Brake Vibration

An article from the June, 2005 issue of Grassroots Motorsports

General Information

How to prevent brake vibrations
Story by James Walker Jr. From June, 2005 Grassroots Motorsports

The year was 1962. The Beach Boys had just released “409,” an all-American, high school tribute to four-speed transmissions, dual quads and Posi-Traction. Their follow-up 1963 hit, “Little Deuce Coupe,” continued the band’s celebration of mechanical speed. And in 1964 the same group brought us “Fun, Fun, Fun,” clearly a lyrical homage to the Ford Thunderbird and the magical opposite-sex-attracting powers it possessed.

However, the title to their 1966 smash, “Good Vibrations,” clearly indicated that Brian Wilson was not a brake engineer.

Quite simply, brake vibrations are never a good thing. In fact, a common saying in the brake industry is this: “The best brake system is an invisible brake system.” Let’s try to understand why this can sometimes be so difficult to achieve.

Wouldn’t It Be Nice?

You press the brake pedal, and your car slows down. There’s no squealing, no shaking and no vibration. Please fasten your lap belts, close your tray tables, and return your seat backs to the upright and locked position—you have arrived at brake utopia.

Unfortunately, brake utopia can sometimes be in another ZIP code from you.

Pick your favorite brake system malady: brake roughness, pulsation, shudder, hot judder, shake, vibration, or the all-time favorite, rotor warping. To the brake engineer, these all have slightly different meanings, but to the average enthusiast they are all simply a pain in the—well, they’re a pain and we’ll leave it at that.

While usually not a detriment to brake system effectiveness at first, none of these conditions can be considered desirable. Few vehicle problems are as annoying as a pulsating brake system. And if they’re ignored long enough, these problems can have legitimate performance impacts.

So what causes these conditions, and what can be done to prevent them in the first place? We’ll get there, but first we have to go back to brake school for a quick refresher course.

Be True to Your School

What does the brake system do? Put aside the small details, and you find that the brake system’s primary responsibility is to convert the kinetic energy of the car in motion into thermal energy, also known as heat. If there is available tire friction (contact patch force), the car will certainly decelerate (this in response to the brake torque generated during the energy conversion). However, the brakes do not stop the car. That’s the tires’ job.

No tire friction, no tire force, no deceleration. Hello, tire wall. Thud!

If we now look in more detail at the brake pad and rotor interface, we discover that this is where most of the energy conversion takes place. It is the friction between the brake pad and the spinning rotor that creates heat while simultaneously building torque in the rotating assembly, and over the next few paragraphs we will be dissecting this dynamic interface.

Round, Round, Get Around, I Get Around

Who takes the time to worry about how the stationary brake pad and the spinning rotor generate friction? Odds are this question has never passed through your head, but it is paramount to understanding brake vibrations.

Brake pads engage in two distinctly different types of dynamic friction: abrasive friction and adherent friction. The details should be left to the Ph.D. community, but in general the two modes operate as follows. In the purely abrasive mode, friction is generated as a result of interference between the microscopic high and low spots on the brake pad face and the spinning rotor. In very gross terms, this is similar to holding a block of wood on a belt sander. As the high and low spots are slowly machined away (much slower than the wood on the belt sander, of course), the breaking of their molecular bonds creates a force that resists the rotation of the rotor. It also heats up the materials involved.

Presto! We have converted kinetic energy into thermal energy by breaking a bunch of molecular bonds. Not too surprisingly, this is the mode that most people naturally envision when asked to explain how brake pad friction “works.”

Adherent friction is quite different in nature. In the adherent mode, pressure and temperature collaborate to deposit a thin layer of brake pad material, or a transfer layer, on the rotor face. Subsequently, as the caliper squeezes the brake pads against the rotor, the pads contact the transfer layer, not the rotor itself.

As the pressure increases, molecular bonds are then very quickly formed between the similar materials of the brake pad and the transfer layer. Just as quickly, however, those very same bonds are broken as the rotor continues to move relative to the brake pad. As a result, heat is generated, and the brake pad material wears away.

While the concept of adherent friction may be difficult to grasp, just sing it to the tune of the Beach Boys 1964 hit “I Get Around” to permanently ingrain it in your subconscious: Bond-bond, form a bond, I break a bond, yeah, bond-bond, form a bond, I break a bond… I form a bond… I break a bond….

In summary, abrasive friction can be found between the brake pad and the rotor itself, slowly wearing away both materials, breaking bonds, and generating heat and torque in the process.

With adherent friction, however, the rotor never actually wears. Why? Because all of the bonding-breaking action is occurring between molecules of the brake pad material, only the pad itself wears away over time (in theory, anyway; see the adjacent sidebar for the rest of the story).

