what causes brake fade?

Back when I was much younger; my buddy and I drove 60’s mustangs around. Of course we had to experiment with braking hard and often, just to get our brakes to stop working. Its pretty weird, stomping on the brake with all your might and barely slowing down.

What’s the mechanism for this? Does the brake fluid “boil” so the vapor can’t transfer the force to the pads. Does the coefficient of friction on the pads somehow mysteriously change when they get really hot?

What up wit that?

Wiki says both, though lists your second reason first.

Didn’t 60’s Mustangs have 4-wheel drum brakes?

When I read the OP’s title, I immediately thought of disc brakes when I read the word “fade”.

From what I remember in high school physics, your breaks turn the rotational energy into heat via friction. If the heat isn’t dispersed, you can no longer stop. That’s why high performance disc brakes have vents in them to stay cooler and stop harder faster longer.

So where does friction go? It seems odd that friction would just stop happening because the objects involved were hot.

The friction doesn’t “go” anywhere. When the material gets hotter, the physical properties of the material change. Including the amount of friction that the material can produce.

This explains it to some degree (from here): "Pad fade occurs for several reasons. All friction material (brake pad stuff) has a coefficient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is the highest. Sometimes you can use the brakes so hard that you get the temperature over the point of maximum friction to where the coefficient of friction curve starts to decline.

“The mechanics of this decline in the coefficient of friction are varied. At a certain temperature, certain elements of the pad can melt or smear causing a lubrication effect, this is the classic glazed pad. Usually the organic binder resin starts to go first, then even the metallic elements of the friction material can start to melt. At really high temperatures the friction material starts to vaporize and the pad can sort of hydroplane on a boundary layer of vaporized metal and friction material which acts like a lubricant. Pad fade is felt as a car that still has a decent, non mushy feeling brake pedal that won’t stop even if you are pushing as hard as you can. Usually it builds somewhat slowly giving you time to compensate for it ,but some friction materials have a sudden drop off of friction when the heat is put on them resulting in sudden dangerous fade.”

I’m sure more thorough explanations are available. From what I’ve seen it is common for the coefficient of friction between two materials to change with temperature, and that applies to more than just brakes.

I do know that brake linings for a vehicle are selected with the temperature factor in mind. For example, race cars have linings that hardly work at slow speeds, but work spendidly when they get hot enough, typically by braking hard at 100+ mph. Ordinary passenger car linings would pretty much stop working at those temperatures.

Isn’t there something about a part in drum brakes expanding, causing it to actually lose mechanical contact with the inside of the drum?

Yes. I once drove a car that had rear drum brakes with the handbrake partially on for some distance on the highway. When I pressed on the brake pedal, it went right to the floor because of drum expansion. After the drums cooled down the braking was perfectly normal.

Expansion aside, drum brakes are more prone to fade because they don’t dissipate heat as well as disc brakes.

How do you know it wasn’t boiling brake fluid? It seems to me that if the drum expanded so much that the shoes couldn’t reach, how was contact maintained with the handbrake?

Perhaps the spring that keeps the pad in the right position as it wears would push the pad out as the drum heated up, reaching an equilibrium position at the full extension of the spring?

I pulled into a shop and inspected the drums while it was acting that way, and they were indeed oversize. The front brakes would apply with a pump of the pedal. That said, I do not know with absolute certainty that there wasn’t also some boiling of the fluid.

The mechanism would move the shoes out to a certain point as the drums expanded. Once the drums got hot enough to lose touch, it would probably take quite a while for them to contract, given the lack of airflow to the drums. As I recall, it was about 30 minutes with the wheels off for them to return to normal.