Modern production cars are equipped with solid blank rotors.Performance brake rotors on the other hand are fashioned as slotted,cross drilled,dimpled or a combination thereof.When the brake pad is pressed against the rotor to slow the car down , the energy of the moving rotor is converted into heat that needs to be dissipated.Also,when the brake pads reach their operating temperatures or higher they start to gas or breakdown,the thermal energy conversion between the brake pads or and the rotors produces a gas discharge that normally gets caught between the brake pad and rotor surface,contributing to brake fade.
The slotted rotors are designed to allow the gas to be channeled into the body of the rotor and out of the centre vent,effectively increasing your clamping power when braking aggressively.
The cross drilled .dimpled,slotted design has nothing to do with heat dissipation as many mistakenly believe.However,it can be argued that there is a degree of heat dissipation but only as a byproduct of its design.The only way to transfer heat is by using a larger rotor whether in diameter or thickness.A larger rotor is dependent on your rim size to fit.As with most big brake upgrades,the use of a larger wheel is required to provide adequate clearance.However, the use of a rotor that is too large will limit its temperature to the extreme lower end of this range. Not only will the rotor temperature not reach the optimum range,it will also be heavier than is necessary.
The mass of the brake system is also unsprung mass.Reducing the unsprung mass has the additional benefit of improved suspension performance,resulting in enhanced ride,handling and acceleration.The added weight disadvantage of larger rotors may be circumvented by a two piece construction where a light centre aluminium hub makes available most of the unusable while holding only the business end (less weight) of the rotor.Performance rotors are mostly larger and or thicker and made with high carbon cast iron materials to avoid potential noise problems with improved thermal conductivity.Some performance rotors are heat treated and finished durable anti corrosive plating (gold zinc or black coating)to prevent corrosion and harmful rust from forming inside the slots.
Moreover,the edges of the slots or holes continuously clean and refresh the pad surface as well as provide increased brake bite.The slots and or holes also lighten the rotor,thereby decreasing its rotational inertia.Slots further expel dirt,dust and prevent ridges development on the rotor surface.Dimpled rotors degas pads and prevent cracking.Both slots and dimpled rotors maintain approximately 96 percent of the friction surface.Cross drilled rotors maintain about 85-93 percent and cross drilled and slotted only maintain 80-93 percent of the friction surface.Generally ,the larger the friction surface,the better the heat transfer and the stronger the component's tensile strength.Slotting is better as it removes less than four percent of the rotor's surface area and reduces the chance of cracking.
If rotor blanks are drilled,they can potentially weaken the integrity of the rotor and if drilling hits a vein it may crack.Drilling can be effective if a rotor is made of a material meant for the application.For example,the Ferrari Enzo and several Porsches and supercars are equipped with cross drilled only rotors.However,these rotors are firstly an oversized design to reduce the risk of cracking and secondly they are constructed from very expensive strong ceramic composites,unlike the cast ones in regular cars.There are also unique materials like aluminium and titanium specifically made for race cars. The advantages of a hybrid drilled and slotted rotors are the same,increased brake bite and a continuous refresh of the brake pad surface.Standard cross drilled rotors have the additional advantage of being lighter but without the ducting running along rotors they are hardly more effective than your standard blanks in dissipating gassed.Cross drilled rotors decrease the co efficient of friction and are prone to thermal shock and cracking.Thermal shock is different from bluing which is a result of thermal stress on cast iron rotors but is normal for brakes that are exposed to hard braking.It will however lead to shorter rotor life.
A larger diameter and thicker rotor will disperse heat more evenly throughout the rotor ,thereby reducing the chance for heat soak.The larger rotors also mean more surface area to be scrubbed,and this translates into less brake fade.Rotors are usually designed to last about three times longer than the brake pads.It is important to check the rotor thickness after every pad changes and replace them if necessary.Rotors,just like brake pads,must be changed in pairs.
A common problem with rotors is warping,the rotor surface becomes uneven.Warping may be caused by misaligned calipers and rotors or worn suspension such as bushes that cause the brake components not to sit properly.You know you have a warped rotor when the steering wobbles each time you hit the brakes.Cracks are another problem with rotors.Perhaps the most common problem is when the brake pad material is unevenly deposited on the rotor friction surface,sometimes visible and popularly known as cementite.In this instance,the rotor can be skimmed to make it usable again.
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