Re: [ihc] About those parking brakes .....

[Steven Stegmann <steve.stegmann@xxxxxxxxxxxxx>]

John,

The reason the parking brakes don't hold at all in
reverse while they do have some small effect in
forward is that the drum brakes are self energising. 
This type of drum brake is called a single leading
shoe design.  Almost every drum brake is like this. 
It has nothing to do with the position of the shoe,
just how the force of the drum on the shoe is
resolved.  Usually it is the shoe towards the front of
the vehicle. 

It is designed so that when pressure is applied to the
leading show, the drum tries to make the shoe move
with the drum.  What happens is that of course it
doesn't really move with the drum but the force
generated trying to move the shoe wedges it tighter
against the drum.  Viola! power brakes!  Virtually all
parking brakes work on the trailing show of the drum,
not sure why.  No self energizing.

This is why drum brakes were always so hard to control
on icy pavement.  Their response is non-linear.  If
you double the pressure with your foot, you get way
more than twice as much braking.  This never happens
with disc brakes because the pads move only parallel
to the axle, never parallel to the rotation.

I had a 73 Toyota pickup with twin leading shoe drum
brakes.  Two wheel cylinders at each wheel.  That was
fun in cold damp weather.

Steve