While the basic operating principles of the brake systems on exotic supercars are essentially the same as those we see on any other automotive brake system, the carbon fibre brake rotor shown on this exotic Italian supercar represents the pinnacle of automotive brake technology. Sadly, though, very few of us will ever encounter the high-tech brake systems of supercars, but many of us are often faced with problems in the brake systems of lesser vehicles that represent the next step down in terms of brake technologies and performance.
These vehicles typically fall into the muscle car category, and the problems that we see most often on these vehicles include brake fade, brake noise, rapid or uneven brake wear, pedal pulsation, and inconsistent braking performance, among others. Thus, in this article, which is the first in a two-part series, we will take a closer look at some of the things that make high-performance brake systems different from "normal" brake systems, as well as what to look out for when servicing high-performance brakes. Let us start with saying that-
The set of brake pads shown above were not designed to improve the braking performance of wannabe street racers; the particular brake pads shown here were developed to satisfy the specific braking demands of a high-performance application. This means that in practice, high-performance brake pad formulations are generally not interchangeable since the efficiency of the brake systems on high-performance vehicles depends entirely on the tribological* effects that the combination of brake pad formulations and rotor designs produce.
* Tribology is highly interdisciplinary science that uses principles and laws from physics, chemistry, biology, materials science, and multiple branches of engineering and mathematics to study the effects of friction, lubrication, and wear rates of disparate materials that are in relative motion. As this applies to automotive brake systems, tribology is the underlying science (or combination of sciences) that OEM manufacturers use to develop both friction materials and brake rotor designs to meet the braking demands of high-performance vehicles.
This is of course not the same as saying that OEM manufacturers do not apply tribological principles to the development of friction materials and brake rotor designs intended for use on “normal” passenger vehicles. In practice, the brake systems on “normal” vehicles hardly ever reach the temperatures at which tribological effects produce symptoms like brake fade, brake shudder/noise, and accelerated wear of friction components.
However, the efficiency of brake systems on vehicles like Ford Mustangs, Dodge Chargers, and high-end BMW’s (among others) that typically weigh about 2000kg, depends entirely on which tribological effect is produced at any given point in a braking event. Therefore, and unlike the friction materials and rotor designs on “normal” vehicles that often produce satisfactory braking performance over a wide range of applications and operating conditions, the friction materials on high-performance vehicles are a) application-specific more often than not, and b), intended to be used in conjunction with a specific rotor design, which is why-
The selection of brake rotor designs shown above is a good case in point. These rotors are not slotted and/or cross-drilled to make wannabe street racers look good. Each rotor design shown here was developed to satisfy the specific demands of a particular application, which means that in practice, rotor designs are generally not interchangeable since the efficiency of brake systems on high-performance vehicles depends entirely on the tribological* effects that the rotor design/pad formulation produce on that particular application. So, what does this mean in practice when we are confronted with say, a late model Ford Mustang that exhibits severe brake fade?
The above question has many answers, each of which touches upon not only different aspects of the overall design of the brake system but also on how well (or otherwise) we understand exactly what happens in the interfaces between brake pads and brake rotors when the brakes on a 2000-kg car are applied during an emergency braking event. Let us start with the easy part, which is explaining why high-performance brake rotors have holes and/or slots in them to aid in -
We all know that cars do not stop because of the friction that is generated between brake pads and brake rotors, and we, therefore, do not have to delve into the mechanics of how kinetic energy is converted into heat. Suffice to say that braking performance of a given brake system is directly proportional to the rate at which kinetic energy is converted into heat (assuming a correct/suitable rotor and pad combination is in use) with the conversion rate being a function of among other things-
* Note that brake rotors, and in particular, ventilated rotors, are generally not heated evenly during braking events: and not even during emergency braking events. In practice, the heat profile of a working brake rotor is cyclical, since any spot that is equal to the contact area between the rotor and pads, heats up when it enters and passes through the pad/rotor interface, and starts to cool down immediately upon exiting the interface area. This heating and cooling cycle is repeated for as long as the rotor rotates against brake pressure, but note that in this context, the word “ventilated” refers to the presence of equidistantly spaced vanes between the two friction surfaces of a brake rotor, and not to the presence of holes or slots in friction surfaces of the rotor.
Thus, having said the above, the primary function of holes and/or slots in brake rotors is to prevent the gasses that are generated during braking from becoming trapped between the brake pad and the brake rotor. These gasses are generated when particularly phenol-based binders and adhesives in the friction material decompose as the temperature of the friction material rises, and in cases where slots and/or holes in rotors are insufficient or incorrectly placed relative to the shape of the pad/rotor interface, the released gasses can lift the friction material off the brake rotor surface.
In practice, this often causes severe brake fade that often progresses into catastrophic brake failure. To prevent this from happening, OEM manufacturers spend enormous amounts of money on tribological research to develop friction materials that offer a high friction coefficient, a reasonable degree of wear resistance to extend component life, as well as a low, or at least, a limited propensity to generate high volumes of gas under extreme operating conditions.
However, since heat is a function of friction, the final formulation of high-performance friction materials is, for the most part, a compromise between the all the various characteristics or properties a high-performance formulation should have. Nonetheless, the adage that says "What you gain in one area, you lose in another", is as true of brake friction materials as it is true of anything else, which means that high-performance friction material formulations typically fall into the semi-metallic, or high ceramic content categories, which are the most expensive formulations to produce.
