Typical Disc Rotor Service Problems

Disc brakes made their first appearance in the 1930s but did not see wide acceptance in the automotive market until the mid to late 1960s.

Today the disc brake is the preferred braking system for stopping the modern automobile and truck. The system is lighter, stops faster, and is more efficient at dissipating heat. It is also much easier to service than the older drum-type systems it has replaced.

Commercial trucks are now utilizing disc brakes as well for all of the same reasons as the passenger car.

Four-wheel discs are becoming the norm for the industry and in the near future will become standard equipment for all passenger cars as well as most light and medium-duty trucks.

Although the technology is not new we sometimes still struggle to understand and diagnose disc rotor problems. Let's take the time to look at a few simple steps to help correctly diagnose and service disc rotors correctly.

  • ROTOR RUN-OUT
Customer Complaint - Vibration or shake in the steering wheel upon brake application.
Run-out is a distortion of the disc rotor and may be attributed to wear in the disc and/or hub. It causes an oscillation in the brake disc rotor and is felt in the steering wheel.

  • DISC THICKNESS VARIATION (DTV)
Customer Complaint - Pedal pulsation upon brake application.
DTV is the difference in the rotor disc thickness measured at several points around the circumference of the friction surface. DTV can be felt in the brake pedal as a pulse.

  • SURFACE ROUGHNESS
Customer Complaint - Excessive brake noise.
Roughness causes either a low or high frequency or vibration of the brake components. So, when the surface finish does not conform to the OEM Specifications, the result is excessive noise.

SERVICE PROCEDURES

NOTE: Always remember, do not replace pads without servicing the rotor, stopping distances can increase by as much as 20% without a correctly serviced rotor.

To service rotors that are not performing up to specification, you can proceed with the following:

  1. Replacement - Not always the most cost-effective way to solve a problem, and even if it is cost effective, it may not by itself solve the problem. If run-out was the original concern and the problem is actully in the hub assembly and not the disc you will have the same situation after replacement. This is why most new car manufacturers require their technicians to true new rotor using an on-car disc rotor lathe after installation.
  2. Machining the Rotor - This can be performed by either an on-the-car bench type lathe. The secret to success is not only to use the correct equipment but to also use the equipment correctly.
  • On-the-car lathe - This is the most effective way to correct rotors mounted to a highly preloaded bearing such as in today's front-wheel drive configurations. By truing rotors "on the car", it eliminates hub and bearing variations thus totally eliminating run-out.

  • Bench lathe - Used in situations where the axle or bearings "float" and are not pre-loaded to "0" tolerances. A high percentage of new cars use rear rotors that are mounted on "floating axles" and are best serviced by bench-type lathes. Earlier rear-wheel-drive vehicles, as well as some light and medium-duty trucks, use conventional tapered bearings in a front hub assembly and are also best serviced with a bench lathe.

After selecting the correct equipment, the correct use of the equipment then becomes important. Selecting the correct adapters and/or mounting hardware is critical to being successful at servicing the rotor.

Closely follow the equipment manufacturer's directions. This is very important and will most assuredly help provide a successful machine operation. If in doubt, always contact the manufacturer for assistance.

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