by Dave Zeckhausen
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.
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.
It is not possible to tell if a drilled rotor is installed properly, simply by observing the drill pattern. Different manufacturers use different patterns with spiral "arms" that appear to lean in either direction, regardless of the internal cooling fin orientation. For example, StopTech AeroRotors, used on their big brake kits, have a drill pattern that appears to lean to the front of the car. Porsche OEM rotors have a drill pattern that appears to lean to the back of the car. In both cases, the internal cooling fins are properly oriented, leaning backwards. But the drill pattern is opposite. This is why Porsche mechanics often install StopTech big brake kit rotors backwards, despite the big "L" and "R" stickers on the rotors! They're relying on the drill pattern, rather than the direction of the internal cooling vanes.
As with slotted rotors, one must check the direction of the internal cooling vanes in order to determine the proper orientation of drilled rotors.