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Why Cross-Drilled/Slotted Rotors and Big Brake Kits are More Ineffectual than You Think
A recent trend in the sports car market is the use of cross-drilled or slotted rotors. You see them on all kinds of cars today, and while they may look nice, few people know the benefits to using them. Many people don’t even know what the slotted rotors are for. Are they just for looks, or do they actually increase your braking performance in some way? There are pros and cons to using them, and that’s what we’ll be focusing on here.
Slotted rotors are generally more reliable than their cross-drilled counterparts. One of the biggest benefits to using a slotted rotor is that the slots help pull brake dust away from the pads. By reducing the debris between your pads and rotors, you allow more of the pad’s surface area to come in contact with the rotor, which means better grip, and better stops. The coefficient of friction is increased, so you’re using less energy every time you step on the brakes. In other words: you can stop your vehicle faster with the same effort. This is why motorsports such as NASCAR advocate the use of slotted rotors on their race cars.
However, there is a tradeoff that occurs to gain this extra stopping power. By cutting into the brake rotor—whether slotting or cross-drilling—you are effectively reducing its structural integrity. So even though this reduced, “unsprung” weight it’s also putting your brakes under stress. To some, that extra stress is worth the extra stopping power.
Cross-drilling was a common practice in the old days of asbestos brake pads—particularly in the 1950’s. When these pads were applied, a resulting buildup of gases got trapped between the pad and the rotor, referred to as “gassing out” or “outgassing.” This decreased the friction between them lowering brake efficacy. To counteract this, holes were cross-drilled into the rotors to give the gas somewhere else to go, and while this may have been effective 60 years ago, we live in a world of carbon-ceramic brake pads that don’t experience this same phenomenon. In today’s cars, if you use cross-drilled rotors under heavy load, such as on the race track, you’re losing structural integrity even more than with a slotted rotor, which makes your rotors much more prone to cracking.
Cross-drilled rotors are considered visually appealing in many situations. When run on cars that won’t see the track, you can run cross-drilled rotors and not have to worry about the cracking mentioned above, because on the street your vehicle won’t generate enough heat to crack them. In other words, your choice of brake pads is purely aesthetic and all about personal preference, until you decide to hit the track and push your car to its limits.
So you have your heart set on one of those new big brake kit upgrades for your street ride. They come with larger, vented, rotors and usually either four or six piston opposing calipers. Despite their high cost, they do not make your car stop faster, or decrease the stopping distance greatly and here is why.
The Physics Behind Stopping
Physics doesn’t discriminate between starting and stopping but instead considers them both changes in kinetic energy. Thus, an object at a constant speed reacts the same as an object that is sitting still. That being said, the Standard Kinetic Friction Equation applies to stopping just as much as starting movement.
In simple terms it says an object in contact with the ground will absorb the most kinetic energy right before movement, or sliding. This applies in the reverse as well—a car in motion will absorb the most kinetic energy right before the wheels lock up. With respect to modern vehicles, this equation states that the only way to make a car stop quicker is to either increase the coefficient of friction with the road, (e.g. upgraded tires) or decrease mass by lightening the vehicle. Note, both of these things have little to do with brakes.
Why They Don’t Reduce Stopping Distance
As the physics lesson states, under braking, your car absorbs the most kinetic energy (movement) right before the tires lock up. This is true for both non-ABS cars and ones equipped with ABS. Abs shortens stopping distance by attempting to keep the brakes on the point of locking- where most kinetic energy is absorbed. Even though they have come a long way, even the most advanced ABS systems react by pulsing braking pressure resulting in a slide, roll, slide, roll pattern.
In a perfect world, a perfect ABS system would perch the brakes at the cusp of sliding without ever actually locking up. Upgraded calipers and rotors in a big brake kit will result in more braking torque, the stopping force applied to the wheels. This will bring about wheel lockup or ABS intervention sooner than with the stock system. This should not be interpreted as a shorter stopping distance. In many instances the larger brakes confuse the stock ABS system which is not tuned to the different pressures required to engage and disengage the brakes.
How They Improve Performance
Big brake systems were not designed to stop your car sooner, but rather, stop it more efficiently and consistently. The larger calipers, vented discs, and exotic-compound pads increase heat capacity and heat dissipation. These upgraded braking systems not only perform well under extreme temperatures, but in most cases perform better when hot as opposed to average street driving temperatures. This is definitely something to consider when deciding on rotor and pad compounds. Because these brakes dissipate heat more effectively, they reduce brake fade. Brake fade occurs under high temperatures, can be caused by boiled brake fluid, and results in a soft pedal and elongated stopping distances. Big brake systems don’t decrease your car’s stopping distance over stock. Instead, they are designed to perform consistently under extreme conditions so that your car will stop just as well the first time.
