Data is recorded as bits onto the surface of a disk in concentric rings called tracks by a read/write head (see hard disk for a description of heads and tracks). Since the head is essentially a single point, it can be said to move "linearly" along the track (even though the track is a circle) and therefore to have a linear velocity as it passes above the disk surface.
If the disk is always rotated at a constant rotation rate, the disk has a constant angular velocity (CAV). Since disks are solid and the circumference of each track varies (depending upon its radius from the center of the disk) the linear velocity must also vary as the head reads different tracks: slower velocity for the innermost tracks, faster for the outermost.
This variation in velocity is often unacceptable because:
- Bits are stored onto the disk surface at a constant rate. If the disk is rotated with constant angular velocity, the bits are therefore recorded more closely on innermost tracks. A head designed for the innermost track may not read bits from the outermost track efficiently; if designed for the outermost track, it may not be able to read the more closely-spaced bits on the innermost track.
- A head designed to "fly" (see hard disk) above the disk surface on an air bearing may impact the disk surface (crash) on inner tracks (due to the reduced velocity) or fly too far above the surface on outermost tracks.
Some disk drive head designs (particularly hard disk designs) tolerate this difference, compromising to the average track velocity. Other designs use constant linear velocity to resolve this problem. Constant linear velocity is achieved by adjusting the rotational rate in order to keep the apparent linear velocity the same for each track, regardless of its radius. That is, the disk rotates more rapidly when the head is reading the innermost tracks and more slowly for the outermost tracks.