Facts about Brake Rotors You Should Know

May 3, 2016
Has it ever happened to you that a shudder or vibration becomes evident when you apply the brakes soon after a brake rotor replacement? Have you ever returned the rotors to the supplier and did they replace them as defective, since it turned out the rotors were both warped? If you can answer “Yes” to these questions, you need to read this post, since it is very likely that the rotors were never warped.

You might have thought that it is friction between the brake friction material and the brake rotors that stops your car when you apply the brakes. If you did think this, you are only partially right. What actually stops your car is the conversion your car’s kinetic energy into heat energy, and friction is only the mechanism that manages that process of energy conversion.

However, how well your cars’ kinetic energy is converted into heat energy depends largely on the grade of cast iron the rotors are made from, and the formulation of the brake friction material and this is where the problem begins, more often than not. To understand the problem of juddering brakes, you first need to know two things, these being that brake rotors and friction materials are not created equal, and that there are two types of friction that determine how well your brakes work. Let us look at the different types of friction first-

Abrasive friction
The cast iron from which a brake rotor is made consists of an amalgam of iron crystals that are tightly bound together in various patterns, with the pattern largely determining both the hardness of the rotor and its resistance to wear, which incidentally, are not the same thing.

Nonetheless, when the rotor and the brake pads are brought into contact under pressure, the relative movement between them breaks the crystalline bonds in both the rotor and the friction material, and this action is what creates heat. However, the crystalline bonds in the friction material, and to a smaller degree, in the rotor are broken permanently, which results in the harder material (the rotor), wearing away the softer, and less wear-resistant material of the brake pads. It is this type of friction that causes both rotors and pads to wear out prematurely, while leaving deep score marks, discoloration, and sometimes surface cracks in the rotor if the rotor and friction material are not compatible.

Adherent friction
In this form of friction, the crystalline bonds in both the rotor and friction material are also broken, but instead of being carried away as dust, some of the friction material is bonded onto the surface of the rotor in a thin but uniform layer. In this process, the composition of the surface of the rotor becomes indistinguishable from the surface of the brake pad. However, the transfer of material works in both ways; some of the rotor material is also deposited onto the brake pad’s surface in a continuing process in which crystalline bonds are broken and reformed on both surfaces on a molecular level. This is the form of friction that imparts a smooth, shiny surface to the rotor.

The process of material transference between the rotor and friction material is normal and an essential part of the operation of the brakes. However, the transfer of material between the rotor and the friction material is not perfectly symmetrical; if it were, the brakes would never wear out but since it is close to being symmetrical, the rotors and pads last far longer than any aftermarket replacement(s) ever will.

So why do some brake rotors and pads wear out so quickly?

The answer involves the composition of both rotors and brake friction material. Car manufacturers invest a great deal of time and money in selecting a grade of cast iron for the brake rotors on their cars that will provide satisfactory, and predictable braking performance without overheating, cracking, fracturing, or warping.

These parameters are collectively referred to as the “thermal efficiency” of the rotor, but this is only half of the story. The other half involves selecting a friction material formulation that will ensure an efficient transfer of material between the brake rotor and the friction material. If the engineers get this selection right, the result is a car that never experiences any sort of brake shudder or brake-induced vibrations- until either the brake rotors, or the brake pads (or both) are replaced with aftermarket replacements.

The trouble with aftermarket brake rotors is however that most manufacturers pay scant attention to the OEM specifications with regard to hardness and/or durability. A further problem is that the formulation of brake friction material is far more of an art form than it is an exact science, and when these two factors are combined, you get either an accelerated rate of abrasive friction, or a transfer of material between the rotor and pad surfaces that is so efficient that brake performance is compromised.

Friction material composition explained

Regardless of the manufacturer, about the only aspect that brake friction materials share is that no formulation contains asbestos in any form. However, current regulations in the North American, European, and Japanese markets allow manufacturers of friction materials to use more than one hundred ingredients in any combination that suits a given requirement or need.

The result of this chaotic situation is that when one manufacturer takes out a patent on a specific formulation for brake pads that fit applications X, Y, and Z, all other manufacturers are forced to develop formulations of their own that offer the same, or at least similar performance and longevity for pads that fit applications X, Y, and Z. This is where you, the consumer, who is very seldom an automotive engineer, comes into the picture.

