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g/auto • Posted by u/road_warrior_rex •

Brake fade on a loaded semi descending mountain grades — drum vs disc comparison?

School question (sorry if this is basic). We're covering braking systems this week and the instructor talked about brake fade on descending grades. He said drum brakes are more prone to fade than disc brakes, but didn't explain why at the physics level.

Can someone explain why drum brakes fade more than disc on extended downhill grades?

💬 3 replies

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u/wrench_monkey •

Real-world observation: a loaded semi with drums on all axles coming down a 6% grade for 10 miles is fighting a losing battle if the driver isn't using engine brake properly. It's not a brakes problem, it's a driver training problem. Teach your drivers engine brake first, service brakes as a supplement.

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u/diesel_dan •

Good question. Drum brakes fade more for two reasons: (1) Heat dissipation — drums are enclosed, trapping heat inside. Disc rotors are open to airflow on both sides, dissipating heat much faster. (2) Self-energizing effect — drum brakes rely on the drum dragging the shoe into contact harder as the shoe heats, which sounds like a feature but at extreme temps the shoe material softens and loses friction. Disc pads don't have this self-energizing geometry.

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u/scan_tool_sam •

Also: disc brakes maintain more consistent pad-to-rotor contact under thermal expansion. Drums can warp and develop hotspots that reduce effective contact area when hot. This is why heavy trucks switched to disc on the steer axle first — steering reliability under fade conditions matters most up front.

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