Practical Automotive Maintenance: Brake Rotors

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Brake Rotors

09.13

00:00	The next obvious component for us to discuss is the brake rotor or brake disc as they're also known.
00:05	This is the components that are clamped by the brake pads to slow the car down.
00:10	By being rigidly attached to the hub, when they're clamped by the brake pads, they also provide a torque to slow the rotation of the hubs, wheels and tyres, thereby slowing the car.
00:20	The brake disc can be thought of a thermal sink that absorbs a lot of the thermal energy produced by the brakes when we slow the car.
00:27	After absorbing the energy, it then gradually releases it to the ambient air as it cools.
00:32	The central part of the disc is mechanically attached to the hub, with the outer surface being clamped by the brake pads.
00:38	The part that's in contact with the pads is known as the swept area of the disc.
00:42	Brake discs can be solid or vented.
00:44	However, a non-vented disc is, so uncommon, we'll struggle to find any modern cars with them and certainly it's rare in anything high performance.
00:52	Solid discs, as the name implied, 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.
01:04	In vented discs, the cooling air flows from the inside to the outside.
01:08	By rotating, a vented disc acts as a sort of an air pump to encourage the flow through them.
01:13	Vented discs come in a number of different styles, but can broadly be broken down into straight and curved vane designs.
01:20	Straight vane discs have the advantage of being cheaper to manufacture.
01:24	Curved vane discs work more efficiently as air pumps, but not only are they more expensive to produce, they're also directional and this means that we need a separate design for the left and the right side of the car.
01:35	The direction the vanes point is critical as the air always flows from the inside to the outside.
01:41	The correct orientation for the vanes is shown here, which helps to promote cooling airflow.
01:46	If the discs are fitted to the wrong side of the car, the discs will not cool as well.
01:51	Materials for brake discs vary widely based on the intended application and budget, but there are three main categories of material used.
02:00	Iron, carbon ceramic and carbon.
02:03	Iron discs are what the vast majority of OE road cars are produced with.
02:07	While iron may sound like a pretty old-school material, it has some big advantages.
02:11	And that's why you'll still find them fitted to many high-end race cars like modern GT3 cars.
02:16	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.
02:31	The main downside of an iron disc however is weight, especially compared to the other two types of discs we'll discuss.
02:37	The next category is carbon ceramic discs, which you'll generally find fitted to extremely high performance road cars.
02:45	Benefits include the ability to work well from cold and a much lighter weight when compared to iron discs.
02:51	They also have an extremely low wear rate.
02:54	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.
03:02	The ability to work well from cold may not be too important in a race car, but it's critical for road cars.
03:09	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.
03:15	The reason for this is that carbon ceramic discs are poor heat conductors.
03:19	This means that they need to be larger in order to reject enough heat to survive.
03:24	Downsides are the very high cost as well as being extremely fragile.
03:28	Due to their nature, while they're very strong, they're also extremely brittle, meaning any impacts may permanently damage them.
03:36	Carbon ceramic discs also aren't tolerant of the high temperatures experienced on track, which makes them unsuitable for motorsport use in general.
03:44	If they're overheated, they become permanently damaged and the wear rate increases rapidly.
03:50	One place you will find carbon ceramic discs being used in motorsport is in some hill climb racing.
03:55	This is because they work well from cold and they don't see the extreme temperatures that will often occur in circuit racing.
04:01	This allows hill climb competitors to benefit from the lightweight advantages carbon ceramics offer.
04:08	The final category to discuss is carbon discs.
04:11	These are normally referred to as carbon carbon, which describes the brake system with a carbon disc and a matching carbon pad.
04:18	You'll find carbon discs fitted to high-end race cars, things like LMP1, GT500 and Formula 1.
04:25	Advantages here are the extremely low weight, low thermal expansion and the ability to survive at high temperatures, while also having a high coefficient of friction.
04:35	Downsides include the eye watering cost, long manufacture time, as well as poor friction characteristics and high wear rate at low temperatures.
