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Analyzing Brake Friction Material

Aftermarket brake pads require a great deal of testing and engineering because they are made for a specific vehicle. A quality brake pad manufacturer will spend a significant amount of time and money developing an application-specific brake pad for each vehicle. This includes simulated and on-the-vehicle testing, both of which are time and equipment ­intensive.

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Visual Clues Ensure Proper Pad Selection

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pads1As a technician, it can be difficult to judge a brake pad before you put it on the vehicle. But, doing your homework and looking for certain ­visual cues can help ensure you make the right choice.

 

As with any product you purchase, the bulk of what you’re paying for when you buy a quality brake pad is research and development.

Aftermarket brake pads require a great deal of testing and engineering because they are made for a specific vehicle. A quality brake pad manufacturer will spend a significant amount of time and money developing an application-specific brake pad for each vehicle. This includes simulated and on-the-vehicle testing, both of which are time and equipment ­intensive.

As a technician, it can be difficult to judge a brake pad before you put it on the vehicle. But, doing your homework and looking for certain ­visual cues can help ensure you make the right choice for the application.

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CERAMIC

The structural properties of a ceramic material are very stable under high temperatures, much like Corning cookware. But this is where the comparison breaks down. The ceramic materials that go into a brake pad are very small strands that are engineered to be a certain length and width.

There are three advantages of ceramic pads in certain applications. First, since the ceramic materials offer stable performance under a wide range of temperatures, they can offer quiet performance. Second, ceramic brake pads manage heat in the caliper better on some vehicles than do some non-ceramic applications. Third, ceramic brake dust does not show up on or stick to wheels like some other brake pad formulations. This could be an important factor to consider if your customer has an expensive set of custom wheels on his/her vehicle.

 

NAO

Non-asbestos organic (NAO) friction materials typically wear more in comparison to harder semi-metallic compounds. It’s hard to generalize about the wear characteristics of NAO and ­ceramic-based compounds, however, since there are so many options and wear rates vary depending on the formula the friction supplier chooses for a ­particular application.

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Different vehicles require different coefficients of friction, so formulas are often “application ­engineered” to deliver the best combination of stopping power, wear resistance, pedal feel and noise control. Most premium-quality NAO and ­ceramic-based linings will provide longer life and wear less than an equivalent set of NAO pads on the same ­application.

 

SEMI-METALLIC

Semi-metallic pads incorporate metal fibers to ­provide structure and friction characteristics. The metals used are typically high-quality steel, copper and other ­exotic metals.

So what else is in the semi-metallic mix? It’s a variety of ­materials like glues, lubricants and structural fibers that are blended together by the manufacturer to create the best performance for that application.

 

APPLICATION/VEHICLE-SPECIFIC LINES

If all of the material jargon is too much to deal with, you can look for a line that touts itself as ­vehicle- or application-specific. These lines can help you leave the material selection up to the manufacturer.

What makes one friction material quieter or better than another? It’s a two-part answer. First, if a friction material is better at keeping a constant coefficient of friction across broad temperature ranges and environmental conditions, it’s probably a quiet pad. A “consistent” friction ­material causes less ­variation in vibrational excitation at the friction ­coupling by having consistent brake torque at ­environmental extremes of ­humidity and ­temperature (-40° F to 500° F).

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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.

 

TO THE NAKED EYE

After you’ve ordered the brake pad and it’s been ­delivered, you can now inspect it with the naked eye. Packaging of the pads can be an indication of the quality. But don’t fully judge a book by its cover. Some of the best pads come in small boxes. This is done to reduce packaging costs and the ­impact to the environment.

More importantly, look at the quality of the overall finish of the pad and the shim materials. Many manufacturers will use a high-quality paint or coating on their pads. This coating is ­designed to resist corrosion and survive high temperatures, and should not easily scratch off.

Some friction material companies are now playing a game of one-up-manship when it comes to what they put in the box. Typically, brake pad companies will include hardware that includes abutment and anti-rattle clips. Some even include the torque specs on the box or new caliper bolts and guide pin boots, if required.

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BACKING PLATES

pads2Because the backing plate creates a foundation for the friction material, it needs to be stiff and stable. If a backing plate is flexing, it means that the ­friction material is not in full contact with the rotor. This can cause longer stops and a softer brake pedal, and it also increases the potential for ­unwanted noise.

Look at the thickness of the backing plate and the edges. A poorly stamped backing plate will have rough edges. Also look at the areas where the pad contacts the caliper, which should be as smooth as possible.

The attachment method of the friction to the backing plate does matter. For some fleet and safety-critical applications like school buses, the manufacturer or insurance company may specify a brake pad that uses a mechanical attachment to ­secure the friction material to the backing plate.

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Mechanical attachment involves some type of ­protrusion from the backing plate to hold the ­friction material. Some backing plates have tiny hooks ­machined into the surface for that purpose.

 

FRICTION SHAPE

pads3Another 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 things like overall length and width, and 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.

 

SHIMS

Look at the shim material on the back of the pad. If possible, remove the shim and drop it gently on a hard surface. If the shim makes a nice “thunk,” it’s made of a good material. On the other hand, if the shim makes a rattling sound, like a dropped wrench, chances are it will not make a good sound insulator.

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Brake shims are not designed for adjusting spacing/distance between the friction material and the rotor. Instead, shims provide multifunction noise control as a component attached to the pad’s backing plate. 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. But, remember, a shim does not do its job if it’s left in the box.

 

THE UNSEEN FEATURES

pads4When you’re buying a brake pad, you’re also ­buying the engineering and research behind it. When an aftermarket brake pad manufacturer is developing or reverse-engineering an application, rarely do they test on an actual vehicle. This type of testing is expensive and time consuming, and the human element can alter the ­results. On the other hand, a brake dynamometer can test brake systems in a controlled ­environment that mirrors the real world. It can run 24 hours a day and measure the performance of a braking system over its entire lifetime.

A brake dynamometer is also more sophisticated in that it can simulate the conditions the brake system will experience in a much shorter time. This means that a brake dynamometer can simulate the mass, inertia and performance capabilities of a vehicle.

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The typical brake ­dynamometer can cost anywhere between $250,000 (used) to more than $1 million. Some brake friction suppliers own dynamometers, while some lease dynamometers from testing companies.

In the engineering world, there is a saying that goes: “One test is worth a thousand expert opinions.” This saying is also true for the brake technician.

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