The disc brake, as we know it, was first patented in , but it failed to gain acceptance in the U.S. until the early s when vacuum assist made the pedal effort acceptable for the driving public. European makers adopted disc brakes in the '50s, rapidly following Jaguar's dominance in the 24 Hours of Le Mans race using four-wheel disc brakes. While exotically sculpted and finished calipers get all the glory, the disc brake rotor is where the rubber meets the road, or more literally, the pad meets the rotor and kinetic energy is converted into heat. The rotor has two critical functions: provide a consistent, high coefficient of friction surface for the pads to engage, and act as a heatsink and at the same time shed that heat into the surrounding air. When talking rotors, mass is your friend since mass helps regulate heat. With aftermarket and high-performance brakes, a lot of engineering goes into rotors from the materials used to the design of the cooling vanes. There's also a lot of science in keeping that heat from migrating into the caliper and subsequently the brake fluid, but caliper design is another story for another day. For now, let's focus on disc brake rotors.
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The first disc brake rotors were one-piece castings somewhat similar to the one-piece rotors specified on most cars and trucks, and many aftermarket applications today. They perform well but carry a burden of excess weight, and they promote greater heat transfer to the hub/axle and bearings. In smaller sizes like the CPP setup on the left, which is designed to be used inside smaller classic OE style wheels, compact disc brakes can provide long-lasting service with far better braking compared to drums. However, as rotor diameter and thickness increase to improve brake leverage and heat absorption and dissipation, a solid rotor will distort in a bell curve shape under heavy use, potentially causing uneven pad wear and excess drag. Those concerns plus the ability to optimize for heat related material expansion led to the development of two-piece rotors like the CPP 14-inch rotor six-piston setup shown on the right.
ADVERTISEMENT - CONTINUE READING BELOWTwo-piece rotors were first used on motorcycles to save weight and allow for the fixed calipers required on the bikes. The use of an aluminum "hat" or "bell" bolted to an iron rotor ring was introduced to the high-end racing classes in the '60s. As with many track-bred innovations, the technology trickled down over the next few decades to more mainstream racing and high-performance street driven cars, and now trucks. Here you can see a two-piece rotor on the left and a more traditional one-piece iron rotor on the right.
There are three common ways of attaching rotors to hats—or bells, as they say in Europe: fixed bolt and nut; T-bolt style floating rotor; and stanchion style floating rotor with anti-rattle clips.
ADVERTISEMENT - CONTINUE READING BELOWBack in the '60s and '70s, the typical way to attach the hat to the rotor ring was with bolts that clamped the hat into threaded holes in the rotor. But a simple set of bolts isn't adequate to the critical task of keeping the hat firmly attached to the rotor. Everyone has seen nighttime pictures or video of brake rotors glowing red under racing cars caused by the enormous heat generated by heavy braking at speed. Those cycles of heat, expansion, and contraction, as well as thermal tempering of the bolts themselves means that the bolts will come loose or fail completely over a relatively short period of time. Using the right type of bolts, along with safety wire, can solve these issues.
The negative effects of heat cycling can be worse on two-piece rotors because of the vastly different rates of expansion between cast iron, steel, and aluminum. Aluminum expands at roughly twice the rate of cast iron and grade-8 steel fasteners. Early users of two-piece rotors learned that the bolts had to be safety wired to avoid catastrophic separation of the rotor and hat. Safety wiring was the standard for keeping bolts in place in the '60s, and these complex and time-consuming safety wire schemes were all that stood between finishing a race or having the equivalent of a 150-mph flywheel come loose under the car. The modern fasteners used by companies such as Baer and Wilwood lock in place without the need for safety wire, but people still like the warm-fuzzy feeling of security it offers. Plus it looks cool.
ADVERTISEMENT - CONTINUE READING BELOWThere are other issues with using conventional bolts. Because of the expansion rate differential and the stress relaxation it causes, SAE J429 grade-8 bolts shouldn't be used at temperatures above 800 degrees, grade-5 not above 450, and plated bolts should not be used above 250 degrees. Iron and steel glow red at 900 degrees. Your rotors need to be fastened according to how much heat you'll be generating. So use the right fasteners and torque them to what the manufacturer stipulates. The bolts supplied by companies such as Baer and Wilwood are designed to handle the heat and stay locked in place even under extreme heat.
