HELP ASAP! (lol)

bit noobish of me but been playing forza horizon 2 on the xbox one, and noticed the telemetry menu. on of the tabs is -Damage- and i see you can damage the

-tyres (tyre wear)
-suspension system (springs & shocks)
-body (chassis & frame)

BUT! whats the wee brake disk looking thing directly behind the wheel??? i thought at first it was either brakes or hubs but there currently 100% damaged and theres no differnce in braking or power to the wheels, they spin fine and everything! like im a mechanic and have no clue its probably obvious lol

thanks anyway