Brake rotor warping - yet again!

["John Fielding" <johnf@domain.elided>]

Hi All,

First off may I wish everyone a peaceful and safe Xmas & New Year and many more happy
hours reading the AD in 2003.

To the brake rotor saga.

I think many people are confused what the symptons of brake rotor warping and other
related faults are.  A warped brake rotor causes the caliper pistons to be forced back
into the calipers as the rotor has some run out.  Run out is simple to check with the
wheel off and a magnetic stand on a dial test indicator.  If the rotor is warped it should
be replaced.  What causes rotor warping?  There have been postulations, myself I think the
biggest culprit is inferior grade cast iron or manufacturing methods.

There are also other faults connected with brake rotors which lead people to believe the
rotor is warped.  Change the rotors for new ones and the problem goes away.  Reason: the
rotor was warped.  Maybe not.

There is another common fault which is to do with the varying porosity of cast iron.  The
minute particles of scrubbed off pad embed into the rotor surface.  As long as the
porosity is uniform this doesn't normally cause a problem.  However, if the porosity
varies the amount of pad material also varies.  This gives a sympton similar to rotor
warpage, but it isn't quite the same.  True rotor warpage causes "pad-knock-back" as it is
commonly called.  Varying porosity cause the brakes to judder or snatch as the rotor
rotates against the pad.  Knock-back can be felt through the brake pedal and the steering,
not as well on a servo or power steering assisted car as the older ones with plain braking
and steering systems.  Pad varying "stiction" doesn't cause any sensation in the brake
pedal but the vehicle judders under braking and can be felt through the steering. As the
pad material builds up on the rotor the symptons get worse.

Often a change to a different make of pad will eliminate pad stiction.  Resurfacing the
rotors will also make the symptons go away as you now have a virgin surface for the pads
to rub against.  On my race cars the brake pads are made by EBC and are a compositon of
carbon fibre and kevlar.  The pads are much softer than the normal ones and can operate at
higher temperatures which equates to superior stopping power, less fade and close to zero
rotor wear.  All of this at a higher cost when new pads are required.  However, there is
never any squealing or judder no matter how hard you brake.  In fact EBC guarentees that
brake squeal or judder will not occur provided the rotor surfaces are good when the pads
are fitted.

My wifes car (Honda Ballade) has normal Mintex pads and I had the rotors resurfaced when
the front brakes and suspension were overhauled a couple of years ago.  This car suffers
from pad stiction and the way I cure it is to take the car for a serious thrash down the
hill nearby and do a few emergency stops.  My wife is too gentle on the brakes and this
causes the stiction to reoccur every few months!

At first I thought the rotors were warped, but a check with a dial test indicator showed
the run-out to be close to zero, certainly not enough to cause the juddering and the rotor
surface looked good.  The wheel bearings were also checked and found to be perfect.  So I
changed the pads for another partly worn set and the problem went away.  Some months later
when I drove the car I noticed the juddering was back.  My wife is particularly
insensitive to noises and feelings on the car, in fact I think the car could collapse
around her and she wouldn't notice!  I tried a couple of crash stops to see if the
symptons would get worse and was surprised to find that they got better.  I asked a friend
who owns a brake surfacing shop what the problem was and he clued me up to the problem.
He had resurfaced the rotors originally and I know he does a good job so I trusted him.
His opinion is that certain rotors need a particular make or composition of pads to work
best.  He doesn't like the newer types of pads with fancy metal.  He sees thousands of
rotors and brake drums a year through his shop and prefers the older fashioned type of
material.  His opinion - not mine, so don't flame me for this!

John
Durban
South Africa
Alfetta 1.8L turbo
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