BRAKES 101

Ultimate performance for your BMW

BRAKE PARTS 101

EVERYTHING YOU EVER NEEDED TO KNOW ABOUT BRAKE ROTORS, BRAKE LINES, and BRAKE PADS.

OVERVIEW

Brake parts are for much more than simply slowing down the car. The control of deceleration forces is a critical part of performance driving and safety, no matter if on the street, track, or competition course. Brakes control the chassis, meaning that appliction of braking forces affect the weight transfer and balance, which in turn affect how and what the suspension and tires do to position the car on the road.

Weight transfer changes the forces on the tires; the very simple example is skidding (or engaging ABS), which signifies that the tires are being asked to do more than they can. While this can lead to a loss of control on the street, in can lead to even more dangerous control failure in the higher speed and quicker-changing situations on the track. Maintaining ideal chassis control through braking is not only a function of the driver's input, but the design and type of braking components on the car.

ROTORS

For simplicity in this explanation, we will address the standard iron brake rotor and leave carbon and ceramic rotors aside.

MATERIALS

The specific metal used for brake rotors is nothing all that fancy, but the details and manufacturing are the key components to quality and performance. All iron brake rotors are cast from a formulation called "gray iron". This chemical composition of this iron has a specific carbon and silicon content which causes it to have a graphitic microstructure for very high stiffness, high heat capacity, and most importantly high thermal conductivity which allows for rapid cooling.

Although gray iron would seem to be a simple alloy, the fact is that the methodology and care used in the alloying and casting process are what separates a good quality rotor that provides long service life, smooth operation, and no failures from a cheaply-made product that may wear faster, cause vibration and shimmy, and even crack or break under hard use.

DESIGN Aside from being just a plain iron disk, there are various surface treatments done with varying results. The most common and most effective treatment is slotting, where grooves are machined into the surface of the rotor. Slotting has multiple purposes. First, as a safety feature in street-driven cars; when driving in rain or running through deep puddles, a rotor that gets wet will instantly transform that water into steam which can exert enough pressure from expansion to push the pad off of the rotor surface. The slots act as a pathway to release that steam quickly, eliminated the pad float problem. The second purpose is similar, but more of a feature for track or race use; when brake pads get hot enough, they can "off-gas", which is to release a gas as a product of extreme heat. The off-gas effect is similar to the steam effect earlier, lifting the pad from the surface of the rotor. The slots perform the same function and give the gas a quick escape path, allowing the pads to remain in contact. Thirdly, a combination of safety and good maintenance, is that the slots act as a visual wear indicator to show how much rotor life is left. With modern cars, including all BMWs, the allowable wear depth is often approximately 1mm or just over. With the slots machined to the maximum wear depth, a very quick visual inspection shows how much rotor life is left. With UUC rotors that are slotted to the minimum wear thickness, it is simple to tell the rotor is worn out when the slots are worn away.	Slotted rotors:

	UUC slotted rotor.

Drilled rotors:	The second most common treatment is drilling, where holes are drilled through the rotor all across the surface. Unfortunately, the applications where this is most commonly usedare the applications that most likely should not be using it. Drilling a rotor serves only one legitimate purpose, to remove weight from the rotor. True race rotors that are lightened in this way have large holes, almost big enough to put a pencil through. These race applications are usually short duration (such as auto-x) as the drilled weight reduction reduces the rotors cooling ability. For faster track use, the reduced cooling capacity is substantial and certainly not worth the weight reduction. For street cars getting drilled rotors just for "racy looks", the fact is that it's a braking performance downgrade. The reduction in cooling capacity is due to multiple factors, each one of them substantial. First, the way that a rotor cools itself is through the internal venting, viewable from the rotor's edge. The internal vanes and air passages act as a centripetal air pump, drawing air in at the center and pumping it through to vent at the edge, cooling the rotor internally. With holes drilled all through the rotor, there is no pumping action whatsoever and therefore no cooling effect. The second major issue is the weakening of the rotor disk itself caused by drilling holes; the introduction of this perforation is similar in effect to what is seen with the perforation line between sheets of household paper towels, a point where the basic material breaks apart easily. Initial stages of this are visible on many cars, exhibited as a spiderwebbibng of small cracks between the holes. Catastrophic rotor failure is definitely not a fun thing to experience in any situation. Between these two problems, excessive heating and structural weakening, drilling is a treatment that should be avoided.

Race-only rotor with lightening holes.

Drilled rotor showing stress cracks.

