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Heeltoe's Take on Brakes

  • Posted: 08-03-2007 12:52 PM
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Heeltoe’s Take on Brakes

Perhaps, before outlining my beliefs here, maybe I should mention a little about myself. My name is Marcus di Sabella. I have been directly involved in the automotive performance and repair arena since 1997 as a service technician, parts consultant, direct retailer, and customer support agent. I have been indirectly involved in the industry (as a consumer) since 1994. I have BS in Manufacturing Engineering Technology program at Cal State Long Beach as of Summer 2007. I am speaking in this article to and from the perspective of street sports driver.

Brake Rotors
What are we talking about here? Cast iron. Rotors are made out of cast iron because this material possesses many of the properties necessary for brake rotors to function. They can handle high temperatures and they are great at dampening vibration. They are also relatively inexpensive to produce. In fact, a basic, run-of-the-mill cast iron rotor is downright cheap to produce. It is not expensive to make a rotor that will "get the job done." And yet, so many of us are having trouble with brake rotors warping and having brake pad material build up on the rotor surface causing a violent shaking in the steering wheel. If stock rotors were adequate, why is this happening?

There are three main reasons why a brake system will cause a shimmy under braking.
1- Rotors warping
2- Brake pad material buildup
3- Inconsistent rotor hardness

Let's look closer at these points. Learning more about them will help us in fighting them.

Rotors warping (and cracking)
This issue is caused by heat. Not just too much heat, but also too much change in heat. Braking involves friction between the pads and rotors. Average around the town driving will not create excessive heat, but braking for sustained periods, lightly, or heavily, will. Descending down a hill is a good example. A small or average hill will not cause an issue, but coming down a mountain certainly will. Riding the brakes for long periods of time will cause considerable heat buildup. Or prolonged braking when exiting a freeway off ramp. Sustained braking like this will cause the pad material to get very hot, and this heat is dissipated into the rotor. Brakes can get so hot, in fact, that the material itself will start to break down. The average brake rotor will start to crack at the microscopic level. Once these cracks start, they begin to grow. Pretty soon they are visible, and then the rotors are dangerous to use. The important thing to know is, just because the cracks may not be visible, they can still be present on the rotor!

The warping occurs when the rotors get extremely hot, and are not cooled down evenly. The rotor material can get so hot that it actually becomes softer. If it is not cooled down slowly and evenly, it will actually re-solidify out of its original shape. Flat faces are no longer flat, and the rotor is no longer true. The rotor is then warped.

I once read a very interesting and enlightening article by an author named Carroll Smith. In his
The "Warped" Brake Disc and Other Myths of the Braking System, he calls out the phenomenon of warped brake discs as a myth. In fact, he indicated just about any inconsistency in brake rotor surface is caused by brake pad material being unevenly deposited on the rotor surface. I do not agree; I do not believe brake rotor warping is a myth.

Blaming every shimmy on warpage is not correct, but I believe it is an existing problem. I believe that brake rotors DO warp, albeit there needs to be a fairly severe circumstance. I have seen rotors so out of true that no less than 4-5 passes with a brake lathe were needed to take down the high spots. Whether or not these rotors were actually warped is unknown, but there was certainly more going on here than just built-up pad material. Rebedding with newer more aggressive pads, as suggested by Smith, was not going to fix these car’s rotors.

In a racing situation, where brakes are upgraded in spec and used consistently, repetitively, and with considerable pressure, warping should not be a problem. I venture to say that Mr. Smith’s article was written from the perspective of a racer. In real-world driving, often times brake disc heat is built up to similar temperatures as a racing situation. However when racing, there is generally time for the brakes to cool down before the next corner. A modern car’s ventilated rotors will pump air through them as they spin, and they do so faster and faster with the car’s acceleration. By the next turn, brakes have had a chance to cool down to a point where they are effective. While by no means cold at this point, the rotors are not overheated and unusable if the materials have been upgraded properly (recall we are referring to a racecar here).

A racetrack is a controlled environment where setups can be fine tuned to the driver, track, and car. I am not surprised that in a racing situation a rotor would scarcely ever be warped. But the street is a different story. We simply cannot design one system that will function for all drivers in a given car, all over the world, and that is why OEM braking systems are inadequate for enthusiast and/or discerning drivers.

Brake pad material buildup
The first time I read this article, I had not heard of brake pad material buildup on the rotor face as being any sort of issue at all. I now know, through experience, that Mr. Smith was correct in this assertion. I have come to expect that most of the time a brake shimmy is experienced, it is because of brake pad material buildup on the rotor face. Getting pad material on the rotor is important. That is a key part of how brakes work. But the bedding or breaking in process of brake pads is such a critical part of pad installation. If done improperly, or if not done at all, the results are disheartening.

