Brake Fade

[David in San Antonio]

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.

[Bruce_MO]

I always understood that one contributor to fade was the heating of the metal brake shoe results in expansion of the shoe in such a way that the radius of the shoe decreases a bit. So the shoe makes less contact with the drum (which has expanded). So there’s a double effect of reduced lining contact to drum. Can’t recall where I “learned” that, as it was long ago. I can’t disagree with any of your other comments.

[nkaminar]

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The expansion rate of cast iron is 5.8 X 10 -6 in/in/degree F. So if the drum gets heated 500 degrees hotter the expansion of a 11 inch diameter drum is about 0.032 inch or 0.016 on the radius. The shoe is going to expand too which will make up some of this difference. (You can think of the drum and shoes as one body that is heated uniformly. Not really true but a good way to think about it.)

The expansion rate of steel is slightly higher about 7.0 X 10-6. The steel drums on the Model A were thin compared to the cast iron drums so there was less material for the heat to be dissipated into.

Part of brake fade is due to the expansion of the drum but another big factor is the change in the coefficient of friction of the brake shoe lining. As the lining gets hotter the coefficient of friction decreases. The point where the coefficient of friction decreases is called the knee in the coefficient.

Some gas is generated when the brakes get hot but this is not a factor in the loss of braking.

For more information, and some ideas on how to improve your drum brakes, see https://en.wikipedia.org/wiki/Drum_brake