Brake Rotor Runout

Feb. 9, 2024
Whenever servicing brake rotors, it's important to take a few measurements. Measure lateral runout, disc thickness and measure for thickness variation.

Whenever servicing brake rotors, always take the time to take a few measurements, even if the customer hasn’t complained about a bouncing brake pedal. Measure lateral runout, disc thickness and measure for thickness variation.

Poorly machined or abused rotors, or rotors that have been warped as a result of uneven or over-tightening of the wheel fasteners can easily create a pulsating brake pedal. Aside from the annoying feel of this, if the pedal is pulsating/bouncing, this means that the pads are not in a consistent, full contact with the disc surface — which results in a varying contact patch between the disc and pad, which reduces braking efficiency and potentially generates hot spots on the rotors which will increase the chance of warping. 

Measuring lateral runout is a simple process, and there’s no reason to avoid the task, whether you’re dealing with used or even new rotors. Why check new rotors? While the new rotor may be fine, mounting it to its hub and checking it may allow you to discover a runout issue with the hub. 

With the wheel removed, install all of the wheel fasteners to secure the rotor to the hub. Installing only two or three fasteners can result in an erroneous runout reading, which can easily fool you into thinking that the rotor has more runout than it actually has. This is especially critical when dealing with thin-hat rotors. Uneven and incomplete deflection at the hub can easily result in warped discs that display excessive runout. 

Always install and fully torque to specification all of the wheel’s fasteners. In order to avoid damaging the nut (or bolt) seat surfaces, it’s also a good idea to install conical washers between the fasteners and rotor hat surfaces. As an example, by installing only two nuts on a five-bolt hub, you might obtain a runout reading of, say, 0.005-inch. By installing three nuts the reading might be 0.003-inch. But by installing all five, the reading may be 0.002-inch, which may be within manufacturer’s spec. (Always refer to the vehicle maker’s runout specifications.) Torque all fasteners to equal value.

Mount a dial (or digital) indicator to a stationary area that doesn’t move in relation to wheel rotation (frame, strut, etc.) Indicator mounts are available with magnetic bases or clamp-on designs (usually featuring a vise-grip). Position the indicator’s plunger at 90-degrees to the disc surface, and push the plunger in to provide about 0.050-inch preload. Then zero the gauge face. Ideally, the indicator’s plunger should feature a small roller bearing at the tip to provide a consistent reading. The plunger tip should be placed about a half-inch inboard from the outer edge of pad contact. Slowly rotate the rotor and locate the low spot, then zero the gauge again. Using a Sharpie, make a reference mark on the disc at the lowest reading location. Slowly rotate the rotor, observing the gauge, noting the highest reading. The difference represents the amount of runout. Vehicle manufacturer specifications may vary, but as a rule of thumb, the maximum allowable runout is about 0.001-inch to 0.002-inch for most applications. Depending on the rotor design, you may be able to correct for runout using an on-car lathe, or you may need to simply replace the rotor. However, before replacing a rotor that you suspect of having excess runout, make a matchmark on the rotor hat and a corresponding wheel stud, then remove the rotor and reinstall at the next clockwise position and re-check runout. You may have a stack-up situation where combined tolerances between the rotor and the hub are creating the excess runout. Continue to relocate the rotor on the hub, checking runout with each change. You may be able to install the rotor with the high point of the rotor aligned with the low point of hub runout, thereby potentially correcting the runout issue. 

In a situation where you find a lateral runout condition (where a stack-up of tolerance between the rotor and hub exists), you may be able to easily correct this without replacing parts by using a tapered correction shim between the rotor and hub. These shims are available to correct problems from initial runout of 0.003-inch to 0.006-inch.

Disc thickness

Rotor disc thickness variation is also known as parallelism. Both sides of the rotor must be parallel to prevent pedal pulsations.

