to, for example, 20 gears on a 2×10, right? The jumps will be almost twice as big (10 gaps compared to 19).
But this isn't how it works. On a front derailleur system, you tend to shift the front, select gears from the option for that chainring. You only shift the front to shift the range of available gears, not to fine-tune a gear. Using the front to fine-tune a gear ratio used to happen, way back when when people ran 5 or 6 tooth cogs in the rear, and then mostly for bike touring where taking the time for an extra front shift wasn't a big deal. But in racing it's mostly been shift the front only when necessary.
Given a typical 10% jump between cassettes, and a 35% jump between chainrings, the front chainring adds only around 3 effective gears. So a 1×11 is similar to a 2×8.
Consider the following two systems:
There's a great on-line gear calculator which allows these sorts of comparisons. Click here to see the result, which I reproduce here:
There's a 13.3% jump from 15-17 for the 1×11, which is slightly uncomfortable, compared to a slightly smaller 12.5% jump from 16-18 for the 2×8, but otherwise the two systems are very comparable. And this gear range, while hardly relaxed, is certainly enough to cover the vast majority of road races I've done, and most of the hill climbs.
The advantages of the system are weight and ease of use. You lost the weight of the front shifter mechanism, cable, housing, and front derailleur. That's all nice. But the big win is never worrying about a front shift. They take time, they're error-prone, and more complicated. The only downside is chain angle on the extreme gears: it's not as bad as cross-chained gears on a 2×10 or 2×11, but not that much better.
The effect on cross-chaining on drivetrain resistance has historically been overstated, however: measurements show the effect is relatively small except when things start rubbing. So this may be an acceptable price, especially if the extreme gears are used only rarely.
So what would I run? I'd use the 11-28 as described, then use a 48 chainring for "road races" (which I don't do much of any more), a 44 chainring for more double century type events and rides with steeper climbs, and a 36 chainring for steep hill climbs. Swapping rings shouldn't be bad since there's no need to adjust the front derailleur since there would be no front derailleur. The downside might be the need to change chain length, depending on rear derailleur capacity. Note the rear derailleur on a single-ring SRAM system doesn't work the same as on a multi-ring system: no need for dual-parallelogram since the derailleur never needs to accommodate differences in effective chain length due to front shifts. But I suspect it should still be possible to do chainring swaps without chain length adjustments.
Bicycle Quarterly volume 13 number 1 has a nice review of the SRAM 1×11 cyclocross system. The article notes there that the 11-28 cassette had a 19-22 jump (16%) (SRAM page). I'd rather go 15-17-19-21-23 and get the 13% jump from 15-17, I think, with the single ring. With a double chainring if you're looking for a small-20 or small-21 you are probably on a relatively steep hill where the grade changes often enough that being off from optimal isn't that big a deal. On a single-front system, however, this cog range is more likely to come up during riding on a steady grade. Fortunately Shimano cassettes provide this gear combination.
Unfortunately we're down to only one ProTour team on SRAM (it's more popular with women's teams). But I guarantee we'll see this in action at Paris-Roubaix. It's just too good for that race.