Tech Stuff

Luxury Car and High Performance Brakes

Some high performance rotors employed by OEMs feature an exotic disc construction formula such as carbon fiber-infused material. The point is that not all rotors are simply made of “cast iron.”
<p>Some high performance rotors employed by OEMs feature an exotic disc construction formula such as carbon fiber-infused material. The point is that not all rotors are simply made of &ldquo;cast iron.&rdquo;</p>

Owners of luxury and high performance vehicles expect topnotch braking performance. In addition, these customers likely have an appreciation for high-quality parts, workmanship and attention to detail. As a result, they are willing to spend what is necessary to have their vehicle’s brake job done properly. This customer base represents a solid profit opportunity by giving them what they want.

Premium-priced high-performance luxury cars tend to push the envelope in terms of braking system performance. While a disc/drum or disc/disc system found on any production vehicle is designed to provide safe and reliable braking, luxury performance cars tend to be outfitted with “spirited” driving in mind. As engine power increases, accompanied by the potential for higher speed operation, there’s more demand on the brake system, requiring the system to meet these challenges.

As a result, expensive luxury/high performance vehicles tend to take advantage of braking components that provide a higher level of performance. Rotors tend to be larger in diameter, and sometimes made of more exotic materials than are found on less expensive cars.

Many OE high-end luxury/high performance braking systems feature larger diameter rotors and multi-piston calipers for superior stopping power under higher speed situations, providing a larger pad contact area and often utilizing fixed caliper designs that feature multiple opposing pistons. In addition, innovations such as brake pad wear sensors and electronic parking brakes are more common.

After all, when the customer is paying big bucks for his or her new ride, they expect all of the latest bells and whistles.

Brake pad materials are routinely selected by OEMs to maximize braking performance. With regard to high-dollar luxury and high performance vehicles, customers expect superior braking capabilities, and the vehicle makers tend to utilize the best materials available, regardless of cost, in order to maintain maximum braking performance and to avoid issues of squeals, premature wear and/or brake dust accumulation on incredibly expensive alloy wheels.

When dealing with this demanding market, you simply cannot arbitrarily replace pads with whatever is readily available from a local supplier or at the lowest cost.

Pads intended for high performance European and some domestic vehicles likely use a semi-metallic or low-met material in order to generate a higher coefficient of friction. Some ultra-exotic applications employ carbon/carbon rotors and pads or other unique application-specific pad compounding.

The evolution of semi-met materials has come a long way over the past few years to accommodate the higher energy generated during braking.

Depending on the application, high performance fitments may feature ceramics, Aramids or some application-specific semi-metallic formulation.

Stopping power is of obvious importance on any vehicle application, but for “spirited driving,” the customer will expect and demand consistent braking with a high degree of fade resistance, so don’t compromise.

Bear in mind that these customers are not going to be as budget minded as others, so regardless of cost, always select the highest-performing pads for these discriminating customers.

The point we’re making is that high-end/high performance applications require pads that are designed for specific makes and models.

Rotors

High performance vehicles (this may include both late-model OEM vehicles that push the legal limits in terms of horsepower, torque, handling and braking, as well as custom vehicles built with these attributes) require high performance brakes. As part of the system, brake rotors must be able to withstand the rigors of performance use. In addition to potential rotor wear, issues of concern involve cooling channel rust. As these heat-evacuation channels become clogged, the rotor tends to absorb more heat. As heat increases, the chances of disc cracking, wear and warping increase.

If you discover rotor warp, it’s best to replace the rotor(s) rather than resurfacing. Machining the disc reduces its thickness, making it even more susceptible to warping and cracking due to overheating. When you’re dealing with a mega-dollar vehicle, don’t cut corners. Buy the best rotors available that meet the same material and design criteria as the original equipment rotors.

Premium performance rotors may feature a high carbon content. Citing Centric Parts’ offerings as an example, a proprietary blend of molybdenum and chromium is used in the casting process, which boosts braking power, reduces potential squeal, resists stress cracking under extreme braking use and aids in heat dissipation.

Aftermarket performance rotors are offered in a variety of designs. Examples shown here (from left to right) include slots, slots and cross-drilled holes and cross-drilled holes only. Note that non-friction surfaces are e-coated to prevent rust and to maintain a like-new appearance.
<p>Aftermarket performance rotors are offered in a variety of designs. Examples shown here (from left to right) include slots, slots and cross-drilled holes and cross-drilled holes only. Note that non-friction surfaces are e-coated to prevent rust and to maintain a like-new appearance.</p>

From an appearance standpoint, this formula also resists oxidation/rust, making the rotors looking good for a longer period of time, as the increased chromium and carbon content is also more resistant to oxidation.

