In Part 1 of this series of articles, we stated that not many of us would regularly (if ever) encounter the type of brake technology shown here on an exotic Italian supercar. Nonetheless, this particular example of advanced brake technology is relevant to this article for two reasons; the first is that it shows an application-specific brake rotor and the second is that it shows an application-specific brake calliper. So why is this important, given that very few of us would ever work on brake systems like this?
The answer to this question involves the fact that even though we do not always realise it, the brake systems on the high-performance muscle cars we do see regularly also depend on application-specific brake rotor designs and friction material formulations to work as intended. In this article, then, we will discuss high-performance brake rotor design, and how a particular brake rotor design contributes to consistent braking performance on high-performance vehicles. Let start by explaining-
Before we get to specifics, we should say something more about the brake rotor shown at the beginning of this article. In this particular case, the rotor is fabricated from a composite material that consists of various forms of carbon, and various metals that include iron, chrome, nickel, and others in powder form. These types of matrices are most commonly bound by Kevlar and other carbon-based fibres that are distributed evenly, but in a specific pattern throughout the matrix. The actual process of fusing the matrix into a solid material is a highly exact and proprietary science, the details of which are never shared with anybody outside of the manufacturing process.
Nonetheless, the final product is light, immensely strong, and thermally stable, which properties characterise the principal differences between these kinds of brake rotors and the heavy, but homogenous brake rotors we see on the high-performance vehicles we do encounter regularly. However, observant readers will have noticed two major differences between the composite brake rotor on the exotic car, and the cast-iron brake rotors in the image shown immediately above.
In case you missed it, the holes in the composite brake rotor are, a) smaller than the holes in the cast-iron rotors, and b), the holes in the composite rotor are arranged in a random pattern. By way of contrast, the holes and slots in the cast-iron brake rotors are both bigger than those in the composite rotor are, and are arranged equidistantly from each other. You may be wondering where this is going, but the point is that these differences exist for specific reasons, the most important being-
Although the brake rotor on the exotic car is made from an enormously strong composite material that is thermally stable, the fact is composite materials are not homogenous, which means that when composite materials fail despite their inherent structural strength, they fail where two disparate materials in the matrix meet. Therefore, the size of the holes and the way the holes are arranged in the composite rotor is calculated to eliminate the potential for fracture lines to develop between adjacent holes under severe operating conditions.
Ventilated cast-iron brake rotors, on the other hand, are typically made of a single casting to form a homogenous product. Moreover, cast-iron brake rotors are typically not subjected to either the high braking pressures or the operating temperatures that obtain in exotic brake systems. Therefore, bigger holes do not present a significant potential for fracture lines to develop between adjacent holes during operation of the brakes under conditions that are considered normal for typical high-performance vehicles.
Gas cushion venting
The various tribological effects* that occur between ceramic brake pads and brake rotors that are made from composite materials are beyond the scope of this article. However, the tribological effects that occur between ceramic brake pads and cast-iron rotors are central to the topic of gas cushions in terms of what they are, how they develop, why they develop, and how they are vented to prevent issues like brake fade, brake noise, and even friction catastrophe**.
* 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.
** Friction catastrophe refers to a condition in which 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.
Before we get to the specifics of how gas cushions affect braking performance, it should be noted that friction coefficients of brake friction materials are not always proportional to either brake pressures or brake interface temperatures. Therefore, there is no single set of circumstances under which a gas cushion will, or could form on any vehicle, although an excessive brake interface temperature is often the trigger that initiates the formation of a gas cushion. Having said that, let us look at the specifics of-
While the exact chemical compositions of advanced friction material (or any friction material, for that matter) are closely guarded trade secrets, what is known is that advanced friction materials contain relatively large proportions of substances such as modified rubber granules, phenols in various forms, and various carbon-based fibres to improve the structural strength of the matrix.
