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 work.
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.
A new tool called BrakeView digitally maps the surface of brake rotors to display thickness variations, rotor runout, warpage and other problems. This is not a do-it-yourselfer tool, but it's pretty cool.
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.
HOW TO REPLACE BRAKE ROTORS
If your old rotors are worn out (too thin), warped, cracked or severely rusted, they must be replaced. Bad rotors are unsafe and should not be reused. Replacement rotors are available in various price ranges. Economy rotors (usually made in China) are the least expensive, but may not delivery the same braking performance or durability as somewhat higher priced premium rotors. Premium rotors meet or exceed all OEM requirements and deliver like-new braking performance. They use a higher grade of cast iron, and some use a special vacuum degassing process to assure the best possible casting. The cooling of premium rotors castings is also carefully controlled during manufacturing to assure the best temper and uniform hardness. Most premium rotors also have the same cooling vane configuration as the OEM rotors they replace.
Rotors should be replaced in pairs (both front, both rears or all four depending on wear and condition). This helps maintain even braking side-to-side.
Wash the new rotors with warm soapy water and a brush BEFORE you install them to remove all traces of grease, oil and machining residue.
When mounting the new rotors on your vehicle, clean the wheel hub with a wire brush or circular brush and drill to remove all rust and dirt. Any debris or roughness between the rotor and hub may cause rotor misalignment, resulting in a wobble, shudder or pedal pulsation when braking.
Tighten the wheel lug nuts to specifications using a torque wrench. Tighten the nuts in a star pattern sequence, alternating crosswise so the wheel and rotor are evenly loaded. Uneven torquing of the lug nuts can distort the rotor, causing it to wear unevenly resulting in a brake pedal pulsation when braking.
WARNING: DO NOT USE OIL, GREASE, ANTI-SEIZE OR LUBRICANTS OF ANY KIND WHEN TIGHTENING LUG NUTS!
Proper torque on lug nuts is very important for three reasons. One is to keep the lug nuts from loosening up and the wheel coming loose, another is to prevent distortion of the brake rotor behind the wheel, and a third is to prevent broken studs. A torque wrench should be used for final tightening of the lug nuts, and the nuts should always be torqued to the recommended specifications.
CAUTION: Torque specifications for lug nuts are always for CLEANand DRY studs and lug nuts. That means no oil, no grease, no anti-seize and no lubricants of any kind. Any of these products will reduce the friction between the threads. This may seem like a good thing to prevent rust and frozen lug nuts, but the reduction in friction means a much higher percentage of the applied torque (up to 25% or more) will go toward loading the lug nuts. The end result may be brake rotor distortion or broken studs!
Wheel studs should be cleaned with a wire brush to remove rust and dirt BEFORE the wheels are mounted. If the lug nuts are heavily rusted or have damaged threads and won't turn easily on the studs, replace the lug nuts. The same goes for any wheel studs with damaged or badly corroded threads. And remember to mount the wheels DRY with nothing on the threads.
For more information about the dangers of thin rotors, visit BadBrakes.net.
Affinia Group has filed a petition requesting the National Highway Traffic Safety Administration (NHTSA) to begin rulemaking toward adoption of a first-ever Federal Motor Vehicle Safety Standard for brake rotors.
The new Safety Standard that Affinia seeks from NHTSA would require all rotors sold in the United States to meet minimum performance standards for structural strength and crack-resistance under rigorous laboratory testing. No such mandatory standard exists in the U.S. today, although rotors are a critical component of a vehicle's most important safety feature, its brake system.
The proposed rule would also for the first time require rotors to be stamped with identifying markings, including a "DOT" (U.S. Department of Transportation) symbol representing the manufacturer's certification that the part meets the new standard.
Until recently, manufacturers of replacement brake parts consistently designed and built their products to match the design specifications and to meet or exceed the performance and durability of the Original Equipment parts that they were meant to replace. Meeting or exceeding OE specifications thus came to be understood by consumers and auto technicians alike as the de facto standard for replacement brake parts.
Unfortunately, within the last year and a half, a number of companies have been importing and distributing aftermarket brake rotors that are lighter, thinner and cheaper than their OE counterparts. Affinia says it also has identified numerous instances in which importers or distributors of lightweight rotors have falsely asserted in advertising or on their Internet sites that the rotors meet OE specifications and performance levels. The end users of these rotors ( retail do-it-yourself customers and technicians who work in vehicle repair shops) have no way of knowing that such assertions are not accurate. Thus, they often will select a replacement rotor based on price, on the assumption that all replacement rotors provide an adequate level of performance, durability and safety.
Affinia says it believes that the only appropriate remedy for this significant public-safety risk is a federal standard that all rotors must meet. Over the years, the automotive industries of North America and Europe have developed separate, but roughly comparable, laboratory testing procedures and criteria for rating the strength and crack-resistance of brake rotors. Affinia would in general prefer adoption of the stricter European criteria, but believes that even adoption of the domestic standards would be welcome because, as the company said in its petition to NHTSA, they "would assure that the worst offenders among lightweight rotors could no longer be sold in this country."
After years of wrangling, the Society of Automotive Engineers (SAE) has developed a brake rotor test standard called J2928 for testing a rotor's ability to withstand cracking caused by repeated thermal cycling. The test requires a rotor to withstand 150 heat cycles without developing dangerous cracks that could result in the structural failure of the rotor on a vehicle.
The new J2928 brake rotor test procedure is a voluntary standard that any brake manufacturer can use to test their rotors. Successfully passing the test means the rotor can safely handle the kind of heat stress commonly encountered when driving without cracking or undergoing undesirable dimensional or structural changes that might result in rotor failure.
To pass this test, rotors must be made of high quality cast iron with consistent metallurgy. Low quality rotors that are made by some offshore manufacturers do not have good metallurgy and may crack or fail when subjected to too much heat. The purpose of the J2928 test is to separate the sheep from the goats, and to help consumers identify quality rotors that can meet the new test standard.
Look for wording that says "Meets J2928 test standards" on the rotor box or packaging as an indication that you are buying a quality rotor that will provide years of safe driving.
For more information on this subject, see SAE J2928, a Hot Topic.