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Brakes

Double-Disc Ground Rotors

 

Many of the brake rotors supplied by Autospecialty, as well as by OE manufacturers, have a double disc ground finish. This finish is easily identified by the non-directional or "swirled" friction surface.

Double disc grinding is the only procedure that ensures perfect parallelism and virtually eliminates runout and taper of the rotor surface. This finishing procedures, required by all OE manufacturers, is accomplished by using specialized CNC precision grinding equipment costing nearly $750,000. Tolerances are held to +/- .001 of an inch.

Many automotive technicians make it a practice to check runout on a new rotor before installation. This procedure is usually done using a brake lathe and a dial indicator. However, brake lathes are incapable of matching the tolerances used in the rotor finishing for these reasons:

  1. The lathe arbor is only supported on one side.
  2. The arbor has runout at the unsupported end.
  3. The ends of the spacers and double taper adapters are not machined perfectly square. When a rotor is mounted on the lathe and the arbor nut is tightened, the spacers and adapters may shift, causing the rotor to shift on its axis. This actually introduces runout and will give the illusion that a perfect rotor is warped.

THIS PHENOMENON CAN BE DEMONSTRATED ON ANY BRAKE LATHE BY THE FOLLOWING PROCEDURE.

  1. Mount a rotor (a double-disc ground rotor is preferable) with appropriate adapters and/or spacers and tighten the arbor nut. Note the torque used on the arbor nut.
  2. Mount a dial indicator solidly to read runout near the outer edge of the friction surface.
  3. Locate and mark the low and high spots on the rotor by rotating the rotor 360 degrees. Also, mark the location of the arbor in relation to the marks on the rotor.
  4. Loosen the arbor nut and, without moving any spacers, adapters, or arbor, rotate the rotor 180 degrees and retorque the arbor nut.
  5. Repeat step 3. If the low and high spots are in the same locations in relation to the mark on the arbor then the arbor and/or spacers and adapters are causing the rotor to exhibit runout. The low and high spots will appear to be reversed on the rotor.
  6. To determine the cause of the runout, check the unsupported end of the arbor with a dial indicator. A couple of thousandths of an inch here will translate to several thousandths on the mounted rotor.

(from Autospecialty catalogue)

 

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