Unusual weather dry spells can contribute to brake noise. Be sure to use new hardware installed correctly and proper lubricant in all the right places. A coarse rotor surface creates “mountain peaks” where the new pads ride. This reduces surface contact, which causes higher pedal effort, excess wheel dusting, and noise until the surfaces smooth out and comply with each other. The smoothness of the rotor face is measured in roughness average (RA); the lower the number, the smoother the face.
Aggressive braking after a brake job can cause overheating of the friction material, which brings the binders (glues) to the surface, known as glazing. This harder surface is prone to noise and can cause higher brake pedal effort until it wears away or is sanded to a flat surface. In many cases, the pads must be replaced when this happens.
Expected Brake Pad Life
It is difficult to predict just how long brake pads will last in terms of mileage or time because of varying driving habits and conditions. If a vehicle is used for towing or in constant stop-and-go traffic, the brakes generally require more frequent service. Younger drivers may go through pads more frequently due to last-minute stops. Some vehicles may require front brake service as much as three times more than rear brake service due to weight transfer and system design.
You may notice more brake dust on the front wheels than the rears when washing your vehicle. This is especially true on some European vehicles because many use more aggressive silicone-carbide brake pads resulting in more rotor wear and dark brown or black dust on the wheels. Some owners wax their wheels to minimize the dust formation.
If the front pads are severely worn or you find that they wear out frequently, be sure the rear brakes are working properly. Make sure the brakes aren’t dragging due to a misadjusted pushrod in the master cylinder, too much fluid, or a misadjusted brake light switch at the brake pedal. Brake hoses with an internal restriction can cause the brakes to stay on, causing abnormal wear.
In rare cases, an ABS issue can contribute to excessive pad wear. For example, a mechanical problem within the ABS modulator on some vehicles can eliminate rear brake function and cause accelerated front pad wear.
Rotor Inspection
Be sure to examine the rotor surfaces for deep grooves and excessive wear. Some rotors have internal cooling fins and some are solid. Most front rotors have fins because they run hotter than the rear brakes. If your vehicle has a brake pulsation when stopping, you may have excessive rotor thickness variation (also called parallelism), which can develop due to lateral rotor runout. Machining or replacing the rotors is required to fix this issue.
You may also need to replace the rotors if they are badly worn or the thickness is below the minimum specification. A micrometer and dial indicator are required to properly measure your rotors for overall thickness and variation. The typical amount of runout allowed by many manufacturers is 0.003 inch, and thickness variation standards are even tighter at 0.0005 inch. As you can see, tolerances are pretty critical for proper brake operation.
Your local auto parts store should be able to measure the thickness and compare it to OEM specifications found in the vehicle service manual to make a recommendation as to whether the rotor can be machined or must be replaced. The service information often lists the nominal thickness, which is the thickness of the rotor when it was new. It also may list a “machine to” thickness. This is the minimum thickness that the rotor can be after it is turned on a lathe. If the rotor has deep grooves, measurements should be made at the deepest point.
If the pads are worn unevenly it could be an indication that the caliper or bracket needs attention. The slides could be binding or a pin or slider bolt could be seized. Further disassembly should reveal the culprit. A slight amount of normal uneven wear does occur on some systems due to caliper flexing.
A micrometer or dial caliper can measure the remaining rotor thickness to determine if it is still within the manufacturer’s safety tolerance. A micrometer can also be used to measure the thickness at several locations to compare them and determine thickness variation (also known as rotor parallelism).
If service information only lists a minimum thickness, the rotor is generally machined to about 0.030 inch larger so that it is still within tolerance after it “wears in.”
You may also find specifications cast into the rotor or listed online. Be sure not to purchase poor-quality rotors if yours need replacement. Some shed iron particles that can embed in the new pads and cause noise. Impurities in the cast iron and improper heat treating during manufacturing are the common reasons for this. You don’t want to have to do the job again in a couple of months.
If the rotors are within specification and you have no pedal pulsation or deep grooves, some manufacturers approve simple pad replacement only. This “pad slap” practice is often debated among professionals. Some shops have no problem with this approach while others machine or replace rotors on every brake job. One reason for this is that most shops do not possess a micrometer that can measure down to 0.0005 inch to determine if the thickness variation is within specification. They suggest replacement or machining to ensure that it is corrected if it is out of specification. Other shops feel that if brake pulsation does not exist when test driving, a pad slap is an option.
Another concern is that every time a rotor is machined, it has slightly less mass and therefore could run hotter during braking. Some professionals are concerned about the quality of the replacement and suggest that the machined OEM rotor with less mass is still better than some brand-new inferior replacements.
You also want to inspect the rotor surface for evidence of overheating and cracks. Bluing and hot spots can develop and are reasons to replace rather than machine.
A dial indicator can be used to determine the amount of rotor lateral runout. It is attached to the suspension system or caliper bracket and a dial gauge placed on the rotor surface. As the rotor is rotated slowly, runout causes the needle to deflect.
Rotor thickness specifications are available in the vehicle service manual and are also cast into the rotor in most cases. This specification is generally the minimum thickness, not the amount the rotor can be machined down to.
New or Rebuilt?
There seems to be the trend toward a throw-away world. Sometimes this is because the replacement part may cost less than repairing the old part. It is common with engines, starters, alternators, and even transmissions. Years ago, brake calipers, wheel cylinders, and even master cylinders were commonly rebuilt at a workshop. Most of these parts are now replaced with new when they fail due to the labor required and limited availability of parts.
Many shops replace rotors and drums rather than machine them. The choice is not always an easy one when faced with a grooved or pulsating rotor. Some shops feel that some replacements from overseas may be of lesser quality and choose to machine the originals, but others find that the price is close enough to the machining cost and have had success with replacements, so they go that route.
High-quality rotors and drums are available in the industry from name-brand manufacturers, but it is not always easy to tell if you are buying a quality piece. Just remember that as in many things, you get what you pay for, and doing the job over because a part failed might cost you much more in the long run. Quality components are especially important if your vehicle is used to haul heavy loads or pull a trailer. ■
On-Car Lathes
Lathes for machining rotors on the vehicle have been around for quite a while, but they are becoming more popular. Most shops have a lathe that can turn both rotors and drums,