The bike's headset is one of those less-obvious parts that are easy to overlook, but crucial.
The headset is a collection of cups and bearings that fit into your bike's head tube (the shortest tube at the front of your frame, through which the fork connects to the stem and handlebars).
Most headsets will feature a top and bottom bearing, which enable your handlebars to turn smoothly for accurate steering. These bearings are secured via various cups/races and seals, the design and number of which will vary according to frame and headset type, a top cap and bolt, and an expanding 'star nut' that fits inside the fork steerer tube.
A correctly-fitted headset will allow the fork's steerer tube to turn smoothly and freely inside the frame without any excess movement back and forth (play) Along with the bottom bracket the headset is one of the bike's two main bearings and ideally should provide a long life of smooth and trouble-free performance.
Tough riding conditions – wet weather and mud especially- as well as the demands of some riding disciplines can shorten the life of a headset, which is where extra investment in a high-end unit with quality sealed bearings could pay off in the long run.
When your headset is working as it should, you won't even notice it, but old, worn bearings will hamper your steering (giving you a crunchy, gritty feel when you turn the bars) and a loose headset can damage your frame and potentially be dangerous.
For MTB and road bikes there are numerous different types of headset available, choice of which may largely be determined by the type of head tube on your frame.
Historically, there were two most important headset standards - threaded and non-threaded or 'Aheadset'
A Threaded Headset: -
A Threadless Headset: -
A Nice Comparison: -
The older type of threaded headset – recognisable by its two large nuts between the stem and the top of the head tube – fit forks with threaded steerer tubes, typically of 1” diameter. However these are increasingly rare and now threaded headsets are generally only found on older bikes, vintage-style commuter bikes (the Dutch-type city bike, for example) and on some bikes at the budget end of the range.
Threaded headsets feature an adjustable bearing race screwed onto the fork steerer and secured by a locknut, meaning two special spanners are required to adjust the bearings.
The threaded headset has largely been superseded by the threadless or Aheadset type. Rather than having nuts which screw onto a threaded fork steerer, the threadless headset is used in conjunction with a stem that clamps to the steerer tube by means of one or more pinch bolts. The Aheadset type of threadless headset features a special star-fangled nut (SFN) which is installed inside the fork's steerer tube. The headset's top cap meanwhile sits on the top of the steerer tube and is connect to the SFN by a threaded metal rod. Tightening this rod with an Allen key secures all of the elements – top and bottom races, bearings and seals - and eliminates play in the headset.
Recent years have seen a number of new developments in terms of threadless headsets, with several different types available. The most important ones to know about are integrated, internal, standard and tapered.
Also of importance are head tube size, headset type and bearing/seal type.
Your replacement headset will likely be determined by your frame type – a frame and fork setup requiring a threadless, integrated, tapered headset, for example, will limit your options – but it’s useful to know about the different types so the choice won't seem so confusing.
Head Tube Size
When choosing a new headset it's important to match the size to the diameter of the fork steerer, which in turn will correspond to the inside diameter of the head tube. The most common steerer diameters are 1”, 1 1/8”, 1.5” and tapered.
• 1”: Typically, the older-style threaded headsets will be found on bikes with a 1” threaded fork steerer (although 1” threadless headsets are available).
• 1 1/8”: The standard fork steerer diameter for most road and MTB bikes is 1 1/8”.
• 1.5”: Some modern gravity bikes or speciality road bikes use 1.5” fork steerers and correspondingly larger head tubes for extra strength and front-end stiffness.
• Tapered: A new trend has emerged in road and MTB for tapered head tubes with a steerer diameter of 1.5” at the bottom and 1 1/8” at the top. This results in better stiffness up front and also enables frame designers to use larger-diameter down tubes, with stiffness benefits in the bottom bracket area. Tapered head tubes are designed to be used with forks that have tapered steerer tubes, and with appropriate headsets, but you can get reducers to enable 'standard' 1 1/8” parts be used in frames with tapered head tubes.
NOTE: The diameter of the fork steerer and the internal diameter of the head tube are different. However sometimes people regard them as interchangeable, and may refer to a head tube designed for the 1 1/8” steerer standard as a “1 1/8” head tube”. Strictly speaking, this isn't necessarily accurate as a frame designed for a 1 1/8” fork may have head tube with a larger internal diameter to accommodate an internal or 'zero stack' headset. If you're looking up frame geometries you may see reference to a '44mm head tube' – this is what they are talking about.
There are different types of threadless headsets, the most important being integrated, internal, and standard.
• Integrated headsets are used on frames where the head tube is machined so that the top and bottom bearings can fit directly into the frame, eliminating the need for extra bearing cups and enabling the bearings to sit flush with the frame for an attractive, clean look. Because they have less parts, they are a little lighter than other headset options. They are often used on road and MTB racing bikes and are especially common on bikes with tapered head tubes, as the lack of a lower external bearing race means more surface area for the down tube joint and a neater, more aerodynamic interface between fork crown and head tube.
• Internal headsets are for bikes without machined head tubes and require aluminium cups to be pressed into the top and bottom of the tube so that the bearings can sit into them. They are similar to integrated headsets – and sometimes referred to as 'semi-integrated' – but the difference is that the bearing races are separate components, rather than being integral parts of the head tube. It can sometimes be hard to distinguish whether your bike is using an integrated or internal headset.
• External headsets are also known as 'standard' headsets. This is the traditional type of threadless Aheadset which has upper and lower bearing races pressed into the frame, with the bearings positioned outside the head tube (as opposed to inside, in the case of integrated and internal headsets). These can typically be found on bikes with 'straight' 1 1/8” head tubes (as opposed to tapered models).
NOTE : Headset type is not an indicator of quality. Integrated or internal headsets are not 'better' than standard ones, but are rather designed to work with particular schools of frame design, and materials such as aluminium and carbon fibre which require the use of large-diameter, thin-walled frame tubes, as opposed to steel or titanium bikes which use smaller-diameter tubes.