Duplex brake
The duplex brake is a type of inner-shoe drum brake .
A duplex brake has two cams or wheel brake cylinders, each of which controls an approaching brake shoe. The self-reinforcement of the brake is greater than that of a simplex brake, in which only one of two shoes comes on, i.e. a "servo effect" is developed by "pulling in" the braking effect, while the second brake shoe "runs off", i.e. can push itself out of the braking process and also less worn. If the vehicle drives backwards, the brake has a weaker effect because both jaws then slide off.
Duplex brakes were often installed on high-performance motorcycles and automobiles from the 1950s to 1970s, before disc brakes and brake boosters became popular.
Some English motorcycles of the 1960s and the motorcycles from Münch have double duplex brakes, in which two brake shoes, i.e. a total of four shoes, are actuated by four cams on both sides of the brake anchor plate. However, this type of construction hardly offers any advantages because the overall width of a drum brake makes it necessary to make the shoe sets individually narrower, so the area of the brake linings can hardly be increased with this. In addition, the power distribution (with motorcycles back then with a Bowden cable ) is more difficult and more prone to failure.
The "servo brake", in which one jaw is actuated directly and the second jaw "pushes in front of itself" as the wheel rotates, has greater force amplification than the duplex brake. Therefore, their braking effect is even greater than that of the directly actuated brake shoe. It was common among farm tractors.
The self- reinforcement depends on the coefficient of friction of the drum and brake lining. Even small differences in the coefficient of friction, e.g. B. by moisture, rust film, dirt o. Ä. Therefore, especially with duplex and especially with servo brakes lead to greatly different braking effects with the same actuation force. This means that it is very easy for these types of brakes to pull crookedly.
Because the coefficient of friction decreases with increasing temperature, drum brakes, and especially the types with increased self-reinforcement (duplex, servo, duo servo brakes) have more fading than disc brakes; when the brakes are warm, the actuation force for the same braking effect is greatly increased.
Adjustment of motorcycle duplex brakes with coupling rod
For a good braking effect and, above all, a precise pressure point, an exactly synchronous setting is important for duplex brakes. The following method has proven itself for brakes with a coupling rod:
Use is made of the fact that if the brake is not set perfectly synchronously, the brake anchor plate is pressed a little in the direction of the weaker seated shoe when the brake lever is pulled.
A dial gauge is therefore attached to the immersion tube so that the button can radially contact the brake anchor plate in the area of a brake shoe (not a cam). When pulling the brake lever you can now see whether the anchor plate is moving minimally in the direction of one or the other brake shoe. To synchronize, the coupling rod is adjusted until the clock remains (largely) at rest when the brake lever is pulled.
