Rx For Rotors
The Prescription For Disc Problems Can Vary
With front-wheel drive being the norm on most vehicles built today, including the increasingly popular light trucks and sport/utility vehicles, it follows that service technicians are seeing more rotor-related problems coming into the bays.Generally speaking, disc brake problems are mainly related to improper fits and distortions of flat surfaces. Rotor problems can originate with the rotor itself, but other components and the environment of the brake rotor can also contribute to its early demise.
When there are problems, they can be the result of using composite rotors rather than full-cast rotors. A full-cast rotor is a one-piece cast-iron rotor; composite rotors consist of a stamped steel insert cast into a cast-iron braking surface. Composite rotors are lighter in weight, but they flex more than cast-iron; they also aren't able to absorb as much heat or as many vibrations as the full-cast -- two factors that can cause noise during braking.
Excess Vibration
Apart from the problems originating with the rotor being composite, a number of factors may also affect rotor performance in the brake system.Lateral run-out is the wobbly movement of the rotor from side to side during rotation. It can be caused by poor mating of flanges between the hub and rotor when they are assembled as a unit. This problem can also be caused by excessive rust and contamination preventing a clean fit of the rotor on the wheel, or even by improper torquing. If wobbling becomes excessive, the rotor will knock the pads back and can result in chatter, excessive pedal travel, a pumping or pulsating pedal and vibration during braking. In addition, this condition can be caused by a warped rotor or a loose wheel bearing.
When checking for run-out on hubbed rotors, first tighten the wheel spindle nut to a snug bearing adjustment with all end play removed. (For hubless rotors with the existence of end play, the bearing will have to be replaced.) Tighten the nut to the specified rotor checking torque. Fasten a dial indicator to the suspension at a convenient place so that the indicator stylus contacts the rotor face about 1 inch (25mm) in from its outer edge. Set the dial at zero. Check the Total Indicator Reading (TIR) while turning the rotor one full revolution. If the rotor is warped beyond the run-out specifications, it must be machined or replaced and the wheel bearings readjusted. Make sure rotor run-out is below specifications before reinstallation. Also be sure the hub is clean of rust and contamination before replacing the rotor, using a steel brush to remove contaminants. Always use a torque wrench to tighten the wheel nuts to prevent rotor distortion that could result in brake pulsation.
Parallel Sides
Parallelism refers to variations in rotor thickness, or how close both sides of the rotor are to being parallel to each other. This may be caused by run-out, which is why turning or replacing the rotor may not solve the problem. Lack of parallelism or excessive variations in rotor thickness can cause pedal vibration and pulsation, excessive pedal travel, grabby braking action and front-end vibrations.Check for parallelism with a micrometer. Measure rotor thickness at four or more equally spaced points about 1 inch in from the edge, or at the middle of the pad contact area. The difference between the largest and smallest readings should be compared to the specification for the vehicle -- usually less than .0003"-.0005".
The Proper Finish
Surface finish is the smoothness of the rotor friction surface and is measured in micro-inches. In a service situation, actual micro-inch measurement is impractical and unnecessary. It serves only to determine the grit of a grinding wheel that may be used.Often there is a change in perceived braking effectiveness that occurs after rotors have been refinished, the semi-metallic disc pads have been replaced, or both. This may be due to improper rotor surface finish.
When refinishing the disc brake rotor, ensure that the brake lathe is in good operating condition and all tool bits are sharp. Vibration dampeners or special adapters should be used as required, particularly on composite rotors.
Recommended brake lathe settings for obtaining the correct rotor surface finish are provided at the end of the article.
A secondary finishing operation must be performed to get the correct surface finish. Using 120-grit sandpaper, sand each rotor surface for a minimum of 60 seconds using moderate pressure while the rotor is turning to achieve a swirl pattern on the surface. After sanding, wash both surfaces with soap and water to dissipate the surface magnetism and remove all the fine metallic dust from the surface pores, which could aggravate squealing if left on the surface.
It is important that the rotor surface have a very smooth finish (10 to 70 micro-inches). Rotors -- and drums, for that matter -- are like phonograph records: the wider and deeper the groove, the more noise problem the vehicle will have. And today's front-wheel-drive cars, because they are so much lighter than cars of 15 or more years ago, are more sensitive to the finish on the rotor -- there's no longer the weight of the vehicle to suppress noise. The noise is just amplified as it goes into the chassis.
When dragging a fingernail over the rotor surface, it should feel glass-smooth. This smooth, flat surface ensures the maximum contact area of the semi-metallic pad face with the rotor surface. If pads only contact the high points of a rougher surface, the brake stopping performance will be affected.
