ON A DIME: BRAKE TECH
Brake Pads and Their Friction
Many will insist that the brake pads are the simplest part of the brake system. They are very, very wrong. Their creation and the material they use to create friction against the brake or drum surface is very complicated. Let’s talk about brake pads and friction materials.
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.
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 break 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.
Many will insist that the brake pads are the simplest part of the brake system. They are very, very wrong.
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”.
…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.
As the piston of your caliper begins 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 Pad Materials
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.
Now that you see the complication involved with brake pad friction, that’s got to leave you wondering about rotors and their design, right?