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Disc brakes have now become mainstream in the world of bikes. They provide smooth, powerful braking in a wide variety of conditions. They have also added some extra complexity. There have been a few different mounting options through the years as disc brakes have evolved, now there are two major mounts. Here's a basic run down on disc brake mounting.
The two major mounting standards right now are:
51mm International Standard (or "I.S.") - A disc caliper is attached to the frame/fork with two bolts that are 51mm apart (center to center). The bolts aim at the wheel.
A flaw in this design is that the distance from the center of the disc (the hub axle) to the caliper mount is different on the front and back brakes - meaning you need to have two unique calipers or adapters for the front and rear brakes.
 | | Rear 51mm IS Mount |
 | | Front 51mm IS Mount |
 | | Direct Fit 51mm Caliper |
74mm Post Mount - A disc caliper is attached to the frame/fork with two bolts that are 74mm apart (center to center). The bolts are screwed directly down into the frame/fork - the bolts are parallel to the plane of the wheel.
The beauty of this design is that the center of the rotor to the mounting surface of the brake posts is equidistant for the front and back - meaning as long as both post mounts are designed for the same size rotor, for instance 160mm rotors, you would be able to mount two of the exact same calipers onto your frame and fork - NO ADAPTERS!! But the bike industry isn't completely there yet...
 | | Rear 74mm Post Mount |
 | | Front 74mm Post Mount |
 | | Direct Fit 74mm Caliper |
A short history of disc brakes... Around 2000-'01, disk brake mounting was a real mess. There was 22mm Hayes Direct Mount (used on Trek and Gary Fisher bikes of the era, plus others that I'm sure I don't know of), a 68mm Manitou Post Mount, along with this new-fangled 51mm IS and 74 Post Mount (and others that I'm sure I have just tried to forget). There was about about a six year period that the 51mm IS mount was the dominant one on frames and forks - Manitou being the major exception, they stuck to their guns on the 74mm Post Mount. So some calipers were made to directly mount to a 51mm-IS, some calipers were made to directly mount to 74mm-Post. That's where the adapters came in, to make one work with another - or to lift the caliper up for a larger rotor size.
Currently... Many fork manufacturers are switching to or have completely switched over to post mount, and now some frames are doing the same. Again, the potential advantage to post mount is that you use exactly the same caliper for the front and back, no need to worry about adapters.
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; "Rear 51mm IS Mount")
; "Front 51mm IS Mount")
; "Rear 74mm Post Mount")
; "Front 74mm Post Mount")
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