The hub flange is ground zero for pulsation problems. The rotor will magnify any runout in the flange but how do you know when to replace a flange, stub axle or an entire hub unit? The answer can be confusing and will take some math and a micrometer.
All flanges have some degree of runout. This is why there are tolerances in the service specification and in manufacturing. Most vehicles can tolerate a certain degree of runout before the driver begins to feel pulsation.
A flange may have up to .005” of runout and the bearings may have zero end play or noise. It is impossible to see damage with the naked eye.
This is why measuring with a dial indicator as part of a brake job is important. Damage to the flange is usually found on the backside of the flange where it meets the shaft and inner race.
How Did Damage Occur?
A flange can be damaged by curb strikes, installation errors or even corrosion. Extreme or catastrophic flange damage due to a curb strike is rare. But, some curb strikes can tweak the flange and cause runout. In most cases, the wheel and knuckle will deform before the flange or stub axle.
If a hub or bearing is over torqued, it can damage the flange and cause runout. It can also damage the bearings and the axle shaft. This type of runout cannot be corrected by re-torquing the axle nut. The damage is permanent.
Pitting and corrosion where the rotor mates to the flange can induce runout. If the corrosion is too great, replace the flange or bearing. Also, the flange can be damaged by excessive and uneven lug nut torque.
How Much Is Too Much?
When diagnosing runout in a flange, a few things have to be taken into consideration before replacement. First, take into consideration the vehicle. On a small vehicle, .003” of runout in the flange may be too much. On a full-sized pickup this may be acceptable.
Some OEMs may have a specification and replacement criteria for the flange runout but many do not. Always check for service information.
Another consideration is the rotor. If you are using an on-the-car brake lathe, there is only so much meat on the rotor to remove to compensate for lateral runout. If too much material is removed, it could create a rotor that is thermally unbalanced.
Most on-the-car brake lathes with automatic runout compensation will not allow the user to continue if the amount of lateral runout is too large. Either the machine will cite an error with the rotor or mounting of the lathe.
One option to save a flange is the rotor correction plate or shim. 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. Change the rotor position 180 degrees on the flange and check runout. If the high spot changes 180 degrees, the rotor could be OK or ready to turn after the bearing is shimmed.
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 you 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. 
Replacement
As a rule of thumb, runout greater than .005” for a light vehicle is a sign that the flange may be damaged or out of specification and further corrective action must be taken.
If all attempts have been made to correct runout and it can’t be brought below .002” or recommended specification, the final option is replacement of the flange, stub axle or wheel bearing hub unit.
