Why are Rotors not completely round?

[Atlantis Blue Emira]

Silly question, but was cleaning my Emira and noticed the rotors are NOT completely round.

Can someone explain this to me from a physics standpoint (or like I was in 7th grade)?

 

[VL3X]

It's for balance and normal on these. Not sure exactly why some have flat spots on the edge and some don't. Maybe it's specific to certain batches.

 

[Eagle7]

This is an issue for drilled rotors. Unless you remove exactly the same amounts in exactly the same location across from one another, you'll get an imbalance. You can't just randomly start drilling holes around the rotor. This was one of the things I liked about RacingBrake's patented rotors. They put the holes in the slots, in a very specific pattern so the rotor is balanced. Here's a couple of shots I took when I got a set for my Camaro.

They're located in Fullerton, California. If anybody is near there, and is willing to visit them, I'll contact Warren the owner (if he's still there), or his son and set up a meeting. I would love to see them make a set of these for the Emira, but they need a car there to look at and measure.

 

[slotus]

As stated above that is how they are balanced. Instead of adding weights like a wheel, metal is ground off.