The automotive industry is absolutely packed full of myths, misconceptions, and sometimes even lies. While some are in the realm of marketing, and some are in the realm of maintenance, all bug the absolute crap out of me. I could rant for hours on this stuff, so I decided to curtail my rage to just one segment of the industry for this article, and tackle three myths within the wide category of brakes.
OK… *Takes deep breath*. Let’s get into this.
Marketing Gripes – What Does “Better” Mean?
Brakes are one of those weird areas where “better” might not be “better” for you. More expensive doesn’t necessarily mean an improvement, and buying brake pads above what you need could make your life miserable, or even be dangerous in extreme cases. Unfortunately, marketing around the brakes industry doesn’t really reflect this fact, and the layman could be forgiven for wanting to spend a little extra to get a better brake pad that might be safer, last longer, or create less dust…, just to end up with the exact opposite of what he wanted, and be charged a premium in the process.
So let’s brake down (pun intended) what you actually need, and when not to overpay:
If the marketing copy is to be believed, the more you pay, the better the brake pad is. Well that’s the problem. Better for what? For who? Let’s say you just picked up a new Mustang GT. This is your daily and weekend fun car to take to the canyons every once in a while, and maybe the occasional track day or autocross. You’re going to want some good pads that can handle high heat without fading out to nothing, right? Nice! A set of $250 racing pads would do great on the track!
But what about the other 98% of the time you’re driving your car? Well that’s where we get into my issue here. A high performance track pad is going to be great at what it does, but as a side effect of that, they are often loud, create a lot of dust, have poor bite when cold, and tend to wear rotors more quickly. As a daily driver, those sound more like headaches than advantages to me. Of course if you stick with stock or OEM equivalent pads, then you’ll have the opposite problem of them fading quickly and glazing when you actually put some real heat into them. That’s where a good balance needs to be struck. Something like the EBC Greens or Yellows ride this line well.
“Better” isn’t necessarily always “better.”
Drilled VS Slotted VS Plain and Boring
So why do these serious performance cars like the GT350R, or even Porsches, have drilled rotors? The simple answer is that they’re strong enough that they won’t be a big problem for most drivers on the street, but if you really push them to their limits, they will crack long before straight rotors will. If they are pushed hard, but not to the extreme, you get some pretty interesting wear patterns as well (more on that below).
So where did drilled rotors come from then?
They actually were borne of serious racing, funnily enough. Back in the age of asbestos brake pads (ignorance isn’t always bliss), there was an issue of outgassing with the pads under high heat. Basically the bonding agents in the pad would evaporate and create a thin layer of gas that prevented good contact between the pad and the rotor. The rather rudimentary solution at the time was to just drill a bunch of holes in those suckers and go racing — and that worked pretty well for the time.
The issue is that these days, we no longer use asbestos in our brake pads (no, not even Raybestos pads, despite the name), and with how far material science has advanced in this industry, outgassing is no longer the issue it once was.
“OK,” you’re saying to me, righteously indignant, “but everyone knows they cool better than straight rotors! That’s why manufacturers still make them!”
Yes, drilled rotors tend to run a little cooler than straight faced rotors, but there are two caveats to that:
- The face of the average drilled rotor can have a reduced surface area of 10%-12%, which means overall less area for the pad to grip onto, and less friction applied in total, meaning not as much heat generated. The cooling provided by those holes being there do make a slight difference, however…
- Due to those drilled holes catching air, the surface cools unevenly, leading to cracks. The cast iron of your rotors expands and contracts with heat, and the metal around those holes cools faster than the solid surfaces around them. This means that as the rotors cool, they are contracting at different rates around the surface of the rotor. What’s even worse is that the areas of the rotor that experience the biggest heat differentials are also, by design, the narrow spaces between the holes of the rotor, where there is less material to spread the stress out across.
Like I mentioned, slotted rotors are in fact found quite often in motorsports, even among serious race teams. There are a few reasons for that, namely the ability to provide runout for water, dust, and other debris (hence their popularity in rally). On top of this, they provide a way to wipe the pad’s surface to help with things like glazing, and to maintain a properly bedded pad and rotor. This is in addition to more structural rigidity maintained in the rotor (though they will still crack before straight rotors).