04:44	While they are capable of working well at high temperatures, if they're overheated, the wear rate also increases quickly.
04:50	With the exception of properly cared for carbon ceramic discs, brake discs can be thought of as a consumable.
04:57	They wear out and lose thickness in the same way that brake pads do.
05:01	Each disc has a minimum thickness and once this has been reached, it must be replaced.
05:05	Mounting options for brake discs can vary considerably by application.
05:10	In normal OE road cars with iron discs, the brake hat is usually integral with the disc.
05:15	It's all one piece.
05:16	Generally, the hat will locate on the central spigot of the hub and have holes in it to allow the wheel studs to pass through.
05:23	A single piece disc has the advantage of being cheaper to manufacture.
05:27	But the main downside is it can lead to vibration problems when the discs run at high temperatures.
05:33	This is due to the normal thermal expansion that occurs with the disc and the higher the operating temperature, the worse it gets.
05:39	This can lead to stresses and distortion in the disc that can cause vibrations and unsteady braking forces as well as uneven pad wear.
05:47	These are annoying at best and dangerous or destructive at worst.
05:51	The most common way for this to be dealt with is by using what's called a floating hat.
05:56	This simply means that the hat and the disc are made from two different materials.
06:00	Most often an aluminium hat with an iron disc.
06:03	To some extent, the disc is usually allowed to float radially and axially on the hat while still being able to react to braking torque.
06:11	This means that as the disc heats up and expands, because it's not rigidly constrained to the hat, it can float and find its own natural running position between the pads and the caliper that doesn't cause problems.
06:23	There's a huge range of floating disc mechanisms with each disc manufacturer having at least one and often multiple different floating mechanism designs across their range.
06:32	This usually makes a hat specific to the make and model of the discs.
06:37	A second benefit of the floating disc is that less heat is transferred from the brake disc to the hub and therefore the wheel bearings, which helps extend their life.
06:46	At the same time, because less heat is transferred to the hub, the average disc temperature will increase.
06:52	The final thing to discuss before moving on is drilled and slotted discs.
06:56	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're there and how they work.
07:05	Starting with drilled discs, this is where a series of holes are drilled through the disc surface in a given pattern.
07:11	Although in some cases they can be cast with the holes in place too.
07:15	The original purpose for these holes was to allow gas that's emitted from the brake pads to escape.
07:20	Without a place to go, this gas will reduce the coefficient of friction between the disc and the pad.
07:26	However, modern high performance pads don't tend to suffer as badly from this gassing as they used to.
07:31	The major downside of drilled discs is that the holes act as stress concentrations, which shortens their lifespan as cracking begins to show.
07:39	In some places drilled discs remain in use, but a lot of times this is more to get the high performance look rather than being for a functional purpose.
07:48	Instead, with modern iron discs, we're more likely to see slots machined into the surface.
07:53	These can be straight or in arcs with each manufacturer having their own style.
07:58	The slots have a few different functions.
08:00	The first is to deal with the reduced, but still present gassing from the brake pads.
08:04	While slots are not as effective as holes at clearing the gas, they're simply less of it to clear as they used to be.
08:10	Another advantage of slots is that it helps to prevent the glazing which can occur at high temperatures for some brake pads.
08:17	This results in a lower coefficient of friction.
08:19	The slots help to wipe away the damaged pad material to expose new material underneath.
08:25	Slots also help to increase the amount of bite for the brake pad.
08:29	Each manufacturer will tout their design with being the most effective, but in the end it just seems that having slots is more important than the exact design used.
08:38	While slots are another type of stress concentration, they are much less of a concern compared to a drilled disc.
08:44	Slotted discs don't generally cause problems with cracking provided they've been well manufactured.
08:50	In summary, brake discs act as thermal sinks for our braking system, absorbing the thermal energy created by braking friction.
08:57	They're available in a range of different material with iron being by far the most common.
09:01	Discs are available with different cooling and mounting options and weight is an important factor that affects the car negatively in multiple ways.