This is the first in a series of posts dealing with bicycle steerer bearings (i.e. fork and headset bearings). It contains explanation (with pictures) of basic parts and terms, that will be used in the following posts explaining bearing types and standard dimensions. First thing explained will be fork, then head tube, then headset (i.e. steerer bearings). Only parts important for headset (steering bearings) are noted – for other parts (such as brake attachments etc.) are explained at this link.
Table of contents:
..1.1. Fork types (in terms of headset and attachment type)
…..1.1.1. Threaded forks
…..1.1.2. Threadless forks with a steering column of a uniform width – “ahead” type
…..1.1.3. Threadless forks with a steering column wider near the crown (tapered)
…..1.1.4. Mounting threadless forks (both ahead and tapered)
2. Head tube
..2.1. Head tube bores
…..2.1.1. Head tube for external (also called “standard”) headset bearings
…..2.1.2. Head tube for integrated headset bearings
…..2.1.3. Head tube for internal headset bearings
3. Headset bearings
..3.1. External headset bearings (“External Cup” – EC)
…..3.1.1. External headset bearings for threaded fork steering columns
…..3.1.2. External headset bearings for ahead forks
..3.2. Integrated headset bearings (IS)
..3.3. Internal headset bearings (“Zero Stack” – ZS)
Bicycle fork is used to connect the front wheel to the bars and to enable steering by rotating the bars (and the front wheel). Picture 1 speaks for itself. Important fork parts and dimensions are marked.
Steering column and fork crown diameters are measured on the outside (outer diameter), best done using Vernier calipers. Fork crown sometimes comes with an integrated race for headset bearings, while more often the race is bought with a headset and pressed onto the fork crown when mounting the headset and the fork.
Basic division is threaded forks and threadless (also known as “ahead”) forks.
Threadless forks come in two versions: with steering column of a uniform diameter along the length – and with a steering column that has a wider diameter at the crown than at the rest of the column length. Forks with a wider steering column diameter near the crown are called “tapered” forks.
Easily recognized by the threads on the top part of the steering column. Threadless forks with a steel steering column of an appropriate diameter can be converted to threaded by cutting threads on the steering column.
Threaded forks are attached to the head tube (and the bearings are preloaded) using two (lock) nuts – one with a cone to press against the bearings and another to lock the first one in place.
Easily recognized – no threads and uniform (not changing) width steering column.
These forks are called “tapered”. They are similar to ahead forks, except that steering column is visibly wider at the bottom (near the crown). Tapered fork design is commonly used on modern frames.
Threadless forks (both ahead and tapered) are held in place with a stem (that holds the bars as well on the other end). A special “star” is inserted into the steering column. The star has threads for a bolt in the middle. A cap is then screwed on top, to moderate the bearing preload – pushing the stem and spacers (optional, but common) below – which then preload the bearings. Once a desired preload is achieved, the stem is tightened against the steering column to hold it all in place.
Head tube is a part of a bicycle frame (a tube) where the steering column (fork tube) goes through. Headset (steering bearings) is placed on top and the bottom part of the head tube. Important dimensions are marked in picture 7.
Depending on what kind of bearing (cups) the head tube was designed for, there are three types of bores: for external (or “standard”), integrated and internal bearings.
Easily recognizable by having the same (inner) diameter along the whole length. Cups are pressed into the tube, as will be explained in chapter 3. that deals with headset cups. Top and bottom bore diameters are always the same.
These tubes have specially shaped top and bottom bores to support headset bearings. Bearings are usually inserted (without the need for pressing) directly into the head tube, less common is a design with cups that support the bearings. Deeper part of the bearing support is cut at an angle of 45 degrees (older 36 degree standard now considered obsolete and frames are no longer manufactured with that standard).
This system, without the need for pressing in headset bearing cups cuts bicycle manufacturers’ production costs (headsets are cheaper and take less time to assemble them on the bike). Since head tube bores are not manufactured to strict enough tolerances, these bearings can sometimes damage the frame, because they don’t sit tightly enough inside the head tube, which also “acts” as a bearing cup.
With this system, bottom headset bore can have a greater diameter than the top one, which requires a tepered fork.
Besides “internal“, this system is also called “Zero-stack“, “semi-integrated” and “low profile“. Head tube is of relatively large outer diameter (about 50 mm usually), to allow bearings to fully sit inside the head tube. Bearing cups are, like the external ones, pressed into the head tube, but here they go practically all the way in, with just a thin “lip” left wider to prevent them from going all the way in.
With this kind of head tubes, it is common that bottom headset bore has a greater diameter than the top one, i.e. tapered forks are used.
There are three types of headset bearings, matching each of the three head tube types: external, integrated and internal. Each standard’s bearing layout (parts) will be explained in further sub-chapters.
External headset bearings come in two types: for threaded fork steering column and for threadless (ahead system, not for tapered). They are referred to as External Cup (EC).
Picture 11 shows the parts of a typical bearing of this type:
There are also versions with roller bearings instead of ball bearings, but principle and basic parts are the same.
Unlike threaded steering column bearings, these bearings use a bolt to adjust the preload – and the stem is then tightened to the steering column to hold the adjusted preload and the whole system in place.
System of adjusting (and maintaining) the preload is the same as for external ahead bearings. All the other parts are also practically the same, except that head tube acts as the (top and bottom) cup. That is the bearings are placed (i.e. “slip-fit”) directly into the head tube, without inserting, or presing any bearing cups in. Bottom bearing can have a bigger diameter than the top one (for tapered forks).
Typical integrated headset (for a tapered fork) is shown in picture 13.
Internal headset bearings use cups that are pressed into the head tube and go almost all the way in – only a thin widened part of the cup stays outside, siting flush with a head tube end. Internal headsets are also called: “Zero-stack“, “semi-integrated” and “low profile“.
They have the same parts (by function and layout – i.e. stacking order) as ahead bearings, just the cups are differently designed to fit almost completely inside the head tube.
Overview of the three headset standards for threadless forks – ahead (external), integrated and internal – is shown in picture 15.
This sums up the fork and headset parts important for steerer bearings. Further posts will explain important dimensions and standards.