Brake System Design and Optimization: Cooling
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Cooling
12.24
| 00:00 | - So far in this course, we've talked a lot about how the kinetic energy of the car is absorbed as heat energy when we apply the brakes. |
| 00:09 | In this module, we'll talk about how that absorbed heat is rejected by the brakes and the different ways of achieving that. |
| 00:17 | The majority of the energy absorbed by the brakes is transferred to the brake discs and this extra energy results in the temperature of the discs rising. |
| 00:27 | While brake discs are designed to do this, they can't just continue to absorb infinite energy. |
| 00:33 | In order to stay at reasonable temperatures they must reject that thermal energy themselves. |
| 00:39 | This rejection in thermal energy is otherwise known as cooling. |
| 00:44 | Where the different parts of the brakes transfer energy to their surroundings, thereby lowering their own temperature. |
| 00:52 | There are three heat transfer mechanisms and all three play a part in cooling the brakes. |
| 00:58 | The first mechanism to discuss is conduction. |
| 01:02 | This is heat transfer between components that are in physical contact. |
| 01:06 | Heat that's generated at the disc pad interface is transferred through the disc into the hub, upright, calliper, wheel bearings, wheels, tyres, suspension and everything else that's in physical connection. |
| 01:20 | The next is convection. |
| 01:23 | The heat transfer through a fluid. |
| 01:26 | In a motorsport context, the fluid we're focusing on is ambient air whereby directing it onto and through the different components like the brake discs we can transfer thermal energy into the air stream which carries the energy away, lowering the temperature of the brakes. |
| 01:43 | The last is radiation which is heat transfer without any physical contact. |
| 01:49 | For context, this is the same process that allows us to feel the warmth of the sun. |
| 01:55 | Radiation becomes becomes more significant as temperature rises, in the high temperatures we see in motorsport applications, this becomes extremely relevant. |
| 02:04 | There are two main perspectives we need to look at when considering the amount of cooling required. |
| 02:11 | Reliability and performance. |
| 02:13 | For reliability, it's simply a means of making sure that each component stays within its safe limits so we don't have a mechanical failure. |
| 02:22 | For performance, as we learned in the brake pad module, there's a temperature operating window we need to aim for in order to maximise the performance and consistency. |
| 02:32 | As we already discussed, the discs are responsible for absorbing the bulk of the energy in the brakes and in our discussion on cooling, we'll mainly be focused on the discs. |
| 02:42 | Though all the parts in the system need to be considered from a cooling perspective. |
| 02:47 | Not only do the pads and discs have safe temperature limits but the calipers, brake fluid, wheel bearings, wheels, tyres and even things like your TPMS sensors all have limits and can be damaged by being exposed to high brake temperatures. |
| 03:04 | If the callipers are overheated, the piston seals can begin to break down and at the same time the callipers are also transferring heat to the brake fluid. |
| 03:14 | Like everything else, the fluid has a maximum safe temperature limit before it will start to cause problems. |
| 03:21 | Our callipers, pads, wheels and tyres will be cooled from some extent through having the air flow past over them. |
| 03:28 | However the most powerful cooling parameter we have at our disposal is the amount of ambient air we can supply to the brake discs. |
| 03:37 | Due to the fact that modern vented brake discs work naturally like air pumps to draw the air in from the centre and exhaust it through the outer edge of the disc even without any extra air supplied the discs will cool themself to some extent. |
| 03:53 | On a street car, this is likely more than sufficient in most cases. |
| 03:58 | In a motorsport application though, the airflow through the disc, often isn't enough and extra ducted air needs to be supplied to the inside of the disc. |
| 04:08 | The purpose of brake ducts is to increase the mass flow of air through the disc to increase the amount of convective cooling. |
| 04:18 | The source of the air needs to be as cool and high energy as possible. |
| 04:22 | For the front brakes, this normally means taking air from the front of the car in a place where the air supply will be as clean and cool as possible. |
| 04:32 | Generally somewhere on the front bumper or air dam. |
| 04:35 | From the inlet, the air is ducted throughout the bodywork and directed to an outlet that's placed in the inner ring of the brake disc which is the cooling inlet for the disc. |
| 04:46 | This ducting of air from the front of the car to the disc can pose a significant challenge which is most problematic on road based cars where there is little space to fit anything additional. |
| 04:58 | Modern cars tend to have everything packaged pretty tightly as it is. |
| 05:03 | Running brake ducts through the wheel wells can often result in tyre to duct contact, particularly with the wheels steered at full lock. |
| 05:13 | This is something that needs to be avoided as the duct material will not last long with anything rubbing on it. |
| 05:20 | Regardless of how we route the ducting through the engine bay, wheel wells or otherwise, it's important to keep the cross sectional area the same throughout the duct. |
| 05:30 | Keep in mind if you have to change the cross sectional shape of the duct anywhere for clearance, a reduction in cross sectional area will hurt your flow through the duct. |
| 05:40 | The required cross sectional area of the ducting depends on many factors but it ranges from a single 50mm diameter duct per side, right up to multiple 100mm ducts per side in extreme cases. |
| 05:54 | In order to size or upgrade your ducting, it's best to make use of some temperature data which we'll discuss in more detail later in the course. |
| 06:03 | The material you use for any solid parts of the duct is up to you but the flexible sections, particularly any parts that are exposed to high temperatures should be made from proper brake ducting hose. |
