Brake System Design and Optimization: Discs
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Discs
13.03
| 00:00 | - After calipers, the brake discs are obviously the next component for us to discuss. |
| 00:05 | These are the components that are clamped by the calipers and brake pads to slow the car. |
| 00:11 | By being rigidly attached to the hubs, when they're clamped by the brake pads, they provide a torque to slow the rotation of the hubs, wheels and tyres, thereby slowing the car. |
| 00:22 | The brake discs can be thought of as a thermal sink that absorbs a lot of the thermal energy produced by the brakes when we slow the car. |
| 00:30 | After absorbing the energy, it then gradually releases it as it cools. |
| 00:34 | The central part of the disc is mechanically attached to the hub with the outer surface being clamped by the brake pads. |
| 00:42 | The part that's in contact with the pads is known as the swept area of the disc. |
| 00:48 | Brake discs can be solid or vented, though while some modern front wheel drive vehicles do use a non vented disc on the rear, it's unlikely that we'd find them on anything high end or with a focus on performance. |
| 01:01 | Solid discs, as the name implies, are made from a solid section whereas vented discs have both swept faces separated by a hollow central section that allows air to pass through the middle for cooling. |
| 01:15 | In vented discs, the cooling air flows from the inside to outside. |
| 01:20 | By rotating, a vented disc acts as a sort of air pump to encourage the flow through them. |
| 01:25 | Vented discs come in a number of different styles but can be broadly broken down into straight and curved vane designs. |
| 01:35 | Straight vane discs have the advantage of being cheaper to manufacture. |
| 01:39 | Curved vane discs work more efficiently as air pumps but not only are they more expensive to produce, they're also directional. |
| 01:48 | This means that you need a separate design for the left and right sides of the car. |
| 01:53 | The directions the vanes point is critical as the air always flows inside to outside. |
| 01:59 | The correct orientation for the vanes is shown here which helps to promote cooling airflow. |
| 02:06 | If the discs are fitted to the wrong side of the car, the discs won't cool as well. |
| 02:11 | Materials for brake discs vary widely based on the intended application and budget but there are three main categories of material used. |
| 02:20 | Iron, carbon ceramic and carbon. |
| 02:23 | Iron discs are what the vast majority of OE road cars are produced with. |
| 02:28 | While iron may seem like a pretty old school material, it has some big advantages. |
| 02:33 | And that's why you'll still find them fitted to many high end race cars like those found in modern GTE and GT3 classes. |
| 02:41 | Some of the benefits include low cost, good mechanical properties, the ability to absorb a lot of thermal energy, being dimensionally stable at high temperatures, good wear resistance, a wide working temperature range and a simple manufacturing process. |
| 02:58 | The main downside of an iron disc however is weight, especially compared to the two types of discs we'll discuss. |
| 03:06 | The next category is carbon ceramic discs. |
| 03:09 | We generally find these fitted to extremely high end performance road cars. |
| 03:15 | The benefits include the ability to work well from cold as well as being much lighter weight compared to iron discs. |
| 03:22 | They also have an extremely low wear rate. |
| 03:25 | In some cases, the wear rate can be so low that the discs can be expected to last the entire life of the car if they're properly looked after and cared for. |
| 03:35 | The ability to work well from cold may not be too important for a race car but it's critical for road cars. |
| 03:43 | One of the things that you'll often notice in cars that are fitted with carbon ceramic brakes is that the discs are very large. |
| 03:49 | The reason for this is that carbon ceramic discs are poor heat conductors. |
| 03:54 | This means that they need to be larger in order to reject enough heat to survive. |
| 04:00 | The downsides are the very high cost as well as being extremely fragile. |
| 04:04 | Due to their nature, while they're very strong, they're also extremely brittle, meaning any impacts may permanently damage them.. |
| 04:12 | Carbon ceramic discs also aren't tolerant of the high temperatures experienced on track which makes them unsuitable for motorsport use in general. |
| 04:22 | If they're overheated, they can become permanently damaged and the wear rate rapidly increases. |
| 04:29 | One place you'll find carbon ceramic discs being used in motorsport is in some hill climb racing. |
| 04:34 | This is because they work well from cold and they don't see the extreme temperatures that occur in circuit racing. |
| 04:40 | This allows hill climb competitors to benefit from the lightweight advantage carbon ceramics offer. |
| 04:47 | The final category to discs is carbon discs. |
| 04:50 | These are normally referred to as carbon carbon which describes the brake system with a carbon disc matching to a carbon pad. |
| 04:59 | You'll find carbon discs fitted to high end race cars, things like LMP2, GT500 and Formula 1. |
| 05:06 | The advantages here are the extremely low weight, low thermal expansion, and the ability to survive at high temperatures while also having high coefficient of friction. |
| 05:17 | Downsides includes the eye watering cost, long manufacture time as well as the poor friction and wear characteristics at low temperatures. |
| 05:27 | While they are capable of working well at high temperatures, if they're overheated, the wear rate also increases quickly. |
| 05:34 | With the exception of properly cared for carbon ceramic discs, brake discs can be thought of as a consumable. |
| 05:41 | They wear out and lose thickness in the same way the brake pads do. |
| 05:45 | Each disc has a minimum thickness and once reached it must be replaced. |
| 05:50 | Mounting options for brake discs can vary considerably by application. |
| 05:54 | In normal OE road cars with iron discs, the brake hat is integral with the discs. |
| 06:00 | It's all one piece. |
| 06:02 | Generally the hat will locate on a spigot on the hub and have holes in it to allow the wheel studs to pass through. |
| 06:10 | A single piece disc has the advantage of being cheaper to manufacture but the main downside is it can lead to vibration problems when the discs run at high temperatures. |
