335
Front brake problems
Gordon Hesketh-Jones from Cornwall (Harvest Gold 1904) relates a history
of his various changes to his brake pads and disks and also sheds
some light on "fade" and "warped disks" (July
05)
It did not take
me many months of owning a Factory MGBV8GT to realise that the front
brakes were not adequate for my combination of high-mileage fast
road use during the week, then club rallies, autotests, and sprints
at the weekend. I arranged to visit the Competitions Department
of Ferodo which was then at Chapel le Frith and they recommended
DS11 pads for the front and matching VG95 linings for the rear.
Using these totally transformed the brakes leading to a total eradication
of brake fade, although occasionally some brake squeal. I was already
using drilled disks and found that these and also the DS11s would
last for between 70,000 to 90,000 miles whilst the VG95s seemed
to last forever. In 1999 the brake squeal became unbearable and
as whilst I was converting the front suspension to coil-over-shock
absorber I took the calipers off and had them reamed out. I checked
the disks with a clock gauge but they were fairly new and the surface
was free of ridges.
Having read that
the EBC Kevlar "Greenstuff" pads had been used on racing
motorbikes I fitted a set, as the asbestos-based DS11s were becoming
difficult to obtain. All seemed well but in 18,000 miles the disks
"warped" even though on my past history they should have
had plenty of life left in them. This time a set of EBC's own grooved
and drilled disks were fitted. The EBC pads seemed to wear out quickly
although they did this without depositing great piles of dust onto
the wheels and a new set was needed after just 32,000 miles.
In less than 6,000
miles the EBC disks warped badly during the Danish rally and EBC
gave me a credit for the disks, but this time I went back to Clive
Wheatley's drilled disks as I know he buys them from the original
manufacturer which supplied the Factory. Sadly Clive's disks also
warped, this time after just 7,000 miles so obviously research and
changes were required as the repeated changes of disks was both
time-consuming and expensive. It seemed clear to me that the EBC
pads were totally unsuited to my driving style and usage.
A posting on the
V8 Register Bulletin Board brought a reference by Paul Wiley from
Surrey to an internet article by StopTech LLC based on their 40
years work in professional racing including the famous Shelby/Ford
GT 40s. This fascinating
article runs to seven pages so I will just summarise some of
the facts and conclusions. The web address for the article is at
the foot of this V8NOTE:
There are two
different types of braking friction - abrasive and adherent
friction:
Abrasive friction
Abrasive friction involves the breaking of the crystalline bonds
of both the pad material and the cast iron of the disc. The breaking
of these bonds generates the heat of friction. In abrasive friction,
the bonds between crystals of the pad material (and, to a lesser
extent, the disc material) are permanently broken. The harder material
wears the softer away (hopefully the disc wears the pad). Pads that
function primarily by abrasion have a high wear rate and tend to
fade at high temperatures. Most OEM pads work by abrasion friction
and when these pads reach their effective temperature limit, they
will transfer pad material onto the disc face in a random and uneven
pattern. It is this "pick up" on the disc face that both
causes the thickness variation measured by the technicians and the
roughness or vibration under the brakes reported by the drivers
and often but wrongly described as "warped disks".
Adherent friction
With adherent friction, some of the pad material diffuses across
the interface between the pad and the disc and forms a very thin,
uniform layer of pad material on the surface of the disc. As the
friction surfaces of both disc and pad then comprise basically the
same material, material can now cross the interface in both directions
and the bonds break and reform. In fact, with adherent friction
between pad and disc, the bonds between pad material and the deposits
on the disc are transient in nature - they are continually being
broken and some of them are continually reforming.
Turning now
to the disks themselves, cast iron is an alloy of iron and silicon
in solution interspersed with particles of carbon. At elevated temperatures,
inclusions of carbides begin to form in the matrix. In the case
of the brake disk, any uneven deposits - standing proud of the disc
surface - become hotter than the surrounding metal. Every time that
the leading edge of one of the deposits rotates into contact with
the pad, the local temperature increases. When this local temperature
reaches around 1,200 or 1,300°F, the cast iron under the deposit
begins to transform into cementite (an iron carbide in which three
atoms of iron combine with one atom of carbon). Cementite is very
hard, very abrasive and is a poor heat sink. If severe use continues,
the system will enter a self-defeating spiral - the amount and depth
of the cementite increases with increasing temperature and so does
the brake roughness.
Brake disks
cannot warp - what we have always referred to as a warped disk
is caused in fact by uneven layers of pad transfer or of cementite
build-up on the disks.
OEM pads are
not designed for high-speed use and if you attempt this you
will get brake fade either by exceeding the temperature rating of
the disks, by
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friction material
transfer, or by causing your brake fluid to boil.
