Brembo makes good rotor blanks. If your main purpose is street driving with occasional 'spirited' use, get plain rotors. Slotted rotors WILL wear the pads faster, crossdrilled rotors are a waste of time and money.
For anyone interested in plain vs. slotted vs. crossdrilled, the following might be of interest to you.
1st off, let's just get it out of the way. These days, crossdrilled rotors are for LOOKS ONLY. Let me explain.
Back in the old days (up to about the 70's), brake pad technology still sucked pretty bad. The pad compounds used back then tended to outgas when they got very hot. Occasionally, this gas pressure from outgassing could be severe enough to somewhat 'float' the pad on the rapidly spinning rotor face. Obviously not a good thing as it could seriously reduce the braking torque. Rotors were crossdrilled to allow these gases to have an escape route thus avoiding the 'float'. These days, modern brake pad compounds don't offgas like that when they get hot, removing the original reason for crossdrilling.
Now here comes the sticky part. Why are crossdrilled rotors still common all over the place? The answer is perception and common belief. A lot of sheeple still believe that crossdrilled rotors have a performance advantage so manufacturers offer them. Folks also have misguided ideas of what exactly these advantages are.
Common misconceptions about advantages with crossdrilled rotors;
- They keep the brakes cooler. OK, using simple physics misguidedly turned into pseudo-science, folks will think, well the more surface area, the better the cooling. Yes, this is correct. Vented rotors dissipate heat better than non-vented rotors because there is more airflow over a greater surface area (correct), so the more holes there are in the rotor (eg. crossdrilled), the better it must dissipate heat (wrong). Why is that wrong? Well it isn't wrong,... in a static environment.
If you took two stock-sized cast iron rotors and stuck them in the oven till they both hit 750f (not hard to reach under medium hard braking) and then removed them and placed them in open air to cool, the crossdrilled rotor would in fact cool slightly quicker. This happens for 2 reasons; 1st, the crossdrilled rotor does in fact have more surface area to dissipate heat by radiation and convection. The other reason (which will become more important later) is that with both rotors being the same size, the crossdrilled rotor will have less mass (you just removed material by drilling all those holes). Since any given material has a given specific heat capacity, removal of material mass = reduced heat capacity = less total thermal energy stored and less to dissipate. A 1kg iron ball heated to 750f will cool much faster than a 1000kg iron ball heated to the same temperature.
Now, the theory works in a static environment but how does it work in a dynamic environment? Not quite as well. Now some folks say, well, more holes, more airflow. Nope, not really. See, there's a world of a difference between the channels of a vented rotor and the holes through the face when it's crossdrilled.
A vented rotor functions like an impeller, similar to the compressor in a turbocharger, or the impeller in a hairdryer. As the rotor spins, it accelerates air radially outward through the vanes while sucking air in at the center (just as a hairdryer or turbo does). The faster the rotor spins, the faster (and more) air flows through it. Well how about those crossdrilled holes? The problem is that those holes are axial to the rotation, not radial like the vents in a vented rotor.
The vented rotor as mentioned, uses centrifugal acceleration to move air through the vanes/vents. The axially drilled holes don't really get much airflow at all though. The crossdrilled holes are perpendicular to the rotation of the rotor. If you can't quite picture why there's little airflow through those holes, here's a simple experiment to try or visualize. Take a regular drinking straw and cut it so it's about an inch long. Fill it with powder and then hold it in front of a fan so the ends are completely perpendicular to the airflow from the fan. Yes, the airflow will actually suck some of the powder out of the ends of the straw (due in part to Bernoulli's principle. The faster a fluid moves, the lower it's pressure, and in this case, air = the fluid, but in this scenario, mostly due to the turbulence as it passes the ends of the straw). It will not evacuate all the powder though because there is low pressure at either end of the tube. No air flowing through it the way air flows through the radial vents.
So now we realize that the crossdrilled holes don't really do much for airflow and they're no longer really needed for outgassing, but the possibility of crossdrilled brakes actually being WORSE than plain or slotted? Huh?
Well, it comes back to that earlier bit about mass and thermal capacity. Now this comes into play when the brakes are really stressed hard (on roadcourses, or spirited driving, or even coming down a long descent). As mentioned in my previous post, all brake pads have a MOT (maximum operating temperature). When you exceed the MOT, the pad surface/material actually begins to liquefy/melt. It's actually the binders holding the composite material together that's giving out. It's obvious that you want to keep the brake rotor temps below the pad's MOT. You can always get pads with a higher MOT if you anticipate higher brake temps (like race pads with MOT's to 2000+f), but there's a limit. Pads actually have a temp range. Pads with a higher MOT also have a raised Minimum OT. They don't brake well until the brakes have heated up to the Min OT (which is why race pads are a bad idea for street use).
