Subject: 8c.9  SA Dynohubs
From: Jobst Brandt <jbrandt@sonic.net>
Date: Tue, 2 May 2006 10:25:45 -0700

The Sturmey Archer Dynohub was a good idea in its day but, as many SA
products, remained unmodified from its inception.  With 6v-1.8W
nominal output, it is not a power house but that is not its main
failing, that being that it cannot be disassembled by the user without
permanently losing the magnetic field of its ALNICO magnet, a type of
magnet that demagnetizes when removed from a closed magnetic circuit.

Although the hub has the warning "Do not remove without keeper", few
people are aware of what that means.  If the stator is removed from
the hub, the magnet demagnetizes instantly and permanently.

Nowhere is the term "keeper" defined, a device of which few if any
exist in the field, much less instructions on how to use them.

Building a magnetizer revealed that the archaic Dynohub isn't worth
rebuilding for other reasons.  The heavy steel housing is not a part
of the magnetic circuit which uses a 20-pole ALNICO magnet that in
essence is a ring of siamesed horseshoe magnets radially facing the
stator.  Better hub generators are available today, having hollow
axles with quick release, aluminum housings, and rare earth magnets
that do not demagnetize when removed from their circuit.