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Brake system upgrades won’t gain you any horsepower. But all the power in the world isn’t worth much if you can’t rein it in with confidence when you need to. And while calipers – and their piston count – tend to be the components that grab headlines in the performance brake world, the discs they’re paired with play a crucial role as well. As Mark Fowler of Baer Brakes points out, it’s not just about giving the calipers something to clamp onto.
“Think of the rotor as the radiator of the brake system. Yes, the caliper is clamping the pads onto the rotor, which creates friction, and that stops the car. But that friction creates heat, and one of the rotor’s primary jobs is to function as a heat sink. It absorbs the heat that’s generated from the friction and dissipates it. And the better the rotor can do the job of dissipating that heat, the less of that heat is transferred into the brake pad, the caliper, the fluid, and so on.”
But that doesn’t necessarily mean that using the largest rotor possible is going to provide the best results. “Weight is also an important consideration in a high-performance application – especially rotating mass,” he says. “But you have to be careful about how you’re minimizing weight in a brake rotor.
“You don’t want to just take weight out of the rotor arbitrarily because that can significantly compromise its efficiency,” continues Fowler. “When you use less material, you have less material to absorb and dissipate heat. In some applications that may not be an issue, but in others you might end up with a brake system that overheats prematurely, which can result in boiled fluid, brake fade, and other issues. A true performance rotor can potentially be as light or even lighter than a factory-stock rotor without significantly compromising efficiency, though – it’s about taking the weight out of certain areas strategically.”
The rotor also needs to be properly sized to the caliper and brake pad it’s being used with, of course. “Certain calipers are only designed to work with rotors of a certain diameter and/or thickness,” says Fowler. “For example, our smaller S4 calipers work with 11-inch to 13-inch rotors, while the larger six-piston calipers that we have will work on rotors ranging from 13-inch diameter up to 16 inches. That’s largely dictated by the swept area of the brake pad that’s in the caliper. If you took one of our 6S six-piston calipers and tried to use it with an 11-inch rotor, it’s not going to have a good contact patch between the pad and the friction surface area of the rotor. And if you run a big rotor with a small caliper and pad, the heat is going to be more concentrated on a specific area of the rotor, and that creates the potential for the pad to overheat quicker because of its small surface area relative to the size of the rotor.”
There are other considerations that go beyond size and weight, too. Let’s take a closer look at some of features commonly found on high-performance rotors to get a better understanding of what they do and where these features work best.
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You may have noticed that many high-performance rotors have grooves etched into the friction surface of the disc. While they certainly look cooler than unslotted discs, the slots do more than just enhance the rotor’s curb appeal.
“There are gases and pressures that build up between the pad and rotor when you’re braking,” Fowler explains. “By putting slots in the friction surface, it provides a place for those gases to escape, and that creates more even contact between the pad and rotor.” While some high-performance features commonly found on brake rotors come with compromises, he says that’s generally not the case with slotting. “There’s no real downside to slotting. Some people claim that slots cause more pad wear, but we haven’t seen any evidence that confirms that. Pad longevity comes down to the pad material, for the most part.”
You’ve likely seen cross-drilled rotors before, too. These rotors have holes drilled through the friction surface of the rotor. Like slotting, these holes provide a place for built-up gases to escape. Aside from the visual enhancement it provides, cross drilling also helps lower the weight of the rotor because material is being taken out of the piece. But this feature does come with a few inherent caveats.
“Cross drilling helps reduce pressure in much the same way slots do, and it’s aesthetically pleasing to a lot of people, too. That’s why cross-drilled rotors are offered. On the street and in day-to-day driving, these work fine. But when you get into higher performance applications – track days, road racing, etc. – cross drilling can actually have a negative effect on the rotor’s performance for several reasons.
“First, we talked earlier about how the rotor is the radiator of the brake system,” continues Fowler. “When you start drilling holes in the rotor, it has less material for absorbing heat. While you’re providing a place for gases to escape, the rotor’s overall ‘heat sink’ effect is reduced, so it’s going to potentially run a little bit hotter in a performance application. And as heat is introduced into the system, everywhere that you cross-drill the rotor has a potential for surface cracking to occur because of the stress risers that the holes create.”
This might seem surprising because cross-drilled rotors are often seen on purpose-built race cars, but it’s important to keep in mind that the rotors used in racing don’t see repeated heat cycling; for the most part, they get hot and stay that way. And perhaps more importantly, racers typically consider rotors to be disposable items. They’re replaced far more often on purpose-built race cars than they are on street-driven performance cars, thus reducing the opportunity for cracks to form. Race cars are often significantly lighter than road cars, too, which allows teams to focus on further optimizing weight rather than maximizing the rotor’s thermal efficiency.
“We see these used a lot in circle track racing – sprint cars, midgets, that kind of thing,” Fowler says. “They’ll use a thin, solid rotor on the left front that’s drilled to reduce rotating mass; the left front of the car usually isn’t generating as much heat as a rotor on the rear axle. At the end of the day, you gain a benefit from the reduced weight, but you have to keep in mind that there are compromises involved as well.”
In a one-piece rotor, the mounting hat – the part that slides over the wheel hub – is cast directly into the rotor. In a two-piece design, the friction ring and the hat are two separate pieces. “The two-piece design has several distinct advantages,” Fowler tells us. “The biggest one is weight savings – now you can use a hat made from aluminum, which is going to be lighter than a hat that’s part of the rotor casting. And from a manufacturing standpoint, it allows a company like Baer to offer that friction ring in a number of different applications using hats of different depths, and things like that. With a one-piece cast rotor, you’re limited to whatever the casting is designed to fit.”
A two-piece design also enhances the rotor’s look. A black anodized coating is usually applied to the hat, which prevents corrosion and gives the rotor a snazzier look overall. Fowler also notes that while most rotors are vented whether they’re a one-piece or two-piece design, the vane design can vary from manufacturer to manufacturer.
“Most of the time the vanes are straight up and down, but the majority of Baer’s rotors are directionally vaned. Curving those vanes enhances their ability to work as a centrifugal pump to pull cool air in at the hub and send it up and out the outer edge of the rotor to help dissipate heat. It also increases the length of the vanes, which in turn allows them to absorb more heat than they would if they were straight up and down.”
Given all of this, Fowler says that when it comes to choosing rotors, the first place to start is to take a realistic assessment of how the brakes are going to be used. “You have to really think about what you’re actually going to be doing with the vehicle. Is it a daily driver that just gets commuter miles? In that case, an OE-style drilled and slotted rotor is probably going to be more than enough for that.
“Things get a little more specific if it’s a vehicle that you regularly race at the track,” continues Fowler. “And the type of racing you’re doing matters, too. A road racing car is going to require a more robust brake system than a drag car, the latter of which really only needs to do one high-speed stop at a time.” He says that while slotting will benefit cars that regularly see track days, compete in time trials, or endurance race, he’d shy away from cross-drilled designs for that use-case.
Compatibility with your wheel size and the existing brake system components will also naturally narrow down your options, too. Cost will likely factor into the decision-making process as well, but Fowler strongly recommends not cutting corners when it comes to your brake system.
“It’s always better to be over-braked 99% of the time rather than under-braked 1% of the time. You might be saying to yourself, ‘Well, I’m going to go to the track once every three or four months, and I don’t want to spend that money on something I’m not going to use every day.’ The issue there is that, while the cheaper option might be fine around town, it’s not going to work as well when you’re towing or running at the track, and you’re really pushing it. That’s the one percent of the time when you’re under-braked, and it’s at the moment you need good brakes the most.”
