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

Headsets come in several different diameters, such as 1", 1 1/4", 1 1/8", and 1 1/2" (often referred to as OnePointFive), so a frame's head tube, a fork's steer tube, and the headset's diameter all have to be compatible.
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Traditional bicycle head tubes and headsets are sized for a one inch diameter steerer tube. Many frame and fork manufacturers are now building their parts around a steerer tube with a diameter of 1 1/8 inch. The larger diameter of the head tube and headset gives added stiffness to the steering portion of the bicycle. Many FR and DH bikes are now using the 1 1/2 inch format (or OnePointFive) because of the increased handling characteristics it has when faced with technical or demanding terrain.


Threaded headsets
Threaded headsets are used with threaded steerers and are the traditional headset. There are 8 parts in a threaded headset (from top to bottom):

top cap
lock washer
upper cone
upper bearings
upper frame cup
lower frame cup
lower bearings
crown race

The bearings may be sealed (deep groove ball bearings or roller bearings), tapered roller bearings, needle bearings, or loose ball bearings (sometimes retained in a cage).

The order of installation of a typical headset follows:

1. The steerer tube is cut to the appropriate length and the top inch of the steerer is threaded using a die (an operation normally done by the fork manufacturer).
2. The threads are normally of the ISO standard, one inch by 24 tpi, but other standards do exist. The cups are pressed into the head tube using a special press, to ensure they are square and true. The crown race is pressed on to the fork crown and the bearings are placed on top of the race, after which the steerer tube is inserted in to the head tube.
3. The upper bearings are placed in the upper cup and the upper cone, which incorporates a cover that is screwed on to the steerer. A locking washer is placed on the steerer tube and the top cap is screwed on top of that.

The adjustment of the headset to remove play is as follows: the upper race or cone is screwed down until it contacts the bearings in the upper cup. A slight preload is applied, 1/8 or 1/4 of a turn of the upper cone. The locknut is then tightened and the headset is checked for play and smooth operation. Readjustment takes place as necessary.

The stem, of the quill variety, is attached to the fork using the expander bolt which fits through the stem from the top with a wedge at the bottom. The stem fits inside the steerer tube and can be adjusted to the correct height without disturbing the headset. To free the stem for adjustment, undo the bolt on the top of the stem a couple of turns and give the bolt a sharp tap to disengage the wedge.

Threadless Headsets
The threadless headset is a more recent design than the threaded headset. Like a traditional headset, it uses two sets of bearings and bearing cups. Unlike a threaded headset, a threadless headset does not have a threaded top headset cup or threaded steerer tube. Instead, the steerer tube extends from the fork all the way through the head tube and is held in place by the stem clamped on top.

Tightening a threadless headset requires tightening the preload bolt in the cap on the top of the stem. This bolt is connected to a star nut driven down into the steerer tube that acts as an anchor. The star nut may be replaced by a self expanding wedge in some designs. The bolt compresses the stem down onto several stacked spacers, usually aluminum, which in turn compresses the headset bearing cups. The preload bolt alone is not sufficient to hold the fork onto the bike; after the preload is set, the stem bolts must be tightened to secure the fork in place. The adjustment must be made such that there is no play in the bearings, but it allows the fork to turn smoothly without binding or friction.

In this system, the spacers are important in providing sufficient compression to the bearing cups. Thus the stack height of the stem becomes important. The steerer tube of the fork must be cut to length such that it leaves enough extension for the stem to clamp on to. Bicycle racers seeking the greatest amount of saddle-to-handlebar drop for better aerodynamics will often forgo spacers and cut the steerer tube down to match the headset bearing cup stack height in addition to the stem stack height.


The disadvantage of this is that is does not allow the rider to switch to a different size stem or headset cups with different stack heights. Once the fork is cut to length any adjustments in height mean purchasing a new fork, a stem with more angular rise, or a special adapter that clamps onto the steerer tube and gives a higher clamping position for the original stem. In addition, many riders who may have less flexibility than a seasoned racer may wish to gain more height on the handlebars, reducing the saddle-to-handlebar drop and providing a more upright and comfortable riding position. Thus, many threadless forks are cut longer than necessary, and the remainder of the steerer tube is stacked with spacers that can be interchanged above or below the stem to fine-tune handlebar height. Often these spacers are aluminum or carbon fiber, but titanium spacers are also available.

Integrated Headsets
A relatively new and emergent headset design, integrated headsets do away with the upper and lower cups on threadless headsets and instead seat the bearings directly against the head tube of the frame. Favored sometimes for their aesthetic appeal, integrated headsets reduce the number of parts involved in the headset assembly.

Prominent standards for integrated headsets include Cane Creek's "IS" and Campagnolo's standard, which is nameless apart from the manufacturer name. Chris King, a leading manufacturer of bicycle headsets, offers a vehement argument against the implementation of integrated headsets. The basis of King's argument is that head tubes with bearing "seats" are far from being machined with reasonable precision. The headset cartridge bearings therefore sit somewhat loosely in the head tube of the bicycle (as opposed to being press fit). Without sufficient preload from the headset top cap, the bearings can potentially slide in their seats and easily damage the softer frame material (often aluminum, although some titanium frames are manufactured for integrated headsets). Given enough damage to the frame, there would be no choice but to replace the frame, especially if the frame is made of an aluminum alloy (titanium and steel can potentially be repaired, but usually at great cost to the consumer). King also argues that the integrated headset is largely a cost-cutting measure for many of the larger bicycle manufacturers, since integrated headsets are somewhat cheaper and take less time to install.

Internal Headsets
Sometimes referred to as semi-integrated headsets, internal headsets include all of the parts of conventional headsets, but locate the bearings inside the head tube, instead of outside. Unlike integrated headsets, internal headsets still employ cups between the bearings and the frame itself.

Prominent standards in internal headsets include Chris King's Perdido and Cane Creek's ZeroStack. ZeroStack uses a 44.0 mm internal head tube diameter, whereas Chris King uses a 44.5 mm internal head tube diameter.
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