The Run Down on Run Out
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Brake disc run out, brake disc thickness variation or "non-parallelism," disc-to-axle flange inaccuracies and play in wheel bearings are all root causes of air pumping.
Run-out exists when a given point on the brake disc does not rotate in a constant plane, ie: that given point runs in-and-out or wobbles. This can be caused by inaccuracies in the rotor itself or, at the rear, by the rotor and rear axle flange surfaces not being parallel. If a brake disc has a thickness variation, its inner and outer friction surfaces are not parallel.
Both of these situations, run-out and nonparallelism, cause the brake pistons to move in and out as high and low spots on the brake disc pass between the brake pads. This produces the cyclical, non-brake pressure induced piston movement that gets you air pumping.
The most accurate measurement of brake disc run out and nonparallelism comes when the rotor in question is installed on a brake lathe, a piece of shop equipment used to machine rotors prior to use. A dial indicator is fixed to a magnetic base mounted somewhere on the lathe. The indicator stem is adjusted such that it is perpendicular to the rotor friction surface and touches the surface near the rotor’s outer rim. Both friction surfaces should be measured. If the quantities are different, not only is there run out, but nonparallelism as well, a figure that is derived by subtracting the two run out numbers. In the front, precise measurement of run out can be made on the car as long as the wheel bearing clearance is temporarily reduced to zero by tightening the bearing adjuster nut to 15 ft/lbs. Run out of rear brake rotors can not be measured accurately with the rotors on the car because the effects of rear wheel bearing clearance cannot be eliminated. Rear rotors must be removed and chucked into a brake lathe.
Now, consider wheel bearing clearance. It causes axial movement of the brake disc which can affect the brake pistons the same way as run-out. Obviously, bearings must have some clearance. However, the more clearance that exists; the less rotor run out is acceptable, because the effects of both are cumulative, but yet, cannot exceed the .010-in. limit, otherwise–you suck air.
Proper procedure must also be followed to correctly measure front wheel bearing end play or clearance. One good way of doing this was discussed in Vette’s October ’93 story on front wheel bearings. A more accurate, but more complex alternative is to use Global’s method. A special tool must be made consisting of a piece of flat, steel stock welded across the top of a wheel bearing adjuster nut. This tool is threaded onto the spindle and hand tightened against the adjuster nut. Use a dial indicator on a magnetic base. The base is attached to the section of flat stock such that the indicator stem is parallel to the spindle axis and can touch the rim of the hub just outside of the wheel bearing. Slip a big screw driver into one of the rotor vents. Push on the rotor and watch the indicator.
This method of checking bearing clearance because it minimizes the multiplicative effect of the brake disc’s radius which can cause significant error in the reading.
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