| 06:20 | This is due to the normal thermal expansion that occurs with the disc and the higher the operating temperature, the worse it gets. |
| 06:28 | This can lead to stresses and distortion in the disc that can cause vibrations and unsteady braking forces as well as uneven pad wear. |
| 06:38 | These are annoying at best and dangerous or destructive at worse. |
| 06:43 | The most common way for this to be dealt with is by using what's called a floating disc. |
| 06:47 | This simply means that the hat and disc are made from two different materials. |
| 06:52 | Most often an aluminium hat with an iron disc. |
| 06:56 | To some extent the disc is usually allowed to float radially and axially on the hat while still being able to react the braking torque. |
| 07:04 | This means that as the disc heats up and expands because it's not rigidly constrained to the hat, it can float to find its own natural running position, between the pads and the caliper that doesn't cause problems. |
| 07:19 | There's a huge range of floating disc mechanisms with each disc manufacturer having at least one or often multiple different floating mechanism designs across their range. |
| 07:30 | This usually makes the hat specific to the make and model of the disc. |
| 07:35 | Secondary benefit of a floating disc is that less heat is transferred from the brake disc to the hub and therefore wheel bearings which helps to extend their life. |
| 07:45 | At the same time, because less heat is transferred to the hub, the average disc temperature will increase. |
| 07:52 | In the vast majority of applications, the discs are fitted as part of the outboard suspension. |
| 07:58 | Most often clamped between the wheels and the hub. |
| 08:02 | While it's rare, occasionally we'll find them fitted in an inboard style which means the discs and calipers are mounted to the sprung mass instead. |
| 08:11 | Generally driven by the driveshafts that run out to the wheels. |
| 08:15 | Some things to consider when selecting a disc for motorsport include its ability to absorb enough thermal energy, its weight as well as material choice. |
| 08:25 | As we discussed, for most motorsport applications, your choices are limited to iron or pure carbon with carbon ceramic rarely being suitable. |
| 08:36 | The extremely high prices of pure carbon discs means that in most cases you'll be looking at iron discs. |
| 08:42 | Having a large iron disc is a good thing for temperature stability as it's effectively a larger thermal sink. |
| 08:50 | This means a disc with more thermal mass will change temperature less for a given energy input, relative to a disc with less thermal mass. |
| 08:59 | The obvious downside with a large iron disc is that it'll weigh a lot. |
| 09:04 | There are a few different ways to think about the extra mass. |
| 09:07 | Clearly it's simply more mass that must be accelerated and moved around with the vehicle which costs us performance. |
| 09:15 | However, as the wheels tend to be towards the outside corners of the car, this means we also pay a price in terms of yaw inertia for heavy discs. |
| 09:25 | Yaw inertia being the resistance of the vehicle to rotate about its vertical axis, which is essentially hurting us in direction changes. |
| 09:35 | Unsprung weight is also a big penalty, the effects of this are discussed in great detail in our motorsport suspension course but essentially a high unsprung weight makes it more difficult to maintain good tyre grip on anything other than smooth surfaces. |
| 09:51 | Another significant penalty of heavy brake discs is in rotational inertia. |
| 09:58 | Not only are we having to carry these discs around with us but we also have to accelerate them rotationally every time the car speeds up and slows down. |
| 10:08 | You can think of them as being four extra flywheels fitted to each corner of the car. |
| 10:13 | The final thing to discuss before moving on is drilled and slotted discs. |
| 10:18 | This is something that most people will be familiar with seeing in high performance brake discs but may not be sure of the reasoning for why they are there and how they work. |
| 10:28 | Starting with drilled discs, this is where a series of holes are drilled through the disc surface in a given pattern. |
| 10:36 | Although in some cases they can be cast with the holes in place too. |
| 10:40 | The original purpose for these holes was to allow gas that's emitted by the brake pads to escape. |
| 10:46 | Without a place to go, this gas will reduce the coefficient of friction between the disc and pad. |
| 10:53 | However, modern high performance pads don't tend to suffer as badly from gassing as they used to. |
| 11:00 | The major downside of drilled discs is that the holes act as stress concentrators which shortens their life as cracking begins to show. |
| 11:09 | In some places, drilled discs remain in use but a lot of time this is more to get the high performance look rather than for functional purpose. |
| 11:19 | Instead, with modern iron discs, you're more likely to see slots machined into the surface. |
| 11:25 | These can be straight or in arcs with each manufacturer having their own style. |
| 11:30 | The slots have a few different functions. |
| 11:33 | The first is to deal with the reduced but still present gassing from the brake pads. |
| 11:38 | While slots are not as effective as holes for clearing the gas, there's simply less of it to clear than there used to be. |
| 11:46 | Another advantage of slots is that it helps prevent the glazing that can occur at high temperatures for some brake pads which results in a lower coefficient of friction. |
| 11:56 | The slots help to wipe away the damaged pad material to expose the new material underneath. |
| 12:04 | Slots also help increase the amount of bite for the brake pad. |
| 12:09 | Each manufacturer will tout their design as being the most effective but in the end, it just seems that having slots is more important than the exact design used. |
| 12:20 | While slots are another type of stress concentration, its much less of a concern when compared to a drilled disc. |
| 12:28 | Slotted discs generally don't cause problems with cracking, provided they've been well manufactured. |
| 12:36 | In summary, brake discs act as a thermal sink for the brake system, absorbing the thermal energy created by braking friction. |
| 12:44 | They're available in a range of different materials with iron being by far the most common. |
| 12:50 | Discs are also available with different cooling and mounting options and weight is an important factor that affects the car negatively in multiple ways. |
