Practical Automotive Maintenance: Brake Pads

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Brake Pads

07.00

00:00	Brake pads are the components that are squeezed against the swept face of the brake disc by the brake calipers.
00:05	They are regarded as a consumable item as energy is absorbed by the brakes during a stop, the brake pad material wears and reduces in thickness.
00:13	While the discs are also consumed to some degree, in general the pads will be designed to wear at a much higher rate.
00:20	A conventional brake pad is made up of two main parts.
00:23	The friction material, which is in contact with the brake disc, and the backing plate, which is generally, made from steel.
00:28	Its purpose is to transfer the force from the brake caliper pistons to the friction material and prevent distortion within the pad.
00:36	The friction material is bonded to the backing plate and in many cases there's also some form of mechanical interlocking between the friction material and the backing plate.
00:44	This can take the form of either raised sections of the backing plate that protrude into the pad or the opposite.
00:50	Brake pads are always arranged opposed to each other, which means as the disc is squeezed by both the pads, the compressive forces on the disc are cancelled out.
00:59	This reduces stresses on the disc and the rest of the hub and upright as well as helping with packaging.
01:04	The design and materials of the brake pad are influenced by the intended application and required characteristics.
01:10	As with everything, this is a case of compromise, with no single brake pad friction material being suitable for every case.
01:17	For example, a pad designed for OE use in a street car may work well from cold, have low noise and dust, but if you were to try to use it on a racetrack, it would stop producing friction at high temperatures and you'd struggle to slow down.
01:29	On the other side of the coin, if you tried to use a brake pad designed for an endurance racing application in a street car, the disc would likely suffer from high wear, they wouldn't produce enough friction when you first went to stop the car, and they would be extremely noisy with a lot of high-pitched squealing from the brakes.
01:45	There are a huge range of brake pad materials available and the choice must be made with your application in mind.
01:51	For motorsport applications, this is best done with the help of a specialist supplier who can guide you through the options.
01:57	The first thing we need to understand is the application we're using these for, both in terms of the car and the brake components that you currently have fitted, as well as the type of use the car will see.
02:07	Is it purely a commuter street car? Maybe a performance street car that gets driven hard? Or maybe something that must be streetable, but also survive track days? The brake disc you're using will enforce which style of pad you'll use.
02:19	Iron, carbon ceramic and pure carbon discs all require their own specific material to be used in order to work.
02:26	But with that said, for the rest of this module, we'll focus on the pads used with iron discs, as this is what applies to the vast majority of us.
02:34	Let's take a look at some of the factors you'll need to consider when choosing a brake pad for your application, some of which we've already touched on.
02:41	The ability to work from cold, initial bite, maximum operating temperature, wear rate of both the pad and the disc, amount of dust and noise that'll be produced, how consistent the friction coefficient will be in the relevant temperature range, and how resistant the pads are to fade.
02:57	These are all things we need to consider before going to speak with the brake pad supplier.
03:02	The ability to work from cold is something that's particularly important for street cars, but not, so much for a dedicated circuit racing car.
03:09	Looking at this plot, we can see the brake pad temperature versus coefficient of friction for two very different pad materials, which have generically been labeled as street and race pad.
03:18	We can see that the street pad has a reasonable coefficient of friction from low temperatures.
03:22	This remains relatively constant before dropping off at high temperatures.
03:27	The race pad, however, has a poor coefficient of friction at low temperatures before climbing much higher than the street pad.
03:33	Not only is the coefficient of friction higher, but it maintains it to a much higher temperature range before dropping off itself.
03:39	Initial bite refers to the rate at which the pad builds up friction forces with disc.
03:45	It is a transient property rather than being used to describe a steady state friction characteristic.
03:50	Bite is not just a function of the brake pad makeup, but also the brake disc design as well to some extent.
03:56	To a degree, this is also a matter of personal preference and the specifics of your car.
04:01	Some drivers prefer a pad that has a very aggressive bite.
04:03	However, in a light car without ABS, this can tend to make it very hard to modulate the brakes without locking up.
04:10	The wear rate of both pads and the disc is something that also needs to be considered.
04:14	This must play into the compromise decision matrix, as having a set of brakes that work the way we need them to, but you're destroying pads or discs at an unsustainable rate means you aren't much further ahead.
04:25	There are a wide range of different behaviors available here.
04:28	Dust and noise aren't likely something you'll care about much in a dedicated competition car, but in a car that gets used a lot on the street, this can become quite significant factors for you.
04:38	There are a number of different materials that can be used for the brake pad friction material.
04:42	The categories we'll discuss here aren't hard and fast rules, more guides as to the way pads are usually marketed to help you identify them.
04:50	Pads intended for mild street use tend to be referred to as organic.
04:54	They're popular because of relatively low disc wear and they tend to be quiet and work well from cold temperatures.
05:00	Ceramic style pads are generally based on organic pads, but with some ceramic additives that vary a lot in both the material and the amount depending on the manufacturer.
05:08	These additional materials may help the pads last longer and operate at higher temperatures than pure organic pads.
05:15	Be careful not to confuse the organic pads that have ceramic additives with the carbon ceramic brakes that we discussed in the brake disc module.
05:22	These are two completely different systems.
05:24	Semi-metallic pads are often considered the next step up, suitable for harder street driving.
05:30	While they still generally work well from cold, they tend to have a higher coefficient of friction that tends to be more stable at higher temperatures.
05:37	The downsides are somewhat predictable in that the disc wear tends to be greater with more brake dust and sometimes more noise.
05:44	Carbon and other expensive materials can be added to semi-metallic pads to increase their performance even further.
05:50	This is a difficult class of pad to describe with a blanket statement, as different manufacturers will use a different number of materials to help in both the performance and marketing of their products.
06:00	These sorts of pads tend to have better high temperature performance and will be classified more as a motorsport pad.
06:06	Another feature you might notice when inspecting a set of brake pads are the slots you often find in the face.
06:11	The purpose of these slots is to give the brake dust a place to escape to during operation and allow the friction material to expand with temperature to avoid distortion.
06:20	In some cases, you'll see an extra piece of material that sits between the pad backing plate and the caliper pistons.
06:25	This may be attached to the backing plate or just inserted between them.
06:28	This extra plate serves two purposes.
06:31	It helps reduce the high frequency vibrations that result in brake squealing noise and can reduce the amount of heat transferred from the pad backing plate to the caliper pistons.
06:40	In summary, brake pads are the main consumable element in the brake system.
06:44	Their friction characteristics vary a lot based on both the operating temperature and the specific makeup of the friction material.
06:51	The different pros and cons of each material group can make pad choice a significant compromise.