Subject: 8e.12  Threadless headset
From: Jobst Brandt <jbrandt@sonic.net>
Date: Tue, 2 May 2006 10:25:45 -0700

Threadless headsets are a welcome and major improvement over quill
handlebar stems and large octagonal head bearing nuts required by
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.

Movement at the upper end of quill stems, although small, pumped
perspiration enriched rain water into the interface and on occasion
froze aluminum stems in the steertube.  They became unremovable
because aluminum oxide expands with enough force to cause a bulge in
the steertube.  Such an interference fit often made removal by force
impossible and required machining.

The threadless steertube solved these problems elegantly in that the
stem is clamped rigidly to the outside of the steertube with Allen
screws.  The head bearing is centered on the steertube by a conical
ring that is pressed into engagement by a sleeve beneath the stem,
while clamping the stem locks the adjustment.

Failsafe clamping is important in selecting a threadless stem.  Unlike
the quill stem, where an attachment screw failure caused a loose
handlebar, the threadless stem using only one pair of screws for the
handlebar clamp, can completely separate in the event of failure.
Therefore, a steertube clamp with two screws and the handlebar clamp
with four screws is advisable.

The improvement is most noticeable when converting from a quill stem,
is that the entire bicycle feels more rigid, especially when climbing
hills standing or accelerating.  Maintenance of head bearings and
removal of handlebars, no longer requires removing bar tape or brake
levers, becomes trivial.

A 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
is more a problem for new riders, or rental bicycles that require
adjustable height.