ROTOR RUNOUT

September 12th, 2013
Rotor Runout Title Image

When a brake rotor deviates from its axial plane viewed from the front edge of the rotor, this refers to a "wobble" of the rotor as it rotates. This off-center deviation is called warp, axial runout or lateral runout, all referring to the same problem that results in annoying brake pedal pulsation, often accompanied by steering wheel wobble/vibration during braking.

Rotor runout may or may not be caused by rotor thickness variation. If a rotor thickness check shows no evidence of a variable dimension, the rotor may be rotating off its true axis as a result of other damage. The wheel bearing may be badly worn and excessively loose, which would cause erratic rotor wobble as the vehicle rolls down the road. In the case of a hubless rotor (where a thin-hat rotor mounts onto a spindle-mounted hub flange), check the contact area between the hub and rotor hat. A large burr or contamination (caused by rust or grit buildup) may create an uneven mounting surface that will cause the rotor to rotate off its intended axis.

Another potential cause (one that is common where today's thin-hat rotors are used) is poor wheel installation practice. If, during wheel installation, the wheel fasteners were improperly tightened, it's very likely that the rotor hat may have warped due to uneven or excessive fastener torque. If you suspect this to be the case, a quick check involves loosening all wheel fasteners and retorquing in the correct pattern and at the correct torque values (do this one wheel at a time to keep track of any potential improvement). After the wheel has been properly retorqued, perform a road test to check for pedal pulsation. If the problem has lessened somewhat (or if you're lucky, maybe it's disappeared), you know you're on the right track. If pulsation has lessened but is still present, and you've ruled out bearing and flange-to-hat mating, chances are good that a light cleanup of the rotor on a lathe will provide the final fix. If not, you may be forced to simply replace the rotor.

MEASURING ROTOR THICKNESS

Before measuring for lateral runout, perform a rotor thickness check. This will determine whether or not that rotor is indeed serviceable. Also, you may find gross thickness variations that will readily explain the pedal pulsation problem.

Check the rotor thickness at eight equidistant points around the perimeter of the rotor (divide the rotor into seven pie slices). Never base your determination for rotor warpage by measuring only one spot on the rotor. Variation in rotor thickness will always cause pedal pulsation. That variation might be the result of excess heat buildup that has warped the rotor, or the rotor may be contaminated by isolated thick spots caused by rust or corrosion buildup (prevalent in vehicles that may have been stored for extended periods of time, where the contact area between pad and rotor may have created rust deposits). Use a micrometer to measure rotor thickness, preferably a specialty mic that's designed specifically for measuring rotors. The unit pictured in this article is a Central Tools #6459 digital disc brake gauge, which features one flat anvil and one pointed anvil. The pointed anvil feature allows measurement of the real minimum thickness of a scored rotor, as compared to a mic that has two flat anvils. The flat anvil surfaces will only contact the top, or high spot of any grooves or scoring lines.

Hub Runout ImageMEASURING RUNOUT

A dial indicator must be used to check lateral runout. The dial indicator must be securely mounted to a stationary (and adjustable) fixture. For on-the-car measuring, attach the fixture to a stationary location such as the spindle or control arm. Locate the dial indicator's plunge tip about an inch inboard of the rotor edge, and zero the indicator. Rotate the rotor manually through a 360-degree rotation, watching the dial indicator for changes in runout. The unit shown in this article is a Central Tools #6450 rotor and ball joint gauge. This features an easy-to-use clamp mount and flexible adjustment arm that solidly locks into a steady position.

Specifications may vary among makes and models, but you can probably use .002" to .005" as your maximum runout limit. In the case of a hubless rotor, if excessive runout is found, index mark the rotor to the hub by placing a chalk mark at a stud and at the adjacent area on the rotor hat. Then relocate the rotor clockwise to the next stud position, and perform your runout measurement again. In this way, you're attempting to "match" the rotor to the hub. Due to potential minor deviations on the machined surfaces of both the rotor hat underside and the hub flange, repositioning the rotor may create an "optimum location" that minimizes total assembled runout.

If you want to check runout of the rotor independently of the hub, chuck the rotor on your lathe and perform a dial indicator reading. It's also a good idea, in an effort to remove other variables from the equation, to make a runout reading of the hub flange itself, with rotor removed. If the flange itself is causing the runout problem, you'll be able to isolate the cause.

WHEEL TIGHTENING PRECAUTIONS

To prevent fastener-tightening-related rotor warpage, make sure the studs are in serviceable condition (check for thread integrity and repair/replace studs as needed), and check the condition of the nuts' threads as well. Never use fasteners that are suspect. Also make sure all threaded locations are clean and free of dirt, grit or other contaminants (poor quality or unclean threads can easily result in incorrect torque values). And NEVER use an impact gun with a standard impact socket to tighten wheel nuts. I don't care what someone else tells you with regard to this. The only proper method of wheel installation, especially when dealing with alloy wheels and thin-hat rotors, is to hand-tighten all wheel fasteners.

