Like it or not bigger is better. But we all knew that right; bigger turbos, bigger intercoolers, bigger pipes, bigger rotors. There are two main reasons why larger rotors, all else being equal, are better. The first mechanical advantage: a larger rotor has a larger lever arm than a smaller rotor. So for the same force imparted by the pad onto the rotor you get a larger resultant force at the wheel. Try and imagine a wheel spinning. Now imagine trying to stop it by grabbing the axle versus grabbing the wheel. Which do you think would be easier. That is essentially the same thing as a larger rotor.
The other reason is heat. The advantages here are many. First a larger rotor will typically have a larger mass. More mass means it takes more heat input to raise the temperature of the rotor a given amount. Obviously this will help keep rotor, pad, and fluid temps down or take longer to get them hot … you can look at it either way. Next the larger rotor, which is vented in the front for our cars, can pump more air through its vanes to further enhance cooling. Imagine two turbos, small and big, spinning at the same RPM: ignoring design differences of the vanes you can see how the larger turbo would throw out more air. The same happens with the spinning rotors.
There are often other things about upgraded rotors that help as well. Many of them will have curved vanes rather than straight vanes. The advantages here should be obvious. Some will even have cooling fins inside those vanes to further aide in heat rejection much like a heat sink on a CPU will.
Other rotor upgrades will include Aluminum hats. This is the part that has the bolt holes for your lug nuts and the rotor friction surface is bolted to that in a 2-piece design. The first advantage here is that when your rotors wear out you only need replace the friction surface. Along with reducing the amount of material that is physically being replaced you are also replacing something that is easier to make since it doesn’t need the hub portion cast with it nor drilled to match your center hub nor lugs. This translates into less versions of the same rotor being made. This is why most aftermarket rotors usually have 2 or 3 different bolt patterns drilled into them.
The other advantage to the Al hub is that it reduces heat transfer into the wheels. At first this might seem bad as the wheels can act as heat sinks for the rotors helping prevent brake fade. The problem here is that all that heat entering your wheels increases tire pressure or more specifically increases tire pressure changes from the start of a run to the end. For sustained racing this might not be too much of a problem as things will stabilize, but for something like Autox, where you get time to cool off and then have to run for 30-60 seconds, the tire pressure change can really throw things off.
As a side note: F1 cars really have trouble keeping heat out of the wheels because the brakes get so hot (glowing red) that they sometimes will overheat the wheel allowing it to deform and/or even damage the tire itself. In my old job working for an Air Force lab trying to get government and industry to work together that was one of the things the race car designers asked us for all the time: better insulators and heat resistant materials.
The next issue for rotors is slotting and cross-drilling. There is a lot of hype out there and a lot of "brake religion." Many people swear by these two methods of "improving" a brake rotors ability to slow you down. They both relate to heat and the gasses generated by that heat. The theory is there but I have yet to really see a good test. No one has really shown me back to back evidence of the same rotor with and without these two modifications, or both, performing any differently. However I can attest to the fact that with slotting I saw evidence that "something" was happening on a friends car. It was obvious that brake pad dust was being thrown out and was hitting the wheels directly opposite the slots. What is the meaning of that I have no idea … but at least it shows that "something" is happening.
The theory behind slotting and cross-drilling is that the voids in the rotor surface allow the hot gasses and dust created to escape resulting in better contact between the rotor and the pad. The problem with cross-drilling is it significantly reduces the mass of the rotor. If you remember more mass results in more heat needed to raise the temperature of the rotor … less mass obviously means it will get hotter faster. Proponents of cross-drilling say that by allowing the hot gasses to escape faster as well as the larger surface area created by the drilling over come the reduced mass and goes even further to over all heat rejection. This may or may not be true but there is also another problem. Cross-drilled rotors have a tendency to crack. The drilled holes act as stress concentrations. Some manufacturers claim that they use larger holes and that helps prevent the cranking. The idea is a good one as mechanical engineering says the tighter the radius of a hole/defect the larger the stress concentration is. But I have to question whether it is enough. Other things done to prevent cracking is chamfering the holes. The effect is again to reduce the stress concentration level at the edge of the hole by reducing the angle from 90 degrees to two 45 degree cuts. Imagine drilling a hole straight into the rotor. Now imagine using a larger drill bit and drilling into that hole again but this time only 1/8". The final war in the cross-drilling saga is rotors that have the holes cast in place rather than drilled. The idea here is that there are no residual stresses from the heat of drilling. That is fine but all it would take is a decent heat treatment after the drilling to remove any residual stresses. Sounds like a marketing ploy to me.
