Das 1x1 der Trommelbremsen: Unterschied zwischen den Versionen

The photo below highlights the location of a pivot (blue arrow) which has a snap ring allowing removal and replacement of the brake shoes. A spring is hidden under the brake shoes, except for its ends, highlighted by the green arrows. The spring retracts the brake shoes when the brake is released.

The next picture shows how a lever rotates the cam (red arrow) and spreads the brake shoes. When the brake is in use, the cable housing attaches at the C-shaped fitting which my thumb is holding, and the inner wire of the cable is clamped to the end of the lever. The hole at the end of the reaction arm is for a clamp band that secures the reaction arm to the frame or fork.

The shape of the cam affects the operation of the brake. As the elongated cam of this brake (typical of bicycle drum brakes) turns to apply the brake harder, the mechanical advantage increases. The hand lever must be pulled farther, and the force therefore does not increase as quickly as might be expected. Also as the brake shoes wear, the mechanical advantage increases. A cam with a spiral-shaped surface on each side would result in a linear relationship, more like that of other brakes, but also increased friction, so the cam might not retract the shoes, cable or hand lever.

Bicycle brakes use a simplex mechanism for the sake of mechanical simplicity. One end of each brake shoe is attached to a pivot, hardly moves outward, and wears little. The leading shoe is the one where the brake drum is rotating toward the pivot; vice versa for the trailing shoe. The rotation of the drum tends to make the leading shoe "dig in," pressing harder against the drum, and removing some load from the cam, while the corresponding effect on the trailing shoe tends to reduce its load on the pivot and put additional load on the cam. At first, the leading shoe wears faster, but the cam produces equal displacement of both shoes (give or take the small difference in radius at which the cam acts). For this reason, the force on the two shoes, and the wear, equalize themselves once the brake has worn in.