Brakes 101 – Rotors, Pads, and Fluid

Brake Caliper Nomenclature

The braking system in todays cars is actually a pretty simple setup but with some electronic twists. Prior to the advent of ABS (Antilock Brake System) and TC (Traction Control) there were no electronics involved with the exception of the brake pedal switch. Like everything else, our brakes have evolved for a number of reasons to include driving assistance.

Brake Calipers

When you press on the brake pedal the master cylinder will try to force brake fluid into the caliper. Since there is no empty space, the fluid pushes back, creating pressure. The pistons inside of the caliper will then be pushed outwards which has the primary effect of pushing the brake pad into the turning rotor. This force modulates how much friction is generated between the rotor and pad, generating the necessary braking torque to slow the vehicle.

Caliper pistons are where many calipers differentiate. Calipers can include up to 8 pistons or as few as 1 depending on the design. They can be the same diameter or they can be varying diameters. By altering the number of pistons and size of the pistons the caliper can modulate how force is applied to the brake pad. This force is tuned to give even pad wear which reduces problems down the road.

Other components include caliper piston seals, which seal out road grime, dust, and other particulates from damaging the piston bores. Heat is a primary killer of these seals and it’s common to have to replace them on track vehicles especially. The brake pads slide up and down the caliper pins so it’s good to keep these clean and greased up.

One common problem with multi-piston open-top brake calipers is that they tend to be noisy. Pops, creaks, and squeal are all problems with their own solutions. Popping is handled by the cross spring. It’s a metal spring that acts to locate the pads and prevent them from rattling. Squealing is a noise generated from high frequency vibration generated at the pad / rotor interface. Brake pads will either use shims, grease, or an insulator backing to prevent or dampen the vibrations.

Another issue that’s common to the 4-piston OEM Brembo design is the flexure of the caliper itself during heavy braking. Any distortion of the caliper body due to load paths will cause strange issues that are unwanted. Pad bind, uneven pad wear, and a soft pedal are common complaints. This is due predominantly to the lack of a centered caliper bolt and the other strengthening methods used in a true racing caliper.

Brake Pads

brake pads

Brake pads are the easiest thing to change out on any braking system. They also have a major effect on producing braking torque as well as how the brakes feel and react to driver inputs.

Brake pads can be made of several different types of materials. Here’s a break down of the materials and their benefits.

Semi-Metallic

Semi-metallic brake compounds feature a mix of 30 – 70% metal powder suspended in a resin filler; among other ingredients. These pads are cheap to make and have fairly good wear rates for daily driving. They offer decent NVH performance and moderate temperature stability. The coefficient of friction for this pad type is generally low, around 0.28 to 0.35 which gives long rotor life and gentler bite / modulation.

Ceramic aka NAO (Non-Asbestos Organic)

Ceramic brake pads are a hodge-podge of materials including copper, ceramic compounds, antimony, and other elements. They suffer from pad compressibility and require lower operational temperatures. They are not recommended for a performance application.

Sintered

Sintered brake pads are constructed of molded metal fibers / powdered metal. They produce very high amounts of friction which translates into high amounts of braking force at the cost of modulation. They will eat rotors alive due to their high mu and rigid construction and pad wear can also be a concern depending on rotor hardness.

Carbon

Carbon pads are made of carbon fibers compressed into a resin “catalyst” filler and then baked onto the backer. They are specifically used in conjunction with another carbon friction surface such as Ferrari’s and Porsche’s carbon ceramic brake rotors. They enjoy a warm rotor face and produce excellent brake force with good modulation. Pad life is moderate depending on rotor temps.

Brake pads are also designed with outgassing in mind. The divisions you see in the pad material are there to give gasses such as steam and resin burn-off a place to exit the contact surface. This improves wet braking as well as braking during high load situations.

Brake Fluid

Brake Fluid Motul 5.1

Brake fluid’s job is to transmit force between the driver and the brake caliper. It is a low viscosity fluid designed to be largely incompressible while also offering corrosion resistence to braking components as well as thermal resistance to boiling. Unfortunately, most brake fluid is also hygroscopic, meaning it absorbs water from the atmosphere. This water is retained in the brake fluid until the fluid is flushed out in your maintenance efforts. (You DO change your fluid right?) If left, this water will eventually cause localized boiling of the fluid and the pedal will become very mushy or even drop to the floor. This is very dangerous and should be avoided at all costs.