Good Vibrations

So, now we can talk about brake vibration. As you read, you will find that the underlying theme will revolve around preventing brake vibration, not curing it. But first, let’s purge the phrase “warped rotors” from our vocabulary.

In nearly every single case, warped rotors are not physically warped at all. The common misconception is that the rotors get hot enough to distort and then, upon cooling, end up looking like a pretzel. Contrary to popular belief, rotors simply do not warp in this fashion.

The vibration that is felt in the steering wheel and floorboard is almost always caused by rotor thickness variation, also known as TV, and the physical pulsing in the brake pedal is nearly always a direct result of the caliper piston extending and retracting as it tries to follow a rotor of varying surface thickness.

Take a second and reread those last two paragraphs. It’s that important.

TV is generally created in one of three ways. For the enthusiast, the most common mode of TV is initiated by an uneven transfer layer of brake pad material on the rotor face.

Improper bed-in of new brake pads and rotors is usually the culprit here, but bringing a car in from the race track without cooling the brakes can also create this phenomenon. Given enough heat and time, most brake pads are more than willing to “go adherent” while at rest and will bond to the rotor in small, seemingly random spots.

Overheating the pad compound can also generate an uneven transfer layer as the pad material breaks down and “splotches” (this is a highly technical term that one should not use without proper training and certification) on the rotor face.

In any case, the uneven transfer layer deposits will wear at a different rate than the surrounding rotor material. On and on it goes until the high spots and low spots on the rotor face are severe enough to be felt in the pedal. How much can be felt? In most cases, even less than 0.001 inch can be downright annoying.

The second most common source of TV begins with the overheating of the rotor itself. If a rotor gets really, really hot, it can develop evenly spaced, localized areas along its face that are much hotter than the surrounding rotor material. These “hot spots” will also wear more quickly, creating a thick and thin wear pattern on the rotor face. As the rotor cools, these thick and thin spots remain and felt by the driver.

Another, less glamorous method of initiating TV is to leave a car parked in the same place for an extended period of time. While it is sitting, a thin layer of corrosion—you know, ferrous oxide, or rust—can form between the brake pad surface and the rotor. As you can probably imagine, sitting in humid or damp environments combined with use of the parking brake can greatly accelerate the corrosion.

When the car is ultimately moved, there will be a localized high spot (an unintended transfer layer of corrosion) on the rotor that will wear at a different rate than the surrounding material. At first the condition is undetectable, but it will only get worse over time as the rotor wears unevenly, creating high spots (thicker areas) and low spots (thinner areas).

Don’t Worry Baby

So now that we know what causes TV and the ensuing brake vibration, what can be done to prevent it in the first place? Don’t worry if you don’t have the answer already—we’re professionals and can help you through this.

First, be sure to follow your manufacturer’s recommended procedure for bed-in when installing new brake pads and rotors. These processes have been developed to reduce the opportunity for uneven brake pad material deposition on the rotor face. See the handy sidebar for more details.

Second, make absolutely sure to run a cool-off lap or two before coming into the pits, and never come in hot and keep your foot on the brake pedal and/or set the parking brake. Any time hot brakes are allowed to sit motionless, molecular bonds may continue to form between the brake pad and the existing transfer layer material. The result is nearly instantaneous TV generation. Would you like fries with that?

Third, during aggressive driving, keep your brakes as cool as possible to reduce the opportunity for hot spots. A set of brake cooling ducts goes a long, long way in this regard. Remember, cool brakes are happy brakes.

Fourth, if your car is typically left outside for extended periods of time, it might be best to select a nonmetallic brake pad. Nonmetallic brake pads (also known as organic or ceramic brake pads) reduce the tendency to generate corrosion between the pad and the rotor. While they are not usually recommended for high-performance applications, they don’t rust as quickly, and over time this may reduce the generation of TV on your garage queen.

Fifth, when installing your wheels and tires, be sure to tighten your wheel nuts in the manufacturer’s recommended pattern and take several passes to reach maximum torque. In some applications, uneven tightening of the wheel nuts can physically distort the rotor enough that during normal driving thick and thin spots may develop on their own.

Help Me, Rhonda

And what if brake vibration is already present in your car? Well, that’s a different story.

In select cases where brake vibration has just begun, it may be possible to remove any uneven transfer layer deposits from the rotor face by using a super-abrasive brake pad for a short while. This is a hit-and-miss strategy, though, and if uneven rotor wear has already started, then it’s too late anyway. Remember, abrasive brake pads cannot make a rotor flat again—they can only smooth off uneven pad deposits.

