The distance from the center of the rotor to its edge (radius) can be thought of as a lever and the caliper in effect pulls on the lever to slow the vehicle. The bigger the rotor, the longer the lever. The longer the lever the more effective the caliper. It is easy to understand then that larger rotors increase braking capacity. If the rotor is also well ventilated, its ability to dissipate heat is improved, allowing the caliper pads to operate at a lower temperature and endure the torture to which they are subjected.
Different pad compounds offer different friction coefficients and the ability to work well at certain temperature ranges. Most often a compromise is reached between friction coefficient and the ability of the compound to operate at the expected temperatures, as a result, a variety of compounds are available to work in different heat ranges depending on the intended use of the vehicle. The higher the friction coefficient of the pad compound, the more grip it has on the rotor, making the brakes more effective.
If we increase the area of the caliper pistons by increasing their diameter, the caliper will apply more clamping force to the pads and rotor, further increasing the braking capacity of the vehicle. Because the hydraulic pressure is constant throughout the system when the brake is applied, the more caliper pistons (or more correctly, the greater the total piston area) we add, the more pressure is exerted by the caliper. More pressure or clamping power is more effective in preventing rotation of the rotor.
Rotors and Heat
In the event that the brake system is used to full capacity even with all of the improvements and the rotor pads are overheated, they will give off gasses as a result of the bonding agents in the pad lining beginning to burn. The expanding gasses from the pad ("out gassing") form a cushion between the pad and the rotor and braking effectiveness decreases rapidly. This condition is one of the causes of "brake fade".
When we drill holes through the rotor from one surface to the other and cut slots into the surfaces of the rotor, we provide channels through which the expanding gasses can escape, increasing the brake efficiency a step further. (For a more detailed discussion on the merits of drilling and slotting see “To Drill or not To Drill”). The rotor must have the necessary mass to absorb the heat generated by applications of the brakes, in effect it is a "heat sink". After the initial temperature "spike" from a hard brake application, the rotor must be able to cool or dissipate the heat ready for the next application. Many methods of cooling have been tried, even water cooling, but most often rotors are air cooled. Increasing the surface area of a given mass allows more contact with the cooling air and so speeds the dissipation of heat. The most common cooling design is referred to as a ventilated rotor and most often is a sandwich of the two pad friction surfaces separated by fins. Separating the friction surfaces and adding cooling fins increases the overall cooling area of the rotor. This allows air to circulate between the hot friction surfaces and around the cooling fins, sucking away the heat. There are designs with more and less fins, staggered fin placement, posts instead of fins, curved fins etc. etc. The most common high performance designs have curved fins or vanes. This design makes the rotor into an impeller pump which actually forces air through itself for super cooling. Cross drilling of the rotor increases ventilation and can be a factor in cooling efficiency as well as dealing with pad out gassing.
A consideration sometimes overlooked is the overall rigidity of the various components of the brake system. If a component flexes or yields under the pressure to which it is subjected, it limits the maximum pressure in the system. Rubber brake flex hoses are known to expand under pressure, some calipers flex under pressure, a poorly designed brake pedal can flex, causing an overall drop in system efficiency. Flexing of components creates a mushy, not solid, feel to the pedal, usually requiring greater pedal travel to achieve a given force at the calipers. Removing flex from the system can make a marked improvement in brake function. The first choice for improvement is typically the flex hoses as most other system flex problems are more difficult to solve for the average enthusiast.
All the brake improvements in the world are not going to help if there is no traction available between the wheel and road. Ultimately, your tires will determine how well your car stops. A brake system of given efficiency will be most effective with the smallest possible diameter tire. In the same way that the larger rotor offers the caliper a longer lever, the smallest diameter tire offers the vehicle a smaller lever. While different (smaller) wheel and tire diameters are frequently an option for racers, most street vehicles are limited to near standard tire diameters for a variety of reasons. Fortunately the trend in recent years toward larger diameter rims and low profile tires has opened up many options in the area of big brakes. In many cases, increasing rotor diameter will require an increase in rim diameter to make room for the rotor. Fortunately, low profile tires keep the outside diameter the same or near the same as the original equipment so increasing the rim diameter usually has no ill effect on braking. Conversely, modern low profile tires are typically of a higher traction rating (or at least high traction ratings are available) which will aid braking.
