Like brake pads, brake rotors don't last forever. The rotors wear every time the brakes are applied. The rate at which rotors wear depends on a lot of variables: the type of brake pads on the vehicle, the metallurgy (hardness and quality) of the castings, how efficiently the rotors cool themselves, the type of driving the vehicle is subjected to, the braking style of the driver (aggressive or easy) and exposure to moisture and road salt.
Semi-metallic brake pads usually contain a very high percentage of chopped steel fiber, so they typically cause more wear on the rotors than most ceramic or nonasbestos organic (NAO) brake pads. On vehicles where rotor wear is a problem, switching to a “softer” friction material (such as ceramic or NAO) may solve the problem provided there’s a choice of friction materials available for the application.
The quality of original equipment rotors can vary from excellent to questionable. Most carmakers insist rotor quality is extremely important. After all, the rotors are part of the brake system and safety is paramount. Yet some original equipment rotors come from the factory with hard spots, inclusions, impurities and other junk in the cast iron that undermines their durability, wear resistance and performance. Good metallurgy is critical because it affects the friction qualities of the rotor as well as its strength, hardness, sound characteristics and even its corrosion resistance.
As a rule, most original equipment rotors used to be designed with enough thickness to go two or more pad replacements. But on some cars today, the rotors are thinner to save weight and cost. Consequently, the rotors may be worn down to minimum thickness specifications (which is usually marked on the casting itself) by the time the first set of brake pads need to be replaced or even sooner in some cases.
The minimum thickness specification is an important dimension because it is the minimum thickness that provides safe braking. As a rotor wears and becomes thinner, it has less mass. This reduces the rotor’s ability to absorb and dissipate heat. It also reduces the strength of the rotor, increasing the risk of cracking or even breaking (rotor failure).
That’s why the thickness of the rotors should always be measured every time the brakes are serviced. If a rotor is worn down to the minimum thickness specification, or cannot be resurfaced without exceeding the dimension, it must be replaced. In some states, this is the law.
Replacing one worn-out rotor usually means replacing both rotors on the vehicle, since they usually experience the same amount of wear. Many brake experts recommend replacing both rotors at the same time (even if one is still “good”) to maintain even braking side-to-side. A significant difference in rotor thickness may cause the brakes to pull toward one side.
Uneven Rotor Wear
Uneven wear is another problem that can ruin a rotor. As long as a rotor runs true and has minimal lateral (sideways) runout (less than .002 inches of wobble as it rotates), it should wear fairly evenly. But several things can cause a rotor to wear unevenly, resulting in thickness variations that produce an annoying pedal vibration when the brakes are applied.
Excessive runout in the rotor or hub can result from a stack up of manufacturing tolerances (or poor quality control). Runout can sometimes be reduced by reindexing the rotor position on the hub, or if that doesn’t work, by resurfacing the rotor on the vehicle with an on-car brake lathe or by installing thin tapered shims between the rotor and hub.
Rotor runout also can be corrected using a conventional bench lathe, but it requires a multi-step process. First, you have to measure and mark the point of maximum runout with the rotor on the vehicle. Then, you have to mount the rotor on the lathe arbor and attempt to duplicate the same amount of runout on the lathe. Then, you can cut the rotor true and remount it on the car in the same index position as before and hopefully the runout will be gone.
Rotor distortion and runout also can be caused by overtightening or uneven tightening of the lug nuts. If somebody zips on the lug nuts with an impact wrench, the uneven loading of the lug nuts can twist and distort the hat section of the rotor, causing the disc part of the rotor to wobble as it rotates. Using a torque wrench to final tighten the lug nuts prevents this kind of distortion.
Dirt or rust between the rotor and hub also can create runout. Cleaning the face of the hub and the inside face of the rotor hat section with a drill-powered circular brush can eliminate this cause of runout.
Hard spots in a rotor casting also can cause uneven wear. The hard spots resist wear while the surrounding softer areas experience more wear. It only takes about .001 inches of thickness variation to cause a noticeable pedal vibration. Resurfacing doesn’t get rid of the problem because the hard spots usually extend far below the surface. Shaving off the high spots may allow the rotor to run true for awhile, but sooner or later uneven wear will cause the high spots to return. The only cure is to replace the rotor.
Corrosion also can kill rotors. All rotors rust to some extent when a vehicle is parked and is not driven for a period of time (like a week or more, especially in a damp environment). Some rotors are worse than others when it comes to rusting. To reduce costs, General Motors reportedly deleted chromium (a rust-inhibiting ingredient) that has been used in some of their cast iron rotor alloys. The result has been some severe rusting problems with GM original equipment rotors. Rust is bad because it weakens rotors over time, and makes the brakes noisy when the vehicle is first driven after being parked. Rusty rotors also increase brake pad wear. The pads will usually scrape the rust off after a few stops, but until then the rust is interfering with the pads and reducing their stopping ability.
