Please take notice of the adjustment mechanism of drum brake 4!
The drum-brake has advantages over the disc-brake because of the lower production costs, the lower weight and the more favourable operation forces in relation to the braking power (self-strengthening), apart from that, due to the generally much longer lifespan of the brake linings, they have longer service intervals. They are enjoying a certain comeback in the compact-car segment, particularly in the price-sensitive area.
The only problem is, that with relatively open wheel-rims, after some time, the unattractive appearance of the drums. The manufacturer, ATE has developed a coating which is an effective protection against corrosion (see picture 1 and 2) because due to the fact that the drums get pretty hot, painting them is not a suitable solution.
In a cast iron or steel drum which is linked to the wheel, here with sprocket-teeth for the ABS-impuls generator, the two crescent-shaped brake shoes are pressed outwards by a spreading mechanism. Their pads provide friction on the inside of the drum. The brake is applied to the wheel because the pads are connected through the anchor plate to the suspension.
In the top picture the brake drum has been dismantled. Right at the top, in the center, the wheel brake cylinder can be seen, which when the brake pedal is pressed, transfers the pressure as a force through two pistons, to the two brake shoes. Because they are hinged at the bottom, they are spread by the cylinder against the drum. In particular the spring which is mounted parallel to the cylinder, pulls them back again, off the drum.
Should the brake pedal move up and down when braking, it may mean that the brake drum is no longer perfectly round but ovallly shaped.
In the case of this Simplex-brake, each of the two brake shoes is applied to the brake drum with differing force. Assuming that the roll-direction is anti-clockwise, the leading shoe brakes stronger because through the friction with the left rotating drum it functions almost like a wedge. The friction with the drum increases this effect. This is not the case with the trailing shoe on the right. This effect is unwanted in the case of ABS because the brake must be rapidly released. This, or to achieve an even wearing down of the pads, may be the reason why the right shoe has a distinctly thicker pad.
On the right hand brake shoe, the spreading lever for the hand brake can be seen, it's bowden cable comes in from the left and is connected to the bottom. The spreading lever is supported against the left brake shoe by a connecting rod. In addition, this carries the mechanism for the automatic adjustment, i.e., it becomes longer and longer the more the pads are worn down. Thereby, it may not lock the brakes, it must always have a certain amount of play to ensure the flawless releasing of the brakes.
At the time of the drum brakes, low pressures were normal, indeed, the pressure in the braking system should not have been, at any time, lower than this pressure. With approx. 0,3 bar it was just enough to prevent the brake shoes from making contact, at the same time, keeping the response distance short and holding the sealing lip cup seals, with a certain amount of force, pressed against the wall of the wheel brake cylinder. 09/11