Brake rotors are an often replaced item when the brakes are relined because they bear the brunt of the friction created by the brake pads. Every time the brakes are applied, the pads rub against the rotors and create friction. Friction creates heat and wears both the brake pads and rotors. Eventually both need to be replaced.
BRAKE ROTOR WEAR & ROTOR COOLING
Rotor wear is usually much less than pad wear because the rotors are harder. Rotors are made of cast iron for three reasons: it is relatively hard and resists wear, it is cheaper than steel or aluminum, and it absorbs and dissipates heat well to cool the brakes.
The amount of heat that is created at the rotors depends on the speed and weight of the vehicle, and how hard the brakes are applied. A normal stop from 60 mph can easily raise the temperature of the front rotors 150 to 250 degrees. Several hard stops in quick succession can send rotor temperatures soaring into the 600, 700 or even 800 degree range!
If rotor temperatures keep going up because the driver is riding the brakes (as when traveling down a steep mountain) or is driving aggressively, the brakes may get so hot they start to fade. Once this occurs, it takes more and more pedal effort to slow the vehicle. Eventually the point may be reached where the brakes can't generate enough friction no matter how hard the driver stands on the pedal.
Large heavy vehicles like fullsize SUVs and trucks obviously create more braking heat than small passenger cars. Consequently, the rotors on trucks are larger than those on cars. The bigger the rotors, the more heat they can handle. That's why race cars and performance cars typically have oversized rotors -- so they can stop quickly without frying the brakes.
Many rotors have ribbed cooling fins between the rotor faces to help pull air through the rotor for better cooling. These are called "vented" rotors, and they are usually found on the front brakes. Some rotors are "solid" or "unvented" and have no internal cooling fins. These are used mostly for light duty applications or the rear brakes on cars with four wheel disc brakes.
BRAKE ROTOR DESIGN
Rotors also differ in the design of their cooling ribs. Vehicle manufacturers currently use over 70 different cooling rib configurations in their rotors. Some ribs are straight, some are curved and some are even segmented. Some ribs are evenly spaced while others are not. Most ribs radiate outward from the center but others zigzag like a maze. Different cooling rib configurations are used to "optimize" brake cooling on specific vehicle applications, and to reduce harmonics that contribute to brake squeal.
Replacement rotors may or may not use the same rib configuration as the original equipment rotors. A rotor supplier may consolidate rib designs to reduce the number of SKUs needed to cover the market, but this may mean compromises in cooling and noise performance on some applications.
Most economy rotors use a "standard" cooling fin configuration, while most "premium" rotors use the same rib configuration as the OEM rotor.
BRAKE ROTOR MATERIALS
Another factor that affects rotor performance is the metallurgy of the rotor itself. The metallurgical properties of the iron determine the rotor's strength, noise, wear and braking characteristics. The casting process must be carefully controlled to produce a high quality rotor. The rate at which the iron cools in the mold is critical and must be closely monitored to achieve the correct tensile strength, hardness and microstructure.
If the casting process is not carefully controlled, the iron may not form the proper microstructure resulting in a noisy rotor or one that lacks proper hardness. A rotor that is too hard may crack while one that is too soft may wear prematurely. Again, economy rotors may not be made to the same level of quality as premium rotors.
COMPOSITE BRAKE ROTORS
Some rotors have a "composite" design instead of being a one-piece casting. A stamped steel center hat is combined with a cast iron ring to reduce the weight of the rotor. Composite rotors are less rigid than solid rotors and must be supported with adapters or large bell caps when they are resurfaced to prevent chattering and flexing.
If a composite rotor has to be replaced, it should usually be replaced with the same type of rotor as the original. Cast rotors are available as a lower cost alternative for many vehicles that were originally equipped with composite rotors, and some technicians believe cast rotors cause fewer problems. But the hat section of a cast rotor is thicker than the original composite rotor and changes wheel offset slightly. This may have an adverse effect on steering geometry (scrub radius) and wheel alignment.
BRAKE ROTOR SURFACE FINISH
The surface finish on the rotors is also important because it affects the friction characteristics of the brakes, pad seating, break-in, wear and noise. As a rule, most new OEM rotors today have a surface finish between 30 and 60 inches RA (roughness average), with many falling in the 40 to 50 RA range. Some OEM specifications say that anything less than 80 RA is acceptable -- but smoother is always better. Premium rotors typically meet all OEM specifications but some economy rotors may have a rougher finish.
NOTE: New rotors should NOT be resurfaced prior to installation. The rotors should be ready to install right out of the box and require no additional
WHEN BRAKE ROTORS MUST BE REPLACED
Rotors must be replaced if they are worn down to minimum specifications or the discard thickness, or cannot be resurfaced without exceeding the minimum "machine to" specification.
Rotor thickness should always be measured with a micrometer to accurately determine thickness.
Rotors should also be replaced if they have hard spots. Hard spots usually return even after the rotor has been resurfaced.
BRAKE ROTOR RUNOUT
How much brake rotor runout is too much? It depends on the vehicle application. Some vehicles are much more sensitive to rotor runout than others. Generally speaking, the lighter the vehicle and the lighter the suspension, the more sensitive it is to rotor runout. The latest OEM service specifications typically say rotor runout should be no more than .002 to .003 inch. On some vehicles such as 1997-2002 Chevrolet Malibu, 1997-1999 Olds Cutlass, 1999-2002 Olds Alero and 1999-2002 Pontiac Grand Am models, GM allows no more than .0015 inch of rotor runout.
Rotor runout can be measured by placing a dial indicator against the face of the rotor and turning the rotor. If runout exceeds specifications, corrective measures are needed.
Runout can sometimes be reduced by simply repositioning (reindexing) the rotor's mounting position on the hub. If rotating the rotor one or two lug positions either way fails to reduce runout, it may be necessary to (1) shim the rotor with a Brake Align tapered shim designed for this purpose, or (2) replace the rotor and/or hub, or (3) resurface the rotor on the vehicle with an on-car brake lathe. An on-car lathe can minimize runout by cutting rotors parallel to the direction they rotate. GM, Ford and several other vehicle manufacturers recommend on-car rotor resurfacing for fixing runout problems.
