RS Tech Article: Brake Myths Explained

[Lomac]

Disclaimer: While in the middle of writing an article about brakes, I stumbled on a post by Garrett Davis on Oppositelock/Jalopnik. It put my original article to shame, so I'm just gonna more or less copy what he has written. With permission, of course. Some bits have been added, deleted or adjusted by myself.

Brake Myths: What to Expect When You're Expecting (to Stop)

Brakes are sort of the unsung hero of any good car. Besides showing off those wicked sweet drilled and slotted rotors and rattle-can-red calipers, not many people seem to pay much attention to them until there's a problem. Good pads and fluid are not exactly among the more "glamorous" upgrades you can do, unlike those 10 degrees of negative camber you have dialed in or that totally functional and not ridiculous wing you have towering over traffic.

I was recently doing a little research into upgrading the brakes on my new-to-me 300ZX which led me down a path to hours of research on the finer details of how our brakes work, and some common ill informed advice we all see day to day. So I figured I'd do my part for the greater good of Jalophood, and pass some of that information along and hopefully clear up some myths and misconceptions about how those things work that allowed your car stop just short of that 3-series that jammed in front of you at the last second at a light that one time and made you like, super pissed.

1) Drilled and Slotted vs Lame and Normal

I wanted to get this one out of the way first, partially because there isn't actually as much info on these as I expected there to be, but also because it helps to dispel my favorite of these myths.

Quick, picture the hottest new car that you lust after. The one you'd have hanging above your bed if you were still 9 (or maybe now, not judging). Chances are the brakes are huge, flashy, and have more holes drilled in them than Kanye's ego. So those massive drilled rotors must be the pinnacle of braking technology right now, right?

To help answer that question, let's first take a look at the brakes of an F1 car:



Hmm, besides being incredibly beefy and high tech looking, that rotor and caliper combo doesn't really look much different in principle than what comes on a modern Camry. It's a flat disc surface on a side vented rotor. What gives?

Well it turns out that drilled rotors have something of a lasting problem, mostly in that they just don't. Under high stress and heat, those holes create a whole mess of weak points on the rotor, especially on cheap ones that don't drill between the cooling vanes, and instead just drill a nice looking pattern whether that means going through the structurally important vane sidewalls or not. Even with good equipment from reputable brands, after repeated heating a cooling cycles, those effin sweet looking rotors start to look like this:



So why do drilled rotors exist? Funnily enough, they actually did get their start in racing way back when we still used asbestos brake pads. When these wore down at temperature, pockets of gas created by the pad would reduce brake effectiveness during a race. This was called out-gassing, and our modern semi-metallic or ceramic pads do not have this same problem. So now these holes are put there under the guise of cooling more effectively, which is partially true, because drilled rotors do tend to run cooler. Where there is debate, though, is how much of this effectiveness comes from the (not insignificant) lack of surface area to create that heat, and how much is due to good airflow. Either way, there are better ways to cool your rotors that don't involve ruining their structural integrity.

All metals flex and "grow" when heated up, the cast iron rotors on your car are no different. When a piece of metal is repeatedly heated and cooled, it relies on the entire structure to flex evenly along with it. Introducing big, evenly spaced holes just gives the metal more wiggle room to flex on its own as its temperature changes unevenly to the metal around it. Drilling holes means less surface tension to combat this issue. There is no perfect way to heat and cool our rotors completely evenly while driving, so this is just a fact of life. This is why you will never see any serious modern race car running drilled rotors. Go ahead, Google it. I know I did when I first read about this.

So what about slotted rotors? Well a few racing organizations actually do use slotted rotors, Nascar and many LeMans cars included. The idea is that the slots give the dust from between the rotor and the pad a place to go (even though most front pads have this built in), as well as "wiping" your pads to mitigate glazing (more on that later). The problem here is that if you have glazed your pads, you have already heated them up past their intended heat range, and the pad has literally melted against the rotor's surface. To say that these slots will resurface your pads for you is both optimistic and ignoring the fact that you pushed your equipment passed what it was designed to handle.

