Subject: RE: [Healeys] Rear Brakes Locking
About all I can say to this is that if your rear brakes are not gummed up
and locking for that reason, then you are one lucky Healey (insert any Brit
car marquee here) owner. I know about 200 owners who wished their rear
brakes did more than just add dead weight.
I can see Chris' point on a race car, but not on a road car, let alone ones
that don't see 3000 miles a year (snipped from oil change interval thread)
I used to change Sprite rears to Wolsey rears (nearly a bolt on) just to get
a response from the pedal.. ;~)
dave
frogeye@porterscustom.com
1954 BN2 1959 AN5
Porter Custom Bicycles
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www.britishcarforum.com/portercustoms.html
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http://picasaweb.google.com/porterscustombicycles/PorterCustomBicyclesStuff
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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.
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