The vehicle owner may say that under light braking the vehicle pulses to a stop, or under heavy braking the vehicle shudders. Technical service bulletins (TSBs) often describe these symptoms as a “judder.” There are only two causes for judder — lateral runout and disc thickness variation on the rotor’s friction surface.
What could have caused the lateral runout? The causes of the condition range from lugs that were over or unevenly torqued to common corrosion between the hub and rotor.
There are other causes, such as the rotor machined to less-than-minimum thickness and overheating. Correcting the condition requires the machining or replacement of the rotor. Whether you are turning a rotor on or off the vehicle or replacing it, preparation is the key to success.
The greatest cause of lateral runout comebacks is not poor OEM engineering or low-quality aftermarket parts, it is ignorance and carelessness. Don’t take this the wrong way, but some of your instincts and methods of operation could be costing you. By no means is this written from the perspective of talking down to the technician, it is just some self-introspection can even help the best technician. Take the time to perform a diagnostic on yourself. You might be surprised.
You are Only as Good as Your Tools
Both on-the-car or off-the-car brake lathes are precision machines that must be maintained and treated with great care. Dirt and metal chips can shorten the life span of any lathe and lead to comebacks. When was the last time you checked the runout of the arbor and cutting head? Remember, the cut is only as true as the arbor, spindle and adapters.
Look at the condition of your adapters. A weekly cleaning and lubrication should be performed. The adapters should be trued to the arbor on a monthly basis. After they are trued, all nicks and burrs should be removed with a fine emery cloth. Coat the adapters with a light coat of oil.
Today, you can purchase an on-the-car brake lathe that, after it has been attached to the vehicle, will automatically properly compensate for runout. It is quick and easy.
But, wait a minute, most manufacturers have written extensive bulletins on the proper methods of turning brake rotors. All of the bulletins begin with the proper preparation and measurement of the components before turning the rotor. Most dealers are required to have a runout compensating on-the-car brake lathe by the OEMs, and the OEMs are paying the warranty time. So why would they begin a TSB with this “extra” check before turning the rotor? Simple, it is a quality check that helps to eliminate runout problems that could actually be made worse by turning the rotor.
Turning a rotor is a precision operation that requires a clean and rust-free mounting surface. There are tools available to clean the bearing flange and rotor mounting surfaces. They range from a rotating abrasive to hooded sandblasters. Vented rotors can be a source of rust that can range from dust to chunks coming from the vent passages. It is a good idea to knock loose the rust with a wire brush to prevent it from getting on mounting surfaces and turning equipment.
Once the rust has settled, pull out your dial gauge and go to work. The arbor of a bench lathe is essentially the wheel bearing when cutting on the car. The maximum endplay for a bearing is 0.004” (0.100 mm). This is a good reason to check flange runout and endplay before installing the lathe adapter.
The wheel bearing is the heart of the assembly. If the bearing flange has a runout, that runout will be magnified at the rotor friction surface. A runout of 0.0005 inch (0.00254 mm) at the bearing flange could result in a 0.001 inch (0.025 mm) runout at the rotor friction surface. The maximum allowable runout at the rotor friction surface is 0.002 inch (0.050 mm) as stated by most manufacturers.
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 180º on the bearing can check flange runout. 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. Mark the relation of the rotor to the bearing flange. Also, mark the rotor high and low runout spots on the rotor friction surface — the low spot marked as zero and the high spot as 0.005”. Mark the high and low runout spots on the bearing flange with the same method and the rotor friction surface.
Once you have collected the data, the following comparisons should be made. If the endplay exceeds manufacturer’s specifications, replace the bearing and recheck runout. 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.
Hub and Rotor Interaction
How the rotor interfaces with the hub can determine runout. Think of the rotor as the peanut butter in a peanut butter and jelly sandwich and the hub and wheel as the slices of bread. If you push down on one corner, the peanut butter will shoot out the opposite corner. If you tighten lug nuts in a circular pattern, stress is placed on the rotor hat section and bearing flange unevenly.
All wheels should be mounted by using a crossing pattern to tighten the lugs.
Pulling it Together
Most knowledge and learning for technicians about trueing rotors is picked up in bits and pieces from various sources and failures. It is learning with don’ts and very little do’s. The following is a list of how it should be done when installing new rotors or machining old rotors.
Installing New Rotors
Check bearing end play: End play is an indication of bearing condition and adjustment.
Clean the bearing flange: Surface should free of debris for a good dial indicator reading.
Check bearing flange runout: Flange runout is magnified at the rotor friction surface.
Install new rotor: Torque the rotor on the flange to check alignment to the knuckle.
Check new rotor runout: It’s a double check before installing the wheel.
Turning Rotors (off vehicle)
Check lateral runout: Mark high and low spot (a shim maybe required to correct the runout).
Check rotor thickness: Compare to runout for minimum thickness before and after turning.
Clean the bearing flange.
Clean the rotor-to-bearing mounting surface.
Check bearing end play: Should be no more than 0.004 inch (0.100 mm).
Check bearing flange runout: Change rotor position on bearing.
Check turned rotor runout and thickness: It’s a double check before installing the wheel.
Turning Rotors (on vehicle)
Check rotor thickness.
Check bearing end play.
Check lateral runout.
Clean wheel and rotor mounting surface.
Mount lathe according to manufacturer’s instructions.
Machine the rotor.
Check turned rotor runout and thickness.
The steps above may look simple, but used as a framework for basic brake repair, they can go a long way to ensuring quality and reducing comebacks.