Parking Brake Quirks

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dibbons

dibbons

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I have noticed with the parking brake applied, although I can't proceed forward, oftentimes the parking brake does not hold when I use reverse (like when I forget to release it and back from a parking space). Does anyone know the explanation for that? Even my '96 Dodge Dakota does the same thing.

handle.jpg
 
Hey Dibbons,

Pretty sure its because when you put the car in reverse, the smaller front shoe is providing most of the friction, less surface area takes less force to move.
Anybody else??
 
Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing."[5] The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.
So, if going in reverse, it unapplies itself.
 
spl440 said:
Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing."[5] The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.
So, if going in reverse, it unapplies itself.
Click to expand...

Interesting points! Would these forces also be in effect seeing that the handbrake is applied and there is no rotation and therefore no bite? Your description makes perfect sense if the car is already moving forward or backwards but how does it apply with the car stationary?
 
When you go in reverse you are undoing the self energizing effect
 
If I remember correctly, the parking brake lever pushes the leading shoe forward, so would act the same.
 
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Dana67Dart said:
When you go in reverse you are undoing the self energizing effect
Click to expand...

Hmm, ok I think I understand what you are describing about the energising effect, what I don't understand is how the drum brake system distinguishes direction. It would seem to me that the only difference in direction is which shoe is being applied first in the direction of rotation. In the case of reverse, the smaller shoe is first and with less surface area applied to the drum, is more easily overcome by putting the vehicle in reverse or by parking on a hill with it applied.

Interesting post Dibbons!!
 
spl440 said:
If I remember correctly, the parking brake ever pushes the leading shoe forward, so would act the same.
Click to expand...

That might explain it. The wheel cylinder is not used to move the shoes, just the lever and that applies pressure to the front shoe only. Would this be correct??
 
original.jpg

Picture is from a ford truck, sorry. But all about the same, as brake cable is pulled, it pushes forward the primary shoe through the parking brake link, pivoting the rear shoe back against the drum through the lever retainer, applying the brakes.
 
spl440 said:
View attachment 1715251959
Picture is from a ford truck, sorry. But all about the same, as brake cable is pulled, it pushes forward the primary shoe through the parking brake link, pivoting the rear shoe back against the drum through the lever retainer, applying the brakes.
Click to expand...

Yup! Great pic!
 
"It's because parking brakes suck!!"

On a serious note, how do you apply it? It helps if you hold the brake pedal while you apply them. I still remember my old V code RR, which by then (1975 or so) had a 340 in, one day started it up on a cold eve to get off work. Damn thing ended up rolling back down an "almost not an incline" out into a not-very-used street and bumped the opposite curb. Still it could have hit something
 
67Dart273 said:
"It's because parking brakes suck!!"

On a serious note, how do you apply it? It helps if you hold the brake pedal while you apply them. I still remember my old V code RR, which by then (1975 or so) had a 340 in, one day started it up on a cold eve to get off work. Damn thing ended up rolling back down an "almost not an incline" out into a not-very-used street and bumped the opposite curb. Still it could have hit something
Click to expand...

For sure! I never could understand why such a poorly operating system is mandatory here in Canada for Government mandated motor vehicle inspections!
 
spl440 said:
Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing."[5] The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.
So, if going in reverse, it unapplies itself.
Click to expand...

Exactly, and the edge of the applied shoe forces the rest of the shoe into the drum hence the term "self energizing"
This is exactly what can cause loosely adjusted shoes to grab the drum and lock up that wheel completely.
Always called this the "camming in" effect.
 
67Dart273 said:
"It's because parking brakes suck!!"

On a serious note, how do you apply it? It helps if you hold the brake pedal while you apply them. I still remember my old V code RR, which by then (1975 or so) had a 340 in, one day started it up on a cold eve to get off work. Damn thing ended up rolling back down an "almost not an incline" out into a not-very-used street and bumped the opposite curb. Still it could have hit something
Click to expand...

Pressing the brakes first to release a tightly set E brake works too.
 
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