Tech Stuff

Rotor routine -- Rotor guidelines and OE developments

Photos by Mike Mavrigian

Weber, president of Virginia-based Write Stuff Communications Inc., is an award-winning freelance automotive and technical writer with over three decades of journalism experience. He is an ASE-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. He is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an auto mechanic, a shop manager and a regional manager for an automotive service franchise operation.

Brake complaints fall into three general categories: stopping issues, noise issues and vibration issues. While rotor problems can contribute to all three, vibration is the main culprit. Pulsation is due to either thickness variation or lateral runout.

Although there are a variety of causes for runout, one common cause is due to variations between the rotor and the hub to which it is mounted. That is why it is essential that you index the rotor to the hub. To facilitate this, use chalk or a grease pencil to mark the position of the rotor relative to the hub. Most often you need to simply mark one of the lug studs and the corresponding hole in the rotor.

Sometimes the vehicle comes in with a vibration problem following some wheel service. It is not uncommon to find the previous service person did not properly index the rotor. In many cases, moving the rotor forward or back one hole solves the problem. Sometimes, you may have to re-index the rotor nearly 180 degrees from the original position.

Runout should be checked with the rotors on the vehicle, not on a brake lathe. Remove the wheel and tire then reinstall the lug nuts to hold the rotor in place. Turn the nuts over so the flat (non-tapered) side contacts the rotor. Be sure to torque them evenly to avoid uneven clamping force.

Measure rotor disc thickness at least 10 mm or so from the outer edge, preferably about 1 inch inboard. Using a standard flat-anvil micrometer is useful, but care must be taken to obtain an accurate reading.
<p>Measure rotor disc thickness at least 10 mm or so from the outer edge,  preferably about 1 inch inboard. Using a standard flat-anvil micrometer  is useful, but care must be taken to obtain an accurate reading.</p>

Use a dial indicator to check the rotor. Attach the tool to a rigid location such as the steering knuckle; place the indicator lightly in contact with the rotor surface (approximately one inch inboard from the outer edge, and preload the dial indicator by about 0.050-inch, then zero the indicator). Doing so about one inch from the outer edge will show the greatest runout. Then, turn the rotor slowly by hand while watching the dial indicator needle. Generally speaking, if the total indicator reading for lateral runout is 0.002-inch or less, there’s no problem. Of course, it’s best to check your service specs for proper limits, since some automakers may specify a tighter tolerance range.

It’s not a bad idea to check new rotors before buttoning up the job and returning the vehicle to the customer after your test drive. Correcting runout may be as simple as reindexing the rotor, even when dealing with a new out-of-the-box rotor. You do test drive, don’t you? It is especially important after installing new pads or rotors that the brakes are properly burnished before handing the keys over to the customer.

Thickness variation is also known as parallelism. Both sides of the rotor must be parallel to prevent pedal pulsations. Use a caliper to check rotor thickness in at lease four places (preferably as many as 12 places) around the rotor’s circumference.

Thickness should generally not vary more the 0.0005-inch anywhere on the rotor, but check the vehicle’s service specifications to be sure.

In their quest to reduce weight and increase fuel economy, carmakers have been installing the lightest rotors possible. It has gotten to the point that most of them are akin to Bic lighters: You just toss them out after use. The rotor is used up by the time the car is due for its first brake job. Rotors that are at or below the minimum thickness often lead to high pedal effort or long pedal travel.

If you wish to salvage a rotor, you may be able to machine it, preferably on the vehicle. If so, there are three measurements you need to keep in mind: nominal thickness, machine-to thickness and discard thickness. Nominal is the thickness of a new rotor. The machine-to thickness is the limit that will provide safe braking with new pads. Once it reaches the discard thickness, recycle it as scrap. Many vehicle manufacturers do not suggest machining rotors unless any scoring is 0.060-inch or deeper. Leave minor scoring alone. Use a dime as a go-no go gauge. Insert the edge and if the top of Roosevelt’s head goes below the surface the groove is deeper than 0.060-inch.

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