What causes brake fade?
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!
Clyde Kirkpatrick
Santa Barbara, CA
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