AfterMarketNews Brake&Frontend BodyShopBusiness Counterman EngineBuilder Fleet Equipment ImportCar Motorcycle & Powersports News Servicio Automotriz Shop Owner Tire Review Tech Shop Tomorrow's Tech Underhood Service Speedville

Servicing Mercedes-Benz AIRMATIC Suspensions

The Mercedes-Benz AIRMATIC suspension system was introduced in 1999 on the S-Class and has subsequently been used on the E-Class and most of the automaker’s SUVs. The system employs electronically controlled air springs that provide an ideal balance...

Read more...

Mazda: Performing Regular Undercar Maintenance

In this article, we’ll take a look at brake and undercar service on the Mazda vehicle lineup, with the footnote that even though this type of work ­becomes routine when you have a preventive maintenance mindset, good work habits from beginning to end...

Read more...

ZF 8-Speed Transmission Replacement

The ZF 8HP transmission made its debut in 2009, and since its introduction, has been one of the top choices for international car manufacturers. BMW, one of ZF’s largest customers, uses the 8HP across its entire product portfolio. BMWs featuring...

Read more...

The Ins And Outs Of Sanders

Sanders are required tools in today’s collision repair shop. Body techs and painters rely upon them every day to achieve that perfect finish on your customers’ vehicles. Whether you’re prepping a panel for paint or removing imperfections before...

Read more...

Are You Regularly Maintaining Your Equipment?

Technicians who are idling because the welder won’t feed wire, the hydraulic ram won’t pull chains, the booth heater won’t heat or the air compressor won’t compress enough air is a costly mistake, as labor time is the most expensive thing in any...

Read more...

Celebrate 'Back To The Future' Day By Watching The Time Machine Get A 2015 Detail

    For many today is just another Wednesday, but for a lot of people it is more than just your average Wednesday, it is "Back to the Future" Day. It is a day that everyone who watched the cult classic trilogy Back to the Future recognizes...

Read more...

Using Your Oscilloscope: Current Ramp Test Ignition Coils

Regardless of design configuration, the role of the ignition coil is to multiply battery voltage into high voltage. Following Ohm’s law for the conversion of volts to amperes, oil-filled coils generally require 3 to 5 amperes of primary current...

Read more...

Serial Data Bus Diagnostics

Understanding The Function of Serial Data Buses If serial data buses did not exist, a wiring harness would have to be five times its normal size and use twice as many sensors to deliver the same level of functionality and safety we see in the modern...

Read more...

Throttle-By-Wire Codes: P1512 On 2002 GMC Envoy

The PCM continuously monitors the commanded and actual throttle positions. The commanded throttle position is compared to the actual throttle position based on ­accelerator pedal position and possibly other limiting factors, and both values should...

Read more...
Home Brakes Brake Pulsation Questions, Rotors, DTV

Print Print Email Email

Answered by the editors of BRAKE & FRONT END

1. What causes runout?

Rotor runout can be caused by several things: variations in manufacturing tolerances, sloppy resurfacing procedures, a buildup of rust and corrosion between the rotor, hub and wheel, and uneven torque on the lug nuts.

2. What else can cause pulsation problems besides the rotors?

Loose wheel bearings will cause the rotor to tilt in the caliper when a load or side thrust is placed on the bearings. Disc brake pistons require lots of fluid volume and pressure to push the pad against the rotor. If loosely adjusted wheel bearings force the pistons into the caliper, the result will be a low or spongy brake pedal.

3. What exactly is “pulsation” and how does it relate to “warping?”

If a vehicle equipped with floating or sliding calipers has a slider problem which prevents the caliper housing from moving, runout can cause pulsation. The caliper piston will move in and out as the rotor rotates resulting in fluid movement and pedal pulsation. Likewise, fixed caliper vehicles are sensitive to runout induced pedal pulsations. Fixed calipers have pistons on both sides of the rotor due to the stationary caliper housing. Excessive runout will cause piston movement and can result in pedal pulsation due to the runout.

4. What else can cause customers to think their rotors are warped?

Brake roughness is the other component to the equation and it is often misinterpreted and misdiagnosed problem. At the root of brake roughness is a characteristic called Brake Torque Variation (BTV). This can be determined by measuring torque multiple times within one wheel revolution. The brake torque variation is the maximum torque measured within one revolution subtracting the minimum measured.

If BTV is excessive, the vehicle will start to transmit the BTV back to the driver and generate a comeback. The symptoms are pulsation in the pedal.

5. What is Disc Thickness Variation?

People generally equate a high runout value with “warping.” In reality, runout by itself is not a direct or sole generator of pulsation. The characteristic that most directly generates roughness is actually Disc Thickness Variation (DTV). This measurement is the result of measuring the thickness of the rotor surface at multiple spots around the rotor.

The DTV is the largest (thickest spot) minus the smallest (thinnest spot) of the rotor. This action of thick and thin spots passing through the caliper generates the brake torque variation. When the thick part of the rotor is forcing itself through the caliper, the torque of the brake and the pressure in the caliper rise. When the thin spot passes through, the torque drops and pressure drops. Very small amounts of DTV can create a significant problem. Today, new vehicles are typically built with a thickness variation of less than 0.00078”. Thickness variations in excess of 15 microns (0.00059”) can easily generate driver complaints.

6. “Do I have to measure the runout for every brake job? I don’t have the time!”

The bigger question is, “Do you have time for comebacks?” A dial gauge is used to measure runout in the rotor and hub assembly.

The dial gauge is a very robust piece of equipment that can last a lifetime. It is also a good idea to invest in a good magnetic base or vise-grip handle that can be setup quickly.