Turning the rotors can also alleviate the vibration situation, but this may not be a viable long-term solution. If the rotor has been heated to the point that the chemistry of the rotor has changed (specifically, if localized areas of cementite have formed due to heat, yet another topic for the Ph.D.’s), then very quickly the vibration will come back as the softer areas of the rotor face wear away more quickly. (Note that in some cases even turning the rotors may not cure the vibration even for a short time, as the hard spots can deflect the cutting tool making for an uneven cut on the rotor face.)

Finally, the only known long-term solution to purging vibration is to replace the rotors themselves and properly bed-in the new parts, assuring an even transfer layer. It may sound like a brute-force approach, but desperate times call for desperate measures. Just be sure to learn from your mistakes to keep the vibration monster from rearing its ugly head again.

As with most things in life, in the war against brake vibrations, the best offense is a good defense. Good vibrations, indeed.

Developing a Transfer Layer

Q: So if I’m putting new brake pads and rotors on my car, how do I properly develop a transfer layer? Is there a defined procedure I should be following?

A: The process of developing a transfer layer is typically referred to as brake pad bed-in. In general, bed-in consists of heating a brake system to its adherent temperature to allow the formation of a transfer layer.

The brake system is then allowed to cool without coming to rest, resulting in an even transfer layer deposition around the rotor circumference. This procedure is typically repeated two or three times in order to ensure that the entire rotor face is evenly covered with brake pad material.

The procedures that follow are generic and are only intended to introduce you to the theory of pad bed-in. As these procedures are not manufacturer specific, be sure to check with your brake pad supplier for any special considerations related to the bedding-in of your particular rotors and pads.

Adherent vs. Abrasive, the Rest of the Story

Although we have talked about abrasive friction and adherent friction as if they were mutually exclusive, all brake pads operate in both modes, and sometimes simultaneously.

Typically, though, most pads will operate in a primarily abrasive mode when they are cold and will then transition to an adherent mode as the brake temperature increases. This is why some brake pads require warming before they will be operating properly on track—they need to “go adherent” before they exhibit their desired performance.

If you have ever used the ubiquitous Hawk Blue 9012 pads, then you know exactly what we’re saying here. This material operates like a brake lathe (mega-abrasive mode) until it gets hot enough to stop on a dime (ultra-adherent mode). It’s also why you shouldn’t run Hawk Blue brake pads on the street: The brake temperatures will never get hot enough to get out of the abrasive mode, and the rotors will pay the ultimate price.

A final interesting note on adherent friction: If you use primarily adherent pads on your race car, chances are that your rotors will actually be thicker than new when the time comes to replace them at the end of the season. Why? Because of the added thickness of the transfer layer material. The rotors may still need replacing due to cracking or other thermally induced maladies, but rarely are race rotors replaced because they have worn too thin.

Bedding-in Street Performance Pads
For a typical brake system using stock-sized rotors and street performance pads, a series of six to eight partial-braking events from 60 mph down to 10 mph will typically get the brake components warm enough to be considered one bed-in cycle. Each of those six to eight partial-braking events should be made at moderate to high deceleration (about 75 percent of the deceleration required to lock up the brakes and/or engage ABS) and should be made one after the other without allowing the brakes to cool in-between.

Once the brakes have faded a bit and/or you smell friction material inside the car’s passenger compartment, the cycle is complete and you should allow the system to cool by driving at steady speeds without bringing the car to a complete stop.

After cooling, repeat the partial braking event procedure listed above one more time; cool down again, and you’re typically good to go. In some situations, a third cycle is beneficial, but two are usually sufficient.

Bedding-In Club Race or Full-Race Pads
When bedding-in stock-sized rotors with typical race pads, the bed-in procedure needs to be run a bit more aggressively. Usually, a series of seven to 10 partial-braking events from 60 mph down to 10 mph is performed followed immediately by three or four partial-braking events from 80 mph down to 10 mph.

As above, each of the partial braking events should be made at moderate to high deceleration (about 75 percent of the deceleration required to lock up the brakes and/or engage ABS) and should be made one after the other without allowing the brakes to cool in-between.

Once the brakes have faded a bit and/or you smell friction material in the passenger compartment, the cycle is complete and you should allow the system to cool by driving at steady speeds without bringing the car to a complete stop.

After cooling, repeat the partial braking event procedure listed above one more time, adding two or three additional partial braking events from 100 mph down to 10 mph. Follow up with a cooling run, and you’re typically good to go. In some situations, a third cycle is beneficial, but two are usually sufficient.

And Now, a Word From the Lawyers
Note that these speeds are neither recommended nor condoned on public roads. This procedure is designed to be run in a controlled environment such as a race track. While you need to get heat into the system to achieve a proper bed-in, you also need to exercise common sense and take full responsibility for your actions.
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