What are Brake Pads, and How do they Work?

Example of brake pads being tested

As the term "brake pads" suggest, brake pads are essentially pads of friction material that are bonded onto steel backing plates to provide structural strength to the pads of friction material. 

However, brake pads are not created equal; the braking demands of modern vehicles now require that modern friction materials conform to specific performance standards set by vehicle manufacturers to satisfy the specific requirements of modern ADAS (Advanced Driver Assist) systems that depend on the ABS system to work.

These include (among others)  Adaptive Cruise Control, Traction Control, Stability Control, and Automatic Emergency Braking, all of which use the brake system to function. Therefore, manufacturers of brake friction materials have developed specific friction material formulations that produce reliable, consistent, and most importantly, predictable braking performance under all possible operating conditions.

In practice though, brake pad manufacturers are free to use any number of about 400 different compounds and substances to develop friction material formulations.  This list of allowable ingredients is far too long to produce here, but suffice it to say that modern friction materials include friction modifiers, lubricants, fillers, binders, and bulking agents that are mixed and then bound into a matrix under extreme heat pressure to produce the familiar shapes of brake pads we see for sale in parts stores.

As a matter of interest, modern brake pads contain things like various powered metals, granulated rubber, metal chips, ordinary steel wool, finely ground pecan and/or walnut shells, zinc in various forms, and asbestos, which contrary to popular belief, has not been banned for use in brake friction materials.

The actual “recipes” that friction material manufacturers develop and produce are considered proprietary trade secrets and are therefore never published or shared with anybody outside of their own organisations.  Not that this matters much, because the products of all major manufacturers of friction material all perform in much the same way under similar operating conditions, as required by car manufacturers and regulatory bodies in some markets.   

So while all major manufacturers of brake pads produce so-called organic, semi-metallic, and ceramic brake pads for use in light vehicles, the fact of the matter is that organic brake pads made by all manufacturers do not contain only organic materials, and semi-metallic brake pads made by all manufacturers contain only marginally more metal than organic brake pads.

Thus, from the perspective of an average car owner, there are no material or meaningful differences in terms of brake performance between organic and semi-metal brake pads made by any one manufacturer, as opposed to the organic and semi-metal brake pads made by any other manufacturer. 

The only meaningful differences between brake pads that the average car owner should take note of are the different optimal operating temperatures that apply to ceramic-based brake pads vs. organic and semi-metallic brake pads. Ceramic brake pads require substantially higher brake temperatures to work optimally, so from that perspective, ceramic brake pads are NOT interchangeable with other types of brake pads, all of which work optimally at lower temperatures than ceramic pads, which brings us to-

How brake pads work

When a driver applies the brakes in a moving vehicle, enormous hydraulic pressures squeeze the brake rotors between sets of brake pads, and since this action creates a lot of friction between the brake pads and the brake rotors it is easy, to believe it is the friction thus created that slows the vehicle down. However, although friction does play a role in slowing a vehicle down, the friction between the brake pads and the brake rotors is only an intermediate step in the process of slowing a vehicle down when a driver applies the brakes.

So if it is not friction alone that slows a vehicle down, what does slow it down when a driver applies the brakes? The answer to this question is rather complicated because it involves several laws of physics but for the most part, a vehicle slows down when a driver applies the brakes because this action converts one form of energy into another, with friction as the mechanism that does the conversion.

We need not delve into all the laws of physics that are involved in slowing a vehicle down, but suffice it to say that as far as brake pads are concerned, the most important law of physics is one stating that energy cannot be destroyed; energy can only be converted from one form into another. In the case of brake pads, it is the rate at which one form of energy can be transformed into another form that determines how well (or otherwise) the brakes work when a driver applies the brakes. Here is how this works-

When a vehicle moves forward, it possesses kinetic energy, or put differently, a vehicle’s speed is a measure of how much kinetic energy it possesses, which raises another law of physics, which states that when an object moves, it will continue to move until a larger force acting in the opposite direction, stops the object.  

 Thus, if we want to stop a moving vehicle, we have to create a force or form of energy that is larger than the moving vehicle's kinetic energy. A larger force is required because the moving vehicle's kinetic energy cannot simply be absorbed in some way because doing that would amount to destroying energy, which is impossible to do in the real world.

Therefore, by using friction between brake pads and brake rotors, we can convert a moving vehicle’s kinetic energy into heat, which satisfies both laws of physics we referenced above. However, as with many other things in life, the proverbial devil lives in the detail, and in this case, the detail involves not only the amount of heat we generate but also the rate at which the heat is generated.

On a practical level, both of the above factors determine how well the brakes on a vehicle work, but on the other hand, brake systems cannot generate unlimited amounts of heat, nor endure extreme brake temperatures for indefinite periods. Moreover, the overall efficiency of an automotive brake system cannot exceed the ability of the tyres on the vehicle to absorb extreme braking forces without losing traction with the road surface, because of the danger of a driver losing control of the vehicle during braking events.

Thus, from a design perspective, a modern brake system is the result of several important trade-offs between the maximum amount of braking force it can generate, and the brake system's ability to endure high brake temperatures without suffering adverse effects. In addition, the maximum braking force a brake system can generate must be high enough to stop the vehicle safely, while at the same time, not exceeding the tyre’s ability to absorb braking forces effectively while also maintaining traction with the road surface.

From an engineering perspective, however, all of the above requirements must be balanced against the need for the brake system to produce reliable, consistent, and predictable braking forces while extending the useful lives of brake system components in general, and of the brake pads and brake rotors, in particular. We can put this differently by saying although brake pads and brake rotors are designed to wear out, the overall design of the brake system much be such that both brake pads and brake rotors have reasonably long service lives, which usually run to between  40 000km and 50 000km of use, depending on the predominant use of the vehicle.

So while all brake pads stop vehicles through a process of converting kinetic energy into heat, the efficiency of this process in any given application depends on several factors. These include, among others, the formulation of the friction material, the diameters of the brake rotors relative to the diameters of the wheels, the size of the contact area between the brake pads and the brake rotors, and to a lesser degree, the position of the brake calipers relative to the centre of the wheels.

Thus, although all of the above factors influence or determine the overall efficiency of the brake system, it is the formulation of the friction material in terms of its composition that ultimately determines the efficiency of the conversion of kinetic energy into heat, with this process being the basic operating principle of all brake pads on all vehicles.  

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