On a bicycle, this is the tube attached to the crown of the fork and passing through the head tube of the frame. The handlebar, which is used to control the bike, is attached to it via the stem.

To allow the steerer tube to turn smoothly through the bike frame, there are bearings at each end of the head tube. This set of bearings, along with the associated cups and cones, is called the headset.

On a newer mountain bike, the diameter of the steerer tube will be 1 1/8".