17th Annual Technical Forum: Rotors

17th Annual Technical Forum: Rotors

Counterman magazine presents 15 technical and sales topics in an easy-to-read question-and-answer format for the magazine's annual Technical Forum. This article appeared in the August 2009 issue.

Q. Should brake rotors be replaced when the pads are replaced?
A. The need for rotor replacement depends on the condition of the rotors. If the rotors are in relatively good condition with minimal wear, it may not even be necessary to resurface them. However, resurfacing is usually recommended to restore a flat, smooth, like-new rotor surface for the new pads. This will speed pad seating, provide the best-possible friction surface, and reduce the risk of noise or pedal vibrations.

Regardless of the age or miles on a vehicle, rotor thickness should always be measured with a micrometer when the pads are replaced. As rotors wear and become thinner, they also become weaker and are less able to absorb and dissipate heat. This increases the risk of the brakes overheating as well as rotor failure. All rotors have a minimum thickness and/or discard specification that is usually stamped or cast onto the rotor itself. If a rotor is worn down to this specification, or it cannot be resurfaced without exceeding this dimension, the rotor must be replaced.

Rotors can also wear unevenly. This is often due to excessive runout in the rotor, or from hard spots in the surface of the rotor. In many instances, rotors can be distorted if the lug nuts are not torqued evenly. Not using a torque wrench or torque-limiting sticks to final tighten the lug nuts when a wheel is mounted can load the center hat section of the rotor unevenly. This may produce enough distortion in the rotor to cause it to wear unevenly. Dirt or rust that is trapped between the rotor and hub can also cause the same kind of distortion.

Resurfacing the rotors will restore flatness and parallelism between the rotor faces. But if a rotor has hard spots (discolored bluish areas on the surface), it should be replaced because the hard spots often return after resurfacing.

Cracks are another reason to replace rotors. Some minor surface cracking is normal as a result of heat and wear. But deep cracks or ones that are more than an inch long are dangerous because they could cause the rotor to fail.
Rotors should usually be replaced in pairs to maintain the same thickness and wear on both sides.

Q. If the brakes pulsate because of rotor wobble, how do you get rid of it?
A. “Brake pedal pulsation occurring after brake service is primarily caused by thickness variation in the rotor,” according to Bob Milliman, training supervisor for Affinia. “The rotor wears unevenly due to the lateral run-out on the hub, which causes the thickness variation. The pads upon application make contact with the high spot first. Pedal pulsation can occur from 1 to 12,000 miles after brake service has been performed, depending on the extent of the run-out.”

Resurfacing the rotors in place with an on-car lathe can usually minimize a runout problem, and is usually easier than trying to compensate for runout on a bench lathe.

Replacing the rotors with new ones can also eliminate a pulsation if the problem was a bad rotor. But it won’t help if the wobble is due to excessive runout in the hub. So runout should be checked with a dial indicator.

Runout is measured by placing the tip of a dial indicator against the outside face of the rotor, then rotating the rotor one full turn. If the dial indicator shows more then .003 inches (0.076 mm) of total movement, there’s too much runout. On some vehicles, that number may be as low as .002 inches or less.

You May Also Like

Tool Intel – Understanding Air Tool Fittings and Couplers

Why don’t air tools come with fittings installed? Here’s why customers need to buy what they actually need.

Your customers may be using air tools in a variety of circumstances for an even wider variety of jobs. Here's how to help them understand why they need to buy the right fitting for the application.

View Full Diagram Here

There are multiple different sizes and styles, and what one shop uses may not be the same as another. The size and style affect the volume of air they can deliver, a critical point because air tools require a specific pressure and volume for proper operation, and restrictive fittings can limit their performance. Here’s a look at the most common sizes and styles found in most automotive shops, and how you can identify them.

Read the April Digital Edition of Counterman

The April issue contains article designed for technical training, management efficiency and store profitability.

ASE Education Foundation Seeking Outstanding Instructor

Nominations are being accepted for the 2024 Byrl Shoemaker/ASE Education Foundation Instructor of the Year award.

Why Does Engine Coolant Need Replacement?

Two specifications can be used to justify replacement — the condition of the additive package & the freezing point.

Gaskets vs. Seals

Whether your customer asks for a gasket or a seal, you know one thing: They’re trying to stop a leak.

Gaskets and Seals

Other Posts

MEMA Aftermarket Suppliers’ BMC to Host Summer Meeting

The Brake Manufacturers Council meeting will be held on May 31, in Naples, Florida.

Basic Types of Brake Rotors

The most common materials used in brake rotors include cast iron, carbon composite, and ceramic cast. This video is sponsored by The Group Training Academy.

Customer Service: How It’s Done

Customer service should be your number one priority, and it all starts with the greeting.

Tool Intel: Why Are There So Many Screwdrivers?

Screwdrivers come in many shapes and sizes, and they are not created equal.