| 06:16 | This is designed to operate in high temperature environments and is supported by a coil of embedded steel wire that helps it keep its shape. |
| 06:24 | With outboard brakes at least some flexible section is always required especially in the front where the brake will be moving up and down with the suspension as well as the wheels being steered. |
| 06:36 | Again, the routing of the flexible ducting in this area around the suspension arms, upright and brake calliper can present a significant challenge and fitting everything in there while preventing any contact between components is usually a tough job. |
| 06:53 | When routing the brake hoses, it's worth spending time to get this set up as cleanly as possible if you don't want to be replacing your brake ducts regularly. |
| 07:01 | The last part of the brake duct to discuss is the outlet that guides the air into the cooling inlet of the disc. |
| 07:09 | The guiding principle here is that we want to maximise the amount of air that flows through the disc, this means we'll need some sort of backing plate integrated into the outlet. |
| 07:20 | The purpose of this backing plate is to seal the inlet area and make sure that as much of the air from the ducts makes its way through the disc as possible without allowing it to escape into the wheel well area. |
| 07:35 | In most cases, this backing plate will have a circular outer surface almost the same size as the inner diameter of the disc air inlet. |
| 07:43 | We want this clearance to be as tight as possible. |
| 07:46 | We shouldn't need to allow more than 1-2mm of clearance here for a well made duct and some designs will have a backing plate the full diameter of the disc instead. |
| 07:57 | While this can work, it can cause some problems with the inner disc face running hotter than the outer, leading to a thermal gradient which can distort the disc as well as cause cracking. |
| 08:08 | This is in part because less ambient air is able to come in contact with the inner surface of the disc and also because heat radiated from the surface of the disc is reflected back towards it by the full sized backing plate. |
| 08:22 | Hopefully it should be obvious already that we also don't want to aim cooling air at the inner disc face for the same reason we don't want to put a backing plate against the inner surface, this will lead to a thermal gradient across the disc causing distortion and potential cracking. |
| 08:40 | The goal with cooling the disc is to do it in the most uniform way as possible. |
| 08:45 | In an ideal world, the entire disc would operate at a similar temperature at all times which would help to reduce the build up of thermal stresses and resultant distortion and cracking. |
| 08:56 | The ducts must also be sized for the worst case scenario. |
| 09:01 | This being running at the track that is the heaviest on brakes on a hot day. |
| 09:06 | This means that on any other day, other than the worst case we'd be over cooling our brakes which can cause other problems by being outside the temperature window of our specific brake pads. |
| 09:18 | This is managed by what's known as blanking where we intentionally close off some of the duct inlet to restrict the amount of cooling air directed to the disc. |
| 09:28 | If you pay attention to most professional race series, you'll notice this in action. |
| 09:33 | Most often this takes the form of tape being placed partially over the duct inlets. |
| 09:39 | In other cases, there'll be pre made duct inlets of different sizes available that the teams can swap in and out quickly. |
| 09:47 | Often these are held in place with magnets so no tools are required to make a change, further speeding up the process. |
| 09:55 | Engineers will pay attention to the brake temperatures constantly throughout each session by using the brake tempeature history along with previous ambient temperatures and predicted temperatures for future sessions, they'll constantly adjust the blanking as required for each session. |
| 10:12 | Another less invasive type of cooling is an air guide. |
| 10:16 | This is something we'll often see on high performance road cars from the factory. |
| 10:21 | These can be built into either the factory front undertray or most often attached to the front lower control arm. |
| 10:29 | Their job is to redirect the air that would otherwise flow underneath the car towards the disc. |
| 10:35 | In many cases, these can be surprisingly effective if we only need a modest increase in brake cooling. |
| 10:41 | The advantage being that they're a lot less work to integrate and they take up less space than a full brake duct, as well as being fewer parts to make. |
| 10:50 | While it's not directly about brake cooling, it's also worth discussing that in order to maximise the life and performance of your brakes, the best thing we can do is to gradually warm them up before hard use as well as gradually cooling them down. |
| 11:06 | If the brakes are cold, on an average length racetrack this usually means spending two laps gradually building up the temperature before attempting a fast lap. |
| 11:16 | After you're done running flat out, we should spend one full lap at slow speed without putting too much energy into the brakes. |
| 11:24 | Just as heading out and and attacking the brake pedal as soon as you exit the pit lane will result in a high thermal gradient across the disc, resulting in unnecessary stresses, entering the pits and stopping without sufficiently cooling the brakes will result in an uneven temperature spread as well as problems with pad deposits which we'll discuss in a later section. |
| 11:47 | If you can have the discipline to do this, you'll be rewarded with discs that last longer without cracking or distoring as well as getting more life out of your pads and more friction consistency over the entire life of that set of brakes. |
| 12:01 | In summary, brake cooling is critical for both reliability and performance of our brakes. |
| 12:08 | There are a number of ways to cool our brakes but cooling the brake disc is what we tend to focus on. |
| 12:14 | We usually manage the cooling with the brake ducting and can use blanking to help tune the amount of cooling we're getting. |
