[Healeys] Rear Brakes Locking

[WILLIAM B LAWRENCE]

It may seem counterintuitive, but the opposite is true. Because the shoes are
hinged on one end it is only the leading edge of the lead shoe which actually
makes contact with the drum. No need to consider the trailing shoe since the
vector forces are all pointing in the wrong direction for self actuation. The
reason the leading edge is cut back on most applications is to optimize the
contact angle of the friction surface with the drum to establish a self
actuating or servo action. This is dependent on the point of contact and is
the result of the additive vectors formed by the angle of the shoe in relation
to its anchor point and its tangent angle with the rotating drum. A shoe with
a full length lining contacts the drum at a very shallow angle and so the
vector forces needed to establish self actuation are quite weak. When the shoe
is cut back the angle of the contact is deeper and the motion of the drum
against the friction surface tends to increase the braking force. Sorry if
this explanation isn't clear, but it has been a while since my college days
and some of the terminology has faded. The gist is that as the leading edge of
the shoe contacts the drum it will tend to be forced to pivot on its hinge.
The magnitude of that force depends on the contact angle and brake designers
tried to find the correct contact angle to provide optimal leverage thereby
decreasing braking effort. Surprisingly the dual leading edge girling brakes
were probably the optimal compromise. There are some diagrams in one of my old
ME books if anyone is interested, and if I can find it.
Bill Lawrence

> Date: Wed, 20 Oct 2010 08:59:27 +0100
> From: bluehealey at gmail.com
> To: healeyguy at aol.com
> CC: healeys at autox.team.net
> Subject: Re: [Healeys] Rear Brakes Locking
>
> Thanks Perry.
> My instincts suggest that the most effective way of reducing the efficiency
> of the rear brakes would be to cut back the leading edge of the leading
shoe
> only.