Other Concerns
Minimum allowable rotor thickness becomes a concern when a rotor is machined until it is thinner than the specified minimum thickness; this could result in the rotor wearing past the safe discard point before the linings wear out. This, in turn, could cause the pads and caliper piston to be forced out too far. Whenever disc pads require replacement, check the minimum allowable thickness, parallelism and lateral run-out as specified for each brake type, vehicle model or both. After these checks, restore the wheel bearing adjustment.Check wheel bearings by rocking each wheel with the car on a frame lift or jack stands to feel for looseness and evaluate bearing adjustment and possible wear in the steering and suspension components. Remove the wheels and any inspection plates on the caliper. Compare the lining and rotor wear patterns, noting the rotor pattern is more important as the lining shapes itself to the rotor surfaces.
Pads should be replaced and the brakes reconditioned when: the lining is worn to 1/16" above the metal backing plate on integrally molded pads; is 1/16" over the rivets for riveted pads; when the disc pad lining is contaminated with brake fluid or grease; or when it is loose on the metal backing.
Linings with tapered, uneven wear should be replaced if the difference in thickness from one end of the pad to the other exceeds 1/16" (1.5mm) on fixed calipers, or 1/8" (3mm) on floating calipers. This remains true even if the linings have not reached minimum thickness because this condition could permit the pads to become wedged in the caliper.
Grooves in the rotors, created by excessively worn pads gouging the rotor surface, must be machined away to ensure good braking performance and long lining life.
Heat checking of rotors is caused by excessive heat from dragging brakes, the wrong type of friction material and severe braking conditions. If the rotor appears blued or heat-checked, inspect the linings for severe overheating. On some brakes, it is possible to easily remove the pads for inspection without other disassembly; on others, it may be possible to force the worn pads back far enough to inspect the lining surface. Before forcing back the pads, open the bleeder to avoid back-flushing fluid through the system. If the rotor is severely heat-checked, replace it.
Calipers should be overhauled if the fluid leakage is enough to wet the lining or cause a definite loss of reservoir level between vehicle lube intervals. Some lowering of the reservoir fluid level is visible on the disc brake side of the reservoir, possibly due to pistons moving out as the linings wear. Seepage can cause some dampness or staining around the piston, but has no immediate significance. For maximum safety and pad life, recondition caliper cylinders whenever brake pads are replaced.
On-car Lathes
The minimum equipment requirement for any shop doing rotor (and drum) work includes a well-maintained lathe, the correct adapters and sharpened bits. However, as front- and four-wheel disc brake systems have come to dominate the market, more and more installer technicians are adding on-car (OC) lathes to their "required" list of shop equipment.Driving the demand for OC lathes is the advancing technology of vehicles and their braking systems. When you remove a rotor from a car and put it on a bench lathe, you're basically attempting to simulate its placement on the car -- you're assuming the center of the rotor is centering exactly the same way the car is centering. But as cars have gotten more sensitive or exacting in their mounting systems, it becomes much more difficult to simulate that same parallel system on the bench lathe. The system used to mount it on a bench cannot simulate run-out that may be present when the rotor is mounted on the car; it also can cause brake pulsation by cutting slight run-out in the rotor, totally without the knowledge of the installer until the car hits the road. The result is a customer comeback for brake pulsation, usually before the warranty's expiration.
There are two types of OC lathes: the hub-mounted configuration mounts to the rotor hub, combining the tool slide and the rotor drive device into a single unit; and the caliper-mounted configuration with a two-piece design, one that supports the tool slide mounted to the caliper on the steering knuckle and a second unit that attaches to the rotor hub, providing drive during the refinishing operation.
Although both are OC lathes, they are very distinct from each other. Both systems are solidly established in the brake refinishing market and both have their advantages and disadvantages. The hub-mounted lathe eliminates the need for two components and operates on a single drive, yet the technician must perform run-out compensation. The caliper-mounted lathe does not require run-out compensation, which simplifies the setup.
The rotors on virtually every vehicle using rotors may be turned on the vehicle itself, one exception being Jaguars in which inboard rotors are located closer to the center of the axle rather than on the wheel.
Another area of caution: ABS sensor rings. Sensor rings can be on the CV joint or rotor. The ABS warning light can come on if drive line or brake service effected the sensor ring or air gap. If the service technician is not aware of what service caused the problem, considerable time can be spent tracing its cause. If the CV shaft is handled carefully during removal and service, it normally can be installed without affecting the air gap or sensor. If the CV joint with the sensor ring is being replaced, or the entire shaft is being replaced, the replacement should be examined to ensure the sensor ring is not damaged. If the air gap is found to be incorrect, specifications and adjustment procedures are available in vehicle service manuals.
OC lathes, of course, cannot turn brake drums. As long as four-wheel disc brake systems are not the automotive norm, bench lathes will be required in the shop. But it's also expected that OC lathe equipment will become mandatory for service facilities that are serious about the brake business.
Brake Lathe Settings
Rough Cut Finish Cut Spindle speed 150 RPM 150 RPM Depth of cut .005" (.127mm) .002" (.051mm) Tool cross feed/revolution .006"-.010" .002" maximum Vibration dampener Yes Yes The preceding technical article is courtesy of Wagner Brake Products, a part of Moog Automotive.
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AutoInc. Magazine ®, Vol. XLIV No. 9, September 1996