At 01:09 PM 5/16/2012, neil n wrote:
>think the wear patterns indicated that the top 1/3 of shoes were contacting
>drums but I haven't looked in some time.

Look in the archives for Dennis Haynes' discussions on spacers for
the bottom pivot.

If the lining material is of constant thickness, there's no possible
way for it to have full effective contact throughout its life in a
setup with a fixed pivot point.  Whatever point touches first will
start to wear in a small patch which will gradually spread as the
brakes wear in.  The better the initial fit, the larger that initial
patch (or perhaps two patches) will be.  Perfect fit requires that
the lining radius of curvature precisely match the drum in its
present condition of wear, and that the pivot be shimmed until as
nearly as possible the entire lining contacts the drum simultaneously
when the brakes are applied.  This will quickly wear into full
contact.  At the end of service life the cylinder end of the lining
will be worn to minimum thickness, while the pivot end will be
considerably less worn.  The cylinder end of the lining will always
do more work, and because our brakes are set up in a leading and
trailing shoe arrangement the shoe where the cylinder end faces the
oncoming brake drum will do more work.

Back when drum brakes were used for front wheels where most of the
work gets done and cars didn't have vacuum-assisted brakes I believe
they had more elaborate arrangements for some, with double leading
shoe setups with (I presume) two cylinders and such; and there were
brake shops that would individually fit shoes to drums and so
forth.  And my grandmother's (late '50s/early '60s) Studebaker had band brakes.

The (front) disk brakes on my Saab 96 took this wear thing to an
extreme.  They were Lockheed brakes, single-cylinder with a pivoting
instead of sliding arrangement, and new pads looked like a ski
slope.  When they wore down to level, they were used up.

Yrs,
d