So now you have street pads with a limited MOT (some are still better than others). Seemed like we digressed a little from rotors didn't we?
Well here's the tie-in. Whenever you use the brakes, the brake system is merely an energy conversion system. It converts kinetic energy (the car's motion) into thermal energy. It then must dissipate this thermal energy. Every braking system has a rate of thermal dissipation though and if you were ever foolish enough to try touching your brake rotors after coming home from a drive, you'ld have realized that the brakes don't really dissipate heat very quickly.
In hard use, one single maximum brake application from 70 mph can get the rotors and pads to 700+f, but if you drive off again, after 1 full minute, the rotors will still be at 400+f. Now if you had to hit the brakes again, they're not starting from ambient temp anymore. You've already filled them up with some heat which they haven't yet managed to empty.
As you can see, each repeated braking event adds more heat into the system. If the rate you add heat to the system exceeds the rate at which they can dissipate the heat, you will hit the system's (the rotor's) thermal capacity.
See the rotors in the brakes are there for 2 things. One is to act as a surface for mechanical leverage for the pads to act on to retard the car's momentum, and the other important purpose is as a heatsink, to absorb all the heat generated from braking. The more kinetic energy you will be converting to thermal energy, the bigger the heatsink required, so heavier vehicles and faster ones need bigger rotors.
When you hit the rotor's thermal capacity, each subsequent braking event causes the temps to skyrocket because the rotor can no longer absorb any heat. This means that you can very easily shoot past the pad's MOT causing pad fade.
The main purpose of bigger brake kits is to allow for bigger rotors. The primary reason for this is not to increase the leverage/mechanical advantage with larger diameters, since you can increase braking torque simply by using a pad with a higher mu (coefficient of friction). The primary reason is to allow rotors with greater mass and thus greater thermal capacity. This allows not harder braking, but consistent, repeated hard braking without exceeding the system's thermal cap.
With crossdrilled rotors, you have reduced the mass of the rotors and thus reduced their thermal capacity. You will now reach thermal capacity more easily and the miniscule increase in heat dissipation afforded by the crossdrilled holes compared to plain doesn't come even close to making up for the loss of mass.
In professional racing where cast iron rotors are mandated, NO teams use crossdrilled rotors. Aside from the disadvantages of reduced thermal capacity (with no real measured advantage in thermal dissipation), crossdrilled rotors are also more prone to crack and frag. Some teams will use slotted rotors, but NONE of them use crossdrilled rotors.
So if there are no advantages, why are they still all over the place? An insider for a well known sportscar company was charged with testing crossdrilled rotors vs. plain rotors for their sportscars. The tests showed that the plain rotors had equal braking performance, better consistency and endurance, and gave better brake pad life. In addition, not needing to have crossdrilled rotors lowered costs too. So why do their cars still have crossdrilled rotors? When they put the test results to use and put plain rotors on their cars, most potential customers looking at the cars in the showroom commented more or less, "You'ld think for a top-end sportscar, they could at least have installed high-performance crossdrilled brakes". The company realized that most folks are still misguided and that they could potentially lose sales to these misperceptions. So they put crossdrilled rotors back on the cars. They just made sure they were big enough that even with the crossdrilling, they had sufficient mass for the projected application.
As for slotted rotors. Well, most race teams use either plain or slotted. What's the purpose/advantage of slotted rotors? Well if your pads still outgas, they do have an avenue for escape without floating the pads. The slots don't actually remove much material from the rotor and the slots themselves do 2 things. The edge of the slots increases pad bite which provides slightly higher brake torque and the slots sweep the pad face continually under braking which helps de-glaze the pad face. Under high heat, it's sometimes possible for the binders in the pad to secerete and glaze over the pad face which results in a (possibly severe)loss of brake torque. The slots pretty much scrape the pad continuously preventing this. They also keep the pad face even. The downside? All this scraping eats pads quicker.
Plain rotors for street driving, maximum pad longevity and potentially hard use. Slotted for a lot of projected hard use if you don't mind reduced pad life. Crossdrilled.... only for the bling. To impress the clueless and to show the rest of us that you're clueless.
Max