Bolts must be sized so that the sheer plane, the surface where the hat and rotor are attached, is on the shank of the bolt, not the threaded portion. Bolts are weakest at the root of the thread, and bolt strength is reduced almost 30 percent when the threaded portion is placed in sheer. Therefore, the length of the bolt shank is critical, as is the material it is made from in order to have an extended service life and significant margin of safety. Look at the bolts used in your two-piece rotors of choice. Are they sized correctly? Do they have the right shank length? And what material are they made of?
ADVERTISEMENT - CONTINUE READING BELOWLike so many other technologies, aircraft and aerospace requirements led to the development super high-quality bolts and nuts that are specifically recommended for use in circumstances like rotor-to-hat attachment. High-end brake rotors from Alcon, Baer, Brembo, and Wilwood use NAS A286 stainless bolts and nuts. These fasteners retain their clamping properties at temperatures up to 1,300 degrees and are more than twice as strong as grade-8 fasteners.
NAS A 286 bolts come in conventional hex head styles and these unique counter-sunk tri-wing configurations for tight clearance situations. NAS fasteners typically can be reused once for a rotor replacement. The torque specification is double the grade-8 fastener, and while they cost 10 times as much as other bolts, what would you rather have holding your rotors together?
The next evolution in hat and rotor technology is referred to as floating rotors. As rotor size has increased to keep pace with today's faster and heavier cars, the increased rotor mass, greater diameter, and thicker hats has exacerbated the effects of differential rates of heat expansion. To allow for that, rotors, or more often, hats are now slotted to allow the materials to grow radially while being safely clamped to prevent or minimize side-to-side motion. The aluminum hat and iron rotor "float" in relation to each other so that problems don't occur as the metals expand at their own rates when heated.
For street use, with acceptable noise levels (the two parts can rattle under some circumstances), two approaches have been used: T-shaped bobbins that lock the hat to the rotor, and more complex CNC shaped rotor stanchions often combined with anti-rattle clips. The bobbin is more compact and simpler to produce, saving some cost and again allowing for tighter fitments.
ADVERTISEMENT - CONTINUE READING BELOWThe stanchion style is better for eliminating or reducing noise, and due to its increased size and flat (rather than round) sides, it can support 1.125-inch-thick rotors in 15-inch and even 16-inch diameter sizes. For these rotors, most aftermarket brake manufacturers also use aerospace-grade fasteners.
The quality of the iron used in rotors is also important. All of the higher-end brake manufacturers use proprietary formulations in the cast iron rotors. They also control for uniformity and precision with their casting and finishing processes. The typical modern rotor is finished to 0.-inch tolerance. If your high-performance car will see significant track time, look for rotors specified for road racing and extended on-course use with pads to match. Just because all rotors—from a stock replacement to a high-end aftermarket unit—are iron doesn't mean they are all made from the same quality of iron.
The most persistent myth about rotors is that they warp. It takes a lot of heat, the kind of heat used in the casting process, to actually warp a rotor. The two common problems that cause pedal pulsation are lateral run-out and disc thickness variation. Lateral run-out is typically caused by run-out from the hub face, wheel bearing, uneven lug nut torque, or a buildup of rust and corrosion between wheel and hub. Disc thickness variation is most often caused by excess pad transfer, the very thin layer of pad material that bonds to the rotor surface, and uneven wear from an improperly mounted caliper. The fixed mount calipers, radial or ear-mount, found in almost all high-end disc brake systems have to be mounted squarely to the rotor. Most new disc brake system issues are caused by caliper-to-rotor misalignment. The photo shows a caliper and rotor before proper shims have been added. A precisely installed caliper will result in a long-lasting, low-noise, high-performance braking system. Follow your manufacturer's specs precisely.
