Wear and Failure Modes
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 the term 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 false 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.
Read more about this topic: Headset (bicycle Part)
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