You're up to highway speed and suddenly a car cuts across your lane to make an exit. You manage to slow enough to avoid a nasty accident, but the brakes definitely don't feel right. The brake pedal has had this kind of vibration for a couple of weeks, but only at low speeds. Now it feels like you're stepping on a running chain saw, and the front wheels don't want to go where you want them to.
Forget The Guesswork
Pull the wheels for a closeup look. With the wheels off, you can inspect the rotor-lining contact surfaces. First look for a crack that goes all the way through a vented rotor—or at least through one of its two surfaces. A cracked rotor causes severe pulsation, and the only cure is replacement.
Rust at the rotor edges is meaningless, as are moderate scores in the disc's surface, or a few off-color spots on the shiny areas swept by the pads. Be sure to check both sides of the rotor, however. When you pull the wheels, there should be an accessible area on the back side (even if there's a dust shield) where you can inspect the surface. Severe rusting of the lining contact surfaces is a problem. So are thick, deep scores, uneven thickness and lateral runout (warping).
* For background information on how disc brakes work, click here.
If the rotors have never been resurfaced, it is theoretically possible to have them machined to remove significant rust and deep scores (about .060 in. deep). However, many late-model rotors are not very thick to start with, and can't take more than a .030-in. resurfacing. Unless the rotor is at least .060 in. thicker than the minimum thickness stamped somewhere on the rotor, forget trying to save it. Buy new rotors. How can you test the depth of a score? If it's thick enough, insert the head of a dime into it. If the dime goes in beyond the top of the president's head, the score is too deep. If a score is too thin for the dime to slip in, it's harmless. Another test is to run your fingernail across the surface radially. If your nail catches, the rotor needs to be resurfaced. If not, you're probably okay, provided there's enough thickness left on a true-running rotor.
Uniform thickness is critical. Measure each rotor with an outside micrometer at a number of evenly spaced points around the lining contact surface. A variation of .005 in. or more is enough to cause the pulsation you feel.
Lateral runout is caused by a rotor that's uniform in thickness, but is wavy. Uneven thickness and excessive runout often go together. That is, the runout causes the uneven wear on the rotor. So if you catch it early, the rotor may pass a thickness check, but it still could have excessive lateral runout. The high heat (then rapid cooling) from hard braking is a primary cause of runout, but close behind is uneven tightening of wheel lug nuts. Dirt or rust buildup between the rotor and hub also can produce runout.
To check for excessive runout/warping, you need a dial indicator, which you can rent from many auto parts stores. When you pull the wheels, the rotors might be loose on the studs, so refit the lug nuts and tighten them securely. Install the dial indicator, and mount it on the suspension. Then, aim the indicator plunger at the middle of the lining contact surface, at as close to a 90° angle as possible. Set the plunger so it presses lightly against the rotor-lining contact surface, then zero out the dial. Slowly rotate the rotor and watch the dial. If it exceeds .005 in., the rotor is not running true.
You may not have to replace or resurface the rotor. With the rotor off, you can look for and remove dirt or rust buildup. If that's not the problem, you can try re-indexing the rotor. That is, just remove it, rotate it one or two stud holes, install and tighten the lug nuts and recheck. Although the typical rotor just slides off the studs with the wheel removed (some advance spraying with penetrating solvent may be necessary), the caliper has to come off first.
First, remove the bolts that hold the caliper itself to its anchor frame or to the suspension. Then pry the caliper up, lift it off the frame and wire it to the suspension. This will relieve any strain on the brake hose. Many vehicles have "flip up" calipers. Remove one lower retaining bolt, flip up the caliper and slide it off a stud. If the brake shoes stay in the anchor frame, perhaps held by a retaining clip, remove those next.
Finally, unbolt the anchor frame, lift it out and the rotor should slide off. But first, if the rotor is not necessarily being replaced, make alignment marks on the hub section and the stud, so you can reinstall it in the same location. If it binds on the hub, that's normal rust buildup. Just spray the studs and holes and hub/rotor joints with penetrating solvent, rock the rotor with a pulling motion and it should walk off. Clean rust off the hub with ultrafine sandpaper or a wire brush. Clean the mating face of the rotor as well.
If you're working on rear discs, they're likely to have a mechanical actuator for a parking-brake function or a drum-type parking brake built into the hub. With the drum type, the rotor itself will have a deep "hat" section to fit the hub, and the inside of that hat is the drum for the parking brake. Normally, with the parking brake off, the rotor just slides off, but if the parking brake has a self-adjusting linkage, it's possible that it's overadjusted and the drum shoe linings are locked against the inside of the hat. In that case, you can reach in from the back through an access hole (in the hub and also possibly through a dust shield), and back off the adjuster. With the type that uses an actuator on the disc brake lining pads, you may be able to just lift the caliper off. At worst, you'll first have to disconnect the brake cable from the actuator on the caliper.
Surface grooves deep enough to catch your fingernail will need to be removed by machining the brake on a lathe.
A dial indicator will show you the total lateral runout—.005 in. is too much.
If you need a new rotor, install it straight out of the box. Some of the lightweight rotors, made of a stamped steel hub with a cast-iron braking surface, could test as warped unless you check them with all lug nuts installed and tightened.
Note: There are some rotors that are a real bear to remove, so if the vehicle manufacturer indicates special tools or if a press is necessary, a rotor can be machined on the vehicle. You're best off leaving this to a properly equipped pro.
Both Sides, Please
Normally, what happens to one wheel happens to the other. If one rotor is gone and the other is marginal for machining, replace both. However, if one is particularly bad, perhaps the vehicle has been pulling, leaving the other side pristine. There may be uneven brake application, perhaps from a hydraulic or mechanical problem at the wheel. Trace the cause in addition to correcting the rotor problem.
Clean lug nuts and stud threads with a mild solvent and wipe dry. Don't lubricate them, as today's torque specifications are based on clean, dry threads. One of the most important tools you can use is a torque wrench. When you're re-indexing a rotor, be sure to reinstall the lug nuts and torque them to specifications in at least three stages. Use a crisscross pattern for each stage until all are tight.
Removing disc-type rear brakes may require backing off the parking brake adjusters first to relieve the tension of the parking brake shoes.
HOW IT WORKS: BRAKE ROTOR
The brake rotor, the disc in disc brakes, mounts on the hub with the wheel, and both rotor and hub are held by the same studs and lug nuts. When you apply the brakes, the hydraulic device called the caliper clamps the brake shoes (also called pads) against the rotor, and the friction material on the shoes stops the spinning of the rotor (and therefore the wheel). The friction material on the shoes causes the energy of wheel and rotor motion to be converted to heat, and the rotor temperature can soar to more than 1000Â° F in a hard stop. That heat has to be dissipated very quickly so that the rotor can cool down for the next stop. Because front brakes do about 75 percent of the work in a front-drive car (over 50 percent in a rear-drive), the front rotors have to be able to dissipate a lot more heat than the rear brakes, drum or disc. As a result, the front rotors on virtually all cars today have two contact layers separated by a vent area, which resembles a paddle fan, to throw out hot air. This design not only gives each brake shoe a separate disc, but the airflow through the center provides greater cooling. Some high-performance cars also have vented rear discs.