Better Shoe Centralisation
When properly set up RM brakes are very good.
Sometimes however no matter how often they are checked they do not
perform as well as they should. The Workshop Manual covers most
things but problems can arise which are not covered. Before
making any changes to the braking system ensure that they are properly
adjusted and that the shoes and drums are clean. The change
described here has been found to work well but it is presented for
information only. The braking system is one of the most important
system on the car and it must be both reliable and efficient. Do
not make any changes to it unless you really understand what you are
doing and are prepared accept any resulting consequences.
For brakes to work efficiently the brake linings on
the shoes must make full face contact with the brake drum. Linings
which are worn more at one end than the other are clear evidence that
when fitted they were not making full face contact and have therefore
worn unevenly. In this case braking performance will be poor until the
linings have worn sufficiently to give full face contact.
Consider the system used on the RMF (above). The shoes are
pushed towards the drum by the hydraulic cylinders. Assume that the
shoes are not centred properly and one end of the lining contacts the
drum before the other. The shoes are free to slide sideways at both
ends so the end making first contact will push the shoes sideways until
both ends of the linings are making equal contact.
Now consider the system used on the RMB (above). Again the
shoes are pushed towards the drum by the cylinders but this time the
other end of the shoes are constrained to pivot around a circular
fulcrum. There is no provision for the shoes to slide sideways to
ensure that full face contact is made. Even if the shoes are correctly
positioned the end nearest to the cylinder will always move furthest so
unless the other end of the lining started off almost touching the drum
full face contact will not be achieved.
My prewar Riley had Girling rod brakes on all four wheels and even
after careful adjustment they were never a good as I would have liked.
I had a spare set of shoes which I was prepared to sacrifice so I
decided to try an experiment
The picture above shows the overall layout for the
rod system. The adjuster unit is at the top and the expander unit is at
the bottom. At the adjuster end the shoes have a circular cutout which
forces them to pivot around the adjuster thus preventing them from
sliding sideways to centre themselves within the drum. I filed the
cutout so that it was considerably wider and had a flat bottom so that
the shoes could now slide sideways. I did this for all eight shoes. I
then reassembled everything and adjusted up the shoes. I stamped hard
on the brakes a couple of times and checked the shoes adjustment again
before I took the car out for a road test. The improvement in the
braking was immediate and very noticeable.
These modified shoes have been on the car for a few years now and the
braking performance has remained excellent. I no longer fear the car in
front with disk brakes stopping suddenly without warning.
I hesitate to recommend this modification to other Riley owners as
should something go wrong the consequences could be very serious. All I
will say it that it has made a big improvement to the brakes on my car.