The last thing we as technicians should be concerned about, however, is a) the price(s) of high-performance brake pads, and b), the exact proportions of the (typically) 30 or so ingredients that go into the making of advanced friction materials since these details are proprietary information and are never published, anyway. We should therefore only be concerned with making sure that we fit brake pads to high-performance vehicles that a), generate sufficient amounts of heat to activate some ingredients in advanced friction materials, and b), fit brake pads whose friction coefficients are not affected negatively by the various-
This image shows a small section of a brake rotor under test magnified 100 times by a scanning electron microscope. The purpose of this particular test was to examine the various types of film that form in the brake pad/brake rotor interface in a temperature range from about 200 degrees Celsius to about 350 degrees Celsius, which is the range of temperatures that typically obtain in high-performance brake systems during normal use. The small area circled in red shows a small, but deep pit that formed as the result of cavitation conditions that had occurred during the formation of a gas cushion film at 300 degrees Celsius.
While the formation of various types of film (also known as "glazing", on brake pads and rotors is a highly technical subject, it is nevertheless necessary to understand at least the basics of these processes and their possible effects on braking performance if we are to understand the principal differences between normal and high-performance brakes. While these processes largely determine the severity of many, if not all of the common brake system issues on performance-oriented vehicles, one case, which involves the formation of friction films on brake rotors is a desirable characteristic of all high-performance brake friction materials, but let us look at film formation processes in more detail, starting with-
We all know that some semi-metallic and ceramic brake pads are designed to transfer and deposit some friction material onto the surface of a brake rotor. We also know that these deposits can vary in thickness from a few microns to several hundred microns, but what may not be commonly known is that these deposits are not stable in the sense that once deposited onto the brake rotor’s surface, these films remain on the rotors’ surface for the life of the rotor.
While the presence of a friction film on a brake rotor greatly enhances braking performance because the film essentially consists of the same material as the brake pad, friction films represent an intermediate stage between two other processes that continually remove and redeposit the friction film. Here is how it works-
When braking occurs, some material is removed from the brake pad in processes that include shearing, cutting, and deforming of binding fibres in the friction material by asperities on the brake rotor. As the rotor rotates, some debris from the wear process is cast out of the interface while others are partly pressed into microscopic pits and valleys in the rotors’ surface. This process eventually produces what is known as a “primary contact plateau”, but as the interface temperature rises, remaining peaks in the primary contact plateau are deformed and shorn off to fill the spaces between asperities in the primary contact plateau.
At this point in the braking event, the resulting film can have one of two possible morphologies (forms); it can be either loose or granular, or it can be a dense, solid sheet known as a “secondary contact plateau”, which covers the entire contact area between brake pads and brake rotors. In practice though, a stable friction film is typically a short-lived phenomenon because increasing pressure and temperatures can and do, break up and transform the dense film into a loose granular structure, some of which material is subsequently re-transformed into a solid, dense state as the loose granular material again fills up the peaks and valleys in the rotor's surface.
The frequency at which this process s repeated is entirely a function of the peak pressures and temperatures in the pad/rotor interface, which are in turn, functions of both the size of the pad/rotor interface and the formulation of the friction material in use. However, since the formation of friction films is a desirable characteristic of advanced friction materials, all manufacturers of high-performance friction materials go to great lengths to ensure that their products produce friction films that are stable for as long as possible under all foreseeable operating conditions, which brings us to-
One of the more notable properties of oxygen is that it reacts more readily with hot metals than with cold metals, which is why oxidation films on brake rotors typically form at the upper end of a rotor's allowable temperature range. In practice, oxygen reacts with hot surface debris on a rotor surface to form a hard, glass-like film, with the hardness and brittleness of the film depending on both the temperature of the pad/rotor interface and the duration of the braking event.
However, since oxidation films are typically extremely brittle, they are quickly destroyed at high brake temperatures, but the downside is that oxidation films can cause severe brake fade, grinding noises, and even some brake shudder as the friction material is prevented from making direct contact with the brake rotor's surface. Somewhat counter-intuitively, the presence of oxidation films (that reduce pad/rotor contact) can cause increased brake wear rates as the hard material is broken up and passed through the pad/rotor interface.
Liquid lubrication films
This phenomenon occurs when some compounds and materials on the surfaces of friction material decompose under high temperatures and pressures to the point where they are suddenly transformed into liquids. This is arguably the most dangerous type of film to form on brake components because the friction coefficients of both brake pads and rotors drop off precipitously when a liquid lubrication film forms.
Worse, though, the drop off in friction coefficients and subsequent loss of braking performance typically cause drivers to apply more pressure to the brake pedal, which raises pad/rotor interface temperatures even more. This, in turn, increases the transformation of more friction material components into more liquid in a vicious cycle that can lead to a condition known as “friction catastrophe”. This condition simply means that all friction between the pads and rotors is lost, which if it happens, translates into the total loss of braking force on affected wheels regardless of the brake force being applied to the brake system on affected wheels.
The formation of liquid lubrication films seldom occurs when high-performance brake rotors are used in conjunction with recommended or application-specific brake pads. However, the formation of these films is common in cases where a) unmatched brake components are used on a high-performance application or b), in cases where slotted and/or cross-drilled rotors are used on applications where solid rotors are recommended- such as might be the case on most normal passenger vehicles, which leaves us with just-
While this article alluded to gas cushion films and the degassing abilities of some brake rotors, this subject is better suited to the next article in this series, in which we will also discuss some practical tips on what to look out for when servicing the specialised brake systems on high-performance vehicles.