Brake NVH: Part Two

Brake NVH: Part Two

An in-depth look at individual brake components and how each can produce noise and how to resolve the malady.

Last month we discussed what is noise and how a brake system can produce it. This month we will look the individual components and how each can produce noise and how it can be resolved.

The first thing to remember is that all brakes make noise. When the friction material makes contact with the rotor, the coupling causes the brake pad and rotor to oscillate and vibrate. In engineering terms, this is called “force coupled excitation,” which means that the components are locked as a combined system that will vibrate at the system’s natural frequency combined modes of vibration. The driver hears these vibrations as noise. This is “ground zero” for brake noise.

The amount of excitation and the frequency generated can be influenced by variations in brake torque (or changes in the coefficient of friction) across the rotor’s face. As the components heat up, the rotor may develop hot spots that could cause the rotor to have different regions of friction that produce different levels of brake torque. This friction coupling is where most brake noise is generated.

Just like a diagnostic flow chart, it is possible to look at a brake noise complaint and the components to find a resolution. With the right combination of diagnostic knowledge, quality parts and noise solution, the resolution can be productive and profitable.

Friction Materials
What makes one friction material quieter then another? It is a two-part answer.

First, if a friction material is better at keeping a constant coefficient of friction across a broad temperature ranges and environmental conditions, it probably is a quiet pad. A “consistent” friction material causes less vibrational excitation variation at the friction coupling by having consistent brake torque at environmental extremes of humidity and temperature (-40F to 500F).

Second, some friction materials leave or transfer a layer of friction material (transfer film or “seasoning”) on the rotor’s surface that some friction material companies claim can smooth out the rotor surface, thereby causing less excitation and noise at the friction coupling. Also, this transfer layer may not be as sensitive to heat induced brake torque variation.

Another aspect of brake pad NVH is the backing plate. As stated before, the more consistent brake torque, the less excitation and noise. If a backing plate is flexing under the mechanical forces of the caliper, its friction “footprint” on the rotor is changing as the driver modulates hydraulic pressure.

Brake pad manufacturers can either add thickness to the backing plate or change the attachment methods so that the holes for rivets or integral molding voids are minimized. Some attachment methods eliminate the holes altogether.

Another aspect to look at is the shape of the friction material on the brake pad. OEMs tune the shape of the brake pad to the overall system. This includes elements like overall length, width and feature design elements like chamfers and slots. These physical dimensions and design elements can reduce excitation and shift natural vibrational frequencies. Some aftermarket brake pad manufacturers will tune the shape of their pads to suit older vehicles and the characteristics of their friction material blend.

Many technicians are quick to condemn the brake pads in a noise issue, when in actuality the source of the noise is the rotor. The reason why most technicians blame the pads instead of the rotor is that they think that the rotor is unchanging under heat and extreme force. This is not true. A rotor’s shape can become “alive” or “dynamic” under the mechanical forces of braking and heat.

As the mechanical forces of braking act on the rotor, it can actually move and flex. While not visible to the naked eye, this movement can cause excitation and noise at the friction coupling. A poorly designed aftermarket rotor with a non-OEM fin and hat design may not be able to control the dynamic movement. This increased sensitivity to high levels of flex and torsion can be caused by thinner outer disc thickness, overall cheek and hat section geometric ratios and rotor cooling fin design that does not meet the OEM’s design.

These compromises in design cause changes in the friction couplings ability to generate brake torque. This causes the higher levels of vibrational excitation driven by the rotor.

A brake rotor’s NVH signature extends below the surfaces and goes deeper into the microscopic realm of metallurgical/material properties. If the rotor does not have the same mass or metallurgical content as the OE rotor, there may be less mass and inherent damping to minimize rotor dynamic instabilities. If the cooling fin pattern is too spread out, heat may be concentrated at certain points on, and in, the rotor. This can change the coefficient of friction at certain temperatures. This alters the coefficient of friction and can also lead to high DTV, runout and noise.

