ence a pulsation or judder after higher-than-normal brake temperatures are experienced. About 99% of the time, it is not the fault of the rotor; it is the pads. The pad’s friction material has been unevenly deposited around the face of the rotor. This creates uneven braking forces as the dissimilar sections pass by the pads. These types of pulsations or “judders” will typically go away after a few hundred miles.
Parallelism
The main culprit of the chronic pulsation was variations in disc thickness, 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.’ It is also known as the rotor’s disc thickness variation, or 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.
Every time the low spot passes by the caliper, hydraulic pressure at the caliper drops. This produces less braking force as this area passes by the pads. This also can affect braking distances.
Runout
Runout is defined as the amount of lateral (side-to-side) movement of the rotor as it rotates 360 degrees. The specification is usually provided as TIR or ‘total indicated runout.’ TIR is defined as the runout measured on the vehicle or installed runout. TIR includes all factors that can influence the amount of runout. Runout can be in the hub’s flange, the rotor, or it can be caused by improper lug torque.
You also could call TIR the stacked runout of the hub/rotor/wheel assembly. All of the above factors add up to give the rotor’s TIR. This brings up an important point. The average allowable TIR specification for late-model vehicles is between .001-inch and .003-inch. This is the maximum allowed. Some manufacturers have specified .000-inch of runout.
Runout will not cause pedal pulsation on vehicles with floating or sliding calipers, and when the caliper housing is free to move and the runout is not excessive.
Under these conditions, the caliper will follow the runout. The caliper housing will move in and out in relation to the runout. This movement will not cause the caliper piston to move. This is a key point to understand. No piston movement results in a lack of fluid movement in the hydraulic system. 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.
Solutions
The first solution to solve pulsation problems is to prevent them from happening in the first place. For every brake job, runout and DTV should be measured and documented. This includes pad slaps, machining rotors and installing new rotors.
Second, pay special attention to the mounting flange. Taking time to clean the mounting flange on the hub can remove corrosion and debris that can cause runout. Also, pay attention to lug nut torque.
Third, the shop should take time to service the caliper, slides and hardware. If a caliper cannot float or retract the piston, it can result in increased wear that could lead to DTV and pulsation. Some new vehicles use new abutment clips that help to push the pads back from the rotor to reduce drag and allow for less wear on the pads and rotors. Failing to renew these components as part of a brake job could reduce the life of the brake job and increase comebacks.
Fourth, using an on-the-car lathe can help to reduce runout. The main advantage of these lathes is that they are able to cut a rotor in its operating plane. This means the rotor is machined to match the hub.
Fifth, flange runout can be corrected with tapered shims that are available to correct a runout of 0.003-inch (0.075 mm) to 0.009-inch (0.230 mm). A runout of more than 0.005-inch (0.125 mm) at the bearing flange cannot be corrected by the use of a shim. The combination of rotor and bearing flange could prevent the rotor from being turned.
Checking bearing flange runout should be performed after friction surface runout. Changing the rotor position 180 degrees on the bearing can check flange runout. If the high spot changes 180 degrees, 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. Mark the relation of the rotor to the bearing flange. Mark the rotor high and low runout spots on the rotor friction surface; the low spot marked as zero and the high spot as the maximum value. Mark the high and low runout spots on the bearing flange with the same method and the rotor friction surface.
Once a technician has collected the data, the following comparisons should be made:
• Compare bearing flange to rotor runout position. If the shim cannot correct the runout, the bearing should be replaced.
• Check the rotor thickness. The minimum dimension should be stamped or cast into the rotor. There has to be enough thickness to cover the runout without going below the minimum thickness.
Pulsation comebacks cost technicians and shops parts and lost labor. Rolling the dice every brake job by not eliminating runout or trusting in new rotors will cost you more than the time spent measuring with a dial gauge and micrometer.