Some of these substances also serve as friction modifiers and lubricants to reduce wear rates, but the problem with these substances is that they are generally not thermally stable, which is another way of saying that most of these substances start to break down or decompose under high temperatures. In this context, “high temperatures” mean temperatures in the 200 degrees Celsius, to about 400 degrees Celsius range, but as a general rule of thumb, the higher the brake interface temperature, the faster most binding compounds in friction materials decompose.
As a practical matter, when binding compounds decompose, they turn into gasses that include carbon monoxide, carbon dioxide, methane, hydrogen, and others, such as argon, with the proportions of these gasses largely being a function of the friction material formulation. While the formation of these gasses is a normal and expected part of how friction materials degrade, some conditions, such as a rapid rise in both brake pressure and temperature can cause a sudden increase in the gasification of some binding compounds. In some cases, the sudden build-up of gasses in the brake interface can be severe enough to form a dense layer of super-heated gas between the brake rotor and the friction material- this layer of gas being known as a "gas cushion".
In practice, a typical gas cushion has two major effects. The first is that the gas cushion effectively prevents contact between the friction material and the brake rotor, and the second is that it exerts an outwardly opposing force to the brake pressure being applied to the brake pads, and in severe cases, the opposing force can exceed the braking force being applied to the brake pads.
Thus, since friction material cannot absorb the gas cushion, and the crosswise slots in brake pads are neither designed nor intended to vent gas cushions, the only effective way of preventing gas cushions from developing to dangerous levels is to vent all gasses that are generated during brake operation through holes or slots in the rotor’s surface.
However, it is at this point that things become complicated because holes or slots in high-performance brake rotors do not only vent gas cushions- they also serve to either prevent the formation of oxidation and liquid lubrication films* on brake rotors or to remove these harmful and dangerous films when they do form, which brings us to this question-
* See Part 1 of this series for details on these kinds of films.
There is no simple way to answer this question, and especially if your customer demands that you replace his worn-out OEM rotors and brake pads with aftermarket parts, which is a procedure that is fraught with plenty of room for very expensive mistakes, and one that should, therefore, be avoided if at all possible.
This is of course not the same as saying that all high-performance aftermarket rotors and brake pads are substandard and possibly dangerous, and should therefore never be used on a very expensive high-performance vehicle: far from it. However, before you decide to replace slotted rotors with drilled rotors, or slotted/drilled rotors with rotors that sport both slots and holes for whatever reason (even if you are going to install OEM brake pads), we recommend that you educate your customer on the following two points-
The above is saying a lot, so let's break it down into more digestible chunks, starting with-
Part 1 of this series mentioned tribological effects like the formation of oxidation and liquid lubrication films on the surfaces of brake rotors, both of which can cause the sudden, and in some cases, the complete loss of braking performance. Part of how car manufacturers prevent or limit the potential for these films to form is to keep the brakes as cool as possible, but herein lays several, and major engineering challenges*.
* Note that since neither the chemical composition nor the formulation of friction materials is known, this section cannot give specific examples of how brake pressure influences variables such as increases and/or decreases in friction coefficients over time. Therefore, this section can only address brake pad deglazing in general terms to illustrate the importance of installing suitable rotor/pad combinations on high-performance vehicles, since effective brake pad deglazing is achieved solely by the correct combination of the friction material formulation and rotor design.
We mentioned engineering challenges, so consider the following three seemingly contradictory technical requirements, which are presented in a bullet-point form to keep things simple-
On the face of it, the conundrums listed above present automotive engineers with very difficult, if not insurmountable problems in developing brake systems that must deliver braking performance that is consistently far superior to what is expected of brake systems on normal passenger vehicles. Nonetheless, necessity is the mother of invention, and engineers in both the OEM and aftermarket environments have come up with brake rotor designs that fulfil all of the requirements listed above. Note though that this proposition comes with a caveat that consists of three conditions; these conditions being that-
While we have digressed a bit from the main topic of this section, this digression was necessary, since it makes it easier to explain the purpose of holes and slots in high-performance brake rotors. Before we get to specifics, however, consider this example of a-
Note the steep angle of the vanes between the two friction surfaces relative to an imaginary line drawn through the centre of the rotor. Although this angle is not always visible or apparent from the outside of the rotor, it has several critically important functions, these functions being-
In designs like this, the rotor can only work on one side of the vehicle, since the angle and/or curvature of the vanes are designed to "scoop up" relatively cool air from around the wheel hub and to "fling" the cool air out of the rotor through the channels between them. While straight vanes on lesser rotor designs do much the same thing, curved or angled vanes can impart higher velocities to the air flowing through them, which increases airflow rates and rotor cooling significantly.