The only possible exception is the use of a "Torqstik." This is a torsion-bar type anvil socket that is designed to relieve tightening force as the predetermined torque is reached. If you want to follow this approach, you'll need a torque stick for each size nut and torque range (they are available, color-coded for easy identification, in 17mm @ 55 ft lbs; 17mm @ 80 ft lbs; 19mm @ 65 ft lbs; 19mm @ 80 ft lbs; 19mm @ 100 ft lbs; 13/16"@ 100 ft lbs; 21mm @ 60 ft lbs; 21mm @ 80 ft lbs; 22mm @ 120 ft lbs; 22mm @ 140 ft lbs; and 22mm @ 170 ft lbs. Heavier ratings are available in 1" drive for super-heavy-duty applications as well). These Torqsticks are available from many sources. If you're determined to use an impact gun to tighten wheel fasteners, this is the only sensible approach. Always follow the proper tightening sequence:

 
• If installing a four bolt wheel, start at the 12 o'clock position first, then the 6 o'clock, then the 9 o'clock and finally at the 3 o'clock.

• For five bolt wheels, start at 12 o'clock, then the 5 o'clock, then 10 o'clock, then 2 o'clock and finally 7 o'clock.

• For a six bolt wheel, start at 12 o'clock, then 6 o'clock, then 2 o'clock, then 7 o'clock, then 5 o'clock and finally 10 o'clock.

This criss-cross routing enables you to evenly distribute the clamping force across the face of the wheel and rotor hub.  Using a random tightening pattern and guesswork clamping forces is a sure way to create a rotor warpage, ruining an otherwise perfect job.

  • On vehicles with hubless rotors, brake vibration and pedal pulsation may be caused by any of the following...
  • Improperly torqued wheel fasteners (torque uneven, too high, or not tightened in proper sequence)
  • Contamination (rust, dirt, crud) between hub and rotor hat
  • Damaged hub (check runout of rotor on lathe)
  • Extreme heat buildup/isolated hot spots
  • Worn/damaged wheel bearing
  • Excess rust/corrosion buildup in isolated spots on rotor

 

 

RECURRING RUNOUT

Recurring lateral runout can be a nagging problem. The customer may have complained of a pulsating pedal, and shortly after your shop resurfaced the rotor, the vehicle returns with the same complaint. Causes can include a number of possibilities. First perform a rotational torque test on each front wheel. Do this by applying and releasing the brake pedal 6-12 times.

Then rotate the wheel using a beam-type torque wrench on a lug nut, noting the amount of rotational drag indicated on the torque wrench. Next, open the caliper's bleed valve to relieve fluid pressure and re-test for rotational drag.

IF ROTATIONAL DRAG WAS REDUCED AFTER OPENING THE BLEEDER, check for pedal bind. Try to pull up on the pedal. If it moved, re-test the wheel for rotational drag. If drag was reduced, adjust or repair the pedal linkage, brake light switch or cruise control switch as necessary. If this wasn't the problem, check the vacuum booster by pumping the pedal with ignition off. If rotational drag was reduced after eliminating vacuum from the booster, the atmospheric valve is likely sticking, so replace the booster. Note: pull with about 40-60 ft lbs force. However, if the vehicle has ABS, pull the pedal gently, as the rod may be pulled out of its socket.

If the problem is still undiscovered, loosen the master cylinder and pull it away from the booster by 1/4"-1 /2" (don't loosen fluid lines). If this reduced rotational drag, the master cylinder piston is not fully returning. Repair the pedal pivot shaft/linkage, or replace the offending master cylinder, booster or pushrod. Note : on some compact cars, the pedal shaft or the firewall may have been distorted, preventing full pedal return. Check this as a possibility

If the drag was not reduced when the master was pulled away from the booster, check for fluid contamination. This may be indicated by a swollen or slimy rubber diaphragm in the master cylinder. If that's the case, rebuild or replace the master cylinder, and all hydraulic components (calipers, hoses, wheel cylinders) and flush the entire system and replace the brake fluid.

If no contamination is present, trace the fluid lines. Begin by opening the fluid line at the master cylinder. If rotational drag is reduced, replace the master cylinder. If drag was unaffected, open the fluid line below

the combination valve. If drag is reduced, replace the combination valve. If not, open the connection where the steel line meets the flexible hose. If drag is reduced, replace that steel line. If drag isn't reduced, replace the hose.

IF ROTATIONAL DRAG WAS NOT REDUCED AFTER OPENING THE BLEEDER, strike the caliper twice with a rubber mallet to relieve any potential mechanical binding at the piston or at the caliper slides.

If rotational drag was reduced after striking the caliper, rebuild or replace the caliper as needed, and clean and lube the slides. In the case of a twin-piston caliper, make sure both sides have the same type piston (steel/steel, etc.) Also make sure the sliding pins are of the latest style, and use only silicone grease for slide/pin lubrication.

If drag still has not been reduced, check the rear brake function (road test). Accelerate to 20 mph, and carefully apply the parking brake (vehicle must be travelling straight, and you must hold the release button down during this to avoid loss of vehicle control! Perform this test in your parking lot or in a nearby large parking lot if possible). Service the rear brakes as needed to obtain full stopping ability. Adjust the height-sensing proportioning valve linkage if necessary.

Check the wear pattern on the brake pedal. If excess left side wear is indicated, the driver has likely been riding the brakes with his/her left foot, and dragging the pads as a result (increasing heat buildup). If excess right side wear is shown on the pedal, excess brake wear/heat buildup may be the result of towing or other heavy-duty use.

When a rotor warps, even though remachining may regain disc "flatness," the rotor may retain an internal stress "memory" will cause the rotor to eventually warp again. Aside from using a stress relieving process (such as cryogenics or vibrational stress relief, processes that are used in some racing applications to stabilize metals), the only answer is to replace the rotor, if recurring "warpage" continues.