The main piece of braking equipment on a vehicle is the rotor. With the wheel bolts protruding through the rotor, the stopping power of the brake system is transferred immediately to the wheel.
The rotor is a piece of cast iron, carefully machined to exacting specifications and hardness to meet the requirements of today's vehicles. Once they were available only as solid discs, now they can be purchased as aftermarket parts with holes drilled through. Opinions remain mixed as to the superiority of the two types.
Since the invention of disc brakes, solid rotors have been the mainstay of braking technology. Parts manufacturers have improved the metallurgical composition of the cast iron, to increase the hardness, and resist warping due to heat build up. The term solid is not entirely correct when describing brake rotors. The rotor is two discs cast together, and separated by equally-spaced slots and solids.
When the brake system stops the car, heat is generated through the friction of the pads on the rotors. Air is drawn through the back of the rotor, and dissipates the heat through the open slots. This action keeps the rotors, calipers, and pads from overheating, and the resultant reduced braking ability. When rapid overheating and rapid cooling take place, the rotor can be subject to thermal shock, which can result in cracking of the rotor.
Cross Drilled Rotors
Auto racers used to try anything to win, so drilling holes in the cast iron rotors lightened the vehicle. Road vehicles aren't going to gain by the reduction in weight, but they will experience better cooling of the brake components, and faster heat dissipation with cross drilled rotors.
Some brake pads, particularly the cheaper ones, give off a gas as they heat up during braking. This gas is a by-product of the bonding agents used in the pad construction, and excessive heat build-up. The gas creates a film between the rotor and the pad that can cause brake fade, a dangerous condition where the pads don't grip the rotors as they should, and the vehicle won't slow down quickly enough. The drilled holes help that gas to dissipate, reducing the possibility of brake fade.
There is no evidence to confirm that cross drilled rotors will crack more frequently than solid rotors. In fact, in a high-quality, performance rotor, the holes are specifically chamfered to relieve the stresses that cause the cracking.
Like other auto parts, there are good and bad models. In cross drilled rotors, the buyer is advised to stay away from those that have too many holes. While stopping distance is improved because the pads stay cooler and have a better grip, there is only so much surface area that can removed before the safety and structural integrity of the rotor has been compromised.
Comparing Rotor Types
A good set of cross drilled rotors will cost about the same as a good set of solid rotors. Cheap versions can be found in both types, so it's important to consider the comparative quality.
The most important factor to consider when looking for rotors, is the stopping power for the vehicle. If, for example, a solid rotor will bring the car to a stop in 10 metres, and the cross drilled rotor improves braking performance by 30%, that would translate to a distance of 3 metres. That could mean avoiding a collision with the vehicle ahead.
It boils down to individual beliefs. Both sides have statistics and test results to prove their superiority over the other. It's up to the buyer to do the leg work, ask opinions of those they trust, and buy based on need, not price.