Even though drum brakes get little discussion, I have a drum brake question. From the first time I changed drum brake shoes (about 37 years ago) I always knew the primary shoe was toward the front. I also knew the primary shoe had the lining lower on the shoe than the rear facing shoe. Someone just asked me what that does. I hate to say I have no idea. I just did it that way becuause I was taught that way. So, Why primary and secondary shoe and what does it do or not do???
Because the fulcrum is on the bottom, the primary shoe when it’s in contact with the spinning drum’s surface tends to be pulled into tighter against teh surface of the drum by the friction itself. Because it’s pivoting, the area of the shoe farthest from the fulcrum pulls tighter against the surface. The location of the pad is an attempt to somewhat compensate to get more even wear.
Thank You, now I know and it makes perfect sense.
The primary shoe applies its force to the secondary shoe through the adjuster making the self actuating action work. The wheel cylinder piston on the front side applies force to the primary shoe upper fulcrum; the primary shoe friction surface plus the wheel cylinder force is transferred to the secondary shoe through the lower fulcrums via the clearance adjuster; the combined secondary shoe friction force, the primary friction force, and the wheel cylinder force are transferred to the anchor pin by the upper fulcrum of the secondary shoe. The rear wheel cylinder piston pushes against the secondary shoe fulcrum but cannot push the shoe away from the anchor pin. The friction surface on the primary shoe is smaller and of a different hardness (you may notice a different color between the primary and secondary shoe)than the secondary shoe to allow the two shoes to wear out at about the same time.
When stopping in reverse the action is reversed and the secondary shoe moves away from the anchor pin. The self adjuster now moves to lift the pawl that catches the adjuster wheel to take up any excess clearance that has accumulated due to wear.
Really a rather simple but clever design.
So if they were put on backwards the self adjuster would not work?
It would but self adjusters don’t keep the brakes adjusted all that well. This brake system is called a duo-servo system since pressure is applied at the top of the shoes via the wheel cylinder and at the bottom since the bottom of the shoes are not held down solid like import brakes were back in the drum brake days. The bottom of the shoes were held down with pins and there was movement with them allowing the bottom of the shoes to move allowing the primaries & secondaries to do there job.