Raybestos R-300 high performance rotors represent another example of high-carbon metallurgy that improves cooling and vibration-cancelling. High performance aftermarket rotors are offered by a number of manufacturers including Centric Parts, Brake Parts Inc. LLC (Raybestos) and others.

Whether replacing with new OEM or aftermarket components, these vehicles require the highest quality rotors, pads and calipers that provide a direct-fit. 

Today’s quality aftermarket suppliers offer quality and performance that meets or exceeds original equipment.

Rotor coatings

Preventing rust buildup on a brake rotor offers two benefits: to improve visual appeal when the rotor is visible behind an alloy wheel, and to minimize corrosion that can lead to clogging the vents and overheating. While non-frictional surfaces such as rotor hats and vents can be treated with a variety of methods including electro-coating (often referred to as e-coating and is usually black in color), frictional surfaces may be zinc coated. This provides a superior pad bed-in and bite, while maintaining a rust-free appearance for a much longer period of time and use.

These cutaway views illustrate curved vanes. Rotors with curved vanes should always be mounted so that the outboard end of the vanes aim toward the rear of the vehicle.
<p>These cutaway views illustrate curved vanes. Rotors with curved vanes should always be mounted so that the outboard end of the vanes aim toward the rear of the vehicle.</p>

According to Centric, e-coating is an advanced electro-statically applied finish that has been engineered to withstand 400 hours of salt spray testing without corroding. That means much longer service life, especially on icy, salt-encrusted winter roads.

The coatings used on the Raybestos R-300 rotors involves a “black fusion” coating on the hat and a “grey fusion” coating on the disc and vane surfaces rated as withstanding 300 hours of saltwater exposure. Many performance rotors available today feature similar coatings that greatly extend the appearance factor.

If the disc area is coated, it is not recommended to remove this during installation. During installation, it’s advisable not to clean the disc area with a strong solvent such as brake cleaner. Instead, wash the rotors with hot water and Dawn dish washing liquid, followed by a rinsing. This will remove any residual oils and contaminants without disturbing the coating.

Vented and solid rotors

Vaned/vented rotors are designed to release heat from the rotor. As the rotor rotates, the vanes pump cooler air from the center of the wheel, which carries this air through the vanes, picking up heat along the way and removing a percentage of that heat from the rotor. A rotor featuring straight vanes is bidirectional and may be installed on either the right or left side of the vehicle.

If the rotor features curved vanes, it is directional and must be positioned properly to achieve full heat-dissipation performance, mounted so that the vanes curve towards the rear of the vehicle. This causes hot air to be pumped from the center area outwards (pumping air from the inner diameter toward the outer diameter).

Rotor drilling/slotting

An increasing number of high performance brake rotors feature disc slots, cross-drilled holes or a combination of both. While many enthusiasts are attracted to this feature from a visual perspective, these design elements serve a purpose.

Both help to keep the pads clean by providing an escape path for pad residue as the pads wear, and to reduce the gas-ramping buildup between the pad and rotor emitted by pad material resins, reducing the potential “hydroplaning” effect as the pad pushes against the disc and aiding in keeping wheels clean by reducing pad dust buildup and improving brake pad “bite” for superior braking.

Raybestos’ R-300 performance rotors feature an “S-Groove” slot design that simultaneously allows for out-gassing on both leading and trailing ends of the pad’s swept area while maintaining maximum surface contact. The slot design is also non-directional, allowing the same rotor part number to be installed on both sides of the same axle. The high carbon rotor features a “black fusion” corrosion-preventative coating on the hat and a “grey fusion” corrosion-resistant coating on the disc, making them able to withstand 300 hours of saltwater exposure, the company says.
<p>Raybestos&rsquo; R-300 performance rotors feature an &ldquo;S-Groove&rdquo; slot design that simultaneously allows for out-gassing on both leading and trailing ends of the pad&rsquo;s swept area while maintaining maximum surface contact. The slot design is also non-directional, allowing the same rotor part number to be installed on both sides of the same axle. The high carbon rotor features a &ldquo;black fusion&rdquo; corrosion-preventative coating on the hat and a &ldquo;grey fusion&rdquo; corrosion-resistant coating on the disc, making them able to withstand 300 hours of saltwater exposure, the company says.</p>

Slots or holes essentially serve the same purpose in terms of a self-cleaning attribute, with cross-drilled holes also aiding in heat release. If holes are featured, they should include a slight chamfer to reduce the chance of stress cracking.

Rotor disc slots are often machined at an angle relative to the hub centerline, making the rotors directional (dedicated left or right). While some OE rotor manufacturers may specify that slots should sweep away from rotational direction, in most cases, a slotted rotor (whether the slots are straight or curved) should be mounted so that the grooves sweep forward at the outer perimeter of the rotor, allowing the end of the slots closest to the outer disc edge to contact the pads first.

