Headset (bicycle part)

The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame. The tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed into the top and bottom of the headtube. Inside the two cups are bearings which provide a low friction contact between the bearing cup and the steerer.

Traditional bicycle head tubes and headsets are sized for a 1-inch-diameter (25 mm) steerer tube (also known as the fork column). Many frame and fork manufacturers are now building their parts around a steerer tube with a diameter of 1⅛ inch. The larger diameter of the head tube and headset gives added stiffness to the steering portion of the bicycle.

Common sizes

There are a few different types of headset distinguishable by the way the bearings are held in place (with a lock nut or with a clamping stem), or by where they are located (inside or outside the head tube).

Threaded headsets are used with forks that have a threaded steerer tube and are the traditional type (as shown in the above picture). There are eight parts in a threaded headset (from bottom to top): crown race, lower bearings, lower frame cup (pictured as "lower head-tube race"), upper frame cup (pictured as "upper head-tube race"), upper bearings, upper race or cone, washer, locknut.

The order of installation of a typical headset follows. The steerer tube is cut to the appropriate length by the bicycle fork manufacturer and the top 1–2 in (2.5–5.1 cm) of the steerer is threaded using a rolling process. This process assures that no material is lost and the steerer would not be weakened as with a die or lathe cutting. Threaded forks necessitate that the threads on the steerer only use the top 1–2 inches, therefore the forks are sold in varying lengths. If there is a need to use a fork that is too long, meaning the fork steerer is not threaded down far enough, a bicycle mechanic can use a die to extend the threads. This is not recommended if the threads need to be cut farther than an inch or so. As a side note, one should never try to thread an unthreaded steerer after manufacturing as this will weaken the steerer tube. The threads are normally of the ISO standard, 1 in by 24 tpi, but other standards exist. The headtube may be faced and then the cups are pressed into the headtube using a special press that ensures they are square and true. The fork crown may be faced and then the crown race is pressed on to the fork crown with a special press that also makes sure that it is square and true. Then the bearings are placed on top of the crown race, after which the steerer tube is inserted into the headtube. The upper bearings are placed in the upper cup, and the upper race is screwed onto the steerer. A washer, frequently a keyed washer, is placed on top of the upper race and a locknut is screwed on top of that.

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Wikipedia