Heat management is another factor that affects both rotor wear and braking performance. When the brakes are applied, friction generated by the brake pads rubbing against the rotors creates a lot of heat. The heat is absorbed by the disc portion of the rotor, and is dissipated by the cooling fins (slotted vents) between the faces of the disc. Vented rotors are almost always used for the front brakes, while vented or non-vented rotors may be used in the back where braking loads (and heat) are usually less. The number, spacing and shape of the cooling fins also is important because some designs are more efficient than others. Replacing original equipment rotors with ones that do not cool as efficiently because of reduced fin count or fin design may cause the brakes to run hot and increase the risk of brake fade.
Aggressive driving, pulling a trailer, hauling heavy loads or mountain driving all can cause brake temperatures to soar. Over time, this will increase both pad and rotor wear, requiring the brakes to be serviced sooner than would be necessary with more normal or light use highway driving. The pads and rotors on vehicles that spend most of their time driving in stop-and-go city traffic will always wear out much sooner than vehicles which are driven mostly on the highway or in light city traffic.
When brake pads are replaced, the rotors should always be measured to see if they still have enough metal left for safe braking. As we said earlier, if the rotor is worn down to the minimum thickness specification, or cannot be resurfaced without exceeding this specification (or the minimum “machine to” spec if one is provided), the rotors must be replaced.
On the other hand, if the rotors still have plenty of metal and show no evidence of hard spots, cracking, severe grooving or rusting, and there are no complaints about pedal pulsations, the rotors can be resurfaced as needed to restore a flat, smooth friction surface.
In some cases, rotors may not have to be resurfaced when the pads are replaced. If they are relatively smooth with minimal grooving, they may not need to be cut. However, most professional brake technicians won’t take a chance on not resurfacing the rotors for fear the brakes may be noisy or not feel right until the pads are fully seated in.
Resurfacing a rotor obviously removes metal, making the rotor thinner and reducing its remaining service life. Because of this, some consumers as well as some vehicle manufacturers say rotors should not be resurfaced every time the pads are replaced (unless the rotors are badly grooved or uneven). Okay, some people are cheap and are trying to save a buck. We’ll grant them that. But do they really want to cut corners on their brakes? Resurfacing restores a flat, smooth surface that provides the proper friction characteristics, minimizes noise-producing vibrations and allows for maximum pad contact.
Installing new pads on a grooved rotor causes the pads to ride on the high spots of the rotor. Eventually, the pads will wear down and make full contact with the rotors as they seat in. But this increases pad wear and decreases overall pad life. So it could also be argued that not resurfacing the rotors is counterproductive to maximizing brake life.
The surface finish on the rotors also is important because it affects the friction characteristics of the brakes, pad seating, break-in, wear and noise. 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. If rotors are resurfaced, they should be cut to meet these specifications using sharp lathe bits and proper rotation and feed speeds (not too fast!).
Composite rotors (those with a stamped steel center hat and cast iron disc) that are being resurfaced must be supported with bell caps or adapters to prevent them from flexing while they are being cut. A vibration dampener should also be used on all types of rotors to reduce noise and tool chatter.
Just Replace Them?
As inexpensive as many aftermarket rotors are today, it’s quite affordable to simply replace the rotors rather than resurface them. Some auto parts stores offer free resurfacing to customers who purchase pads, while others charge a nominal fee for resurfacing ($10 to $20 per rotor).
New rotors should be ready to install right out of the box. There should be no need to “clean them up” with a shallow cut on a lathe. This may actually produce a worse surface finish and/or runout on the rotors if they are not resurfaced correctly. It will also reduce the thickness of the rotors, shortening their useful service life.
For customers who are more concerned about braking performance and safety than saving a few bucks, recommend premium-grade replacement rotors. Premium rotors typically use the same casting configuration as the original (same number of cooling ribs between the faces and same pattern). If replacement rotors with a different rib design or configuration are installed, it may create cooling and/or noise problems. Premium rotors also have better metallurgy, and are manufactured with tighter quality controls.
Watch out for “thin” rotors that have a wider space and thinner disc faces to reduce weight and cost. These rotors may not cool as efficiently as the original equipment or premium aftermarket rotors.
If a vehicle was originally equipped with composite rotors, the original rotors can be replaced with similar composite rotors or cast rotors. Cast rotors are less expensive and more rigid than composite rotors. But the center hat section of a solid cast rotor is thicker and changes the steering geometry (scrub radius and toe alignment) slightly. This may or many not affect steering and handling on some vehicles. For now, some OEMs (GM) still recommend replacing same with same while others (Ford) say its okay to replace composite with cast.
Finally, whether a customer is having their old rotors resurfaced or are replacing them with new rotors, don’t overlook related sales opportunities such as brake fluid, brake hoses, calipers, brake lubricant and any specialty brake service tools that may be needed to service the brakes.