Always use a dial indicator to verify the amount of runout present in a rotor and hub assembly, both before and after the rotors have been machined. This must be performed even if you are using an on-the-car lathe. By measuring the rotor on the hub, you can also check for play in the wheel bearings, corrosion on the mounting surface and other possible errors. Even if you are putting new rotors on a vehicle, runout should be checked. It doesn’t take much, only about .002 inches on some vehicles, to cause a noticeable pulsation. Also, if you are using a bench lathe, the dial indicator can be used to check to see if you have mounted the rotor or drum properly to the lathe.

7. I drove the vehicle after the brake repair and there was no pulsation problem. What caused the customer to comeback after a few thousand miles with a pulsation problem? Is their braking technique at fault?

Ninety-eight percent of the time, pulsation complaints are not the fault of the driver. The cause of the DTV/Runout and resulting pulsation is a product of the interaction of the rotor and pads over time and how the caliper interprets the runout. But, the main reason why the customer is back is that the runout was not measured in the hub.

The vehicles most susceptible to reoccurring pulsation are those that use unitized bearings. Unitized bearings are preloaded and have zero play. No wheel bearing play results in the runout being “seen” by the brake pads with every revolution of the rotor. The resulting high and low spots scrape the brake pads with every revolution of the rotor. This scraping occurs during not only during braking, but also non-braking.

The long-term result of this scraping is the DTV. Pedal pulsation is the symptom of excessive DTV. The alternating thick and thin spots of the rotor cause the caliper piston to move in and out as the brakes are applied. The caliper piston moves out on the thin spots and in on the thin spots. The piston movement causes brake fluid to move in the hydraulic system and results in corresponding brake pedal movement in the form of a high-speed pedal pulsation.

8. There is not any runout in the rotor, but the customer is still complaining of a pulsation problem. What could it be the cause?

The problem might not be the brake rotors, but the pads. If the brake system is used to the intended limits and the brake pads go beyond their operational temperature limit they will deposit brake pad material unevenly on the brake rotor. Pad material build up creates uneven rotor thickness, which gives you the sensation of a “warped” brake rotor.

Try switching to a more robust brake pad, and clean up the rotors with a light pass on the lathe.

9. Volcanic Bearings?

Many bearings will actually have a machined step in the region the wheel studs are installed. This masks the localized “volcanoing” that occurs in the material around the stud. This micron (1 micron = .000039”) level volcanoing will distort the rotor when the clamp load of the wheel is applied.

10. Will a non-directional finish reduce the chance of a comeback for pulsation problem?

In theory, a non-directional finish will help to burnish the brake pads more effectively. For some friction materials, the non-directional finish will help to deposit a thin layer of friction material on the rotor’s surfaces.

According to GM… In technical bulletin #00-05-22-002 to its dealers, GM says:

“Brake rotors should only be turned when one of the following rotor surface conditions exist: severe scoring with depth in excess of 1.5 mm or 0.060 inch, pulsation from excessive lateral runout of more than .080 mm or .003 inch, thickness variation in excess of 0.025 mm or 0.001 inch, or excessive corrosion on rotor braking surfaces.”

Lug nut torque is also extremely important. Under-torquing only a single lug nut may create as much as 0.003-inch of lateral runout in a rotor!

.0015 inch or less of lateral runout is needed on the rotors to prevent pedal pulsation. If runout is excessive, try indexing the rotor on the hub to see if a change in position helps. Tapered steel shims are also available, or use an on-the-car lathe to cut the rotors in place.

The following two tabs change content below.
  • DIY mechanic

    Can you get a pulsating sensation from rear disk brakes?

    • me

      Yes ,even if you have drum brakes.I had my rear spring set go(corroded),and the rear brake shoe came loose and rested on the drum when I drove and Pulsated when I put on the brakes,also made the drums extra hot,as the brake shoe was constantly rubbing on the drum when I drove,even when NOT applying the brakes.

  • L baker

    my brakes pulsate but not all the time like a warped rotor would, and it corresponds to wheel rotation down to complete stop. Tire shop doesn’t understand it and thinks it related to computer suspension system.

Latest articles from our other sites:

University Of The Aftermarket Foundation Introduces New Website

The University of the Aftermarket Foundation has launched a new website at UofAFoundation.com. The new site features the foundation’s mission, latest news and donor information. The new site also was...More

Bumper To Bumper Hosts The 'Bridge To Excellence Conference' In Little Rock, Arkansas

Bumper to Bumper recently hosted its 2016 Bridge to Excellence Conference in Little Rock, Arkansas. More than 500 people were in attendance, including 392 employees representing 163 stores from Crow-Burlingame,...More

Hunter's Road Force Elite Features Patented Vision System

The new Road Force Elite is Hunter’s fastest diagnostic wheel balancer and performs both a traditional balance and Road Force Measurement in less time than it takes to complete a conventional balance,...More

Streamlight Debuts Lithium Ion Battery and Charger

Streamlight Inc. introduced its 18650 Lithium Ion Battery and Charger, designed to energize the growing line of Streamlight products that accept 18650 rechargeable batteries. The new, dual power input...More

Bumper To Bumper Hosts The 'Bridge To Excellence Conference' In Little Rock, Arkansas

Bumper to Bumper recently hosted its 2016 Bridge to Excellence Conference in Little Rock, Arkansas. More than 500 people were in attendance, including 392 employees representing 163 stores from Crow-Burlingame,...More

ACDelco Expands Professional Reman Alternator, Starter Line

ACDelco has introduced 186 new part numbers to its remanufactured alternators and starter offerings. These additions cover application model years 1999-2015 for GM, Chrysler, Ford, Honda, Toyota, Nissan...More