Headset (bicycle part)

An exploded view of a 28.6 mm (1+1⁄8 in) loose ball bearing threaded headset

Threaded headset with internal bearings on 2016 Brodie Quantum bicycle

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 25–51 mm (1–2 in) 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 25–51 mm (1–2 in), 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″-24 TPI (25.4×1.058 mm), 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.

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 by turning the upper cone and additional 1/8 or 1/4 of a turn. 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's steerer tube using an 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.

Like threadless headsets, threaded headsets can also be internal. Under the internal design, the races and cones are embedded into the head tube, creating a tidy, streamlined appearance. No component is visible between the fork crown and head tube except for perhaps a plastic ring, and only the locknut protrudes from the top of the head tube. In the image shown of the 2016 Brodie Quantum's steering assembly, the lock nut is hidden from view by a plastic cover. The widened top and bottom sections of the head tube contain the bearings. A quill stem inserts into the steering tube.

An exploded view of a 28.6 mm (1+1⁄8 in) cartridge bearing threadless headset

The threadless headset is a more recent design. U.S. Patent 5095770 is owned by Cane Creek Cycling Components and expired on September 29, 2010. Headsets of this type are often referred to by the Dia-Compe (now Cane Creek) registered trademark "Aheadset",[2][3] and are manufactured under license. 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 use a threaded steerer tube. Instead the steerer tube extends from the fork all the way through the head tube and above the headset, and is held in place by the stem clamped on top.

Tightening a threadless headset requires tightening the preload bolt (or cap 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 by gripping the inside of the steerer tube with a downward force. The star nut may be replaced by a self expanding wedge in some designs. The bolt compresses the stem down onto spacers, usually aluminum, which in turn compress the headset bearing cups. The preload bolt does not 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 excessive friction.

In the threadless headset system, the spacers are important in placing the stem and preload bolt in the correct position on the steerer tube. Thus the stack height of the stem becomes important. The steerer tube of the fork must be cut to length such that it leaves at least enough of the steerer tube protruding above the headset for the stem to clamp on to; if the steerer tube is cut longer than the stem, then spacers are used to fill the gap between the stem and the conical compression washer. Bicycle racers seeking the greatest saddle-to-handlebar drop for better aerodynamics will often forego spacers and cut the steerer tube down to exactly match the headset bearing cup stack height plus the stem height.

Cutting the steerer tube to its minimum length prevents switching to a taller stem or to a headset cups with a higher stack height. Any increase in handlebar height after this cut requires purchasing a new fork, a stem with more angular rise (some stems can be turned over for greater height), 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. If an owner simply wishes to replace the headset with one having a slightly larger stack height, it may be possible for a bike shop or framebuilder to mill or re-face the head tube to gain an extra 1-2 mm of stack height without having to replace the fork. Many threadless fork steerer tubes are cut longer than necessary to allow for adjustments and the steerer tube above the stem is stacked with spacers that can be moved above or below the stem to fine-tune the handlebar height. Often these spacers are aluminum or carbon fiber, but titanium spacers are also available.

A relatively recent development, integrated headsets do away with the upper and lower bearing 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" [4] and Campagnolo's standard, which is nameless apart from the manufacturer name. Chris King, a leading headset manufacturer, offers a vehement argument against the implementation of integrated headsets.[5] 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). During use, the bearings, under thrust loads, will rock in their seats and will 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.

Sometimes referred to as semi-integrated headsets, internal headsets include all of the parts of conventional threadless 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 InSet and Cane Creek's ZeroStack. These designs use a 44.0 mm internal headtube diameter.[6][7]

The threaded headset has recently been replaced by the threadless headset on better quality bicycles for several reasons:

• Forks with a threadless steerer tube are cheaper for manufacturers because they can be cut to size at the point of sale and the manufacturer can use the same forks on different frame sizes. By comparison, forks with a threaded steerer tube must be matched with a frame's headset tube length; therefore, bicycle manufacturers need to make or buy a different sized fork for each frame size.
• Regular allen wrenches can be used to adjust threadless headset bearings. By comparison, large, and relatively expensive, wrenches are required to adjust threaded headset bearings; their size generally precludes carrying them on the road.
• Threadless headsets and forks are quicker to install, saving manufacturing costs.
• A threadless headset and fork is marginally lighter than an equivalent threaded headset and fork.
• A threadless stem is more rigidly attached to the forks, giving improved rigidity at the handlebars.
• On bicycles which have not been maintained, water can find its way between the stem and steerer tube of threaded headsets, causing corrosion and seizing.

Threaded headsets have some advantages as well:

• Threaded steerer tubes are matched to the frame at the factory simplifying the adjustment of handle bar height.
• Threaded headsets use a quill stem that can be adjusted vertically over a wider range than typical threadless headset stems.

At least one manufacturer, Cane Creek, offers an after-market threadless headset bearing kits that enables changing the steering axis angle.[8] When all else remains the same, this alters the geometric trail of the bike.

Traditional headset cups are pressed into a cylindrical head tube. Some standard dimensions of the head tube on the frame (for traditional and semi-integrated/ZeroStack) are:

Bike frames with integrated headsets have cups machined directly in the frame, and the bearing sits directly into the frame instead of via separately installed cups. Fewer tools needed compared to traditional bearing cups which need to be pressed into the frame using special tools (or a hammer and wood block).