Composite brake rotors have been around since the early 1980s. When they were introduced, everyone was worried about soaring fuel prices (just like today), and auto makers were scrambling for ways to take weight out of their vehicles so they could meet government Corporate Average Fuel Economy (CAFE) standards. So one of the changes that was made was to replace heavy cast iron rotors with lightweight composite brake rotors.
COMPOSITE ROTORS SAVE WEIGHT
This type of rotor derived its name from the fact that it combined a stamped steel center hat with a cast iron rotor. Thus, it was a composite of two different materials. The new design proved to be about 20 percent lighter than a conventional one-piece cast rotor and saved up to a couple of pounds per rotor.
The composite design also allowed the rotor disc on some applications to be cast out of a special "dampened" gray cast iron for added noise suppression (dampened cost iron is not structurally suited for use in a one-piece cast rotor).
Some of these rotors also featured redesigned cooling fins for better heat management. Some were also directional for use on either the left or right side of the vehicle. Directional rotors are not interchangeable side-to-side because reversing the direction of rotation changes the cooling characteristics of the rotor.
COMPOSITE BRAKE ROTOR APPLICATIONS
Some of the earliest applications for composite rotors were the 1982 Lincoln Continental, 1984 Ford Mustang SVO, 1987 Ford Thunderbird Turbo Coupe, and 1988 Ford Taurus and General Motors front-wheel drive "W" body cars (Buick Regal, Oldsmobile Cutlass Supreme and Pontiac Grand Prix). Since then, the number of vehicle applications has continued to grow.
BRAKE SERVICE PRECAUTIONS
As the vehicles with composite rotors accumulated mileage and came out of warranty, the aftermarket discovered that composite rotors required some special service procedures. Because the stamped steel center hat is not as thick nor or rigid as that in a one-piece casting, the center hat on a composite rotor must be fully supported with special adapters or oversized bell caps when the rotor is resurfaced on a brake lathe. The other alternative is to bolt the rotor to the hub (with the lug nuts reversed to provide better support and to prevent deforming the center hat) and to then resurface the rotor with an on-car lathe.
If a composite rotor is not supported properly, it can wobble and flex creating runout and surface finish problems. Both sides of the center hat must also be clean and rust-free for the rotor to turn true.
Some of the early composite rotors experienced corrosion problems and were recalled for replacement. Ford switched some of its models back to conventional one-piece cast rotors for awhile, and issued a service bulletin (#91-8-9) saying it was okay to replace composite rotors on the 1986 to 91 Taurus and 1988-91 Continental with one-piece cast rotors (P/N F10Y-1125-8). The corrosion problem is mostly history now because the OEMs now apply a protective coating to the stamped steel center of the rotor to inhibit rust. Aftermarket composite rotors typically use stainless steel for the center section to prevent rust, and the circumference of the center section is also dovetailed (notched) for increased strength where it mates with the rotor.
Vibration problems were also blamed on the design of the composite rotor. But the underlaying cause more often than not turned out to be improperly torqued lug nuts. Any rotor can warp if the loading on the lug nuts is not even. That is why an impact wrench should never be used to tighten lug nuts, unless it is used with a torque-limiting socket. Otherwise, a properly calibrated torque wrench should be used to tighten the lug nuts in a star or cross-pattern sequence.
When worn or damaged composite rotors need to be replaced, be careful not to intermix rotor types side-to-side. Rotors should always be the same type on both sides. Replacing a composite rotor on only one side with a cast rotor may create a brake pull. So do not replace a composite rotor on one side of a vehicle with a cast rotor unless the rotors on both sides are being replaced.
Cast replacement rotors for vehicles that were originally equipped with composite rotors are available from various aftermarket suppliers. But other aftermarket suppliers recommend against substituting one type of rotor for another because the cross-section of the center hat on a cast rotor is significantly thicker than the stamped steel center hat on a composite rotor.
The difference may range from 1/8 to 1/4 inch or more depending on the application. This may not sound like much, but it does reposition the wheel slightly further out on the hub. This adds positive steering offset and alters the scrub radius of the steering geometry. The amount of change is not great, but neither is the amount of scrub radius that is designed into many vehicles. Scrub radius affects steering feel, steering effort and steering feedback. It also plays a role in the way braking and engine torque affect steering.
The change created by replacing a composite rotor with a one-piece cast rotor may be enough to alter the scrub radius from negative (which is the case on many front-wheel drive cars) to zero or positive offset. This, in turn, may create a noticeable difference in the way the steering feels and reacts, especially on cars with rack & pinion steering that are especially sensitive to steering feedback.
There is also a concern that substituting a thicker cast rotor reduces the overall length of the lug studs, which reduces the number of threads available for the lug nuts to retain the wheels (especially on thicker alloy wheels).
Suppliers who subscribe to the "replace same with same" philosophy say you are apt to encounter far fewer problems when you install replacement parts that are the same design and function as the original. Those who do not subscribe to this philosophy say there is often room for improvement over the OEM design.
SAE Develops New J2928 Rotor Test Procedure
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.