Poor rotor finish can lead to noise. A huge variable at the service bay level is the quality of the surface finish of a rotor. A rotor finish deviating from OEM design, which was intended to match the recommended friction pad, is a major consideration.

When machining a rotor you have two primary goals. The first is to provide a smooth surface finish for the pads. The second goal is to provide a true surface finish.

The smoothness of the friction surface of a rotor is described in terms of microfinish or RA factor. RA stands for Roughness Average and represents a way to measure the smoothness of a rotor. Most lathes out there, when in good condition and used properly, will yield very acceptable RA factors.

Calipers and Hardware
The caliper influences the friction coupling, as well as being an affected component by the vibrations generated. As stated before, the source for most brake noise is the friction coupling and variations in brake torque, or the coefficient of friction can induce vibration and noise. Even though you have no control over the design of the caliper, it is still important to understand how it can influence brake noise.

The caliper supplies the mechanical force for the friction coupling. The caliper’s ability to keep a consistent “friction footprint” between the pads and the rotor’s face under different levels of mechanical force will determine how well it is able to control noise.

One of the most important characteristics for a brake caliper is rigidity. If a caliper flexes, there is no way possible to keep the brake pad in consistent contact with the rotor. In the case of a single- or dual-piston “floating caliper,” the caliper must be ridged at the bridge and the caliper fingers.

The second property a caliper must have is the ability to absorb and dampen vibration. As pointed out in last month’s article on NVH, more mass can help dampen vibration. In other words, a heavier brake caliper can better dampen vibration. But like all automotive engineering and design problems, only so much weight can be put at a corner of a vehicle before it starts to affect unsprung weight. In some cases, engineers will shift mass to certain areas of the caliper, like the fingers and bridge.

The most difficult item in servicing single- or dual-piston “floating caliper” brake systems is the understanding of tolerances, clearances and freedom of movement and how this relates to brake noise.

In these systems, the caliper and pads ride on slides, pins and bushings or pins. As clamping force is applied by the piston, the caliper hopefully uses equal force to both pads and the caliper is centered over the rotor. In these systems, you have many components sliding and moving.

If a component has too much freedom of movement, it will vibrate and create noise. If a component has a tight fit, chances are it will vibrate and even excite other components around it. This effect is compounded even further when it can influence the friction coupling, like a stuck brake pad. This is why it is necessary to ensure that slides, guides and clips allow movement, but not too much play.

Many clips will lose tension over time. When the spring tension is lost it is then unable to isolate and dampen vibration. Corrosion is another enemy of the floating caliper. Corrosion on the slides and guide pins can destroy surfaces.

Brake Shims or Insulators
When a vehicle ends up in a service bay for brake work, it could be far out of the specifications that the OEM intended. Through normal wear and the replacement of different components, the brake system’s NVH engineering tolerances could be severely out of whack.

In some ways, it is the technician’s job to restore and even “reengineer” the brake system back into compliance. One of the best tools is a brake pad shim or insulator. These items are the best preventive medicine for high frequency brake noise typically described as “brake squeal.”

Brake shims are not designed for adjusting spacing/distance between friction material and the rotor. Shims provide multifunction noise control as a component attached to the friction pad backing plate at the exact location where force coupled excitation is generated and transferred. A quality brake shim can prevent brake noise during the entire life of the brake job and will not dry out or be displaced over time like some lubricants.

Brake shims have the ability to control noise in three ways. First, they prevent and reduce the transmission and amplitude of vibrational forces that cause excitation of the caliper, pad assembly and attached structure. This is accomplished by visco-elastic damping material inherent within the layering construction of the shim and method of bonding to the pad assembly.

Second, a shim can add mass to the brake pad that can dampen vibrations and oscillations in the pad and caliper. They reduce reaction forces transmitted back into the brake piston using elastomer interface coatings on their surface.

