* Source: forums.bimmerforums.com

Installation of a pair or full set of performance brake rotors is pretty simple if you understand the science behind how they function.  Installing a set of brake rotors with a curved set of cooling vanes is a bit different than installing a set with straight vanes, so this is something to be aware of for each unique install.

Other than determining brake rotor orientation, based upon design variance, basic quality control must be performed to ensure proper quantitative variances are kept.  Just as you would check over rotor tolerance numbers when changing out brake pads, new rotors should be scrutinized as well before install.  Excessive run-out and/or surface defects are what the installer is trying to identify, and each individual manufacturer produces literature to help with this identification process.

Back to brake rotor vane design...

Most stock brake rotors use a straight vane rotor design, whether columnar or staggered because these rotor designs are economical to produce.  Cooling may not be superior to a curved vane design but can do an adequate job given a non-abusive braking style.  When installing this variety of brake rotor, there isn’t any way to get it wrong; orientation does not matter.

Curved or "directional" vane rotor designs, on the other hand, might be orientation sensitive; they don't call them ‘directional’ for nothing.  A curved vane brake rotor made for the right side of the vehicle must be installed on the right side of the vehicle, it’s as simple as that.

Many people make the mistake of installing a right wheel designated brake rotor on to the left side of the vehicle and vice-versa because they do not understand how these curved vane rotors work in theory.  They think that these brake rotor vanes are supposed to act like air scoops to cool the brake rotor off, and this certainly is not the case.  These curved vanes are actually used to increase air circulation through the use of centrifugal force and do so by creating a vacuum inside the brake rotor plate sandwich.  Because of this naturally created vacuum, air is constantly being pulled past the brake rotor in question and cools the brake rotor as a result, through convection. 

*Source:  blog.blackdogspeedshop.com

You could actually think of a curved brake rotor design to work much like a fan or turbine in function.  Because of this fact, if you were to install a curved brake rotor on backward with a rotor made for the opposite side of the vehicle, then the curved vanes would cease to function as designed.  Actually, purchasing a set of curved vane rotors and installing them on backward is very much counterproductive, so it is important to that the person installing these rotors are aware of these details.

Drilled and Slotted Curved Vane Rotor Design Installs

Inexperienced rotor installers often make the mistake of improperly installing curved vane rotors with drilled designs, because the drill sites actually run in a backward or opposite direction as the brake rotor vanes in question. 

Not all rotors have drill sites that run in the opposite direction of the curved vanes either, so always check out rotor vane direction for yourself before making any assumptions.  Slotted rotors often run opposite to the direction of the rotor vanes as well, so employ the same determination strategy as listed for the drilled out variety before installing any slotted-curved vane rotor types.