ADVERTISEMENT - CONTINUE READING BELOWSome manufacturers offer rotor and pad combinations that have been seasoned and bedded on a brake dyno under controlled circumstances like this Corvette setup from Pro-System BrakesAlcon. This ensures that your car's braking will be ready to go from the first lap without trying to find the hours and a suitable location to perform a proper rotor seasoning and pad bedding process. This is standard procedure in most racing series, professional and amateur. Of course ,there's a cost involved.
Drilled and slotted vs. slotted only, what is right for my car? Drilled rotors were introduced to combat pad fade caused by outgassing where the pad literally boils away binding materials from the pad. This isn't really a problem with modern performance pads. Selecting the right modern pad formulation can eliminate pad fade, but the higher heat they will generate can cause cracking and accelerated wear in drilled rotors. For truly high-performance applications, specify slot-only rotors. Also, high-performance track pads shouldn't be used on the street since they will never get into their temperature range and will quickly eat your expensive rotors.
The used slot-only rotor on the right is still usable. Surface cracks up to a fingernail width are safe. Slot-only or drilled and slotted rotors should be replaced when the slots have fully worn away or radial cracks at the drilled holes become too large or extend from the hole to the edge of the rotor.
Like we said, there was a time when drilled rotors were needed, but for the most part, those days are gone. Today, drilled rotors (to be fair, most higher-end brake companies don't actually drill the rotors, they are cast that way, which is a much better process) are really more of an aesthetic deal. They look cool, but for heavy track use they can be problematic if cracks start to radiate out from the holes. For a street car that sees little to no serious track time, drilled rotors are still a great-looking option.
The problems that drilled rotors suffer from under hard use is why most track-rated rotors from companies like Baer and Wilwood are slotted only. This lessens the chance of cracks forming under extreme heat, and the slots help clear away brake dust and debris. It also increases the mass of the rotor compared to the same rotor with holes, which helps control heat.
Zinc or E-coat finishes are great for reducing rust buildup on rotors in street applications. The whole rotor is coated, and then the pads "clean" the coating from the swept area of the rotor. However, for track performance, most manufacturers will not apply these finishes as they add time and complication to the pad bedding process since these finishes are much slipperier than the raw iron and the coatings can become embedded in the pads. Also, under extreme heat, zinc coatings will turn to a powdery white color.
ADVERTISEMENT - CONTINUE READING BELOWThe last word in brake rotors are carbon-ceramic rotors. For any application other than F1 or other short-duration, cost-is-no-object racing series where single-use carbon-carbon rotors and carbon pads reign, carbon-ceramic rotors are the highest performance but most expensive option. Carbon-ceramic rotors do not give shorter stopping distances, but they do provide significant weight reduction and much higher fade resistance. Brembo, Chevrolet, and Wilwood offer aftermarket carbon-ceramic rotors and brake systems. Keep in mind that carbon-ceramic brake systems function best when paired with ABS units.
In everyday use, carbon-ceramic rotors will last far longer than iron rotors, but replacement costs are still very high. In OE applications like the Camaro ZL-1 and Corvette ZR-1, track rats often replace the expensive carbon-ceramic rotors with heavier but cheaper iron rotors with the appropriate pads. Carbon-ceramic rotors wear differently than conventional iron rotors as they literally burn off material internally with hard use. If you look closely at the photos, you'll see the hat has a minimum thicknessandminimum weight specifications stamped on it. In heavy track use, the minimum weight is often hit well before the minimum thickness requiring replacement. There are marginal performance gains to be had with these exotic composites, so let your wallet and your aspirations be your guide. Also keep in mind that carbon-ceramic rotors are pretty fragile and can be damaged or destroyed if dropped or hit with a tool.
All the major brake companies offer downloadable brake templates as well as detailed measurement schematics to aid you in buying new wheels or help determine if your old wheels will clear your new binders.
If your brakes fade and you still have a firm pedal, you have exceeded the heat rejection capabilities of the pads and you need higher-performance pads. If your brakes fade and you have a soft pedal or it goes to the floor, you've boiled your fluid. Brake cooling ducts can help either problem, and higher boiling point fluid can help with fluid temperature issues.
