Disc Rotor Installation and Truing
This article will discuss disc brake rotor installation and truing.
Disc caliper brakes slow the bike by converting the speed and energy of the bicycle into heat. The system can generate significant heat from slowing the wheel and bike. Allow rotor and caliper to cool before touching or servicing.
The rotor diameter may vary between models and brands. Common rotor diameter sizes are 145mm, 152mm, 160mm, 185mm, and 203mm. The brake caliper, bike frame, and rotor diameter must be compatible.
The rotor or disc of the disc brake system secures to a disc-specific hub. The common system uses six bolts. Many brands use rotor bolts requiring the use of a 12-point star shaped driver such as the Park Tool PH-T25, or Torx® driver. A mild thread locker is recommended on the bolts. Secure rotor bolts to manufacturer’s torque specifications, typically between 40 and 60 inch-pounds.
Some Shimano® rotors use a splined fitting system with a lockring, similar to a cassette lockring. The spline system of the rotor matches splines at the hub. Use the Park Tool FR-5.2 to secure the lockring to 350 inch-pounds. The Shimano® Saint and Hope rotors require a specially sized Shimano® tool.
The rotor and brake pads should be kept clean of oils and grease. If pads become contaminated, it is best to replace them. When cleaning the rotors or washing the bike, remove wheel and remove pads from bike. Use isopropyl alcohol or similar solvent when cleaning rotor surface of dirt or film. Do not use a solvent or cleaner that contains oils or leaves an oily residue.
Rotors may become bent or warped with use and abuse. Some re-bending may be possible, but rotor replacement is typically the best option. The Park Tool DT-2 Rotor Truing Fork allows you to subtly bend and align the rotor. Watch the wobble at the caliper. Place the long section of the DT-2 over the rotor and pull or push as appropriate. It only takes a small amount of effort to move the rotor. Sight rotor and re-bend as needed.
It is also possible to bend the rotor at the rotor arms. This is useful when there is long bend at the rotor perimiter. Use the short opening in the DT-2. It allows the tool to be at a 90-degree offset to the rotor face.
The DT-3 Rotor Truing Gauge will help speed the sighting and correcting of the rotor alignment. The DT-3 mounts to the TS-2.2 Truing Stand. Mount a wheel in the truing stand, with rotor facing the DT-3. Align the moveable gauge to the outer perimeter of the rotor and spin the wheel. Move the gauge end close to the rotor face and note any rotor movement, similar to truing a wheel with a truing stand.
The DT-3 can be fitted with a dial indicator, the DT-3i.2 Dial Indicator Kit. The DT-3i.2 magnifies the run-out, allowing subtle bends in the rotor to be easily located. The DT-3i.2 reads in increments of .01 mm and uses a “balanced face”, reading both clockwise and counterclockwise from “0.”
Mount the DT-3i.2 to the end of the DT-3. Rotate the dial as necessary to allow viewing of the face. Back the threaded gauge away from the rotor. The DT-3i.2 plunger end will contact the rotor. Use the quick release of the DT-3 to move the plunger of DT-3i.2 to the outer perimeter of the rotor. Continue to press the DT-3i.2 into the rotor to compress the plunger a few millimeters. Secure quick release skewer.
Slowly rotate the wheel and note movement of needle on dial. Locate a section of rotor that is true, with relatively little movement. This section will serve as the “0” reference point for sections requiring truing. Loosen the dial face locking knob and move the rotor face of the dial indicator until the needle lines up with “0.”
Spin the wheel and note any movement away from the the zero mark. Use the DT-2 to align the rotor. The DT-3i.2 will then show relative movement off the zero mark.