At 07:46 PM 9/4/99 -0700, Bud Krueger wrote:
>.... The shoes are asymmetric, i.e., .... There is more shoe showing at
one end of the lining than at the other. ....
>
>Long story short .... The part of the shoe with the most metal not covered
by the lining, i.e., the part with the tapered ends goes into the adjuster
side toward the front of the car. ....
>
>.... I'll bet that Barney can give us a good explanation about why ....
Why sure, glad to be of assistance. Thought you could catch me on this one
late at night, huh? Brace yourself.
Forward motion of the car being most common, think of drum rotationin that
direction. Drum brakes by their nature are self-energizing. When you
apply the brakes, the force of the hydraulic cylinder pushes the shoes out
against the drum. Friction between the drum and the shoe wants to force
the shoes to rotate in the same direction as the drum, so that force is
going in a circumferencial direction along the surface of the drum. The
anchor point for each shoe is an inch or so inside of the drum. For the
leading shoe this is bearing against the adjuster. For the trailing shoe
this is bearing against the abutment at the top. The distance between the
anchor point and the inside of the drum is a mechanical lever arm that
creates a rotational force on the shoe, wanting to rotate around the anchor
point, so the shoe wants to push outwards more against the drum. This is
the self energizing affect.
Now lay a right angle with one leg running through the anchor point and the
other leg being always tangent to the drum. As you move this arrangement
around you notice that at tangent points farther from the anchor point the
tagent leg is more distant from the anchor point, making a longer lever arm
for the circumferential force to work on. That means that the end of the
shoe farther from the anchor point sees a little more force against the
drum, while the end closer to the anchor point gets less force against the
drum.
Locating the friction pad more toward the far end of the shoe effectively
puts more area of lining there where the force is higher, and less lining
at the end near the anchor point where there is less force. This should
lead to more even wear on the lining. If not for this arrangement the end
of the lining farthest from the anchor point would wear faster and get down
the the metal base first. As the linings are shot when the thickness
anywhere along the length approches nil, even wear along the length is a
noble goal.
Did anyone actually understand that? The idea here is that when you
understand the function you don't have to go looking for the pictures any
more to get the proper orientation.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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