Brake judder explained
You, as an average car owner, have no idea what ingredients the brake pads you have just bought contain, or in what proportions those ingredients are present. Similarly, you have no idea if the brake rotors you have just bought are made of the same grade cast iron as OEM parts, but the price charged was good, and you are happy with your bargain.

So, a few hundred miles after the brake replacement (or sometimes, a whole lot sooner), you notice a vibration on the steering wheel when you apply the brakes, but worse- you notice that the vibration gets worse as the brakes warm up. The only logical explanation is that the rotors must have somehow become warped, right? Wrong- here is the most likely explanation of what is happening-

Even the most poorly made aftermarket brake rotors will rarely warp during normal brake operation. However, if the rotor and friction material are not compatible, the transfer of material between the two surfaces will be uneven. In practice, this means that more material will be transferred at some spots between the two surfaces than on others. These spots are usually created, or imprinted on the rotor when the brakes are kept applied during stops when the brakes are hot, which prevents the part of the rotor that is clamped between the pads from cooling down at the same rate as the rest of the rotor.

Since the transfer of material between the rotor and the pads is more efficient when both the rotor and the pads are hot, the practical effect of an unequal rotor temperature is that more material is transferred to the hot spots, than onto cooler parts of the rotor. In practice, the thickness of the rotor is increased in these spots, which is what creates the vibration when the thicker parts of the rotor are forced to pass between the pads when the brakes are applied.

When such a rotor is placed in a lathe and tested with a dial gauge (an instrument to test run-out), the gauge will reveal an amount of run-out that corresponds to the variations in the rotors’ thickness, which is usually misinterpreted as evidence that the rotor is warped. While machining the faces of the rotor to remove the thickness variations might seem to resolve the problem, this is at best a temporary fix, since the problem will return if the same pads are again used on the same rotor.

Why you should stick to OEM brake parts

Since all brake pads use both adherent and abrasive friction to work, the trick in avoiding brake-induced vibrations is to use OEM parts as far as possible. Bear in mind that sales clerks in parts stores are not equipped to provide reliable advice on which friction material formulation is best for your particular needs. Nor are they equipped to advise you on the pros and cons of using other friction materials than those that are designed to operate within the brake temperature range that applies to your vehicle.

As stated elsewhere, friction material formulation is more of an art than a science, but despite that, all major, reputable manufacturers strive to provide friction material that will provide reliable and predictable performance if that material is used for its intend purpose.

As a rule, all reputable manufacturers supply friction material for general street use, but also for heavy-duty applications and competition applications, such as for race cars, and in all of these applications, the manufacturer strives to strike a reasonable compromise between performance, noise reduction, longevity, and the generation of brake dust. For instance, no formulation intended for use in race cars will provide satisfactory braking performance in a family sedan, since the pads will never reach the temperatures at which competition-grade pads are designed to work best.

How to avoid a repeat of the problem

Even if you replace worn or problematic brake rotors and pads with OEM parts, the new rotors and pads need to be bedded in order for a smooth, even layer of material to be transferred between the rotor and pad surfaces. While some manufacturers supply these instructions with their products, others do not, so below is a generic process that will work with all high quality replacement parts-
  • After installation, perform between 8 and 10 gradual slow-downs from a speed of about 35 m/ph, using a moderate brake pressure, but do NOT come to full stop to prevent a chunk of friction material from being transferred onto the rotor, which will almost certainly cause a brake-induced vibration in the near future
  • Make a further three or four slowdowns from about 45 m/ph, using heavy pressure but do NOT come to full stop for the same reason as above
  • Stop the vehicle, and allow the brakes to cool down for at least 15 – 20 minutes, but do NOT apply the brakes during this time for the same reasons as above
  • Do NOT apply light brake pressure over extended distances for at least 300 miles after the initial bedding-in procedure to prevent stripping off the thin layer of transferred material. This layer needs time and repeated applications of heat to become properly bonded to both surfaces, so only use moderate, and occasionally heavy brake pressure during the first 300 500 miles to help “fix” the layer onto both surfaces
One more thing

Only replace brake rotors and pads as complete sets on both wheels on an axle. Do NOT use worn pads on new rotors, or new pads on worn rotors. Doing this will result in localized overheating of both the rotors and the pads, which can lead to brake fade, and possibly, catastrophic brake failure at the exact moment you need the brakes the most.