When it comes to performance upgrades, the brake system is often one of the most misunderstood or misinformed portions of the automotive world after aerodynamics.
The Basic Theory
Let’s start with what your brakes are doing. You need to stop or slow down for that next corner but letting off the gas won’t slow you enough in many cases. In those cases, you need to get on the binders.
When you hit your brake pedal, fluid is sent from the brake master cylinder to your calipers and/or drum wheel cylinder to move a set of pads or shoes against a rotating surface. Those pads and shoes are fitted with a friction material that clamps down on that surface to take kinetic energy, in our case that is wheel rotation.
That then turns that kinetic energy into thermal energy from the friction between the friction material and the rotor or drum surface. This friction causes the wheel to slow until it is stopped. While your tire’s traction will determine how effective your braking is, the coefficient of friction of the brake liner will determine how much bite the pads or shoes will have on the rotors or drums. That thermal energy is then radiated away by air flow over the surface area of the rotor or drum.
How are Brake Pads and Shoes Made?
The friction material of the pads and shoes are mixed with their adhesive prior to being flowed into a mold with the backing plates made of steel. They are then pressed to their friction material shape before getting sent to an oven. They are then baked to very high temperatures with their compound and adhesive. This bonds the backing plate and friction material together in the modern brake pad and shoe. Prior to this, the friction material was pressed into a mold and with openings for rivets, which fastened them to their backing plates.
How are Rotors Made?
Rotors are typically made of cast iron known as grey iron – a type of cast iron with graphite in the mixture and sometimes other compounds such as copper, silicon, or other materials that bond with iron. Early front disc brakes and many rear brakes today are a solid disc. However, these discs can have trouble with dissipating heat fast enough. This is where the invention of the vented disc brake came in to fix that issue.
Both types of discs are molded, but vented discs are done in a procedure known as sand casting. The veins of the vented rotor are made of a separate sand core. It’s placed between the cope (top portion of a mold) and drag (bottom portion of the mold) and the metal flows into the mold. Once the metal cools, the core is removed by hammering it out, using air, or various other methods of removal depending on how the sand cast was made and bound. After that, the rotor is then machined for vehicle fitment before final surface finishing and coating – if a coating is being applied, that is. Drums are usually made in a very similar way with molds.
Now that’s out of the way, we can go into the simplest and, quite honestly, one of the most dramatic changes you can make to your brake system without changing your calipers to something more expensive – your brake pad and shoe compounds. To do that, we need to talk about the friction compounds between the most popular types.
Semi-Metallic Brake Compound
Semi-Metallic, also mistakenly called “metallic”, brake pads are about 30 to 60-percent metal with other synthetic compound mixtures within the friction material. It is a very good material to look for if you do a lot of braking or very heavy braking like you would on a track day. However, because of the amount of metal in the friction material, it can be harsh on rotors. These may not perform well in very cold temperatures as well as they won’t put out enough friction to make any heat and won’t bite. At least initially, once you use the brakes several times, they will begin to generate heat and work as they were intended.
Organic Brake Compound
“Organic” pads are made of materials such as fiber, chopped glass, mineral fibers, and even Kevlar mixtures. Organic pads are usually low-dust, low-noise, and are generally better on the rotors, but they do tend to wear out fast. They are also not good for brake environments that see a lot of heat because of their organic materials. Those materials ablate away as you increase the heat and use the brakes more and more. You’ll usually see these advertised as a “low-cost” or “economy option” brake pad as they are inexpensive to produce over semi-metallic, low-metallic, and ceramic pads. However, because of their compound they are good in low brake temperature environments where the brakes aren’t used that often.
Low-Metallic Brake Compound
The low-metallic pads you see on the market are called so because they don’t contain as much steel as Semi-Metallic Pads. Sometimes no steel at all and those contain a lot of the same mixtures as organic pads. However, they perform better because they are mixed with more copper or other types of softer metals. So, these pads will be a little noisier compared to full-organic, but not as much as semi-metallic. They are also not as harsh on rotors as semi-metallic pads are, but again will wear the rotors faster than organic pads. These are, as you can probably guess, in between when it comes to brake temperature environments. They work far better at low brake temperatures than semi-metallic and can stand higher temperatures than organic pads, but that does depend on the material mixture.
