Being able to diagnose and solve brake rotor pulsation problems is critical to passing the ASE A5 exam for brakes. Pulsation and brake performance problems may not seem to be the most difficult, but they often cause the most headaches for technicians.
Below are sample questions based on the A5 task list, written by the editors of Brake & Front End who have taken the test numerous times.
1. Technician A says he does not have to measure the rotor and/or hub for lateral runout because he is installing new rotors. Technician B says even if new rotors are installed, the new rotor and hub flange should be measured for lateral runout.
Who is correct?
A. Technician A
B. Technician B
C. Both Technician A and B
D. Neither
Answer: B
Always check runout in the hub flange and rotor as part of any brake job. You may think you are saving 10 to 15 minutes by not having to use a dial indicator and a micrometer, but your chance of a comeback can dramatically increase. Also, replacing rotors on every brake job is not a cost effective or efficient way to perform brake service. Runout in the hub and a new rotor can stack up to cause lateral runout that is beyond the specifications.
2. The most likely cause of brake pedal pulsation is:
A. Overheating/warping
B. Lateral runout
C. Suspension damage
D. Disc thickness variation
Answer: D
The main culprit of chronic pulsation is variation in disc thickness variation or parallelism. The two friction surfaces of a rotor are designed to be parallel to one another within a certain specification. The allowable tolerance is known as parallelism, also known as the rotor’s disc thickness variation (DTV).
In order for the pad to stay in contact with the rotor, the piston must extend or be pushed back into the housing as force is applied. This creates the pulsation in the pedal that is most noticeable to the driver.
Runout will not cause pedal pulsation on vehicles with floating or sliding calipers when the caliper housing is free to move and the runout is not excessive. Under these conditions, the caliper will “follow” the runout, which means the caliper housing will move in and out in relation to the runout.
This movement will not cause the caliper piston to move. No piston movement results in no fluid movement in the hydraulic system, and if there is no fluid movement, the brake pedal won’t move or pulsate. So, a key point to understand is runout generally does not cause pulsation. Runout causes thickness variation that leads to pulsation problems.
A driver can feel a pulsation or judder after higher-than-normal brake temperatures are experienced. Ninety-nine percent of the time it is the fault of the pads and not the rotor. The pad’s friction material has been unevenly deposited around the face of the rotor, which creates uneven braking forces when the dissimilar sections pass by the pads. This type of pulsation will typically go away after a few hundred miles.
3. What is the most likely cause of disc thickness variation?
A. Manufacturing defects in the rotor
B. Lateral runout
C. Caliper sticking
D. Fin corrosion
Answer: B
Most aftermarket rotors are manufactured to tolerances far below what your shop’s dial indicator or micrometer can measure. Also, it is near impossible for a warehouse to damage a rotor to the point where it could have excessive runout.
Lateral runout causes the disc thickness variation by allowing the high spot of the rotor to scrape the pad and remove material from both surfaces. Over time, this action will change the thickness in the area that has the most runout.
A sticking caliper can’t cause thickness variation on its own, it can accelerate the wear by not allowing the pad to retract and increasing the contact with the rotor. Fin corrosion is rare and may cause the rotor to become thermally unstable and crack long before the rotor will warp. 
4. Technician A says lateral runout can be minimized by matching the high spot in the hub and the lowest point on the rotor. Technician B says a tapered correction plate can be placed between the rotor and hub to bring the lateral runout into specifications. Who is correct?
A. Technician A
B. Technician B
C. Both Technician A and B
D. Neither
Answer: C
By measuring and marking the high and low spots of runout in the hub and rotor, it is possible to match the high spot of runout in the hub with the low spot of runout in the rotor. This technique can be used to minimize the amount of material removed with an on-the-car brake lathe.
Flange runout can be corrected with tapered shims that are available to correct a runout of 0.003” (0.075 mm) to 0.009” (0.230 mm). A runout of more than 0.005”(0.125 mm) at the bearing flange cannot be corrected by the use of a shim. The combination of the rotor and bearing flange could prevent the rotor from being turned. After checking friction surface runout, check the bearing flange runout by changing the rotor position 180º on the bearing. If the high spot changes 180º, the rotor could be OK or ready to turn after the bearing is shimmed.
Components should be marked as you perform an inspection of the assembly. Check the bearing endplay, and mark the relation of the rotor to the bearing flange. Mark the high and low runout spots on the rotor friction surface, with the low spot marked as zero and the high spot as 0.XX”. Mark the high and low runout spots on the bearing flange with the same method as the rotor friction surface.
5. Technician A says a bench lathe can eliminate total lateral runout. Technician B says this can only be done with an on-the-car brake lathe.
Who is correct?
A. Technician A
B. Technician B
C. Both Technician A and B
D. Neither
Answer: B
A bench lathe machines a rotor in the same plane as the arbor, and it will remove lateral runout in the rotor, but not runout that is in the flange.
An on-the-car lathe can help to reduce runout by truing a rotor in its operating plane on the vehicle. This means that the rotor is machined to match the hub.
6. A customer returns with a pulsation complaint. During the brake job performed 6,000 miles ago, the pads and rotors were replaced. What is the most likely cause of the pulsation?
A. The customer overheated the brakes
B. Brake pads are not the right formulation
C. Lateral runout in the hub was not corrected
D. A caliper is stuck
Answer: C
New rotors will remove disc thickness variation, but they will not correct lateral runout. Let’s say this vehicle had .003” of lateral runout when measured at the outside face of the rotor.
If this vehicle is riding on 205/55R16 tires, in one mile, the high-spot with .003” of runout goes past the caliper approximately 836 times. Over 6,000 miles, that spot on the rotor will go past the pads more than 5 million times. Every time this spot passes the pads, a little bit of the rotor’s material is removed. Over the course of those 5 million revolutions, enough material is removed to create a thickness variation that can be felt by the driver.
Warped rotors are a myth. The heat used to cast the rotor is three to five times greater than the heat produced in the most aggressive braking down a mountain road. Even if the rotor is glowing, it is not even close to casting temperaturers. Most engineers say that rotors will crack long before they are distorted by heat.
Blaming the friction material is not the right answer. By changing the brand of pads without correcting the lateral runout problem, you will have the same results after disc thickness variation develops again.
The biggest cause of lateral rutemperaturesnout comebacks, essentially, is carelessness — not poor OEM engineering or low-quality aftermarket parts. In that way, a technician’s instincts and methods of operation could be the actual issue.