Directional vs. Straight Vane Rotors

Most "normal" cars come with straight vane rotors, with internal cooling vanes that extend straight from the center of the rotor outward, like spokes on a wagon wheel.  (See drawing on lower right)   These rotors are not as efficient at cooling, but they are less expensive to manufacture and car dealers only have to stock a single part for both sides of the car.

Some higher performance cars come with "directional rotors", which have curved or tilted internal cooling vanes.  These curved vanes pump more air through the rotors, resulting in improved cooling efficiency.  Thus, there is a left and a right rotor.  When the directional rotors are properly installed, these internal vanes should lean toward the back of the car - as shown in the diagram on the lower left.

Slotted rotors

As with the plain rotors discussed above, the internal vanes should lean toward the back of the car.  The direction of the slots on the outside of the rotor do not dictate whether the rotor is a left or a right.  For rotors with slots machined in the same direction as the internal cooling vanes, the slots should lean toward the back of the car.  For rotors with slots machined in the opposite direction as the internal cooling vanes, the slots should lean toward the front of the car.  Bottom line - don't use the slots to decide which side to mount the rotor.  Pay attention to the internal cooling vanes.

For slotted rotors with straight internal cooling vanes, the direction of the slots is totally up to you and what you think looks best.  In this case, there is no wrong answer.