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Subject: Threadless Steerers and Headsets
From: Jobst Brandt
Date: October 30, 2004
Threadless headsets are a welcome and major improvement over the quill handlebar stem and the large octagonal head bearing nuts that fit on threaded steertubes. Besides, the quill stem was never adequately attached to the fork, moving radially at its upper end, where it had a loose fit in the steertube.
The need for special wrenches to adjust head bearings was cumbersome, but was more an inconvenience than a functional failing, one for which the threadless design is an ideal solution. Attachment problems, head bearing adjustment, and the greater force exerted on stems with MTB handlebars demanded a design change.
Upper stem movement, although small, pumped perspiration-enriched rainwater into the interface and on occasion froze aluminum quill stems in the steertube. They became stuck and sometimes unremovable because aluminum oxide has a greater volume than aluminum and, at times, expanded with enough force to cause a bulge in the steertube. Such an interference fit can make removal by force impossible and in many cases requires machining.
The threadless steertube solved these problems elegantly. The stem is clamped to the outside of the steertube with one or two Allen screws to give a rigid interface. The head bearing is centered on the steertube by a conical ring that is pressed into engagement by a sleeve beneath the stem, and clamping the stem locks the adjustment.
Fail-safe clamping is important in selecting a threadless stem. Unlike the quill stem, where an attachment screw failure caused a loose handlebar, the threadless stem handlebar clamp can completely separate in the event of failure, if it uses only one pair of screws. Therefore, a steertube clamp with two screws and the handlebar clamp with four screws is preferable.
When converting from a quill stem, the improvement is most noticeable in that the entire bicycle seems to become more rigid, especially when accelerating or climbing hills standing. Maintenance of head bearings and removal of handlebars, without untaping handlebars or removing brake levers, becomes trivial.
The shortcoming is that handlebar height cannot easily be changed without a special stem, one with an articulated extension. This is not a problem for people who know what handlebar height they want. It seems to be more a problem for new riders or rental bicycles that require adjustable height.
More Articles by Jobst Brandt
Next: Fretting Damage in Bicycle Mechanics
Previous: Roller-Bearing Headsets
Last Updated: by John Allen