Let the Chips Fly: ON-CAR & OFF-CAR BRAKE ROTOR SERVICE

Motor,  Jun 2004  by Seyfert, Karl

• E-mail
• Print
• Link

Another potential cause of excessive runout (one that's common with thinhat hubless rotors) is poor wheel installation practice. If the wheel fasteners were improperly tightened during wheel installation, it's very likely that the rotor hat may warp due to uneven or excessive fastener torque.

Measuring Runout

Use a dial indicator to check lateral runout. Mount it securely to a stationary fixture. For on-car measuring, attach the fixture to the spindle or control arm. Locate the dial indicator's plunge tip about an inch inboard of the rotor edge, and zero the indicator.

Runout is defined as the amount of lateral (side-to-side) movement of the rotor as it rotates through 360°. The average allowable specification for latemodel vehicles is between .001 and .003 in. This is the total indicated runout specification, as measured on the vehicle. Total indicated runout can be affected by several factors-hub runout, cleanliness of the hub-to-rotor mating surface and wheel lug torque.

Runout problems on hubless rotors occasionally can be corrected by indexing the rotor. Place a chalk mark on a stud and at the adjacent area on the rotor hat. Now relocate the rotor clockwise to the next stud position before repeating your runout measurement. Repositioning the rotor may minimize the total assembled runout caused by minor deviations on the machined surfaces of both the underside of the rotor hat and the hub flange. It may take a few tries to find the best match-up.

Rotor runout can be measured independent of the hub by removing it from the vehicle and mounting it on an offcar brake lathe. Likewise, hub runout can be measured without the rotor in place, to remove as many variables as possible during a complicated runout diagnosis. If the hub flange or rotor are causing the runout problem, you'll be able to isolate them in this way.

On-Car vs. Off-Car Rotor Resurfacing

When used properly, both on-car and off-car brake lathes can provide excellent results. On-car lathes were originally developed for vehicles with "captive" rotors, and others known to be sensitive to runout problems. In fact, some vehicle manufacturers specify on-car brake lathes only for resurfacing the rotors on their vehicles.

On captive rotor systems, the rotor is mounted behind the hub. Removing the rotor means removing the hub, which may then be difficult to mount on an off-car lathe for accurate machining. Unless you compensate for the rotor runout that exists on the vehicle, you can't guarantee a true-running rotor when it's reinstalled. When possible, rotors on these vehicles should be machined in place.

Shims are also available to compensate for runout on rotors that have been machined on an off-car lathe. The shims are available in a variety of sizes, and fit between the hub flange and the rotor hat. Shifting the position of the shim "zeroes out" the assembled runout.

On-car lathes are designed to compensate for runout problems, and do not require major disassembly of the hub and rotor on captive rotor brake systems. Some even compensate for runout automatically, although setup still requires a properly trained operator.