A headset is a shortened term for the headset bearing, which allows the fork (and its connected handlebar) to rotate within the head tube (the short vertical tube at the front of the bicycle frame). Depending on how it's mounted in the frame, there are different types of headsets. The first bicycles had screw-on headsets (so-called classic headsets). The next solution to appear on the market was the "threadless" headset, where the thread on the fork's steerer tube is replaced by a nut driven into the steerer tube (called a star nut). The most popular type of this type of headset is the a-head standard . Other available solutions include integrated (drop-in) and semi-integrated (ZeroStack) headsets. Dividing them by bearing type, we have machined, ball, and needle bearing headsets. There are, of course, two bearings: one mounted in the upper part of the head tube and the other in the lower part. Nowadays, tapered headsets, meaning they have a variable steerer tube diameter, are also available. The lower bearing has a diameter of 1.5", which positively impacts frame stiffness. Headset selection is determined primarily by the frame's size parameters, including the inner diameter of the head tube, the outer diameter of the fork's steerer tube, and the shape of the head tube's interior.
