Quote:
Originally Posted by DougW
They won’t self adjust until the tops of the shoes have worn enough to allow sufficient movement at the tops of the shoes for the cable to pull the pawl far enough for the pawl to jump to the next tooth on the adjuster wheel. The position of the bottom of the shoes has extremely little affect on this movement. If the adjusters are too loose (which seems to be very common on new axels) only the tops of the shoes will contact the drums which gives poor brake performance. The tops of the shoes will be close enough to the drum that they will not move enough to activate the adjusters until the top few inches of the shoes have worn down some. Being sure brakes are adjusted properly when new is critical to proper operations for stopping, and proper operation of the adjusters.
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I agree that shoes have to move in order to operate the adjustment mechanism but if the tops of the shoes are too close to the drum surface they are incorrectly "ground" from the factory.
Drum brake shoes, properly prepared, have what's called "heel and toe clearance". A few thousandths clearance at the top and bottom of the shoe when pressed against the drum at the center.
Here's a pic of an improper shoe fit:
When brakes are applied either by hydraulic cylinder or rotating cam as in electric brakes, the leading shoe contacts the drum and rotation causes it to activate the rear shoe forcing it into the drum. The fact that shoes do not have exactly the same radius as the drum makes the brakes "progressive" rather than just ON or Off as they would be if matching exactly.
Brake shoes in a drum brake system have two types of shoes/lining. The shoe at the front is the primary shoe which has a softer, higher coefficient of friction, lining than the rear or secondary (in old days referred to as the Power Shoe). which is the one that does most of the work. The primary merely provides the activating power and is controlled by the cylinder or cam (on electric brakes).
Also note that in the picture, the primary shoe is almost always the one with the shorter amount of lining on it. If you have brakes that are not performing properly it might be a good idea to make sure that the linings are installed in the proper position and that the right side assemblies weren't installed on the left side. This picture is for a Right Side:
For years brake shops actually ground the shoe's arc so it had proper heel and toe clearance but asbestos exposure forced a change. Shoes are now arc'd in the factory (or reconditioning facility) and maximum drum diameters were reduced when refinished (turned).
With self adjusting trailer brakes the usual instruction is to pull trailer at a slow speed and manually apply brakes fully. This causes the shoes to take the shape of the drum and if travel is excessive, the adjuster is turned until brake application doesn't cause ratchet pawl to move enough.
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