There is more misinformation about cross drilled rotors than anything else I can think of on a car. The general consensus seems to be that drilled and slotted rotors offer better performance than “blank” rotors. This is simply not the case.
At one point in time race cars did have cross drilled rotors, and this is probably where the idea that they offer increased performance came from. But if you look at any serious professional race car today, I would be shocked if you found any cross-drilling.
Like everything else, there are advantages and disadvantages to drilling and slotting a rotor. Fortunately, technology has progressed so that there is no longer a need to cross dill rotors and therefore, we don’t have to deal with its disadvantages.
The reason why rotors were drilled in the first place was to relieve the gas that was created when the pad material started to breakdown (burn). Since modern pads don’t gas off any significant amount, this is simply not a concern.
Many people and advertisements claim that cross drilling helps the rotor cool. I’m sure those little holes do help the rotor cool in some regard (possibly not measurable), but the effect in reality is completely insignificant. Furthermore, any benefit of extra cooling is most likely off set by the reduction of the rotors mass due to the drilling which lowers the overall heat capacity of the rotor.
So now that you know that there is no benefit to running a cross drilled rotor, we are left with a major disadvantage. What all of those little holes do is create stress risers and a surface that’s unevenly heated and cooled. The result is that the rotor becomes very easy to crack and makes a catastrophic failure much more likely. The worst situation is when a crack forms and connects between multiple holes – much like a connect-the-dot puzzle. This can lead to a large piece of the rotor breaking free which I can assure you is not good at all.
So why do all those high dollar cars like Ferrari, Lamborghini and Porsche have drilled rotors? Well, because people think it looks cool. The rotors on those cars fail when pushed hard as well, and the professional race teams that run these cars replace them with non-drilled rotors. In my experience, cross-drilled rotors just don’t last as long as a blank rotor. If you ever go to the track and find someone pushing a car hard that has cross-drilled rotors, put your ear near one of his wheels and listen carefully when he gets back to the paddock. You will hear small metallic pings and pops as the rotor cools unevenly. What you will be hearing is the sound of the cracks forming….
So what about slotting?
Slotting serves the same purpose as drilling but doesn’t weaken the rotor as much. Slotting is also advertised to “wipe” the surface of the pad preventing glazing. I don’t know how much of this is true or how much of an effect this has, but the real world result of this is that your pads don’t last as long. Typically, if you glaze your pads, you have already surpassed the heat range of the material and the pad will most likely have “melted” onto the rotor surface creating an uneven transfer layer. I don’t see how slotting is going to prevent this.
So what really happens when the rotor is “warped”?
When you’re on the brakes and there’s a pulsing sensation or vibration, we typical call this situation warped rotors. Surprisingly, I have placed a dial indicator on so-called warped rotors to find that they have no run-out what-so-ever.
What does happen is that the layer of pad material on the rotor surface builds up unevenly, and also, the metallurgy of the rotor can change states. The layer of pad material on the rotors surface, if unevenly distributed, will create hot spots. If these spots get hot enough, it can form cementite in the rotors metal – a rough iron carbide formation that creates a lot of friction, but is terrible at dissipating heat. The cementite formation can get so bad and cause so much friction that even when you are off the brake pedal completely, because your pads are always in contact with the rotor ever so slightly, it can create a vibration when driving normally. I have even mistaken this vibration as my tires being out of balance.
Uneven transfer layer buildup and cementite is what produces “warped” rotors, not run-out or distortion*. Cementite is a problem with iron rotors, rotors made of other materials like carbon do not suffer from this problem.
Vibrating can also be caused by a crack in the rotors surface. If you have a vibration that only appears during hard or extended braking, it may be a crack. The reason for this is that the cracks opens up when the rotor is hot and closes when it’s cold. You may never even know that there is a crack if you never build heat in the system…
*Let me digress a little bit – There is surely some uneven dimensional change (warping) to the rotor if you have a localized hot spot. But this seems to be only temporary and when the rotor cools, it returns to its normal flat state. I suppose you could drive through a puddle with very hot brakes and cause a permanent measurable change, but it must be rare. Also, yes, it is possible to have an actual warped rotor or one that was not cut square. In those cases, yes, they will cause a vibration.
If I have my rotors resurfaced, will that fix the problem?
In my experiences, no. When I have had my rotors resurfaced, it only cured the vibrations temporarily. Within a few months, they return even if I haven’t been doing any hard driving. So what gives? Most likely, parts of my rotors had turned to cementite and it was thick enough where resurfacing did not remove it all. Even if there was a small area left after resurfacing, that one spot will create a hot spot which will grow in fairly short order. This is also proof that the vibrations are not coming from a warped rotor.
Food for thought:
- So actually there is one benefit to cross-drilling and slotting but I didn’t want to mention it since it’s also insignificant – that benefit is in wet weather. It turns out that drilling and slotting either give a place for water to evacuate like the tread on a tire, or allows steam to gas through kind of like what drilling was intended for. I don’t know what really happens, maybe both. But either way, the initial bite tends to be better in the wet.
- Many people have such strong convictions about rotor warping that they wont believe anything I say in this post. I dare those people to put an indicator on a ‘warped’ rotor and actually find the distortion in the rotors surface. Rotors do tend to have run-out even when new, but you would never know it as long as the run-out is in tolerance; so, don’t mistake some run-out for warping when the rotor had it from the day it was installed and when the pulsations in the brake pedal only started recently. Also, don’t let some fool mechanic cut your brand-new rotors to ‘straighten them out’ or to get rid of the in-tolerance run-out, this will just shorten the rotors life.
- In general, cutting rotors is only a band-aid.
- There is different levels of cross-drilling. Some rotors have many more holes per inch than others. The ones with a high density of holes suffer more than ones like the rotor at the top of the page. The results are the same when pushed hard.
- On the street, drilled rotors are ok, but can create noise. The thing that kills drilled rotors is fast heating and cooling cycles over a wide temperature range. This is why no one uses them on race cars.
- The transfer layer on the rotors surface is typically invisible in all but extreme cases. When the pad is overheated, it can leave large visible deposits on the rotors surface.
- Rotors with curved vanes are significantly more efficient than those with straight vanes.