Measure rotor disc thickness using a micrometer. (Avoid using a caliper. A micrometer will provide a much more accurate measurement). Avoid making any decision based on only one measurement location. Even if one measurement location is within the allowable thickness, measuring for thickness variation at several spots may locate a thickness that is too close to minimum. This is sort of a double-check of findings that result from checking runout. Generally speaking, allowable thickness variation should be no more than 0.0005-inch. (Some OEs may spec a tolerance range of 0.001-inch to as little as 0.0004 inch.) Again, refer to the manufacturer’s specification. Thickness limits usually can be found on the inside of the rotor hub hat. 

The thickness measurements should be taken approximately 0.40-inch (10 mm) from the rotor’s outer edge. 

Measure rotor disc thickness at a minimum of six locations, but preferably at eight equidistant points along the rotor — at 12 o'clock, 45 degrees, 90 degrees, 135 degrees, 180 degrees, 225 degrees, 270 degrees and 315 degrees. Record all measurements. The total thickness variation equals the maximum thickness minus the minimum thickness. If total variation is beyond the manufacturer’s specification for that vehicle, the rotor may be resurfaced (only using an on-the-car caliper mounted lathe) — or replaced.

It’s best to monitor both runout and thickness variation at the same time.

Checking lateral runout and thickness variation applies to all vehicle applications, and should not be limited only to vehicles that exhibit a brake pulsation issue. Ideally, these checks should be an integral part of any complete brake job. 

Hot spots 

In the days of old (not that long ago), brake pads were made of relatively soft materials that contained asbestos and other material mixes. They tended to wear the rotor evenly. Today, many pad materials contain ceramic, designed to transfer a small amount of pad material to the rotor. If the rotor has no excess runout, the transfer is evenly deposited on the disc face. If the rotor has excessive runout, the transfer takes place unevenly, resulting in higher build-up at the point(s) of higher runout. This results in hot spots, resulting in varying degrees of friction between the pad and rotor (causing a “slip-stick”). The excess, or uneven buildup can also be caused by improper caliper piston return, rusted or sticking caliper slides or even worn/loose hub bearings. If you see uneven bluing wear on the discs, suspect this uneven pad transfer, resulting from too much runout or the aforementioned caliper issues. Remember: excessive rotor runout can be caused by worn or loose hub bearings and may not be caused by the rotor itself.

Pre-installation wash

Whether you’re dealing with new or used rotors, always clean the disc surfaces thoroughly. While spraying brake cleaning solvent is viable for certain applications, it’s not the ideal final-cleaning product for brake rotors. Prior to installing any rotor (new, used or reconditioned), each rotor should be washed with hot soapy water (as opposed to brake clean solvents, which tend to dry too fast) to remove any metal fragments left over from the machining process. These small particles can impede the pads/shoes bedding process and cause noise or affect the brake pedal feel and stopping distance. Many brake experts and even some leading brake manufacturers specifically recommend the use of hot water and Dawn dishwashing liquid and a nylon bristle brush. While other cleaning agents may work, the Dawn brand is very popular and seems to work reliably for removing oils and particles from the machined surfaces. You can certainly follow up with brake cleaning solvent, but don’t rely on the solvent alone. Seriously, wash with Dawn and then dry.

In most cases a new rotor is going to be installed, but if we just machined it, either on the vehicle or a lathe, again, it’s critical to wash the rotor clean with mild soap and hot water to remove all the metal debris and graphite from the previous brake pads and/or the machining process. When not properly removed from the surface of the freshly machined rotor, materials left over from machining — including those too small to be visible by the naked eye — can be transferred to the new pads, and this can result in noise, uneven braking and a host of other issues. Many times the problem is fixed by changing the pads that are contaminated with the small machining particles with a new set of non-contaminated pads. But the actual issue was improper cleaning of the machined rotor.  

Even when you’re dealing with new rotors, cleaning the rotor disc surfaces is imperative to remove deposits and/or anti-rust materials that were applied at the factory prior to shipping. Removing all foreign material exposes the microscopic surface finish peaks and valleys to provide proper friction material transfer onto the disc surface.

