The headset is the set of components on a bicycle which provide a rotatable interface between the bicycle fork and the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. 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.
Traditional bicycle head tubes and headsets are sized for a 1 inch diameter 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 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" or 1 inch. This may have a fork crown (The base of the fork steerer tube) of a number of different dimensions. Milling may be necessary to make some headsets fit.
- 26.4mm (ISO)
- 27.0mm (JIS)
- Other sizes are uncommon, but do exist.
- 1.125" or 1 1/8 inch
- 1.25" or 1 1/4 inch
- 1.5" or 1 1/2 inch, as used in the OnePointFive International Standard.
- Cannondale Headshok. Although a Headshok steerer is very close to 1.5 in it is actually 1.5625 in. The Headtube dimensions for 1.5" and Headshok are very similar, differing only in the minimum press depth.
- 1.5 inch to 1 1/8 inch conical headsets (MTB only, 2009 onwards). The lower bearing is 1.5 inches for increased stiffness and the upper is 1 1/8 in for reduced weight.
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 for use with threaded steerers 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, upper frame cup, 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 and the top inch or two 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. This operation is done by the fork manufacturer. Threaded forks necessitate that the threads on the steerer only use the top 1-2 inches, therefore the forks are sold in varying lengths (this is one reason threadless forks have become so popular with manufacturers as they only need be made in one size). 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 cut the threads farther down. 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 for the reasons mentioned. The threads are normally of the ISO standard, 1 in by 24 tpi, but other standards do exist. The headtube may be faced and then the cups are pressed into the headtube using a special press, to ensure they are square and true. The fork crown may be faced and then crown race is pressed on to the fork crown, again to make 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 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 top of the upper race 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 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.
The threaded headset has been replaced by the threadless headset on recent quality bicycles for several reasons:
1. Threaded forks need a threaded steerer and bicycle manufacturers need to make or buy in a different sized fork for every frame size. In comparison, threadless headsets and forks are cheaper for manufacturers because they do not need to be threaded and different sized frames can use exactly the same forks.
2. Threadless headsets and forks are quicker to install, saving manufacturing costs.
3. A threaded headset and fork is marginally heavier than an equivalent non threaded headset and fork.
4. A threadless stem is more rigidly attached to the forks, giving improved rigidity at the handlebars.
5. On bicycles which have not been maintained, water can find its way between the stem and steerer tube, causing corrosion and seizing.
6. Large and relatively expensive tools are required to adjust the bearings in a threaded headset, and these are not easily carried on the road.
The threadless headset, often referred to by the Dia Compe registered trademark "Aheadset", (lead illustration) 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 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 this 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. Bicycle racers seeking the greatest 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 a switch to a different size stem or headset cups with higher stack heights. Also once the steerer tube is cut to its minimum length any increase in handlebar height 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. Thus, many threadless forks are cut longer than necessary, and the steerer tube above the stem 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.
A relatively recent development, 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 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). 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 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.
Head tube preparationEdit
Traditional headsets that use pressed-in bearing cups reference the top and bottom faces of the head tube. Head tubes often have to be machined to acceptable tolerances, especially on metal frames after welding or brazing, as the heat can distort the tube dimensions. Some composite frames may also require it if glue resin or other contaminates are on the head tube faces. A specialized tool, called a head tube facing and reaming set is used to ream and/or face the head tube ends. Facing is often the main objective as the average head tube inside diameter is usually unaffected, but a reamer may still be used to clean out paint overspray or to convert a 28.8 mm JIS head tube into a 30.0 mm ISO. More expensive, higher-precision headsets often benefit from or even require a properly machined head tube to allow for proper adjustment and bearing life.
Additionally, some newer headsets intended for downhill, freeride or other types of demanding riding have deep-insertion cups (nominally up to 1 in). Some frames may require reaming to this depth prior to installing the headset, although it is crucial for the bicycle mechanic to inspect the inside of the head tube carefully (or contact the manufacturer) prior to having this procedure done, as there is a risk of permanent damage or at least voiding the warranty of some frames. Frames with damaged or ovalized head tubes can sometimes be salvaged with this repair.
Wear and failure modesEdit
Headsets on bicycles without fenders are exposed to water and grit thrown off by the front tire, which causes rust and rapid wear. Better headsets use rubber lip seals or "O" rings (dirt skirt) to try to keep water out, with varying degrees of success. External Neoprene bands with a Velcro fastening are available to wrap and protect the lower race, and are removable for cleaning. Some cyclists remove the fork and reassemble with a section of old inner tube over the lower race, which performs the same function, albeit with less convenience.
On bicycles ridden only in dry conditions and/or with fenders, the normal failure mode is a progressive notchiness in the steering, described as "indexing" in the bicycle world, caused by pitting of the races or false brinelling, although this stems from a misunderstanding of the cause; true brinelling is caused merely by pressing the ball axially into the race, and it is almost impossible to replicate this damage even by striking the fork crown repeatedly with a hammer. The pits are by far deepest at the front and back of the head tube, and are actually caused by flexing of the fork blades, which is transmitted to the steerer tube. This misaligns the bearings and causes fretting, a small amplitude, large stress movement which tears metal from the races at the points where the balls rest.
The solution is to have a 45 degree interface in the headset where this flexing movement can be accommodated, preserving the relative alignment of the races and allowing the ball bearings to take pure axial and rotational loads. Shimano cartridge bearing headsets do this by allowing the cartridges to move relative to the pressed-in cups, while Stronglight roller bearing headsets, and most threadless headsets, now have loose upper and lower races which can move relative to the cups. Modern headsets, therefore, rarely suffer from brinelling.
A less common headset failure is really a frame failure; the head tube can stretch, allowing a headset cup, which is supposed to be a tight interference fit, to become loose in the tube. Lugless frames are most vulnerable; in a lugged frame the lug reinforces the top and bottom of the head tube and generally prevents stretching of this type. A loose cup can be fixed with a retaining compound such as Loctite 660, and some manufacturers produce slightly oversize cups to cope with this situation. It is usually the lower cup which is affected.
In order to provide a cable stop for front cantilever brakes or centerpull brakes, a hanger may be incorporated into the headset, either as part of the washer between the top race and the lock nut in the case of threaded headsets, or as part of a spacer between the top race and the stem in the case of threadless headsets.
|Search Wikimedia Commons||Wikimedia Commons has media related to: Headset (bicycle part)|
- Headset types and standards, from Park Tool
- headset crib sheet by Sheldon Brown
- Videos on adjusting or ovehauling Headsets by bicycletutor.com
- How to put together an 'Aheadset' Interactive assembly guide by www.biketreks.netde:Steuersatz (Fahrrad)