Why disc rotors are so freakin close to frame ?

[mattr]

Even with standardized parts within a team, the clearances are simply too tight between a rotor and pads to simply rely on the same hub rotor combo, as a slightly undersized spacer, hub shell mounting face, or an out of plane locknut on a QR wheel will throw things into disarray unless you check each and every set.

TBH I suspect that within a team they'll have a set of go/no go gauges for the disc and caliper positions. One to locate into the drop out and caliper and another to drop over the locknut and disc face (doubled up front and rear of course). From that you can actually shim every single disc and caliper exactly the same.

If they aren't doing this, they need their heads looking at.

[stormur]

can't see relation between braking and caliper movement. braking force direction is obvious here, and 1 plane only.

One plane? Go back and look at the forces in three dimensions. Then the geometry of the caliper and mounting faces.

I don't have to go back anywhere. Perhaps you should. It is one plane force. Rotor is moving in one plane. pads are moving in 1 plane. Calipers doesn't move at all. It's not floating system.

[mattr]

How do you think the caliper resists being dragged around the axle once clamped to the disc?
There is a bending moment between the braking face and the mounting face of the frame. The bigger the offset the greater the moment. Until either the frame flexes (squealing brakes, odd wear patterns, relatively rapid fatigue failure) or the caliper moves.
Easiest way to minimise this bending moment is to reduce the offset as far as is physically possible. By putting the disc close to the frame.

As an added bonus you get improved bracing angles for the NDS spokes and more space for whatever system you use to mount the disc.

These moments and reaction forces are another reason for one piece calipers, you strengthen and stiffen a point where the flex would be noticeable and possibly annoying.

[TheRookie]

Just to add to this, my frame won't take Novatec hubs as it puts the disc into the frame, Shimano hubs are fine as the disc is further inboard, so word of warning if you have a tight clearance on Shimano, don't buy a Novatec!
I've also noticed on the other side that the 2nd smallest sprocket on a Novatec hub aligns pretty much dead on with the smallest in a Shimano, of course cartridge bearings take less space than cup and cone and a wider flange spacing is mostly good (except for aerodynamics), but it can cause issues!

[clipsed]

Yes on my Fuji cx frame it actually hit my frame, I had to put a washer in between frame and wheel to push out the frame to get some clearance! It did surprise me, a lot.

[mattr]

New MTB here in the summer, the complete shimano front end wasn't compatible with the RS fork.

Well, until i filed half a mm off the mount and caliper mounting tab.