Diameter and Radius
The diameter, or disc size is the measurement across the face of the disc.
The radius is the measurement from the centre of the disc to the outside.
This is the distance between the front and back faces of the rotor.
Flange inner diameter
This is the size of the hole through the middle of the disc, on the rear side.
This is the size of the hole through the middle of the entire disc.
Pad swept area
This is the depth of the disc face which can touch the pad.
This can be measured by the depth of the bell mounting holes.
This is the distance between opposite holes where the rotor bolts to the bell.
This measurement is how deep the bolt face of the rotor sits, compared to the front face of the disc.
If the bolt face sits higher, it is a positive step. If the bolt face sits recessed, it is a negative step.