Go check out the brake rotors on that cool exotic or big performance SUV. Check out all those little tiny holes in them. It’s called cross drilling. The holes are supposed to make you stop faster, look more handsome, and leap over tall buildings in a single bound. But how do they work? And more importantly do they work?
The theory behind cross drilling is something called pad gassing. The idea was that when you were braking hard, the pads would build up a layer of gas and dust in between the rotor and the brake pad. When that happened, the theory went, you would all of a sudden lose braking force until that gas could dissipate. If you put holes in the rotor, instead of having a flat face, the gasses could escape through the holes instead of being trapped. Suddenly your brake pedal would be harder and you could brake faster.
Pad gassing was a big problem back in the days of asbestos pads and tiny rotors. Today, though, brake manufacturers like Friction Master and Brembo say that the pads don’t gas like that anymore.
New compounds don’t cause the layer that could cause you to suddenly lose braking. Well, at least on performance pads. It’s still possible if you take your boring daily driver on cheap pads to the racetrack, but at that point, it’s probably not at the top of your list of problems. New pads can also gas during break-in, which is why your car stops so poorly until the pads are bedded in properly. But under normal circumstances, good pads don’t gas.
So why are they still on cars like the RS5 and the GTI Clubsport S? That’s because in today’s cars they have a new function.
Modern SUVs and performance cars are big and heavy. And they have really big brakes to compensate for that. With those giant pads and open face wheels, the brakes can have a large amount of water that gets on the surface of the rotor. Drive through a puddle, and you’ll notice the complete lack of braking the first time you try and stop. Some automakers even have systems that pulse the pads lightly against the rotors when the wipers are going.
The cross drilling helps some of that water escape, which can help maintain braking in heavy rain.
The other benefit of cross drilling is a small amount of extra cooling. That’s thanks to the extra airflow through the holes. It’s somewhat offset by the reduced mass of the rotor caused by the holes, but there is still a net benefit.
The sharp edge of the holes has a benefit too. If they’re located correctly, they wipe the surface of the pad. That can reduce pad glazing and that rumbling feel in the pedal that everybody thinks means the rotors are warped. It helps maintain consistent performance over the life of the pad, though it does also cause them to wear a bit quicker.
So the holes used to have a use in race cars. Which is how they ended up on street cars, where the original use no longer applies. But they do do something on modern cars, other than just look pretty, so they’ll probably stick around for a while.
What about any downsides to the holes? Well, before you run out and get the cheapest cross drilled rotors for your hard-parking track car, there are a few things you should know.
For a start, the holes can crack. Primarily, that’s caused by cheap rotors where the holes are actually just drilled through the surface. Higher-end manufacturers like Brembo don’t just drill a simple hole. They have a more complicated shape that can be difficult to see completely. That reduces stress and the risk of cracking. They’re also carefully spaced so that the pads aren’t left with high and low spots and there aren’t so many holes that it compromises the structure. Any of those issues can leave you with a shattered rotor.
So the drilled brakes on that car outside do something. But not what you probably thought they did. And they can do the same on your car, but only if you by the good stuff. Brakes, like tires, shoes, and mattresses, aren’t the place to cheap out.