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]

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

 

[bra290cja@gmail.com]

Interesting how these "chats" wonder around and mostly repeat info that has never been vetted . Hummm.
Remember the Steel Brake drum expansion rings that were added to the original steel drums to address the fade issue ? They worked as well as they did because they targeted the issue . The rims of the drums do not "Expand" dimensionally equally throughout the circumference . The drum has a greater expansion as you move towards the outer edge of the drum . Because of less material and therefore less rigidity . In other words, the surface contact area decreases as the drum heats . Less area equals less braking . This is very well spelled out in the application of the fix which was the after market expansion rings that restricted the outer edge growth and thus helped to maintain the contact area . Ever wonder why cast iron drums historically had a spring wrapped around the outer circumference . Figure this out and you will have graduated into the realm of understanding brake fade causes .

 

[P.S.]

Interesting how these "chats" wonder around and mostly repeat info that has never been vetted . .

This discussion ended a year ago after nkaminar gave a perfect answer, based on sound engineering. I'm an engineer and see no flaw in the answer nor the math used to arrive at that answer.

 

[Richard Knight]

What nkaminar said. Maybe "less material" should have been in bold type? From the factory the Ford steel drums are not very thick compared to iron drums. Many have been turned or ground and worn so are extremely thin. The ability to absorb and dissipate the heat is greatly reduced to the point of brake fade.

 

[CT Jack]

I agree with Neil regarding the change in the coeffient of friction beteen the drum and brake lining material during braking. The change in the surface properties of the metal drum when contacting the brake lining during the thermal cycle created by braking is small compared to that of the brake lining material. The thermal conductivity or heat transfer of braking lining material compared to the metal drum is poor. Depending on the brake shoe lining material properties used on traditional bake shoes, they generally lack the capability to dissipate or conduct heat and tend to rapidly develop surface characteristics such as hardening or glazing. Once this occurs the coeffiient of friction is greating reduced resulting in a condition called brake fading. Many of the modern day disc brake system components utilize materials that can both readily conduct heat and wear during severe braking cycles. Items such as ceramic pads and air cooled rotors help to achieve less fading. Again, the key physical principle controlling factor affecting braking is the kinetic energy produced during the braking cycle and the dissipation of the resultant thermal affects produced.

 

[KMBeers]

As I see it the thermal expansion of the steel brake shoes and the CI or steel drum are going to be similar since they are both in contact with the friction material and seeing the same heat. However brake friction materials show an increase in kinetic friction coefficient between certain temperatures but then experience a sharp drop at very high temperatures. This is the basis for brake fade. Depending on the material, that drop off temperature varies significantly. This is why high end brake systems on many modern road cars as well as race cars use carbon material for the brakes that have kinetic coefficient of friction +3X standard brake materials and they don’t fade until the temps get really high.