Why Do Drum Brakes Lock Up When It Rains?

[HavenFitz]

Drum brakes lock up when it rains because the contact area between the brake drum and brake shoes rusts and sticks together after being exposed to rainwater. Here is more information about wheel lock-up: 1. Introduction: Wheel lock-up refers to the sudden locking of wheels when braking, causing the vehicle to lose steering control, typically occurring in vehicles without ABS (Anti-lock Braking System). 2. Phenomenon: In cars without ABS, if the brake pedal is pressed hard while driving, the wheel speed rapidly decreases. When the braking force exceeds the friction between the wheels and the road, the wheels lock up. Fully locked wheels increase the friction between the tires and the road, potentially causing skidding and loss of control under high inertia. If the front wheels lock up, the driver cannot steer the vehicle. If the rear wheels lock up, skidding is more likely. 3. Main Hazards: Wheel lock-up occurs due to excessive braking force between the brake pads and discs or drums. However, due to the vehicle's inertia, the car continues moving forward, with the tires sliding rather than rolling, making it impossible to change direction and leading to accidents. Even with low inertia, on crowded roads or when following too closely, sudden wheel lock-up and vehicle stoppage can easily cause rear-end collisions. If the front wheels lock up before the rear wheels, the car loses steering ability. If the rear wheels lock up first, it can cause skidding.

[GideonFitz]

I found that drum brakes tend to lock up in rain mainly due to their sealed structure. The brake drum encloses the brake shoes like an iron can, allowing rainwater to seep into the gaps but preventing drainage. During my last repair, I noticed that cast iron brake drums rust when exposed to moisture, creating uneven surfaces. When the brake shoes press against these rough drum walls, the friction increases dramatically, causing lock-up. Additionally, the adjusting springs inside drum brakes are prone to sticking when wet, making the brakes hard to release. I've seen many older cars parked in the rain with rear wheels completely locked, requiring hammer strikes on the drum housing to get moving. Actually, regularly cleaning accumulated water from the brake drums can alleviate this issue.

[VonRicardo]

The issue of drum brake lock-up is related to material properties. The brake shoes used in drum brakes contain asbestos or metal components, which are highly absorbent. After rain, the shoes absorb water and swell, directly jamming inside the brake drum. I've measured that soaked shoes can expand by more than 0.5mm, completely eliminating the original clearance. Cast iron brake drums are even worse—just eight hours in the rain can form a layer of rust. Rust particles get stuck between the shoes, like pouring sand into gears. When I drove trucks, parking overnight in the rain would guarantee lock-up the next day. Later, I adapted by pumping the brakes a few times before parking to fling out the water, which improved the situation. Ultimately, it’s the combination of a sealed space and water-absorbing materials that causes the problem.

[Valerie]

Preventing drum brake lock-up hinges on maintenance. After each car wash, remember to step on the brakes a few times at low speed to fling water off via centrifugal force. During rainy seasons, avoid using the parking brake when parked; securing the vehicle in gear is safer. I inspect the drainage holes of the brake drums every month, using a wire to clear any clogged holes from mud. Last time, I found the return springs of the brake shoes were rusted, so I replaced them in advance to avoid jamming. Choosing brake pads also matters; ceramic composite materials are more waterproof than traditional asbestos. These small practices can significantly reduce the chance of lock-up in rainy weather.