Slotting is at least a slightly better solution though again no one has proved to me it does anything other than make money for the companies that do the modification to "blank" rotors made by other companies. The idea with slotting is you still get the "venting" effect of the slot sweeping hot gases away from the rotor/pad interface but because the slots don’t go all the way through the rotor you have much less material removal and stress concentration areas. All very sound arguments for slotting versus cross-drilling. Doesn’t answer the question of does it really work but at least it gets rid of the reasons I think cross-drilling is bad.
For those that say, "well porches come with cross-drilled rotors so they must be worth it," think again. First off talk to any Porsche owner who really drives their car and you will find they deal with cracking problems all the time, and second don’t underestimate the need to put purdy looking things on a car that cost $70k to make it sell better. Just because cross-drilling looks hella dope doesn’t mean it works better. Just like painting your brake calipers may look good, it doesn’t help with heat rejection … notice how Porsche still paints their calipers a nice pretty yellow.
So how do you get bigger rotors? Normally there is nothing you can do since you can’t just throw larger rotors on your car; it is a system with the calipers. Well AEM has a solution. They use spacers for the stock calipers to move them out and allow you to use larger rotors which in their case also have more vanes which are curved. They also come cross-drilled and slotted. You know how I feel about that but you have little choice. On a 2g AWD you go from a 10.75" to a 12.5" diameter rotor (10" to 11.75 FWD). Not a bad increase. There are wheel fitment issues that need to be considered but the kit will work with the 97+ stock 17" wheels and it will work with some aftermarket 16" wheels; the FWD kit will fit any 16"+ wheel.
The stock calipers really aren’t that bad and the pads aren’t that small so this is really a decent upgrade for those of you that aren’t ready to step up to upgraded calipers. Read the Calipers section to find out your choices there and why I think this is a better "next step" for the money than a few other kits that exist. The AEM kit can be had for $380 for the front, and $250 for the rear. That gives you the rotor, hat, and spacer bracket for the calipers … associated bolts and washer are of course included. The install is fairly easy though in some cases it requires a little filing/grinding on the stock parts to make things fit. I had to on mine but it was easy and I have no fear at all that it compromised the parts integrity. It isn’t even noticeable from the outside and does not prevent going back to the stock setup. The only problem I might say with this kit is that rotor replacement cost is a bit steep as AEM is the only source right now and you are forced to pay for all the drilling, slotting, and zinc washing. If I had my way I would buy blank rotors … and don’t think I’m not searching :)
Finally there IS a difference in rotor material quality, don’t let anyone convince you different. Now I’m not going to say that definitely this rotor or that rotor made in this country or that country by this manufacturer or the other are all bad. But there is absolutely the possibility that the alloy composition of a given rotor can be better or worse than another as well as the heat treatment. The thing you have to remember about material specs is that they have a range, which allows for variations in the composition of the alloying elements while still giving the BASIC material properties. How tightly controlled that range is during production can result in 1) a good or bad rotor and 2) varying quality from the same manufacturer. In addition, because there is that range it is possible for a manufacturer to vary the specs within that range to selectively accentuate certain properties of that alloy; not to mention deaccentuate either inadvertently or purposefully for cost. So buying rotors manufactured by a reputable company is a good idea but don’t assume that they are all bad; do some research. Also realize that when it comes to your rear rotors, they simply don’t take that much abuse. I have been running, "Chinese knock-offs," in the rear for 2 years now to include HEAVY auto-x and road racing and have not had warping or cracking problems at all.
As a quick aside that is why there was (they are unfortunately trying/have succeeded in eliminating) Mil-Specs. Mil-Specs were designed to ensure a very strict compliance to an alloys composition, ensuring exact performance characteristics as well as constancy between smelting runs. That is one of the reasons things cost so much in the military … but if you think about it do you want to play Russian roulette with your tank’s armor platting? Or your plane’s wing spars? No you want to know damn well that the material used in your critically designed part will perform exactly how you intended. That isn’t so important on a car where they are usually over-designed in the first place to save money on the research costs involved in minimalist design, but in a commercial jet-liner or an F-16 you tend to expect things to be exactly what they need to be.
So where do you get rotors? Just about anywhere you can. Open your phone book if you want a local shop (always good for emergencies) and surf the net. Just remember that Autozone and Pep Boys are not going to be your supplier of choice. Look for an independently owned parts and performance shop and start making phone calls. Personally I have had great luck in quality and price from http://www.brakeco.com Their prices are good as is their service. They sell everything from OEM Quality rotors to "Chinese-knock-offs" as well as the pricier slotted and cross-drilled stuff. No I don’t get a kick back from them and don’t go mentioning my name to them when you email / call … other than my last name being very memorable if not also unpronounceable they will have no idea who I and my web page are :)