A bicycle brake is used mostly when rolling forward, though also to prevent the bicycle from rolling backward when stopped facing uphill. Which shoe is the leading one depends on whether the wheel is rolling forward or backward. For this reason, the force on the two shoes, and the wear, equalize themselves once the brake has worn in. 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). 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. The leading shoe is the one where the brake drum is rotating toward the pivot vice versa for the trailing shoe. One end of each brake shoe is attached to a pivot, hardly moves outward, and wears little. 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.īicycle brakes use a simplex mechanism for the sake of mechanical simplicity. Also as the brake shoes wear, the mechanical advantage increases.
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The hand lever must be pulled farther, and the force therefore does not increase as quickly as might be expected. As the elongated cam of this brake (typical of bicycle drum brakes) turns to apply the brake harder, the mechanical advantage increases. The shape of the cam affects the operation of the brake. The hole at the end of the reaction arm is for a clamp band that secures the reaction arm to the frame or fork. 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 next picture shows how a lever rotates the cam (red arrow) and spreads the brake shoes. The spring retracts the brake shoes when the brake is released. A spring is hidden under the brake shoes, except for its ends, highlighted by the green arrows. The photo below highlights the location of a pivot (blue arrow) which has a snap ring allowing removal and replacement of the brake shoes. The hub shell with the drum is at the top and the brake shoes on their backing plate are at the bottom. The photo below shows a Sturmey-Archer drum brake partially disassembled. Most automotive drum brakes are operated by hydraulic pistons, but bicycle drum brakes use a cable with a linkage to a cam that pushes the brake shoes outward. When the brake is released, springs pull the brake shoes inward, away from the brake drum. The brake shoes are mounted on a backing plate which is attached to the hub axle and to a reaction arm connected to the frame or fork to resist torque. The drum is part of the hub shell, or is attached to it, and slows the wheel.
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Their pressing outward against the inside of the drum produces friction. How does a drum brake work?Ī drum brake has two brake shoes opposite one another, actuated by a mechanism which pushes them apart at one or both ends. Special, large drum brakes have been used as drag brakes on tandems for heat dissipation on downhill runs.Ī Shimano Rollerbrake is a special kind of drum brake and is covered in a separate article. Since the torque from the brake is opposite that from a freewheel, the threads are right-handed, like those for the freewheel.Ī front drum brake may be combined with a hub generator, and a rear drum brake, with an internal-gear hub. Drum brakes are popular on utility bicycles in countries with wet weather much less so for recreational and racing use, where rim brakes predominate and disc brakes are becoming more popular.Ī drum brake may be an integral part of a hub, or may thread onto the left side of the hub the same way a freewheel threads onto the right side. Drum brakes were once the most common type for motor vehicles, though disc brakes are now more common. A drum brake is a hand-lever-operated hub brake with brake shoes that press against the inside of a cylindrical drum.