Third, a good brake shim or insulator can act as a thermal barrier to ensure consistent temperatures across the entire face of the pad. This can help ensure consistent brake torque.

High-quality brake shims are frequency and temperature engineered multi-layered products using varying grades of metal, viscoelastic polymers/bonding materials, elastomeric rubbers and fiberglass.

High-quality brake shims usually start with a high-quality metal plate. The shim manufacturer will select a grade of steel with the right hardness, thickness and dampening properties.

The elastomeric rubbers are applied to the steel in a controlled process that insures the correct depth. These materials are then vulcanized to the metal to ensure that it will endure the harsh environment of the braking system.

Engineers tune these layers to give the best NVH qualities for the specific brake system noise fingerprint or signature.

If the shim is not making contact with the brake pad, it will not do its job since the pad/caliper assembly undergoes complex dynamic vibrational deformations due to superposition of the combined system natural frequency mode shapes that must be controlled by a properly attached insulator.

Before you install any brake pad or shim, take time to look it over. Look at the overall finish and appearance of the parts, test fit the part to see if the tolerances are correct. Some of the best brake pads have the insulator already attached to the pad set. This can help insure the effectiveness in three ways.

First, by putting the shim on at the factory it can prevent the technician from forgetting to put them on the pad in a dirty shop environment. Second, some pad and shoe manufacturers will use a heated hydraulic press that can properly cure the adhesive. Third, the brake pad manufacturer can use a process like peening and riveting to make sure the shim will perform properly.

If the shim/insulator is not already attached to the pad, drop it on a hard surface and listen. If the shim makes a clinking noise and bounces around, chances are that it will not reduce noise and vibration. If the shim makes a solid noise when it is dropped, it probably will do a good job at insulating against noise.

If a set of pads or shims do not look “right,” do not take a chance by installing them. It is easier to return opened unused brake pads then those that have been slightly worn.

Rotor Sprays
In recent years, aerosol cans of what looks to be silver spray paint with the promise of stopping brake noise have been introduced to the aftermarket. The spray contains a coating that has particles that help to fill in irregularities in the rotors surface.

Does this stuff work? Yes.

How? It is very simple. The coating that is sprayed onto the rotor ensures that the rotor surface has uniform friction levels. This uniformity in the surface can reduce vibration and excitation at the friction coupling. But these sprays are not solutions for worn out brake parts or poor-quality replacement pads and rotors.

Why do lubricants work to reduce some brake noises? It is a simple answer. When a caliper finger is lubricated at the point it touches the brake pad, the lubricant creates a boundary layer that separates the vibration of the brake pad from exciting the caliper finger and the caliper. This is one approach to solving NVH problems that can have its limitations.

Lubricants do not dampen forces by adding extra mass like a brake shim. Also, lubricants can not fill in pitting on brake slides. Additionally, lubricants do not insulate against vibration and is effective for some frequencies.

Using the right lubricants on brake systems can work wonders. Staying power is what special-purpose brake lubricants have over other lubricants. Specially formulated, high-temperature brake grease can withstand the heat, and also not harm rubber seals or plastic bushings.

Petroleum-based lubricants should never be used for brake assembly work because mineral oils can cause seals to swell and fail. Never use ordinary “chassis grease” on brake caliper sleeves and bushings. Chances are that within 500 to 1,000 miles the chassis grease will break down and form a gooey mess that will cause the caliper to seize.

High-quality moly-lube should be used on all metal-to-metal surfaces and a high-quality silicone lube on all metal-to-rubber parts. Some silicone lubricants can be used on slides because they are formulated to withstand high pressures and heat.

Special thanks to Dr. Chris Griffen Of Eagle Pitcher’s Wolverine Gasket Division for his insights into the world of NVH and brake shims. Eagle Picher’s Wolverine Division supplies of brake shim material to brake component manufacturers for new vehicles and the aftermarket. Griffen’s knowledge of brake noise made this article the first of its kind in the aftermarket.

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