With that said, note that OE and aftermarket rotors are available today with a special “E” coating that prevents (or at least dramatically delays) surface rusting. An E-coated rotor has the appearance of being painted with a gray or silver paint. This coating is applied to the entire rotor, reducing the chance of visible rotor edges and hats from turning brown. The coating also prevents rusting in the cooling vanes and the rotor’s rear hub face. A coated rotor package will likely include a caution to avoid cleaning this coating off. It’s best to avoid any solvents on the rotor, even on the disc surfaces. If you’re dealing with brand new coated rotors, just make sure that no oils, dirty fingerprints, etc. are on the disc's surfaces. At the risk of repeating, only clean by simply washing in hot water and Dawn. While the E-coating will likely not be harmed with the application of a quality-brand brake cleaning solvent, you’re safer by only washing and rinsing. By all means, never use a strong solvent such as lacquer thinner to clean a coated rotor, as this can ruin the coating.

ABS glitch after hub replacement 

Here’s a useful tip from Raybestos. This applies to all ABS-equipped vehicles. The ABS light may be on and/or false ABS activation may occur following wheel bearing hub replacement on only one side of an axle.

If you diagnose a bad hub bearing on one side of a vehicle and the ABS wheel speed sensor or tone ring is integral to the bearing, you may need additional repairs to restore proper ABS functioning. In many cases, replacing one hub bearing will cause the driver to feel ABS false activation when coming to a slow stop on dry pavement.  

False activation is usually described as a pulse in the brake pedal when not expected. The pulsation comes from the ABS valves cycling the supposedly locked up wheel. This is due to the difference in signal strength from the wheel speed sensors (WSS) side to side. The problem is usually associated with air gap difference or wiring and/or connector integrity. In many cases, removing the WSS from the other side, if possible, and cleaning the mounting surface may repair the problem. The rust buildup actually lifts the WSS from the bearing, increasing the air gap and weakening the signal. Another possible issue is play in the bearing causing sine wave frequency change and or AC voltage variation. The new bearing will have little to no play, while the remaining hub has acceptable play but can still affect signal strength. Again, the difference in signal from side to side may be enough to trigger false activation. If WSS is integral and not serviceable, replacing the hub bearings in pairs may be the only answer. The ABS is activating as designed, so no warning light will be illuminated in most cases. At least discuss this with the customer to prevent unnecessary surprise repairs in the future and prevent the dreaded “it never did this before you worked on it” conversation.

In some cases, the issue may not be caused by an air gap concern. Citing certain 1996-2005 Audi models as an example, the ABS light may be on under light braking when the wheels are not locking up. Check for damaged wheel speed sensors or mismatched tires. Out of specification wheel speed sensor air gaps are not the likely issue. The most likely problem will be tiny cracks in the ABS tone rings found at the outer CV joint, or inside the rotor or wheel hub. The best way to identify a crack or damaged tone ring is to use a lab scope. With the scope attached to the sensor, turn the ABS ring. A normal wheel speed sensor will show a smooth, round uninterrupted sine wave that increases in amplitude and frequency as the wheel speed increases. If a crack in the tone ring is present, a notch or a flat spot on the wave form will be present. If a scope isn’t available, clean the tone ring and inspect carefully with a bright light. Cracks normally form at the base of the teeth. 

Note: While shop time is obviously valuable, the use of an impact wrench is not recommended for wheel installation. Especially in the case of alloy wheels and thin-hat brake rotors, use only a calibrated torque wrench, and always follow the proper tightening sequence in order to evenly spread the clamping load across the hub face.  

Rotor resurfacing

The rotor’s purpose is to provide a rubbing surface for the friction material and to dissipate heat during braking. And, in order to have trouble-free performance, the rotor surface must be smooth regardless of whether it has a directional or a non-directional finish. 