I should mention that one legitimate benefit of drilled and slotted rotors (besides attracting the hunnies, y'all), is in wet weather conditions. The gaps give the water a channel to run out, much like the tread on your tires for better wet weather braking. Also, none of this is to say that blank rotors don't crack. They do, just not nearly as often and under more extreme conditions.

2) Aw, shit! My rotors are totally warped, Bro!

Nope. They're not warped and they never were warped. You know what? You're warped. (Yeah, go ahead and tell mom, I don't even care!)

I'll admit, part of the reason why I started doing all this research in the beginning was because I was pretty sure my brakes were warped and I wanted to get new rotors. At first I found all the typical tips and info, but because I'm a loser who researches the absolute shit out of everything, I stumbled upon some people calling bullshit on the whole warped rotor idea. Seriously, I had to look all the way down to like, the bottom 3/4ths of a Google search page to find this stuff. That's how deep I go for you guys. Anyway, getting to the point...

Here's a quote from professional racer, Carroll Smith:

"...in more than 40 years of professional racing, including the Shelby/Ford GT 40s – one of the most intense brake development program in history - I have never seen a warped brake disc."

This is from an extremely interesting write up that he did for StopTech years ago, and if you want to get really in depth with this stuff, I suggest you check it out along with some of the other technical papers they have published on their site, some going deep into the physics and chemistry of it all — and by deep, I mean half the stuff flew about 7.5 feet right over my head. I'll give you a more simplified version of what's going on when your rotors feel and have been measured to be warped. Pay attention, there will be a quiz later.

First, we need to establish how your brakes work

When applying the brakes on your car, the caliper squeezes two brake pads to the spinning rotor, creating friction against the rotating mass of your tires and slowing you down. There are two types of friction at play here, abrasive friction and adherent fiction. Abrasive friction literally breaks the bonds of the crystalline structure of the pad and even the cast iron of the disc, creating heat. Mohs Law tells us that the harder material (ideally the disc) wears away at the softer material of the pad as the two materials rub together. Picture a sanding pad against a board. Same concept.

Adherent friction is where some of the pad material literally adheres to the opposing surface as they scrape by each other, creating a thin and (ideally) even layer of pad material on the face of the rotor. That material can continually break its bonds and transfer from surface to surface back and forth between the disc and the pad, continually breaking and reforming like they were bouncing across political parties between elections (heyoo!).

Good performing pads need to strike a balance between these two types of friction. A primarily abrasive pad will have a quick wear rate and will fade at high temperatures as its structure weakens and gives, no longer stiff enough to be abrasive. A primarily adherent pad will result in too much build up, as it is not abrasive enough to scrape the disc clean and uniform, and requires much higher temperatures to be effective. Between these two is where the balance needs to strike for good street pads — something that can handle being ridden all the way down a long hill without fading to nothing once you reach the bottom, and something that can still stop your car effectively on a cold morning. This is why a racing pad that requires high heat to work effectively and can be so dangerous on the street.

So how do my brakes "warp" then?



So going back to that adhesive friction stuff, if a pad is not properly broken in (yes, this is a thing), the material that transfers between the pad and the disc can do so in a seemingly random, uneven fashion creating islands of deposits that keep growing, leaving high and low spots on the disc. Another problem is if you hold the pad against the rotor after intense braking or coming down from a high speed, the pad can literally leave a print of material on the disc like the image from StopTech above shows.

The other way your rotors can feel warped happens when your discs develop heat spots. Modern cast iron rotors are an alloy of iron and silicon mixed with particles of carbon. At high temperatures, spots of silicon carbides form and create uneven hot spots, growing in temperature faster than the iron around it. Once this temperature reaches up around 1300 degrees Fahrenheit, the cast iron around that area begins to form cementite, or iron carbide which is very dense, abrasive, susceptible to cracks, and conducts much more heat than the cast iron around it. Once the cementite forms, continued use will just heat up those spots, causing them to heat the iron around them and form even more cementite. It's a vicious cycle.

Resurfacing your rotors can remove the cementite if you catch it early enough, but it's very unlikely and most times you'll be back in the shop after a few months getting them replaced all together. It's a band aid fix for a larger issue, and honestly isn't even really all that cost effective over a set of decent replacement rotors and the knowledge of how to keep this from happening again.