Everything is a trade-off, however. The (relatively minor) disadvantages of slotted rotors is that they tend to wear out pads more quickly, and in a daily driver, they can result in some noise when coming to a stop in a quiet car. Depending on the slotting pattern, this can be sort of a growling sound, or with some, under really hard braking, a quick thumping not unlike ABS lockup.
“So then why do all of these brands put drilled rotors on their performance cars rather than plain or slotted rotors?”
Because they look cool (marketing), and because most drivers generally won’t find the limits of those fancy looking drilled rotors — even in their 500+ horsepower speed machines. Doesn’t that just kind of make you sad to know? I wish more people would use the performance they pay so much for.
My last myth comes in two forms:
“What the hell, my new brakes are squeaking! They must be defective,”
“I just replaced my brakes a few months ago and they’re already warped? They must be defective!”
What does it mean to bed-in your brakes?
I’m sure most of you are familiar with this process, but if not: when you buy a set of brake pads, especially higher performance pads, they will come with instructions on or in the box on their specific bed-in procedures. This is a process to “mate” your pads to your rotors with gradually increased heat cycles to create a thin, even film of brake pad material on the surface of your rotor. Let’s get into some detail here:
The brakes on your car work by way of two forms of friction – abrasive and adherent:
Abrasive friction: As the pads are pressed against the spinning rotors, the crystalline structure of the pad and even the cast iron of the disc break down, transferring kinetic energy into heat, slowing you down.
Adherent friction: The material of the pad breaks apart and reforms, bonding to the surface of the rotor. This process saps energy away from the turning of the disc, spending it to create that bond, as well as create heat. This is the method of friction that is used to bed your pads to the rotor.
All modern brake pads use both types of friction, just to varying degrees depending on the application. Semi-metallic pads work through primarily abrasive friction, and therefore are tougher on rotors and create more dust in a trade-off that results in the ability to operate effectively at temperatures. Organic and ceramic pads primarily use aderhent friction, the trade-off being that they’re easier on rotors, have better cold performance, and are quieter at the cost of high temperature performance.
Now that we have this established, how does this pertain to the brake judder you’re feeling in your brakes?
Well if your rotors have not been bedded in properly, or they were overworked and lost that bedding, then you can have uneven pad deposits on the surface of the rotor. At first, this just means uneven drip across the surface, causing the pads to grip, then slip, then grip, then slip, etc, and that’s the judder you’re feeling as you brake. At the extremes, if this is not corrected, this can result in uneven rotor wear, and a “warped” rotor. I use quotes there, as the term is a bit of a misnomer, though that is a topic for another time (I covered it here years ago).
What about new brakes squealing?
The confusion here comes from the fact that most brake pads come with a metal tab that will rub against the surface of your rotor to indicate that your brake pads have worn down to a certain point (as I’m sure you’re familiar with). So now when most people hear their brakes squeal, they think that they already need to be replaced, but that is often not the case.
What’s happening here is that the conditions are just right for your rotors to vibrate as they pass through the clamping pads, not unlike how a bow being dragged across the strings of a violin work — the main difference is a violin is a lovely sounding instrument, and a squealing brake rotor tends to roar in the key of “ouch.” Not pleasant.
Proper bedding of your rotors can go a long way in reducing this effect, but there are a few other factors that can contribute here. For instance, a layer of rust on your hubs when you install your brake rotors can let them sit against the hubs slightly unevenly, or with some wiggle room, allowing them to vibrate as they turn. Another reason could be your brake pads shifting and allowing play in the contact between the rotor and caliper, allowing the vibration. This can be solved using brake pad shims or some of that brake pad lube they always try to sell you at your local auto parts store.
Pro tip: A little bit of anti-seize works great, and is cheaper than that stuff they’re trying to get you to buy (assuming you don’t already have some on hand).
Article courtesy of Street Muscle Magazine, written by Garrett Davis.