Flat Mount
Flat Mount is the road and gravel brake caliper mount, created by Shimano in 2016. It's more compact than Post Mount: the caliper is held by bolts 34 mm apart (M5 thread), and on the rear brake the bolts pass through the frame from below. It looks cleaner, lighter and integrated on road frames. In exchange, it takes smaller rotors (typically up to 160, sometimes 180 mm with an adapter). It isn't compatible with Post Mount without an adapter.
If your bike is road or gravel with disc brakes, it's almost certainly Flat Mount: the clean mount that tucked the caliper into the frame.
What it is and why it took over
Instead of two protruding posts, the caliper bolts almost flush to the frame/fork. The front uses a reversible adapter plate (140/160); the rear M5 bolts enter from below the chainstay and thread into the caliper, requiring the exact length. It's lighter and more aero, ideal for road, but limits rotor size.
Key data
What it fits
It fits Flat Mount road/gravel calipers (Shimano, SRAM, Campagnolo) and the correct-size adapter plate. It won't accept Post Mount calipers except with tall adapters that negate its advantage. The rear bolt must be the right length for your chainstay thickness.
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Frequently asked questions
How far apart are the Flat Mount bolts?
34 mm center to center, with M5 thread. That's what distinguishes it from Post Mount (74 mm).
What rear bolt length do I need?
It depends on your frame's chainstay thickness: measure it and add the thread engagement into the caliper (about 5–7 mm). That's why it isn't universal.
Can I fit a 180 mm rotor on Flat Mount?
On the front, usually with an adapter; many rear frames cap at 160 mm. Check your frame/fork maximum.