New disks
and pads need to be run in
The bonding resins in the pads must be burned off relatively slowly
to avoid both fade and uneven deposits. The procedure is several
stops of increasing severity with a brief cooling period between
them. After the last stop, the system should be allowed to cool
to ambient temperature. Typically, a series of ten increasingly
hard stops from 60mph to 5 mph with normal acceleration in between
should get the job done for a high performance street pad. During
pad or disc break-in, do not come to a complete stop, so plan
where and when you do this procedure with care and concern for
yourself and the safety of others. If you come to a complete stop
before the break-in process is completed there is the chance for
non-uniform pad material transfer or pad imprinting to take place
and the results will be what the whole process is trying to avoid.
In terms of stop severity, an ABS active stop would typically
be around 0.9 G's and above, depending on the vehicle. What you
want to do is stop at a rate around 0.7 to 0.9 G's. That is a
deceleration rate near but below lock up or ABS intervention.
You should begin to smell pads at the 5th to 7th stop and the
smell should diminish before the last stop. A powdery grey area
will become visible on the edge of the pad (actually the edge
of the friction material in contact with the disc - not the backing
plate) where the paint and resins of the pad are burning off.
When the grey area on the edges of the pads are about 1/2"
deep, the pad is bedded. Note that pad break-in procedure can
vary between manufacturers so if fitting non OEM pads, ask your
supplier for the recommended procedure.
Is there a
"cure" for discs with uneven friction material deposits?
The answer is a conditional yes. If the vibration has just started,
the chances are that the temperature has never reached the point
where cementite begins to form. In this case, simply fitting a set
of good "semi-metallic" pads and using them hard (after
bedding) may well remove the deposits and restore the system to
normal operation but with upgraded pads. If only a small amount
of material has been transferred i.e. if the vibration is just starting,
vigorous scrubbing with garnet paper may remove the deposit. As
many deposits are not visible, scrub the entire friction surfaces
thoroughly. Do not use regular sand paper or emery cloth as the
aluminium oxide abrasive material will permeate the cast iron surface
and make the condition worse. Do not bead blast or sand blast the
discs for the same reason. The only fix for extensive uneven deposits
involves dismounting the discs and having them Blanchard ground
- not expensive, but inconvenient at best. A newly ground disc will
require the same sort of bedding in process as a new disc. The trouble
with this procedure is that if the grinding does not remove all
of the cementite inclusions, as the disc wears the hard cementite
will stand proud of the relatively soft disc and the thermal spiral
starts over again. Unfortunately, the cementite is invisible to
the naked eye.
From a personal point of view I can now see that the worst
thing you can possibly do is to arrive "hot" at a rally
control and sit there impatiently with your foot on the brake pedal
whilst the Noble Navigator gets the card time-stamped - this will
guarantee the fatal maximum transfer and even "imprinting"
of the pad outline onto the disk.
The use of race pads on the road can invalidate your insurance
so although I wanted better braking I had to watch the insurers'
requirements. EBC now offer their "red" and "black"
series for competition use and the original "green" pads
for road use. They say that their most recent pad formulations not
only have better thermal conductivity but also include a mild abrasive
to gently scrub the discs to prevent high spots, but frankly I never
want to see an EBC pad or disc again. I also looked at the very
comprehensive websites of Wilwood and Hawk but whilst they could
offer very expensive full disc/pad 4 caliper sets, I could see no
simple pads to suit my car.
Having worked out that the EBC pads were at the root of my expensive
problems, I visited the Ferodo website www.ferodo.co.uk from which
I learned that the DS range still exists although no longer based
on asbestos and is now made in Belgium. Their DS3000 range is for
full race applications but the DS2000 and DS2500 are specified for
fast road use and seemed to suit my requirement, however only the
DS2000 comes in our "823" pad size. You cannot buy these
pads at normal car part wholesalers - only from a number of specialist
dealers around the UK. A list of the dealers is on the website and
the people I contacted at Tavistock were very knowledgeable and
helpful; they commented that by modern standards, the 823 pad was
quite small in respect of our car's weight and power.
With the DS2000s in place I tried the above running-in process
on the previous set of disks but they were probably too far gone
in respect of material transfer so yet another set of Clive's
drilled disks were fitted then run in just before we went off on
the 5,800 miles of V8 GOES EAST trip.
Marvellous - no squeal, no fade, constant pedal feel, and worry-free
braking. There was slightly more brake dust on the front wheels
than with the EBC pads but not sufficient to worry about. After
8,000 miles the pad wear seems to be negligible. All I have to do
now is to find out where I can buy sufficient garnet paper to clean
up the three sets of drilled disks stored in the roof of my workshop.
Once this has been done I should have enough sets of disks to last
for the rest of my life.
Paul Wiley contributed some useful comments on the draft of this
note and provided several useful websites as references - see below.
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