Drum-brake self-adjusters

Question

I was reading about Automotive Brakes (Chapter 52, Automotive Mechanics by William H Crouse, Donald L Anglin, 10th edition). And in a section about drum-brake self-adjusters, it is said that the incremental self-adjusters are attached to the secondary shoe of a duo-servo brake; The adjustment is made when the vehicle is moving backwards and the brake is applied. And then the mechanism of how the adjusting lever and adjuster spring follows the text.

What I don't understand is why the secondary shoe? It only seems reasonable since the adjuster is doing its work while the vehicle is in backward motion. But then, why the adjustment only happens in backward motion?

Answer 1

The primary shoe is normally operated directly by the piston and the secondary one by the cylinder sliding in the back plate.

But this varies widely as there are versions using one cylinder with two pistons and others with two separate cylinders and pistons where both shoes are leading instead of having one leading and one trailing, twin leading shoe brakes provide more braking for less pedal force due to the self-servo action.

Without you providing the diagrams showing exactly how these are designed it is difficult to provide more information, but you have enough to give you material for further research.