Brake Fade and You [size](not really a myth, but I'm still going there. Deal with it)



We all know what brake fade is, and have likely experienced it in one form or another, but what isn't always clear is what exactly is happening to make our brakes less effective after hard use. I've covered a bit of this already, but I'm going to go into more detail. Why? Because I feel like it, god damn it! And I want this article to be so long that it alienates 62% of readers and only leaves the Jalop-elite still with me. So if you're still reading this, congrats, you're even less productive at work than most of your peers. Oh, and don't forget to flush once you're done — Stan from accounting complained the other day. I know, I hate Stan too. I digress...

There are three reasons why your brakes fade, and they are not necessarily mutually exclusive. I already covered why the more "average consumer" friendly pads lose their abrasiveness after being overheated, but I'd like to talk about pad glazing and the effects of overheating your brake fluid. More specifically, why some brake fade leaves your pedal feeling healthy and firm, but with decreased effectiveness — and why other times your pedal will feel squishy and soft, forcing you to pump the pedal to get any performance out of the system. (Feel free to add your own innuendos to the previous few sentences, I'll wait.)

Glazed Pads

Pad glazing is when the surface of the brake pad literally melts and hardens, leaving a dense, smooth material that slides easily across the rotor. This will result in a normal pedal feel, but requiring much more effort to achieve adequate braking. A glazed pad has lost its porous, almost flaky surface and instead is left with an almost glassy, hard face for it to slide on. Properly bedding in your pads to the manufacturer's spec can prevent this. Most performance oriented aftermarket pads will come with a guide, but a broad one can be found here.

Last but certainly not least, we find ourselves on possibly the most neglected and ignored item on any car, possibly only losing out to the blinkers on your average BMW (now an optional extra!):

Brake Fluid

The brake fluid sitting in your car is actually one of the most important fluids in there to keep track of. A lot of people, myself included for the majority of my years, never really consider brake fluid as a consumable item. It's not until you learn a bit more about hydraulic fluid and racing brakes that you realize that brake fluid is something you should be watching more closely.

Probably the most common form of brake fading you'll come across shows itself through a squishy brake pedal. This happens when the brake fluid in your calipers literally boils from the extreme heat being generated under hard braking. The reason this is a problem is that brake fluid is chosen specifically for its incompressibility and once the fluid boils, it generates air bubbles. As we well know from the glory of forced induction, air compresses a lot and quite readily. Once those air bubbles form, each push of the pedal puts more energy into compressing the air and less into squeezing the rotor, sometimes to the point where even putting the pedal all the way to the floor won't stop you adequately.

Once your brake fluid has been cooked, pumping your brakes can return some pedal feel, but bleeding your brakes is the only way to now remove that air from the system. Still, once your fluid has boiled, you should change it out for some fresh stuff at your earliest convenience, because boiled hydraulic fluid has lost much of its heat resistance and will boil much easier the next time.

Another thing to keep in mind is that brake fluid is extremely hygroscopic, meaning it will readily absorb moisture if left exposed. This is a problem because adding water to the mix substantially decreases the temperature at which the fluid will boil. As little as a 3% water content can bring the boiling temperature of the fluid down by as much as 150 degrees Fahrenheit — or about 30%. Water will find its way into your brake system over time, especially if you live in a humid area, which is why even under normal daily driving conditions, you should still change out your brake fluid semi-regularly.



There are of course tons of things I left out, enough that I might have to dedicate a whole separate article to them, but this takes care of the more basic and important stuff. A few caveats to include here: One, I really don't know how much of all this applies to something like carbon-ceramic brakes like on the GT up there. This is still fairly new territory, and pretty much all the info I gave up there applies to the much more common cast iron discs. Second, all this talk of brakes and their effectiveness leaves out the importance of tires and suspension. Your brakes can only do their job as well as the tires are able to stick to the surface, something that proper suspension geometry and damping can greatly aid.

[The_AK]

I recall reading this on Jalopnik the other day and made me think about all the ricer days people spitting broscience here and there. They should do an article on aerodynamics too.