In addition, some performance rotors feature a mixture of slot angles (forward angle, rearward angle and straight) as a design element.

Unique slotting configurations are also offered, such as Raybestos’ R-300 S-Groove slot designs that are non-directional (non-axle-specific).

Multi-piston calipers

Many high performance brake calipers will feature multiple pistons. As opposed to a single-piston caliper, this provides greater and more even distribution of pad application force along the length of the pad, reducing or eliminating the potential for tapered pad wear. Some performance calipers will feature equal-diameter pistons, while others will feature pistons of progressively small-to-large piston diameters.

When dealing with a caliper that features a staggered variation of piston diameter, the caliper must be mounted so that the smaller piston end is closer to the entrance or “attack” of the rotor, with the larger piston end toward the exit path of the rotor. The smaller piston end creates the beginning of the clamping force and the larger piston end provides slightly greater clamping force, the combination of which helps to compensate for pad taper wear.

A multi-piston caliper that features different-diameter pistons must be mounted so that, as noted earlier, the rotor travel path hits the smaller pistons end first. This means that the calipers must be dedicated for the right or left side of the axle.

This staggered piston diameter design also helps to alleviate any potential for pad resin gas “pressure ramping” effect.

This pressure begins just after the point of attack as the pad meets the rotor, and continues to progressively build along the pad until it can escape at the exit end of the pad. Depending on the brake pad compound, as the pads compress onto the rotor, heat builds and resins in the pads react and gases are released to the pad surface, building a sudden “pressure ramp,” which forces the pads away from the rotor, which pushes the pistons back into their bores. A staggered piston setup uses the larger diameter piston(s) at the exit portion of the pad to counteract this gas-push by applying greater force behind the pad. If the rotor disc design includes a series of slots or holes, this provides a faster escape path for these gas buildups, which function in unison with staggered piston calipers.

In an effort to eliminate pad squeal, vibration-damping shim layers are featured on pad backing plates. Shown here is an example of Raybestos’ Element3 pad that utilizes multiple shims of steel with vulcanized rubber in-between and a thermo-set adhesive, the combination of which serves to dampen vibrational harmonics.
<p>In an effort to eliminate pad squeal, vibration-damping shim layers are featured on pad backing plates. Shown here is an example of Raybestos&rsquo; Element3 pad that utilizes multiple shims of steel with vulcanized rubber in-between and a thermo-set adhesive, the combination of which serves to dampen vibrational harmonics.</p>

Performance brake fluid

It’s time to pay attention to brake fluid, especially from a performance standpoint.

All too often, a brake system is filled, bled and then forgotten. Age, humidity, operating temperature and potential air ingestion can begin to take its toll.

There are two basic types of brake fluid: poly glycol and silicone. Glycol fluid (DOT 3, DOT 4 and DOT 5.1) is “hygroscopic.” That means that the fluid is capable of attracting and holding airborne moisture.

NOTE: Do not confuse DOT 5.1 with DOT 5 fluid. DOT 5 fluid is silicone, while DOT 5.1 is simply a high performance DOT 4 type fluid with a slightly lighter viscosity and generally a higher wet boiling point.

As water begins to be absorbed into the fluid, this has two effects: it begins to lower the wet boiling point, which diminishes the fluid’s ability to obtain solid and dependable braking performance. Water absorption also creates the potential for holding water inside the system, which can contribute to corrosion of steel brake lines and can lead to wheel cylinder and caliper piston sticking, especially for vehicles that are stored for extended periods.

Because of the propensity for glycol fluid to absorb moisture, it’s important to change the fluid on a regular basis. Brake fluid does not last forever... it needs to be changed to maintain proper braking performance.

Silicone brake fluid (DOT 5) is not hygroscopic, so it does not absorb moisture from the air. Silicone fluid is also not harmful to painted surfaces, as compared to glycol fluid. That’s why some owners prefer silicone (to prevent paint damage in the case of a leak or spill) and because many people assume that, since silicone does not attract and absorb water, that it will protect the braking system from corrosion.

While silicone does not “hold” water, any airborne moisture (humidity in the air) can still enter the system and be carried on the surface of the silicone fluid.

While the use of silicone fluid definitely helps to prevent internal system damage, it’s not a cure-all to prevent moisture contamination.

The real downside of silicone is that it may aerate/foam under rapid braking situations, such as might be experienced in repeated and rapid modulation of the brake pedal at high speed. Foaming results in air globules, which leads to an unexpected soft/lower brake pedal. For this reason, silicone fluid should also never be used in an ABS-equipped brake system.

