Most crossbars and roof racks give their dynamic weight load, which is the weight they can carry when you're driving (i.e. if your cross bars have a dynamic weight load of 165 lbs (75 kg), you shouldn't put anything heavier than that on your bars when you're driving). Because other forces (wind, braking, bumps, etc.) are at work when you're driving, the static weight load (weight limit when you're parked) is actually much higher than the dynamic weight load.