COATING Gray iron, like any standard iron, rusts easily. This can easily be seen on any car after a rainy day, rusty spots on the rotor surface where it got wet. Many original equipment applications with an uncoated rotor will show a heavily rusted central part of the rotor around the hub, the area that is not contacted by the brake pads. With how enthusiasts care for their car's appearance, this rusty central ring is ugly and unsightly. To prevent this, a quality rotor will have an anti-corrosive protective coating. The least expensive way to do this is with a zinc (silver color) or cadmium (gold color) plating process. Typically these plating methods work decently, but may not last the full lifespan of the rotor such that rust will eventually appear in that central area. A better coating that does last the lifetime of the rotor is black e-coating (electro-coating, a process that permanently embeds the black color into the iron), which also makes the rotor center blend in better with any color of wheel.	Black electro-coating in center:

	Unsightly rust in the center and on edges of typical uncoated rotor:

UUC rotors are made from a highly refined gray iron alloy to maximize cooling capabilities and strength as well as provide maximum service life. All UUC DirectFit rotors feature black e-coating and are available with either a plain surface or slotted surface. See all UUC BMW DirectFit rotor applications, [ CLICK HERE ]

BRAKE LINES

Stainless steel braided brake lines with protective outer layer:	The brake calipers are operated by high pressure hydraulic fluid. The fluid is pumped into the caliper from the brake master cylinder (actuated by the driver's foot on the brake pedal), which causes pistons inside the caliper to squeeze the pads against the rotor. This system operates under a very high pressure, up to 1200psi. For an enthusiast, precise and accurate control of brake application is critical for chassis control. The brake lines are the connection between what happens at the pedal and what happens at the brake caliper. This often-overlooked part of the braking system can be easily and affordably "tuned up" with a performance brake line, designed as a teflon tube core, wrapped in braided stainless steel, and then covered in a protective wrap of clear flexible plastic. The original brake lines are rubber and can expand under the normal pressure of use, especially as they get older. This expansion of the brake line negatively affects the brake caliper piston travel, and is felt as mushy pedal feedback. Mushy pedal feel and response affects how accurately the driver can control the application of the brakes. Additionally, as the rubber lines age, they weaken and can even crack and fail. Visible pre-failure cracking of a rubber line can begin in as little as 5 years and will eventually lead to complete failure of the brake line, loss of hydraulic pressure, and therefore loss of braking capability. UUC stainless steel brake lines are fully DOT-compliant, made in the USA, and cost less than purchasing new rubber lines from BMW. See all UUC stainless steel brake line applications, [ CLICK HERE ]

Original equipment rubber hose cracked from age (6 years):

BRAKE PADS

SO MANY TYPES OF PADS, HOW TO CHOOSE? Even more important than rotor choice is brake pad choice. Pads come in a wide variety of types specifically for an equally wide variety of uses... anything from mild street use to full-on racing, there's a brake pad choice to maximize the desired results. What most people want from street pads is a combination of low dust, quiet and smooth operation, and long service life of both the pad and the rotor. Modern technology has delivered the ceramic compound as an excellent way to achieve these results. As an additional benefit to the enthusiast driver, ceramic pads have a higher maximum use temperature than common street pads of even just ten years ago. The Dual Technology Ceramic pads are an excellent example of this type of pad ([ CLICK HERE ] for more details). An enthusiast driver's needs for a brake pad might shift to something a little more aggressive. By aggressive, the characteristic is more of a stronger initial "bite", how the pad reacts to low brake pedal force application and also an overall stronger grab of the rotor (referred to as coefficient of friction, or Cf). This type of material is often referred to as semi-metallic. The differences in this more aggressive material will likely result in more brake dust and faster rotor wear. Temperature and coefficient of friction characteristics really are the basis of brake pad selection, and are what separate street pads from true track or race compounds. As shown in the chart below, street pads work well at normal temperatures associated with daily driving, but begin to drop off in performance and even fail right at the low end of the range that track/race pads are really starting to become most effective. Race pads are an entirely different type of material than street pads. As with any genuine "race" part, steet use is not recommended... in fact, the very different temperature range and expected pad function for street use is very different than what race pads are designed for, making a race pad work very poorly in street use. Race pads are designed for a much higher operating temperature and do not function well when cold. While the temptation is there for the enthusiast driver to put race parts on a street car, in many instances such as this, it's a bad idea. There are many race pad formulas, with each one tailored to a specific heat range, type of car, type of track, and response characteristics. For more details and examples, [ CLICK HERE ].	DTC pads:

	Race pads can withstand temperatures that make rotors glow: