What Causes Brake Fade?
Clyde Kirkpatrick
Santa Barbara, CA
Most of us who have owned British sports cars, especially those fitted with drum brakes, have experienced the frustration of brake fade, and that helpless, empty feeling after a series of high speed stops of pushing harder and harder on the brake pedal, only to have the car slow down with the agility of an ocean liner. This is most distressing in a race or rally when the car you are 'slip streaming' decides to slow down for the next turn early! You have also heard many people exclaim, "Yep, the brakes overheated"! Well why should heat cause such a loss of braking efficiency? Let's look at a typical drum/shoe brake system.
When the pedal is depressed, the brake shoe is pushed up to make a dragging contact with the inside surface of the rotating drum. This (hopefully) stops the car. In doing so, it generates tremendous heat from the friction. What takes place next is the mysterious fandango known as 'brake fade', often followed by expletives from the driver we can't print here. Here's what happens: The brake lining material is made up of compressed particles, usually fiber asbestos. Sometimes, in high performance or competition linings, it also includes bits of other materials (metallic sintered linings, etc.). Whatever the composition, and it can vary from different manufacturers, the whole mess is held together by adhesives. It is these adhesives, when the temperature rises, that cause all the problems. The high heat actually vaporizes the adhesives into a gas. As most of us who stayed awake during sixth grade science class remember, when a solid element is heated into a gas, there is expansion. This expanding gas creates an opposing force between the brake drum and brake lining, trying to force the two surfaces apart. The harder you push the pedal, the more heat there is generated. The more heat,the more vaporizing gas, and the resultant increase in brake fade. In the old days of affordable, do it yourself SCCA racing, there were many attempts made to cool the brakes. People ventilated the backing plates, drilled them full of holes, purchased finned brake drums, cut air scoops in the front wings, or ran flex hose from behind the grille through the wheel well to point at the brakes. One trick used by crafty competitors was to take a hacksaw and cut diagonal grooves part way into the brake lining surface to provide channels for the gas to escape, kind of like the tread pattern of a tire providing escape for water when the rack is wet. How much this helped is questionable. It did increase the wear rate however. None of the above modifications would seem logical for normal street driving, unless you live on top of a very high mountain and get brake fade just going to the mailbox.
I have used replacement brake linings from Moss Motors for many years and find them exceptional for both every day street use as well as hard driving club events. It pays to stick with a supplier that is as enthusiastic about these wonderful old cars as we are. The best cure for brake fade, of course, is disc brakes, but for those of us with older cars who refuse to march to anything but the beat of drum brakes, take heart. Next time you are careening downhill, somewhat out of control, just tell your navigator the car has a bad case of gas!
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