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

Back to Contents listing

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.

www.stoptech.com/whitepapers/warped _rotors_myth.htm
www.turnfast.com/tech_brakes/brakes_balancing.shtml
www.teamscr.com/grmbrakes.html
www.apracing.com/car/brakepad/performance.htm