So what types of fluid are there, and what is this DOT 3 / DOT 4 stuff?

DOT 3 Brake Fluid

This fluid is polyethylene glycol (PEG) based and meets a Government standard that specifies a dry boiling temp of at least 205 °C (401° F) and a wet boiling temp of at least 140 °C (284 °F).

DOT 4 Brake Fluid

This also is a PEG based fluid and meets a Government standard that specifies a dry boiling temp of at least 230 °C (446° F) and a wet boiling temp of at least 155 °C (311 °F).

DOT 5 Brake Fluid

This is a silicone based fluid and is NOT hygroscopic. It meets a Government standard that specifies a dry boiling temp of at least 260 °C (500° F) and a wet boiling temp of at least 180 °C (356 °F).

DOT 5.1 Brake Fluid

This is another PEG based fluid and IS hygroscopic. It meets a Government standard that specifies a dry boiling temp of at least 260 °C (500° F) and a wet boiling temp of at least 180 °C (356 °F), just the same as the above DOT 5 fluid.

As a replacement I would highly recommend a DOT 5.1 fluid. This is easier to switch over to compared to your OEM fill than a silicone DOT 5 fluid and offers the same performance.

“Wet” fluid performance is simply the same fluid with 3.7% water by volume. It simulates the fluid after it has absorbed water over time. It does not mean that in wet weather your brake fluid will perform worse.

Brake Rotors

Brake rotors are a pretty important component of the braking system. They are responsible for generating the braking force when the brake pads bear down. That braking force creates a lot of heat, and it’s the job of the rotors to soak up that heat and expel it into the surrounding air. Here’s some aspects of brake rotors and what role they serve.

Brake Rotor InfoRotor Hat

The rotor hat on most OEM vehicles is cast into the entire rotor assembly, hence the name, a 1-piece rotor. This is a cheap to produce and simple solution for brake rotors.

However, on higher end sports cars and aftermarket brake systems the rotor assembly will consist of two individual components, the rotor hat and the rotor ring. This is called a 2-piece rotor and offers a number of significant benefits but are considerably more expensive. The major benefit is the reduction in weight due to the usage of lighter materials, namely aluminum, for the hat portion. This can save upwards of 3 – 4 lbs. per rotor which is not only static weight, but also rotational and unsprung weight. This means faster acceleration, better suspension performance and ride quality, and better braking.

One Piece vs. Two Piece RotorsRotor Diameter

The rotor diameter is a very important aspect of the rotor and brake system performance. A bigger rotor diameter means:

  • Greater Braking Torque Potential
  • Better Heat Capacity and Faster Dissipation
  • Heavier Weight
  • Longer Life

An immediate downside to rotor diameter is an increased rotational and unsprung weight. In fact, the further away from the hub center-line the rotor mass gets, the more it affects acceleration negatively.

Rotor Thickness

Rotor thickness primarily affects two things. How much heat energy the rotor can absorb, and how quickly it can shed that heat energy from air flowing through the rotor vanes. One thing to note is that while rotor thickness can be the same between two different rotors, it doesn’t mean they will have the same performance. The amount of void space, vane design, and number of vanes can all affect how well the rotor performs. The more metal in the rotor, the heavier it will be, but the greater amount of heat energy it can store. Conversely the less metal in the rotor, the lighter it will be and the less heat energy it can store, but it will tend to shed the heat faster.

Rotor Face

On most OEM applications the rotor face is featureless. This is commonly referred to as a “blank” rotor. However, on some OEM applications and aftermarket rotors you’ll find dimples or holes drilled through the rotor face. This is known as a drilled rotor face.

Likewise, you may also see slits or cutouts on the rotor face in various design patterns. This is known as a slotted rotor face.

Both the drilled and slotted rotor faces are designed to allow gasses put off by the brake pads to escape. While modern brake pads don’t have significant out-gassing, it is slightly beneficial to run slotted rotors for wet conditions where water vapor and steam are present. The common downsides to slotted rotors is the propensity to wear out brake pads quicker than a blank rotor. However the slotted rotors tend to suffer from far less cracking issues, which is a common problem on drilled rotors. Particularly if the holes were actually drilled into the finished rotor rather than cast into the rotor prior to the heat relieving process.

 

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