Brake System Design and Optimization: Pads
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Pads
11.36
| 00:00 | - Brake pads are the components that are squeezed against the swept face of the brake discs by the brake callipers. |
| 00:06 | These are regarded as a consumable item. |
| 00:09 | As energy is absorbed by the brakes during a stop, the brake pad materials wears and reduces in thickness. |
| 00:17 | While the discs are also consumable in some degree, in general the pads will be designed to wear at a much higher rate. |
| 00:24 | A conventional brake pad is made up of two main parts, the friction material which is in contact with the brake disc and a backing plate. |
| 00:33 | This backing plate is generally made from steel and its purpose is to transfer the force from the brake callipers to the friction material and prevent distortion within the pad. |
| 00:44 | 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. |
| 00:54 | These can take the form of either raised sections of the backing plate that protrude into the pad or the opposite. |
| 01:01 | Brake pads are always arranged opposed to each other which means as the disc is squeezed between the pads, the compressive forces of the discs are cancelled out. |
| 01:11 | This reduces the stress on the disc and the rest of the hub and upright as well as helping with packaging. |
| 01:17 | The design and materials of a brake pad are influenced by the intended application and required characteristics. |
| 01:23 | As with everything, this is a case of compromise with no single brake pad friction material being suitable for every case. |
| 01:31 | For example, a pad designed for OE use in a street car may work well from cold and produce lower levels of noise and dust but if we would try to use it out on a racetrack, it would stop producing friction at high temperatures. |
| 01:45 | From the other side, if you tried to use a brake pad designed for highly stressed endurance racing application on a street car, the discs would likely suffer from high wear. |
| 01:55 | As they wouldn't produce enough friction when you went to stop the car for the first time and they would be extremely noisy with a lot of high pitched squealing from the brakes. |
| 02:04 | There are a huge range of brake pad materials available and the choice must be made with our intended purpose in mind. |
| 02:11 | For motorsport applications this is best done with the help of a specialist supplier that can guide us through the options. |
| 02:18 | The first thing we need to understand is the application we're using these for. |
| 02:22 | Both in terms of the car and brake components you currently have fitted as well as the type of competition you'll be taking part in. |
| 02:31 | Is it purely a street car that gets driven hard, something that must be streetable but also survive at track days, a rally car where the brakes must work well from cold, or a heavy circuit racing car that must survive for hours on track. |
| 02:45 | The brake discs we're using will enforce which style of pad we use. |
| 02:49 | Iron, carbon ceramic and pure carbon discs all require their own specific materials to be used in order to work. |
| 02:57 | But with that said, for the rest of the module we'll focus on pads to be used with iron discs as this applies to the vast majority of competitors. |
| 03:07 | Let's take a look at some of the factors we need to consider when choosing a brake pad for our application. |
| 03:13 | Some of which we've touched on already. |
| 03:15 | The ability to work from cold, initial bite, the maximum operating temperature, wear rate of both the pad and the disc, amount of dust and noise that will be produced, how consistent the friction coefficient will be in the relative temperature range, and how resistant the pads are to fade. |
| 03:35 | These are all things we need to consider before going to speak with a motorsport brake specialist. |
| 03:40 | The ability to work from cold is something that's important for street, hill climb and rally cars but not so much for a dedicated circuit racing car. |
| 03:49 | Looking at this plot, we can see the brake pad temperature vs coefficient of friction for two very different pad materials which we've generically labelled as a street and race pad. |
| 04:01 | We can see that the street pad has a reasonable coefficient of friction from low temperatures. |
| 04:07 | This remains relatively constant before dropping off at high temperatures. |
| 04:12 | The race pad however has a poor coefficient of friction at low temperatures before climbing much higher than the street pad. |
| 04:20 | Not only is the coefficient of friction higher, it remains to have a much higher temperature range before dropping off itself. |
| 04:28 | Initial bite refers to the rate at which the pad builds up friction force with the disc. |
| 04:34 | It's a transient property rather than one used to describe a steady state friction characteristic. |
| 04:41 | Bite is not just a function of the brake pad make up but also the disc design as well to some extent. |
| 04:48 | This has a big impact on the feel of the brakes and although this is fairly subjective it's still a very important factor in the overall brake performance as it ensures that the driver is confident and comfortable with how their vehicle brakes. |
| 05:02 | The wear rate of both the pads and the discs is also something that needs to be considered. |
| 05:07 | This must play into the compromised decision matrix as having a set of brakes that work the way you need them to but you're destroying the pads and the discs at an unsustainable rate, means you aren't much further ahead. |
| 05:21 | There are a wide range of different behaviours available out there. |
| 05:24 | Dust and noise aren't likely something that we'll care much about in a dedicated competition car but in a car that gets used a lot for street use, these could become quite a significant factor. |
