Effective brake Rotor Radius

Hello,

I had a question about calculating the effective brake rotor radius, which is used for lockup calculations:

1. I am confused about the exact placement of the calliper parts on the brake disc. How do I visualise where the pistons, brake pad etc. are positioned on the rotor when the calliper is mounted.

2. I read online that the effective brake rotor radius is defined as the distance between the centre of the rotation and the centreline of the calliper pistons when assembled to the fixture. How do I calculate that?

Thank you!

Examine a used brake rotor. You can determine where the pads are wearing on the rotor. Typically the caliper pistons will be centered on the pads.

So if the Outside Diameter of the rotor is say 12", then it has a radius of 6" (Diameter / 2). If the brake pad is 2" wide (so the width of the worn area is 2"), then the inside diameter would be 8" (12" - 2x 2"), or a radius of 4" -- the average radius is (6"+4")/2 = 5" so this would be your effective radius.

Similar math would be (rotor diameter) - (pad width) / 2 or (12" - 2") / 2 = 5"

Just adding to what David said...

The outer edge of the pad should be more or less in line with the outside of the disc.

Centre of pressure will be in the centre of the pad - assuming it's perfectly stiff - which it's not but close enough

So effective radius = disc radius - (pad radius/2)

Of course, the real value can be off by a few mm, but it shouldn't make any practical difference