How does brake pad material become unevenly distributed? I am going to put Smith’s words into my own combined with information gathered from the Racingbrake Forums. I will iterate that I am speaking from the perspective of a street sports driver, not a track driver, although similar concepts apply to both. I wish to quell criticisms from racers who would scrutinize hyper-technical points which do not apply when seat belts do not exceed 3 points.

Brake pad material gets built up on rotors when the pads overheat. Simple as that. Many drivers overheat their pads without even knowing it. In fact, you can overheat stock pads and rotors without ever feeling fade. Long gradual stopping, hill descent, and general bad habits all cause overheating. When the pads reach temperatures outside of their ideal operating range, the compounds they are comprised of act unpredictably by depositing themselves onto brake rotor surfaces unevenly.

Allow me to illustrate a scenario. You are on the expressway traveling 80 mph (that’s pretty average, right? Well, it is in Southern California, traffic notwithstanding). You approach an exit which as a fairly gradual s-curve and a medium range length. So, you start to apply the brakes to slow the car down. Most people, I notice begin braking once the tires leave the expressway, and that braking continues, consistently and evenly all the way to the stopping point. Once stopped, the foot remains planted on the pedal until it is time to drive again. Sounds reasonable, but think about what is happening to your brakes:

Once the pads contact the rotors with the initial braking pressure, temperatures begin to rise. The gradual even braking is applying friction to the system causing the heat to grow and grow. By the time you get to the stop, odds are you have not lifted from the brake pedal, or at least not for long, and have maintained the heat in the brake rotors the entire time. Odds are, you got them pretty hot and they are borderline overheated. Now at the stop, you foot is holding the hot pads to the hot rotor in one place with pressure. Likely, with more pressure than is needed to hold the car still. In the high heat condition of the system, you are now transferring brake pad material onto one spot on the rotor. Continuing this practice will cause uneven pad material distribution and a shimmy in the steering wheel.

Smith and Racingbrake both give tips on remedying the situation. I am going to give my two cents on preventing it. First off, don’t use the brakes to slow the car. Rather, think of the brakes as a way to change the vehicle’s speed. Much the way an accelerator pedal changes the car’s speed incrementally from one speed to another (neighborhood-15mph, cit-35mph, highway-55mph, freeway-65mph), use the brakes to slow the car in the same manor (freeway-65, turn at offramp-45mph, stop approaching-15mph, stop). By braking in stages and removing your foot from the pedal in between, you reduce the car’s speed while allowing the brakes to cool down throughout the process. When you come to a stop, the brakes should be cool enough to avoid issues with pad material buildup.

Second, use more brake pressure. When we feel acceleration (going, turning, braking) forces in a car we tend to try to quell them (well, not all of us) to maintain a calm and comforting drive. Usually the influence of a Double-X-chromosomal human passenger will propagate this tendency. But believe it or not, if you squish your soles deeper into the brake pedal travel you will notice not only a greater braking efficiency, but longer pad life, less squeaking, and less risk of vibration. USE the brakes. Don’t be afraid of them. Getting more friendly with the brakes will also aid in the incremental braking method outlined above. Tell your wife to relax. That’s what seat belts are for.

Third, please please please research pads before buying. Brake pads are like Gins; they are all different and everyone thinks theirs is the best. In reality, the right pads for you and the rotors you have are as individual as you are. Bite, heat range, life, dust, noise; they are all factors to consider when buying a pad. I tend to stick with one brand, Hawk Performance, because they make a pad for just about any application in a number of compounds. Pay attention to the uses and recommendations of the manufacturer. There is something out there for everyone, but simply going with what your buddy has, or what the forums say, my be doing you an injustice. Don’t buy racing pads for the street, and don’t abuse street pads. And bed the brakes in properly. Everyone has their own opinion on methods (mine is here) but whatever method you choose do it as best as you can.

Conclusion
So what have we learned here? We learned what problems exist with brake rotors (besides fade and brake fluid boiling, but those issues exhibit more on the track than the street). We learned about some of the issues with pads when combined with temperature extremes. We discussed ways that you can improve your luck with brake pad bedding and selection. Brakes are hot topics that are widely misunderstood. There are many details and facets I have not touched on. But in a subsequent writing I will outline my recommendation on features in order to ensure that your braking component selection is made wisely with your best interests in mind. Look forward to my next braking article centered on the scientific and economic reasons for opting for the Racingbrake line of brake rotors!


Marcus di Sabella


About the Author

Marcus di Sabella Marcus is the founder of Heeltoe Automotive. He's been working with cars (mostly Honda cars) since 1996, and has been providing enthusiasts with excellent products, services, and web experiences since 2002. He's been published in Honda Tuning, and holds a degree in Engineering Technology.

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