Headset (bicycle part)
The headset is the component on a bicycle which provides a rotatable interface between the bicycle fork and the bicycle frame itself. The short tube through which the steerer of the fork runs is called the headtube. A typical headset consists of two cups which 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.
Headset sizes
Traditional bicycle head tubes and headsets are sized for a 1 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.
List of common sizes
- 1"
- 1.125"
- 1.25"
- 1.5" As used in the OnePointFive International Standard[1].
- Cannondale Headshok. Although a Headshok steerer is very close to 1.5" it is actually 1.5625". The Headtube dimensions for 1.5" and Headshok are very similar, differing only in the minimum press depth.
Threaded headsets
Threaded headsets are for use with threaded steerers and are the traditional headset (as shown in the above picture). There are 8 parts in a threaded headset (from bottom to top); crown race, lower bearings, lower frame cup, upper frame cup, upper bearings, upper race or cone, washer, locknut. The bearings may be of the cartridge type (deep groove balls bearings or roller bearings), tapered roller or needle bearings or loose ball bearings (sometime retained in a cage). The order of installation of a typical headset follows. The steerer tube is cut to the appropriate length, the top inch of the steerer is threaded using a die, an operation normally done by the fork manufacturer. The threads are normally of the ISO standard, 1" by 24tpi, but other standards do exist. The cups are pressed into the headtube 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 headtube. The upper bearings are placed in the upper cup and the upper race, is screwed on to the steerer. The washer is placed on the steerer tube and locknut 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 handlebars are attached to the fork using the expander bolt through the stem with a wedge at the bottom, which 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 race 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 compress 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 allow 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 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 headtubes with bearing "seats" are far from being machined with reasonable precision. The headset cartridge bearings therefore sit somewhat loosely in the headtube 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 headtube diameter, whereas Chris King uses a 44.5 mm internal headtube diameter.
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