Hey Bill.
Now that you describe the forces that are acting, it becomes clear
why trimming the trailing edge is the way to go. Thanks for chiming in
because my 'Smarts' was telling me that leading edge of leading shoe would
be the best way to go. Clearly not!! That could make things worse or even
cause the brakes to grab on. That would not be good.
Thanks.
On 21 October 2010 02:15, WILLIAM B LAWRENCE <ynotink@msn.com> wrote:
> 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
>
_________________________________________
(______________ Alan Bromfield_______________)
(_______\____1957-BN4____/________)
(__tinyurl.com/healeyforum _)
_______________________________________________
Healeys@autox.team.net
Donate: http://www.team.net/donate.html
Suggested annual donation $12.75
Archive: http://www.team.net/archive
Forums: http://www.team.net/forums
|