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Old 08-29-2024, 12:10 PM   #1
David in San Antonio
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Join Date: Sep 2022
Location: San Antonio Texas
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Default Brake Fade

I think there is a tendency for people who try to be helpful to repeat what they’ve read online, not realizing the source material is inexact, misleading, or untrue.
The purpose of this post is to hear back from someone who has the engineering or technical background to confirm or correct the following:
When disk brakes became more common on American cars I read an explanatory article.
It said
1. the retardation effect is based on the physics of kinetic energy (rotation of the drum) being converted via friction to thermal energy (heat) which is transmitted from the shoes to the drum, then from the drum to surrounding air.
2. When the brake drum is heated to the point where it cannot absorb further thermal energy, and air circulation is unable to carry it away as fast as the thermal energy is being put into the system, there is no further retardation effect. That is brake fade.
3. Boiling the brake fluid is a related but separate problem (obviously not an issue with mechanical brakes).

Since joining the Model A community I’ve read and heard repeatedly that
4. brakes fade when heat causes the drums to expand away from the shoes thereby reducing the friction.
5. Cast-iron collars added to steel drums restrict the expansion, and
6. cast iron drums are stronger than steel drums so they do not expand away from the shoes as much.
7. This “expanding away from the shoes” explanation seems questionable because it could be solved by arranging the brake shoes to have additional travel.
8. If that article I read six decades ago was accurate, the cast iron collars and drums simply have greater thermal mass and perhaps enhanced ability to transfer heat to the surrounding air.
What parts are true / incomplete / misleading / inaccurate?
Thank you.
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David in San Antonio
Late ‘30 Deluxe "Wretched Roadster"
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Last edited by David in San Antonio; 08-29-2024 at 12:17 PM. Reason: A phone makes a lousy word processor.
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