The two contenders for the spinning arm design.  Both are vastly lighter
than the bicycle hub (and much less expensive too).  A penny is under the
shaft on the right for comparison.  The black hub unfortunately doesn't
provide free rotation along the bolt shaft axis.  This would mean the arm
would experience friction when a load was placed along the length of
the arm axis.  This is unacceptable.  The bearing shaft on the left has a
nut and two bolts.  It has no friction along the bolt axis, and seems overall
to be more durable.  It's also off the shelf at most hardware stores for
about $5.00.