Rear rotors groove on top. Looks Like factory design.

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2023 CX5 Turbo
I just checked my rear brakes at 56K miles. The pads still have over 33% left. I did look at the rear rotors too and there is a groove or split in about a quarter of the outside edge of the rotor. At first I thought it was a defect however both rear rotors had the same exact groove. This would be on the top looking down. Anyone know if what purpose they serve? The rear rotors are original. Rotor1.jpgrotor 2.jpg
 
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