The hey-day of the drum brake is coming to an end. This is also valid - although somewhat delayed - for utility vehicles. Despite the fact that the various drum brake types are still part of the trade-school examinations, the Duplex- and Twin-Duplex brakes have dissapeared decades ago. As long as the drum brake is still around, only the Simplex- and the Duo-servo-assisted drum brake are, in any way justifiable.
By clicking on the buttons you can see various types of drum brakes. Provided that the drum moves in an anti-clockwise direction, the Simplex drum brake has the trailing shoe to the right and the leading shoe to the left. The shoe is called leading, if a certain point of the rotating drum passes the spreading mechanism (in this case the wheel cylinder) before passing the anchor-pin side. This creates a wedging effect, which produces a relatively large braking force in spite of only little pressure on the brake (self enhancement). With the trailing shoe the effect is the opposite.
The Duplex brake has two leading shoes in the roll direction (i.e. on the left). Reversing however, causes these to wear. You can see a good example of a Duplex brake here. With the Duo-Duplex brake both brake shoes are leading in both directions. Indeed, in this case, two brake cylinders, each having two pistons are also necessary. The Duo-servo-assisted brake is the most elegant, because it is simpler and utilises the wedging effect of the first leading shoe for the second shoe as well. The small, but significant difference to the Simplex brake, is that the two brake shoes are connected to each other by a tongue. In addition, a stopper on the wheel cylinder is necessary. Depending on the type of stopper, the self-enhancing feature is effective when driving forwards and when reversing.
Since the appearance of pneumatic- and hydraulic brake servo-assistance, the self-enhancement is less important. On the contrary, on the front axle it is definitely undesirable, because unequal friction forces on the right and left may be strengthened. The brake has a tendency towards lopsidedness. Nowadays, where only disc brakes are used on the front axle, the problem doesn't really exist any more. 02/09