| 05:36 | Just as important as the maximum coefficient of friction of the pad is the consistency. |
| 05:41 | In an ideal world, the coefficient of friction would be constant across the entire temperature range. |
| 05:48 | This would give us consistent braking torque regardless of the operating temperature which relates back to the concept of mechanical brake bias. |
| 05:57 | It's completely normal to have different operating temperatures at the front and the back of the car. |
| 06:02 | This is expected as each brake circuit is absorbing different amounts of energy, is made up of different components and has a different amount of cooling. |
| 06:12 | This makes the stability of the coefficient of friction extremely important. |
| 06:17 | If we have one end of the car with the coefficient of friction changing a lot over he required operating temperature, our mechanical brake bias will change as you move throughout a lap or a stint. |
| 06:30 | Obviously something we want to avoid. |
| 06:32 | Here's another plot of temperature vs coefficient of friction but this time with a whole lot more real life brake pad compounds. |
| 06:40 | We can see that depending on our application and ability to keep the brakes in a given temperature window, we can have extremely different friction characteristics between compounds. |
| 06:52 | Like we touched on earlier, there are a number of different materials that can be used in a brake pad friction material. |
| 06:59 | The categories we'll discuss these in aren't hard and fast rules. |
| 07:03 | In reality a single brake pad compound can include over 30 different ingredients so these are more as guides as to the way that brake pads are usually marketed to help identify them. |
| 07:15 | Pads intended for mild street use tend to be referred to as organic, these are popular because of their relatively low disc wear and tendency to be quiet and work well from cold temperatures. |
| 07:27 | Ceramic style pads are generally based on organic pads but with some ceramic additives that vary a lot in both material and amount depending on the manufacturer. |
| 07:38 | These additional materials may help the pads last longer and operate at higher temperatures than pure organic pads. |
| 07:46 | Be careful not to confuse the organic pads that have ceramic additives with the fully ceramic brakes that we discuss in the disc section. These are two completely different sections. |
| 07:58 | Semi metallic pads are often considered the next step up, suitable for harder street driving. |
| 08:04 | While they still generally work well from cold, they tend to have a higher coefficient of friction that tends to stay more stable at higher temperatures. |
| 08:14 | The downsides are somewhat predictable in that the disc wear tends to be greater with more brake dust and sometimes more noise. |
| 08:22 | Carbon and other expensive materials can also be added to the semi metallic pads to increase their performance even further. |
| 08:30 | This is a difficult class of pad to describe with a blanket statement as different manufacturers will use a number of different materials to help both their performance and marketing of their products. |
| 08:42 | These sorts of pads tend to have better high temperature performance and will be classified more as a motorsport pad. |
| 08:50 | Further to this, pads can be made from different processes like resin bonding or sintering and this is a further impact on their performance. |
| 08:58 | Sintered pads are those produced using the sintering process where metal particles are fused together under high heat. |
| 09:06 | This creates a stronger more durable pad than the organic and semi metallic pads that are generally constructed using resin to bond the fibres together. |
| 09:16 | Ceramics and other materials can also be used in a sintered pad which have the same benefits and drawbacks as their resin bonded counterparts. |
| 09:25 | Another feature that you might notice when inspecting a set of brake pads are the slots you'll find in the face. |
| 09:33 | The purpose of these slots is to give the brake dust a place to escape during operation and allow the friction material to expand with temperature to avoid distortion. |
| 09:42 | In some cases, you'll see an extra piece of material that sits between the pad backing plate and the caliper pistons. |
| 09:50 | This may be attached to the backing plate or just inserted between them. |
| 09:55 | This extra plate serves two purposes, it helps reduce the high frequency vibrations that result in the brake squealing noise and can reduce the amount of heat transferred from the pad backing plate to the caliper pistons. |
| 10:09 | One final thing to note is pad size. |
| 10:13 | The common misconception here is that bigger pad area means more braking power. |
| 10:19 | But if we consider the friction force which we know is equal to the coefficient of friction multiplied by the normal force, being the clamping force between the calipers, both of these remain unchanged by the pad area. |
| 10:33 | So we can see that an increase in the brake pad area has no effect on the frictional force generated between the brake pads and the disc. |
| 10:43 | This increase in area actually results in a decrease in pressure between the pads and the discs at the same clamping force. |
| 10:51 | And that does a few things, most importantly lowering the operating temperature of the pads and reducing wear. |
| 10:59 | The increase in pad volume from both the additional area and thickness means they have a greater thermal capacity and will last longer assuming the pad compounds are the same. |
| 11:10 | That covers just about everything we need to know about pads so in summary, brake pads are the main consumable element in the brake system. |
| 11:19 | Their friction characteristics vary a lot based on both operating temperature and the specific makeup of the friction material. |
| 11:27 | The different pros and cons of each material group make pad choice a significant compromise. |