For high performance applications where the potential for extreme braking heat is likely, a variety of brands and specification level brake fluids are available that offer higher dry and wet boiling temperature performance.
<p>For high performance applications where the potential for extreme braking heat is likely, a variety of brands and specification level brake fluids are available that offer higher dry and wet boiling temperature performance.</p>

Silicone brake fluid can have a very undesirable effect on ethylene propylene rubber found in most brake systems, as silicone fluid may tend to cause seal swelling. In addition, silicone may tend to be somewhat compressible when heated or in high altitudes, resulting in changes to the pedal travel.

Primarily due to silicone’s benign characteristic in terms of preventing damage to a painted surface, it remains popular for many show cars that are not intended to be driven routinely at higher speeds and potential hard braking.

If the vehicle is stored for periods and rarely driven, and never exposed to performance driving, silicone fluid is certainly an option to consider.

But for your customers’ daily driving and especially for performance driving, stick with DOT 3, DOT 4 or DOT 5.1 glycol fluid that offers a high wet boiling point.

Brake fluid boiling points

As the brake system absorbs airborne moisture, this will lower the boiling point of the fluid. The rated dry boiling point represents the point at which fluid will boil when fresh, with no absorbed moisture.

The lower or “wet” boiling point represents the expected performance of the brake fluid after it has been in the car for approximately one year.

For a routinely driven street vehicle, the wet boiling point is more important than the dry, because the brake fluid is going to be in use for much longer periods. In addition to lowering the boiling point, water in the system will eventually corrode the metal surfaces within the system.

High-dollar luxury performance cars often feature premium-performance brake components such as multiple-piston calipers and large diameter rotors. This example is featured in 2009-2012 Mercedes-Benz AMG vehicles, featuring 6-piston calipers and ceramic-compound discs. You’re not going to find these at your local parts store.
<p>High-dollar luxury performance cars often feature premium-performance brake components such as multiple-piston calipers and large diameter rotors. This example is featured in 2009-2012 Mercedes-Benz AMG vehicles, featuring 6-piston calipers and ceramic-compound discs. You&rsquo;re not going to find these at your local parts store.</p>

While all brake fluids feature corrosion inhibitors, these inhibitors eventually break down and become less effective over time. Glycol brake fluids should be completely drained and replaced, on average, every two years or 24,000 miles in order to maintain optimum fluid performance, regardless of how much the vehicle is driven or stored.

Silicone brake fluid offers very high dry and wet boiling points. However, silicone tends to be more compressible because it will absorb more air than a glycol based fluid, and will infuse itself with air. The dissolved air results in a slightly spongy pedal feel.

Especially from a performance standpoint, it’s recommended to always select a brake fluid that’s rated with a dry boiling point of at least 450 degrees Fahrenheit.

The two most important aspects of a performance brake fluid are compressibility and its rate of moisture assimilation.

Compressibility refers to pedal feel... how firm the pedal feels when the brake pedal is depressed (also assuming the system is properly bled of air).

Different brake fluids can dramatically alter pedal feel, with no other system changes.

Obviously, a braking system generates frictional heat, as the friction material is forced against the surfaces of a rotor disc. This heat is quickly absorbed into the fluid, elevating its temperature.

The published “typical” dry and wet boiling points indicate at what temperature the fluid will begin to boil.

The dry boiling point represents the point at which fluid will boil when it’s fresh and contains no water. The wet boiling point indicates the temperature at which the fluid can begin to boil when it contains as little as 1% to 2% moisture.

The older the fluid, the more moisture it potentially holds, and the boiling point diminishes accordingly.

For maximum braking performance, choose a brake fluid that offers high dry and wet boiling points, and consider replacing brake fluid on a regular basis.

As a glycol brake fluid gradually and inevitably absorbs airborne moisture, its boiling point begins to decrease.

A moisture factor of as little as 3% or so can allow the brake fluid to boil at under 300 degrees F, which will lead to reduced braking effectiveness.

Regardless of the vehicle type or performance level, maintaining the brake system’s fluid purity is important both to maintain system performance and to reduce internal contamination.

If the customer expects maximum braking performance, the brake fluid should be changed according to the vehicle manufacturer’s recommendation (or if no recommendation is listed, changing fluid every one to two years would be a rough guideline).   ■

Examples of performance brake fluid specifications

Brand/model                              Dry boil point                     Wet boil point
Centric STR 600                          594 degrees F                     404 degrees F
Centric STR 660 Ultra                 622 degrees F                     404 degrees F
Brake Man Hi Temp 577 fluid      577 degrees F                     300 degrees F
Castrol SRF                                 590 degrees F                     518 degrees F
Castrol LMA DOT 3/4                  446 degrees F                     311 degrees F
ATE Type 200 DOT 4                  536 degrees F                     388 degrees F
  (same with amber tint)
Ford Heavy Duty DOT 3              550 degrees F                     90 degrees F
  (inexpensive, but must be changed frequently)
TBM DOT 5.1 Xtreme 6               612 degrees F                     400 degrees F
  (high rejection to moisture absorption)

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