If you've been paying attention, you've most likely noticed that brake rotor diameters keep growing. Some of this is so they don't look puny when stuffed inside the 19- and 20-inch wheels popular today, but there's a performance benefit, as well. If you recall your high school physics, then you know all about the leverage effect. As the caliper moves father from the axle center it can exert more leverage on the rotor, which helps braking. There's also a benefit in that the larger rotor will have more mass for better heat control. The downside is that the larger, heavier rotor will add more to your unsprung and rotating weight numbers.
I was born and raised in Southern California and have always been into classic cars. My first car was a .5 Mustang that I remember tossing Centerline wheels on and giving a midnight blue paint job to. It was a straight six, which most likely kept me alive. After high school I picked up a Camaro and spent all my money at Super Shops fixing it up. It rolled on 16-inch IROC wheels, which in the s was pretty cutting edge. I'm old enough that Big Daddy Ed Roth pinstriped my car in his driveway. In I was hired on as Technical Editor for Popular Hot Rodding and later I did the same gig at Camaro Performers where we built a Camaro called Bad Penny (Google it). Later I was editor for Vette and Super Chevy Magazines before ending up here at HOT ROD as a Senior Tech Editor.
I've had the pleasure of building and being involved with quite a few cars from the above-mentioned Bad Penny Camaro to a Camaro called Track Rat and a 1,000 horsepower Wagon rolling on a Roadster Shop chassis. I've owned a few ZL1/1LE Camaros, a couple of Porsche GT4s and currently I'm working over a sweet LS-swapped Chevelle wagon to see if we can make this 4,000 beast handle on track. I live in SoCal with my partner April where I still spend way too much time thinking about the next big project.
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Brake rotors are a vital component of a vehicle's braking system, influencing stopping power, safety, and overall performance. While brake rotors serve the same basic function of providing a surface for the brake pads to create friction and slow the car, some brake rotors are specifically intended to improve performance, cooling, and prevent brake fade.
Drilled rotors, slotted rotors, and a mix of drilled and slotted rotors are some of the most well-known types of brake rotors that improve performance. This detailed guide includes everything you need to know about these brake rotors, including their features, pros and cons, and how they compare to each other to help you choose the best one for your car.
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Brake rotors, often known as brake discs, are big metal discs mounted behind your wheels. When you press the brake pedal, they work with the brake pads and calipers to slow down and stop your car. When the calipers press the brake pads against the rotors, the friction causes the wheels to slow down and brings the car to a halt. Standard brake rotors are solid discs, while drilled, slotted, and hybrid drilled and slotted rotors provide superior performance under high-performance conditions.
Brake rotors are crucial to vehicle performance and safety for several reasons:
Heat Dissipation: When you brake, a lot of heat is produced. The brake rotors help spread and get rid of this heat so that your brakes don't get too hot and fade.
Friction Surface: The brake rotors give the brake pads the surface they need to stop the car. A clean and smooth rotor surface makes sure that the brake pads work well.
Stability and Control: Quality brake rotors keep the brakes working the same way under any circumstance so you can stop safely even when the roads are wet, you’re driving something heavy, or you're moving too fast.
Manufacturers design brake rotors with holes (drilled) and slots (slotted) to improve performance. These modifications enhance cooling, remove debris, and improve braking efficiency under extreme conditions. High-performance and heavy-duty vehicles often require these specialized brake rotors to prevent overheating and maintain consistent stopping power. So, standard brake rotors can suffer from heat buildup, gas accumulation, and brake fade under extreme braking conditions such as high-performance driving, towing, or racing. Here are the specialized brake rotors with unique designs:
Drilled Brake Rotors: Feature holes drilled through the rotor to improve heat dissipation and water evacuation.
Slotted Brake Rotors: Feature machined grooves to enhance pad contact, remove debris, and maintain friction.
Drilled and Slotted Brake Rotors: Use both of these features together to get the best cooling and dirt removal.
These modifications help prevent common braking issues, such as overheating, brake pad glazing, and loss of stopping power.
