Quality issues with brake rotors are one of the most frustrating to deal with in the modern independent repair shop.
To illustrate, I once found myself dealing with a brake squeal complaint on an early ’90s Ford Taurus. Although the brakes had recently been replaced, the straight-line solution had always been to resurface the rotors and install premium pads and a new hardware set. After completing the work to my surprise the brakes squealed as I backed the Taurus out of the service bay. Obviously, the only items left to replace were the “new” brake rotors!
Another quality-related rotor issue cropped up years ago when I uncharacteristically installed a pair of second-line brake rotors on a Ford Ranger four-wheel drive pickup. A few days later, the owner returned complaining that his automatic hubs wouldn’t engage while guiding a hunting expedition in the mountains.
Clearly, a seemingly minor complaint can turn into a major issue when it concerns the owner’s ability to earn his living. To make a long story short, I spent an agonizing four hours carefully disassembling, inspecting and reassembling what turned out to be a fully operational automatic hub.
But could something be wrong with the new rotor? A careful inspection revealed that the outer wheel bearing race had been machined one eighth-inch deeper into the hub than the original. Because the inner part of the automatic hub was mounted on the wheel bearing retainer nut, the automatic hub simply wouldn’t engage under some operating conditions.
While the above case studies underscore the importance of not taking brake rotor quality for granted, the most common and difficult-to-solve brake performance issue is, of course, the noise, vibration and harshness (NVH) complaint. In most cases, NVH complaints are caused by friction material and brake pad mounting issues. Auto manufacturers, for example, have introduced application-specific friction materials and pad mounting designs that optimize braking performance while hopefully minimizing NVH complaints. When brakes are replaced, the OE pads are usually replaced with generic pads that may not meet the OE standard. In other cases, NVH complaints can be traced to sloppy machining and assembly procedures. For the remainder of NVH complaints, the brake rotor itself might be at fault simply because the new replacement rotor doesn’t measure up to original equipment standards.
SALVAGE YARD SOURCING
During the 1960s, ’70s, and ’80s, brake rotors were most frequently sourced from auto salvage yards. Because early brake rotors were built with a relatively large amount of machinable stock, the average brake rotor could be resurfaced a number of times before reaching the discard minimum. In addition, the massiveness of the rotors themselves helped to dampen most of the NVH generated by asbestos pads. Consequently, after the original rotors were worn out, original-equipment salvage yard replacements could usually be found that would last the vehicle through the remainder of its service life.
During the early 1990s, the brake rotor mass had been drastically reduced to help control vehicle weight. While a light-weight rotor reduces unsprung suspension weight and the rotating weight of the wheel assembly itself, the rotor can't absorb as much heat nor can it dampen as much NVH as its more massive predecessors.
Unfortunately, salvage yard replacements have become less of an option because most light-weight rotors contain barely enough machinable stock for a single resurfacing operation. To meet the demand for replacement rotors, aftermarket brake manufacturers began offering new rotors and rotor assemblies through their regular distribution networks. In many cases, new rotors are manufactured by OE suppliers and are identical to the original equipment rotor in dimension, design and materials. In other cases, many rotors manufactured in non-OE and off-shore facilities are look-alike replacements that differ substantially from OE in materials and design. Consequently, not all new rotors are created equally nor do they perform equally.
BASIC CAUSES OF NVH
Contrary to popular belief, brake pads do not glide smoothly along the surface of the brake rotor when the brakes are applied. Instead, as force is applied to the brake pad via the hydraulic caliper, the pad tends to vibrate or chatter against the rotor.
This condition can easily be duplicated on most vehicles by placing the engine in gear and gradually releasing the brakes. As the wheel slowly turns, the pad will vibrate against the friction surface of the rotor. Because most people quickly release the brake and apply the throttle, this particular vibration noise usually isn't noticed.
In contrast, higher-frequency pad-to-rotor vibrations are usually dampened by the mounting hardware on the caliper and the brake pad itself. On the other hand, when the caliper and pad fail to dampen this vibration, an irritating high-pitched brake squeal begins to emanate from the brakes. The NVH factor can also be complicated by a very thin layer of boundary gas that forms between the brake pad and rotor. The high-pitched squeal is usually the most common NVH complaint and also the most difficult to deal with because the problem is usually caused by design factors in the caliper mountings, friction materials, rotor and pad construction, or brake rotor materials.
DESIGN FACTORS IN ROTORS
Because of their huge mass, the conventional hub-style rotors containing the wheel bearings and seals are the least prone to NVH complaints. Most NVH complaints on these designs could be traced to inferior friction materials or the lack of OE noise-dampening shims behind the brake pads.
During the 1990s, sealed-for life wheel bearings on front wheel drive cars brought composite hat-style rotors into vogue. The composite rotor consists of a stamped-steel hub with a cast-iron friction surface molded onto its outer flange. Jobbers and service dealers quickly discovered that brake lathe bits cutting into the cast iron stock caused the stamped-steel hub to vibrate like a bell and ruin the rotor. A market quickly developed for lathe attachments that would dampen these hub vibrations.
To eliminate the machining and NVH headaches of composite rotors, most replacement rotors are made of cast iron from the hub out. Occasionally, however, some manufacturing shortcuts are taken that aggravate NVH issues.
For example, the number of fins contained inside a vented rotor might be reduced, which causes the rotor to overheat during hard brake applications. In addition, reducing the number of cooling fins also reduces the structural integrity of the brake rotor itself, which leaves the rotor more prone to warping or cracking. In other cases, due to uneven internal cooling, hot spots may develop in the rotor that causes NVH to develop during normal use. Last and perhaps more rare, the quality of the cast iron used is such that the rotor isn’t frictionally compatible with the brake pads.
ROTOR SERVICING PROCEDURES
At this point, it is important to understand that any rotor will fail if correct service procedures aren’t followed.
For technicians, the most common error is failing to torque the lug nuts to their specified value in a criss-cross or star-type pattern. It may sound like a simple thing, but failing to follow this procedure creates terrific mechanical stresses within the rotor that will later develop into warpage and hot spots that cause NVH complaints. And that means brake trouble later.
In addition, modern friction materials need a very smooth, clean rotor surface to seat against. Consequently, all brake rotors must be resurfaced as smooth of a surface as possible. The torn surface metal left from machining should then be polished with a 120-grit or equivalent sanding or abrasive pad. The final step is to wash the rotors in soap and water until all loose metal particles are removed.
QUALITY VERSUS COMEBACK
The brake replacement business is an unusually competitive and price-sensitive part of the service market. Shops therefore often use second-line or white-box parts to become more price-competitive and generate more brake service volume. The downside of reducing quality is, of course, generating more NVH complaints. In too many cases, the cost of dealing with comebacks exceeds the savings generated by using second-line or white-box brake rotors. Most quality-conscious shops will, therefore, use either OE or premium aftermarket rotors to ensure that the customers will stop quietly and safely.