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Headsets & Stems
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. more...
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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" or 1 inch;
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" it is actually 1.5625". The Headtube dimensions for 1.5" and Headshok are very similar, differing only in the minimum press depth.;
Headset types
There are a few different types of headsets 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
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 order of installation of a typical headset follows. The steerer tube is cut to the appropriate length, 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 manufactures 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" by 24tpi, 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.
Read more at Wikipedia.org
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