Thus, fitting a rotor with angled or curved vanes to the wrong wheel creates a situation where ambient air is drawn into the rotor from the outer circumference of the rotor and forced towards the centre of the rotor, and since the air is already hot there, rotor cooling is seriously compromised.
So, assuming that the rotor is fitted to the correct side of the vehicle and that the recommended brake pads are in place, let us look at-
As described above, the primary mechanism that cools directional brake rotors is the vanes between the friction surfaces that draw air through the rotor as the rotor rotates. In practice, the cross-drilled holes in a high-performance rotor contribute very little, if anything to the primary cooling process and their only purpose is to vent gasses that develop in the brake interface to the centre of the rotor, from where they are extracted by the rotating vanes.
Unlike holes, slots in brake rotors are not very efficient at venting gasses in the brake interface, but then again, slots are not intended to vent gasses. The primary purpose of slots is to act as “cutting edges” to remove microscopic amounts of material from the pads’ surface every time a slot passes through the brake interface.
Doing this assists in removing oxidation and other types of film that form on friction material surfaces at very high temperatures to ensure optimal contact between the surfaces of brake pads and the brake rotor at all times. Moreover, since the material that is shaved or removed from brake pads by the edges of slots is typically granular, the relatively large cross-sections of slots are instrumental in preventing removed material from becoming trapped in the brake interface.
However, it should be noted that slots in rotors cause rapid wear of friction materials, and it is common to see new brake pads worn down to their minimum thickness in less than 30 000km of use.
The term “hybrid” refers to high-performance brake rotors that have holes and slots in them. As a practical matter, neither cross-drilled holes nor slots on their own can vent gasses and remove surface films from rotors and friction material. Therefore, using rotors with both holes and slots offers the best of both worlds, so to speak, since these rotors can both vent gasses effectively, and maintain optimal contact between rotor and pad surfaces.
Note though that the slots in hybrid rotors cause about the same high rate of brake wear as rotors that have only slots, which begs this question-
When you have to make this decision, the most important thing to keep in mind is not the cost of the replacement rotors. The most important thing is the fact that the manufacturer of the high-performance vehicle you are working on had spent an enormous amount of money on designing, developing, and testing brake rotors and friction materials that work on that vehicle.
Therefore, OEM rotors and friction materials should always be the first choice, but having said that, many aftermarket suppliers of brake parts supply both rotors and brake friction materials that are claimed to meet, and in some cases, exceed OEM specifications in terms of fit, form, function, and performance. It is not for this writer to say whether or not all of these are true (or otherwise) in all cases, but what this writer can say is this-
If an aftermarket supplier claims to produce high-performance brake rotors that, at a minimum, meet OEM specifications, that rotor must be identical to the OEM product in all respects. The aftermarket product must, therefore-
In the real world though, it is impossible to say if all the products of any aftermarket supplier always tick all the boxes in all cases, however, the golden rule and guiding principle in selecting high-performance brake parts should always be the standards and specifications set by the OEM manufacturer, which brings us to this-
Experimenting with high-performance brake parts on wannabe street racers may be permissible up to a certain point, but experimentation or guesswork should never be part of any brake service and/or replacement procedures on any true high-performance vehicle. Therefore, play it safe and protect both your customer and yourself by only using approved high-performance brake replacement parts, because unlike guesswork, tribology is an exact science that does not forgive preventable mistakes.