Ceramic Brake Compound
The ceramic brake pad and is one that uses ceramic compounds along with some other metals like copper in its mixture. They provide the lowest dust and noise and have the lowest wear on the brake rotors. While they can take higher temperatures, they are not a desirable choice for a high-performance option. Reason being is that they don’t get rid of heat as well as metallic-based friction materials.
These are the best choice for the show car that sees street duty and wants decent braking performance. However, there are also new ceramic compounds coming out of Germany that do feature more metal in their mixtures. This means that show and performance drivers can have their cake and eat it, too. This idea of a true high-performance ceramic brake pad is coming down the line.
Full Metallic and Exotic Brake Compounds
Finally, there is the true metallic brake pad compound, usually found in racing brakes. They are made of sintered metals with little to near zero synthetic materials in the friction lining. They have a very high fade resistance and very high-temperature tolerances. This also means they are noisy and are very harsh on rotors as well as require a higher temperature to begin to operate properly. There are also exotic material brakes made from Carbon Fiber, however these types of rotors need to generate high-temperatures to operate and are best suited for harsh race track environments.
Shapes and Design Features of the Friction
Brake pads come in many shapes for many reasons. While caliper and packaging design plays a significant role, it’s not the only reason.
A common design among street brake pads is the chamfer, an angled cut seen at the ends of the brake pad friction material. However, there are several ways a chamfer is done. This is designed to prevent high-vibration areas around the edges of the brake pads when they contact the rotor. This reduces the noise and vibrations you can feel while stopping when compared to a brake pad without the chamfer. A pad with a straight edge design on the braking area usually causes a high pitch squeal from a phenomenon called “tip drag”.
As the piston of your caliper begin to push the brake pads into the rotor, the pads begin to bend and fluctuate. This happens in microns of an inch but can create the high frequency squeal as the pad tips bounce against the rotor. This bouncing can create glazing on the rotors and even increase rotor wear. A straighter edge has the tendency to bounce and grab more than chamfered edges which can lead to noise and can cause pad lift. Pad lift is where the friction material lifts off the backing plate and this can cause moisture to build up, leading to de-bonding from corrosion, corrosion of the backing plate, and brake pad failure.
Cuts in the Friction
The center cut on the brake pad friction that you see in this picture is designed for three reasons: flexibility, cooling, and venting. Even with the chamfer, the pads will still move and bend on their backing plates. If a solid piece of friction material is used on a pad that moves quite a bit, it can lead to chunking of the friction and even full pad failure. The slot also helps the hot gasses that build up to vent out and help prevent the pads from overheating in normal cases.
This same venting will allow the incandescent material, the unburnable debris from pad wear and road pickup, that builds up as your pads and rotors wear to vent out and away from the rotor and pad for optimal braking performance. In some cases, pads can have more than one cut for the same reasons. It all depends on the design requirements of the caliper and backing plate interaction and venting requirements.
Rotor faces come in four distinct type: solid, slotted, cross drilled, or slotted and drilled. How do each of those work and what are the advantages of each?
Solid Face Rotors
A solid face rotor will be the most rigid and can dissipate heat very well. It can take a little more abuse and can also be resurfaced easily from “warping”. It’s the simplest design that all OEs take advantage of because it doesn’t require extra machining or complex work to build or mold it. While it’s simple, it’s still very effective in most high-performance brake systems where pad gassing and debris clearing isn’t an issue.
Slotted Face Rotors
A slotted faced rotor is designed to keep some of the rigidity and heat dissipation of the solid rotor but create a space for gasses and incandescent materials to be wiped away from the friction lining.
Gasses come from the natural breakdown of the adhesive that holds the brake friction to the brake pad as it heats up from use. This gassing creates a bearing surface, like how an air gap works in an old distributor, and creates a form of brake fade because the gasses can’t be compressed. The slots transfer those gasses away from the friction and rotor surface along with the incandescent materials to improve braking performance in high-performance applications. A street car normally won’t see this, but if you track yours then you will and is why a slotted rotor is an excellent choice.
