Brake Tech: Disc Brake Pads, Rotors, and Multi-Piston Calipers
Justin Banner
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When it comes to performance upgrades, the brake system is one of the most misunderstood portions of the automotive world, after aerodynamics. It’s also a universal upgrade, from off road to track days to even drag racing. Once you get going, you need to stop somehow. This article will touch on some of the tech behind your brakes.
Let’s start with what your brakes are doing. When you hit your brake pedal, fluid is sent from the brake master cylinder to your calipers or drum wheel cylinder to move a set of pads or shoes against a surface. Those pads and shoes are fitted with a friction material that clamps down on that surface to take kinetic energy (wheel rotation in this case) and turn it 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 stops. 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.
Now that’s out of the way, let’s 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 pads. Yes, we are only covering the disc brake system. We’ll talk about drums and shoes some other time.
The brake pad liner, the friction material that’s doing the stopping, will be attached in one of two ways: rivets or adhesive. Adhesive has become more common thanks to advances in that technology which have allowed it to take higher and higher temperatures before breaking down. However, that adhesive does still break down and is a cause of brake fade. We’ll touch on that a little later, but know that most brake pads are attached to their metal backing plates by adhesive, though there are still cases where rivets are used.
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-pitched squeal from a phenomenon called “tip drag.”
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 a 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 cause pad lift. Pad lift is when the friction material lifts off the backing plate; this can cause moisture to build up (leading to de-bonding from corrosion), corrosion of the backing plate and brake pad failure.
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.
Now we can talk about the distinct types of pad compounds that exist in the market. There are low-steel, “organic,” semi-metallic, metallic and ceramic compounds. 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 as you would on a track day, but it can be harsh on rotors and may not perform well in very cold temperatures, as it won’t put out enough friction to make any heat and won’t bite.
“Organic” pads are made of materials such as fiber, chopped glass, mineral fibers and even Kevlar mixtures. Organic pads are usually low-dust and 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. You’ll usually see these advertised as “low-cost” or “economy option” brake pads, as they are inexpensive to produce over semi-metallic, low-metallic and ceramic pads.
The low-metallic pads you see on the market are so called because they don’t contain as much steel or sometimes no steel at all, but contain a lot of the same organic formula 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.
Ceramic brake pads use ceramic compounds along with some other metals, like copper, in their mixture. They provide the lowest dust and noise and have the lowest wear on the brake rotors, but can take higher temperatures. However, they are not a desirable choice for a high-performance option, as 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. That being said, there are also new ceramic compounds coming out of Germany that do feature more metal in their mixtures, so this idea of a true high-performance ceramic brake pad is coming down the line.
Finally, there is the true metallic brake pad compound, usually found in racing brakes. These pads are made of sintered metals with few to no synthetic materials in the friction lining. They have a very high fade resistance and very high-temperature tolerances, but are noisy and very harsh on rotors, and 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.
Rotors come in solid disc or vented disc, with most front rotors being vented. The vented 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 that low pressure area, which then creates a low-pressure area behind that to pull in more air. Again, that’s oversimplifying it. Changing the angle of the blades can increase efficiency but will make the rotors directional. There are also multi-blade designs that direct airflow for better hot spot cooling.
Rotor faces come in four distinct types: solid, slotted, cross drilled or slotted and drilled. 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.” 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 an 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.
A slotted face 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, 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 you’ll see why a slotted rotor is an excellent choice.
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 (which can cause heat stress cracks at the drill points) and a loss of rigidity overall for the rotor. However, both of those problems can be reduced or even eliminated with proper metallurgy, design of the rotor and the selection of those drill points in the 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. 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 and 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.
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.
Finally, we come to the working piece of the disc brake system. The caliper is where the brake fluid flows to, forcing the brake pads to move on to the rotor surface. Designs can range from a single large piston to four-, six-, eight- or even twelve-piston calipers, with space between the rotor and the inside of the wheels being the limiting factor. Calipers with more than four pistons (like the one pictured here from R1 Concepts) usually feature a set of pistons that increase in diameter as you get away from the leading edge in the rotation of the rotor (in other words, the pistons closest to the bleed screws, as the top of the caliper is usually at the trailing edge of the rotor). The larger pistons are used to crush incandescent material and prevent pad taper wear from that material and differences in pressure across the pistons.
While you’re looking at this caliper, you’ll notice that the pistons look thicker. These are aluminum pistons that also feature a dust seal around them. The aluminum is made thicker for strength against the brake pads and hydraulic pressure but is still far lighter than full steel pistons, and stronger than phenolic pistons. However, it still produces the same pressure as a thinner steel piston because the diameters are the same. The dust seal is unique and, if you’re running a caliper in the dirt and off road, you might want to find one that features dust seals like this.
The fluid for the pistons is equalized by a small brake line between the caliper halves in this example. However, there are calipers that have this path set into their body rather than using a brake line. On the other side is the bleed valve that allows the tech or user to bleed each side of the caliper. Just like with bleeding from wheel position, you bleed this caliper (and others like it) at the furthest bleeder from the brake master cylinder. So, if this was on the right rear wheel and the master cylinder on the left-hand side, the outside bleeder is opened and bled first before moving to the inside bleeder.
Another thing to consider when it comes to your brake system is your brake fluid. It seems like this is always the last thing on everyone’s mind when it comes to brakes, but it plays a crucial role. Any time you increase your braking temperatures, you want to change to a DOT 4 or racing synthetic fluid. Even a simple change to a higher bite brake pad will increase temperatures to the point where DOT 3 will begin to boil. Even with DOT 4, you want to make sure you bleed and refill your brake system during your track day (at the very least before you drive home). Any time you run your brakes hard and experience fade, check your fluid and bleed the system.
You should also always consider your brake system as a circuit, and any time you change the load – like going to a larger or multi-piston caliper or higher brake temperatures – you should always change the rest of the system to match the new load, just like you would when you deal with electricity and electrical components.
You should also always consider your brake system as a circuit, and any time you change the load – like going to a larger or multi-piston caliper or higher brake temperatures – you should always change the rest of the system to match the new load, just like you would when you deal with electricity and electrical components.
