It is a popular misconception that the slots or drillings in a disc determinate the direction of rotation. In truth, for an internally vented disc, the geometry of the vanes dictates the direction of rotation.

There are 3 different ventilation types in use:
· Straight vanes
· Curved vanes
· Pillars

The straight vanes and the pillars are non-directional, and can be used on either side of the vehicle. The curved vane disc, however, is directional. A curved vane disc must be installed with the vanes running back from the inside to the outside diameters in the direction of rotation. Orienting the disc in this manner, creates a centrifugal pump. The rotation of the disc causes air to be pumped from the center of the disc, through the vanes, and out through the outside diameter of the disc. This enhances the disc’s ability to dissipate heat. On some OEM applications, this standard is not satisfied: the use of directional discs means to manage two codes (left and right) per axle with some extra cost therefore it happens that a car manufacturer can use the same disc with curved vanes on both sides, losing part of the benefit in balance with lower costs.

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