Cross Drilled Rotors
A cross drilled rotor has holes drilled straight across each rotor face that also feature chamfered edges to reduce hot spots at those drill points. This design is for maximum degassing as the venting of the rotor helps pull those gasses away from the rotor surface. The problem you start to encounter with a cross drilled rotor is the reduction of surface area for cooling. This can cause heat stress cracks at the drill points and a loss of rigidity overall for the rotor.
With modern adhesives and pad construction, the requirement of a cross drilled rotor has been reduced to the point that they aren’t used that often in professional motorsports. The exception is environments where having high rotor surface temperatures are needed for brake pad friction effectiveness. In other words, you don’t need a cross drilled rotor on your daily driver. The brake temperatures won’t be high enough for pad degassing and the pads you are using don’t need that much temperature to operate. If you’re using a full metallic compound on your street car, you’re not doing yourself a favor unless you’re heading for a track.
Slotted and Drilled Rotors
The combination of slotted and drilled seeks to gain the advantages of both: the maximum degassing of a cross drilled rotor and the wiping of the friction surface of the slotted rotor while also retaining some of the rigidity from the slotted rotor design. However, if you’re not experiencing any degassing issues with solid rotors, you’re not gaining much in terms of performance from switching to either version. You’ll also lose surface area that helps with cooling your brake rotors.
Both a slotted and cross drilled rotor will be slightly lighter, but only by a few grams at best. Unless you’re in a Formula Car, have reduced the weight of your tires and wheels, losing weight at the rotor isn’t going to be of much use to you and can be detrimental if you don’t buy a high-quality slotted or drilled rotor.
Losing Weight with a Two-Piece Rotor
However, if you want the maximum rigidity but want to reduce weight, you should consider a two-piece rotor with an aluminum hat, like you see here.
The aluminum hat reduces the weight of the rotor significantly since that large mass of metal is of a lighter material. You also gain the ability to change rotor faces and material without changing the rotor hats and this type of hat can allow you to work with a custom design by just changing the hat instead of the whole rotor. This does come at a price increase over a solid hat and rotor but if you’re going for maximum lightness, price usually isn’t a concern at that point.
Discs or rotors of the disc brake system do an equal amount of the hot work of the brake system, but they also do more than just transfer heat. Their face designs help the pads do their job, but what about the issue of rotors “warping?” Well, they don’t warp like a wet piece of board does. What’s happening is that the pads are leaving some of their friction material on the rotor surface under harsh braking.
How a Rotor Cools
Again, rotors come in solid disc or vented disc, with most front rotors being vented. The venting design is a centrifugal (radial) fan type, where – in the simplest terms – the blades create a low-pressure area on the outside of the rotor as it rotates. The high-pressure area between the blades flows in to fill in that low pressure area, which then creates a low-pressure area behind that to pull in more air. Again, that’s over simplifying it.
Changing the angle of the blades can increase efficacy but will make the rotors directional. There are also multi-blade designs that direct airflow for better hot spot cooling.
Notice that “warping” is in quotation marks here. Your rotors do not warp in the sense that wood warps when it gets wet. Instead, what’s happening is that the brake friction material is transferring unequally to the rotor face. This can happen because of unequal temperatures on the surface of the rotor, a hotter spot on the rotor will transfer more friction material onto the rotor surface than the colder spot.
This creates and uneven surface that transfers into the brake calipers and creates the judder and vibrations associated with “brake warping.” When a technician resurfaces the rotor, they are removing that access material along with the rotor surface to create an even face again. That’s not to say a brake rotor can’t warp, but if it does there’s a whole host of other problems going on and usually the rotor will crack and break before that warping happens.
Now, when you go shopping for your next set of pads or rotors, you’ll be more educated without the incredibly conceited wording that many marketing people include in their sales ads and pitches. If you’re driving a daily, get semi-metallic or ceramic pads with normally vented rotors. If you’re racing, get aggressive pads that work in the heat temperatures you’re looking for and either use solid faced or slotted rotors. You don’t need drilled rotors and they will not cool as effectively as solid face rotors and if you’re looking to lose weight, get a set of two-piece rotors with aluminum hats.
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