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".