|Those two threaded holes? That's an early attempt at a disc brake mount.|
Shimano had working disc brakes for bicycles back during the bike boom of the 70's. While the calipers were heavy and clunky, and the rotors were solid steel, they did work. There were concerns , of course, and since forks of the day were spindly and not very strong, disc brakes on bicycles then were typically only mounted on rear wheels. Ironically, Shimano chose to thread on the rotor to the hub, much like a freewheel, and if you think about it, Centerlock brakes are just a minor variation on that theme.
The whole disc brake thing then "went away" for a time, but during the late 70's, 80's and 90's, mountain bikers started tinkering around with disc brakes and by the late 90's, it was going to happen. Early adopters were within the ranks of the Marin clunker gang who used the Shimano disc brakes of the time despite their tank-ish weight. Later in the 80's there was a few attempts at using disc brakes on down hill mountain bikes. The 90's saw more activity with disc brakes, maybe most famously by Mountain Cycle who had an "upside down" fork design which required disc brakes to make it work. Gary Fisher also had a bike with disc brakes at the time, but it was........ahhh.... not very good!
|Most suspension forks now use the 74mm post mount standard developed by Manitou|
Now around about 2014 Shimano unveiled plans to make a new standard called "Flat Mount", which is not unlike what Hayes was trying to do in the late 90's with its 22mm mount. The idea was that the calipers would sit flush to the frame and fork without unsightly adapters or posts sticking off forks and frames. Unfortunately, by the time flat mount came about there were many carbon forks which were using the post mount standard for road applications. To further complicate things, not many custom or high end builders liked the flat mount aesthetic on the rear chain stay, nor were too keen on not using already available IS compatible drop outs which had been refined to look rather good.
Complicating things further is Shimano's lack of interchangeable adapters to fit other types of calipers to flat mount and flat mount calipers to other types of mounts. SRAM has done more in this vein. There are issues with adapters, but one thing flat mount does do is make adapting the caliper to either a 140mm or 160mm rotor an easy affair. You just flip the adapter mount around to use one size or the other. (No 180mm or 203mm rotors can be used with flat mount Shimano calipers.)
|Shimano Flat Mount Disc Brakes|
It also is worth mentioning that along with flat mount disc brakes Shimano also foisted 12mm front through axle for road on us at the same time. This all makes one ask a few hard questions, such as, "What is wrong with 15mm through axles?", (a standard, ironically also foisted on us partially by Shimano), and how about "What is wrong with 74mm post mount, or IS brake mounts we already have?". Don't hold yer breath for any answers to those questions, by the way.
So, in a way, Shimano has brought us full circle back to the flat mount brake, which Hayes proposed in the late 90's. Weird, huh?
The whole disc brake mounting question isn't 100% settled just yet, but it would seem that IS and 74mm post mount will be the realm of off road applications only and this flat mount technology will be the realm of road and gravel. Maybe.......
Then there is the whole through axle thing which is already changing road bike wheels and will, no doubt, upset the apple cart with regard to legacy wheels not working, people wanting certain combinations that won't be possible any longer, and with the brake thing, it will be even worse. Change is often frustrating and painful. This looks to be no different, but we've been through this before with mountain bikes, so welcome to the nightmare......again.