The design of hat type rotors has made the job of servicing the disc rotor much quicker and easier but has also introduced some potentially nasty problems if the rotor is not mounted correctly. When a hat type disc rotor is manufactured, it is machined so that the mounting face and the friction faces are parallel to within a very fine tolerance. The mounting face on the disc rotor and the precision machined face of the hub are designed to allow the disc rotor to be mounted with minimal runout.
During the life of the disc rotor on the vehicle a small quantity of rust scale may appear between these two precision faces. If this scale is not removed before refitting the existing rotor or the new replacement rotor then it will cause the rotor to be mounted unevenly resulting in excessive runout. When the vehicle is driven with this excessive runout it results in uneven rotor wear as the disc intermittently touches alternate pads in alternate places on the disc rotor. As the disc rotor rubs against the pad it wears, so over a relatively short period of time the disc is worn unevenly to the point that the two parallel friction faces are no longer parallel. This is referred to as Disc Thickness Variation, or DTV, the severity of DTV will proportionally determine the severity of brake shudder / pedal pulsation.
To greatly reduce the chance of DTV developing it is very important to clean the rotor and hub mounting faces, including removing any burs that may have formed on the hub face while using removal screws, before installation. It is also good practice to check the runout at the friction face and make sure that the runout doesn’t exceed 0.05mm after installation. Once these things have been done you can refit the wheels and torque the wheel nuts up to the manufacturer’s specifications. You can now be confident that you have minimized the chance of the vehicle coming back with a DTV related brake shudder problem.
Disc rotor ‘B’ shows excessive scale build up which is typical of a rotor mounted on a hub that hasn’t been cleaned properly. The hub contact area is uneven and as a result the rotor had significant DTV develop within less than 2,000 kms. The reference disc rotor, disc ‘R’, shows a typical contact face when fitted correctly to a clean hub. The reference rotor was replaced due to it being worn to its service limit.
This image shows the typical rust scale that develops on the face of a hub. This rust scale must be removed before fitting the new disc rotor and the face of the same hub after cleaning properly ready for the new disc rotor to be fitted.
Disc Brakes Australia