Most often, vehicle manufacturers provide adequate, not excellent brakes on their production vehicles. It is safe to say we don't get what we don't pay for. More expensive vehicles tend to have better brake systems than the less expensive ones for obvious reasons, though this is not always true. What is true is that almost all of them can be improved and that's our business.
High performance brake pads
Probably the least expensive, single most effective upgrade one can perform is a brake pad upgrade. This is an area where one definitely does NOT fit all. There are as many pad compounds as manufacturers of pads and each has its particular niche. For the sake of simplicity, we have broken pads in three basic categories; High Performance Street, High Performance Street/Race combination and Race Only. Street pads have to be able to work well at low to moderate temperatures because street vehicles are driven cold and under normal circumstances don't generate high temperatures. Usually in this category, the pads work well cold and their effectiveness decreases as their temperatures increase until breakdown of the compound bonding agents cause brake fade. Repeated hard brake applications as in most types of racing will quickly overheat these pads making them useless.
Combination pads usually incorporate some degree of compromise to incorporate this flexibility of use. For the street they have to work well enough at low temperatures to be safe and must also be competitively functional at moderately high temperatures under racing conditions. These pads usually increase in effectiveness as they heat up through their upper operating range then fade with overheating. They usually don't work as well cold as a street pad and they don't work as well in the upper temperatures as would a racing pad, the trade off for an "all around" pad. There are many circumstances where these are the best pad for the application.
Many race-only pads do not work cold. There are notable exceptions such as some of the Wilwood race compounds (e.g. 'A', 'H' and 'J' compounds and many examples by other manufacturers). However, these pads are incredibly abrasive when used cold. If used on the street for normal street driving, they will destroy a brand new rotor in one to three months. These pads only become efficient at temperatures not typically reached during street driving. Other race pads provide very little friction until they have been thoroughly warmed-up. You will be in for a huge surprise the first time you go to stop at a stop light and sail right through the intersection as though you had just hit the gas!
Cross Drilled and Slotted rotors
These rotor surface treatments do not automatically guarantee a shorter stopping distance, but can. As mentioned, cross drilling and slotting is useful when the pads become sufficiently hot to emit gasses. Without an escape route, the gasses are trapped between the pad and rotor and actually prevent the pad applying pressure to the rotor (brake fade). For the most part and within the working heat range of the pad, slotting and drilling prevent this from occurring. The extra capacity afforded by these surface treatments can mean the difference between being able to make those last few stops driving down a mountain road or not. They offer that extra capacity for sports drivers and an extra margin of safety for everyone for a very small investment.
We always recommend slotting on the surface of drilled rotors as it has a tendency to keep the pad surface clean and free from glazing. While the irregularity of the rotor surface does not necessarily improve friction, it does help maintain a high friction coefficient as a result of the constant cleaning action of the slots as they wipe the pad, allowing more consistent (non-deteriorating) performance between pad changes.
Steel braided flex hoses
Steel braided Teflon flex hoses are popular and for good cause. These hoses are stronger and in most cases provide the driver with a more firm pedal as compared to the standard rubber type hoses which can swell under pressure. This situation can be aggravated by heat, both radiant (from the rotor) and conductive (from the brake fluid). The stainless steel and Teflon both handle heat very well and tend to maintain a constant cross section under these conditions. Braided hoses are a very worthwhile upgrade and perfectly compliment good pads and rotors.
To Drill or Not To Drill
That IS the Question... Here is a short piece written about the differences between cross-drilled/slotted and slotted only rotors.
Regarding which brakes are better stoppers......
Assuming all the other variables remain unchanged, cross drilled and slotted rotors will almost always deliver better braking figures than other types. The improved cooling effect of these rotors allows effective use of the brake system under extremely severe conditions (as might be seen in racing). The down side to this design is that when these rotors are forced to work to capacity for prolonged periods, they are subject to cracking around the drilled holes as a result of constant overheating and cooling. Until they destroy themselves these are the most efficient. For street use (long track racing), cross drilled rotors are perfect most of the time as the system is seldom used to capacity. The rotors should last a normal life span.
If the rotor is of only marginally adequate specification for the vehicle weight or use, cross drilled rotors can be problematic and require frequent "crack checking" and possible frequent replacement. Under these conditions, slotted only rotors are more desirable. Slotted rotors are less prone to cracking under severe service than drilled. What they lack in efficiency (if any) they make up for in longevity. You will seldom find a high performance short track racecar with drilled rotors, these cars are almost always fitted with slotted rotors for the reasons listed here.