One awkward element seems to have been lost in the collective clamor over the advent of disc brakes for road bikes: the post-mount calipers dominating the market are still essentially a mountain-bike platform. Ever the innovator, Shimano was the first to address this oversight with its BR-RS805 Flat Mount Disc Brake Caliper, which adapts the disc brake standard to a design specifically built for road applications. It's another standard, sure, but it makes sense for bikes with skinny tires — so much so that other manufacturers are already scrambling to mimic it.
The big change with flat-mount is that the calipers bolt from the bottom up. The front caliper requires an adapter, but the rear caliper attaches directly to a flat-mount frame's chainstay. First impressions tell us that it's a sleeker, tidier design, which serves to both clean up the rear triangle and offer some claimed aerodynamic benefits. While we haven't verified the claim of less drag, it certainly stands to reason that fewer wind-catching, parachute-like structures would equate to a smaller footprint in the wind.
Other than the alternate mounting scheme, the flat-mount BR-RS805 is virtually the same as the RS685 caliper model. This starts with Shimano's one-way bleed process, which makes for consistent braking, regardless of conditions and circumstances. There's no waiting breathlessly for the pads to clear the rim of water or debris before grabbing. There's also no panic when you're caught in the drops and confronted with a sudden obstacle like a darting squirrel or a car in a blind driveway — the brakes will still stop — even if you don't have time to get full purchase on the lever.
When combined with Shimano's finned metal pad and Ice Tech rotors, we're comfortable running these calipers with 140mm discs; however, if you're local terrain is all twisty descents and alpine passes, you may want to consider adapters to bump up to 160.