What are drilled brake rotors? They are a series of strategically placed holes drilled through the disc. These holes help dissipate heat, vent gases, and remove water from the rotor’s surface, making them ideal for everyday driving and wet conditions. Here are the pros and cons of drilled brake rotors:
Better Heat Dissipation: The drilled brake rotors’ holes allow heat to escape more efficiently, reducing the chances of overheating and brake fade.
Improved Wet Performance: Water can escape through the drilled holes, preventing a layer of water from forming between the brake pad and rotor, improving braking efficiency in rainy conditions.
Enhanced Gas Venting: Drilled brake rotors allow brake pad gases to escape, ensuring consistent braking performance.
Aesthetic Appeal: Drilled brake rotors have a performance-oriented look, making them a popular choice for sports cars and enthusiasts.
Reduced Structural Integrity: The holes weaken the rotor, making it more susceptible to cracking under extreme braking conditions.
Not Suitable for Heavy-Duty Applications: High-performance, heavy-load, or track-use driving can cause drilled brake rotors to fail prematurely.
Faster Wear and Tear: Over time, the holes can cause stress fractures, leading to a shorter rotor lifespan.
What are slotted rotors? They have machined grooves (slots) etched into the rotor’s surface. These slots are designed to enhance braking performance by continuously cleaning the brake pad surface and improving friction consistency. Here are the pros and cons of slotted brake rotors:
Better Contact with the Brake Pad: Slots keep the brake pad fresh by removing tiny layers of the pad. This keeps it from glazing and provides the best friction.
Effective Debris and Gas Removal: Slots drain debris, dust, and gas buildup, avoiding brake fade.
More Durable than Drilled Brake Rotors: Since slotted brake rotors don’t have structural holes, they are stronger and more resistant to cracking under heavy loads or extreme braking conditions.
Ideal for High-Performance and Heavy-Duty Applications: Commonly used in trucks, SUVs, and race cars that require powerful braking.
Increased Brake Pad Wear: The slots act like a file, wearing down brake pads faster than smooth or drilled brake rotors.
Potential Noise Issues: When you brake, some notched brake rotors make a little humming or whining sound.
Less Effective in Wet Conditions: Unlike drilled brake rotors, slotted brake rotors don’t allow water to escape as efficiently.
Drilled and slotted brake rotors are both designed to enhance brake performance, but they achieve this in different ways. Drilled brake rotors have holes drilled through the disc surface, allowing heat, water, and gases to escape. This design helps prevent brake fade by keeping the rotor cooler, making drilled brake rotors ideal for wet conditions and daily driving. But the holes create weak points in the rotor, which can lead to cracking under extreme stress, such as aggressive braking or heavy towing.
On the other hand, slotted brake rotors feature shallow grooves cut into the surface, which continuously scrape away debris, brake dust, and gas buildup from the brake pad, maintaining consistent friction. Unlike drilled brake rotors, slotted rotors don’t have structural weaknesses, making them more durable and suitable for high-performance applications like racing, off-roading, and towing. However, they tend to wear down brake pads more quickly and can produce additional noise.
Drilled brake rotors are the better choice for daily drivers and wet conditions. For heavy-duty use, performance driving, or towing, slotted brake rotors provide superior durability and braking consistency. Ultimately, the better brake rotors depend on how and where you drive. However, if you want a combination of both benefits, there is a hybrid option: drilled and slotted brake rotors. Let’s explore it!
For drivers who want the benefits of both drilled and slotted brake rotors,hybrid drilled and slotted brake rotors offer a balanced solution. These rotors combine the heat dissipation and water evacuation capabilities of drilled brake rotors with the debris removal and friction consistency of slotted brake rotors. This makes them an appealing choice for performance enthusiasts, high-speed drivers, and those who engage in aggressive braking.
Hybrid drilled and slotted brake rotors have the advantages of better cooling, less brake fade, and steady braking power—even under extreme circumstances. Braking performance is improved by the slots, which guarantee the brake pads maintain firm contact with the rotor surface, and the drilled holes, which aid in the venting of heat and gases. Standard drilled or slotted brake rotors are typically less expensive than hybrid drilled and slotted brake rotors.
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