If the rotor surface is too rough, the brake pads won’t perform from the start. As the vehicle is driven, the driver will likely apply excessive pedal pressure in order to stop. This will lead to overheating the brakes, which can lead to rotor warping and annoying brake vibration and noise. The result: the customer will likely demand that the brake job be re-done. 

  1. Using a rotor micrometer, measure the rotor’s thickness to make sure that machining will not reduce it beyond the specified minimum thickness.
  2. Clean the inner and outer hat area so that it is free of rust and corrosion to remove deposits that would hinder proper mounting to the lathe.
  3. One or two fast cuts may be needed to clean the surface and correct any runout. The last cut should be made with the slowest possible lathe speed to provide a finish that’s as smooth as possible.
  4. A directional finish requires a bit more time. If the machining cut is taken too quickly or if the brake lathe bits are in poor condition, the rotor’s surface will likely have shallow cutting grooves. Microscopic peaks and valleys reduce the surface contact of the pad and result in poor initial stopping power. The customer may also complain of a “thumping” noise due to the pad moving up and down in the caliper on the peaks and valleys as it tries to properly seat itself.
  5. Ideally, once the rotor has been finished, you should use a profilometer to inspect for a smooth finish. (It's a hand held tool that is designed specifically to measure a rotor’s roughness average.) Granted, your shop may not have access to a profilometer, as they can be on the pricier side. But if you really want to be picky with regard to your resurfacing efforts, a profilometer is extremely useful. Most new OEM and quality aftermarket rotors typically have a finish somewhere between 30 and 60 microinches RA (roughness average). 

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 using an on-car lathe. This insures that the rotor is true, considering that any stack-up tolerances between the rotor and hub are corrected at the same time. 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.

A major cause of brake noise is a poor rotor finish. A surface finish of less that 40 RMS (root mean squared) is usually alright — before creating the non-directional finish. Although there are comparison gauges to check the finish, you can get relatively close with a pen and paper. If you can draw a smooth line across the face of the rotor, it is probably okay. A dotted line is not. 

Concerning pad and rotor burnishing (bed-in), new or newly machined rotors benefit from burnishing by 15-20 aggressive stops from about 40-50 mph. Allow time for the brakes to cool a bit between stops. Burnishing can remove surface rust from rotors that have been sitting in a humid environment. It also helps break in new pads. Particles from the brakes' friction material are transferred to the rotor surfaces during burnishing and improve stopping performance. Leaving this step out often results in the customer returning and complaining that the brakes don't work well enough. Bear in mind that some brake pad makers may suggest that no burnishing is required, but it’s always a good idea to bed the pads in yourself to make sure that proper pad-to-rotor transfer occurs.

A hand-held digital infrared pyrometer is a useful diagnostic tool that allows quick and easy measurement of rotor heat, following a test drive. For instance, if the vehicle tends to pull right or left during braking, or if you suspect a brake drag during non-braking, taking a heat measurement of the right and left rotors provides a comparison. For instance, if the left rotor temperature measures 200 degrees Fahrenheit and the right rotor measures 400 degrees Fahrenheit after a series of braking stops, this may indicate a weak braking action on the left rotor (caliper piston or sliding caliper sticking, etc.)

As noted, proper torquing of not just the wheels but all the parts and pieces of the brake system on reassembly is important. Many parts need to be tight but not just impact gun tight. The proper torquing of caliper brackets is essential yet not something many of us do. Take the time to look up the specs. You will be surprised how tight many need to be to prevent unwanted noises.  

If you are machining a rotor, a poor finish can easily create a noise. The rotor’s machined surface finish should be non-directional and of the proper smoothness to allow the pad material transfer which creates the correct coefficient of friction. Improper machining can result in a record player effect on rotors that result in a slapstick banging noise as the pads are pulled away and released, so attention must be paid to bit condition, machining speeds and the final machined finish.

Both wheels and wheel bearings need to be properly torqued to prevent excessive runout. Unevenly torqued or over torqued wheels can easily cause rotor distortion.

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