Modern Tire Dealer

APR 2019

Magazine for the professional tire industry

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Page 35 of 85

B r a k e s M T D A p r i l 2 0 1 9 34 pressure. At higher temperatures, it expands, requiring more pedal pressure and more pedal travel. If the brakes are applied rapidly (quickly pumping the pedal manually during a panic stop), the fluid can tend to aerate and foam, resulting in a drop of pressure. For this same reason, never use silicone fluid in a vehicle that's equipped with ABS. When ABS activates, fluid pres- sure is modulated quickly, which can easily result in the foaming issue. PEDAL PULSATION A variety of conditions can cause a pedal pulsation or vibration problem that can be traced to rotor runout and/or excessive rotor movement from its proper operating plane. Wheel bearing pre-load (looseness of the bearing), rotor thickness variation, rotor runout, hub runout, combined rotor and hub runout and improper tightening of the wheel assembly can each play a part in creating a pedal pulsation. In addition to runout problems, brake pedal pulsation may be caused by varia- tions in brake rotor thickness. Brake rotor thickness variation (not to be confused with rotor runout) causes brake pedal pulsation to occur due to changes in braking force as thick/thin portions of the rotor contact the brake pads. In order to inspect the rotor for thickness variation, measure the rotor with an outside micrometer in eight equally spaced locations. The thickness measurements should be taken approximately 0.40 in. (10 mm) from the rotor's outer edge. Measure rotor disc thickness at eight equidistant points along the rotor (12 o'clock, at 45 degrees, 90 degrees, 135 degrees, 180 degrees, 225 degrees, 270 degrees and 315 degrees). Record all mea- surements. e total thickness variation equals the maximum thickness minus the minimum thickness. If total variation is beyond the specification for that vehicle, the rotor may be resurfaced (only using an on-the-car caliper mounted lathe), or replaced. We've addressed this issue many times before, but it always bears repeating. A pulsating/bouncing brake pedal is likely due to a warped brake rotor or excessive lateral runout. is causes the brake pads to move in and out of their bores as they contact the uneven rotor surface. Checking for runout is simple. First make sure that the brake rotor is fully secured to the hub using all of that wheel position's fasteners, and torque all fasteners to the specification listed for the wheel mounting. You must simulate the installed wheel condition in order to achieve an accurate runout reading. Mount a dial indicator (clamp-on or magnetic base style) and place the indicator plunger 90 degrees to the rotor surface, about 1/2-inch from the rotor outer edge. Adjust the indicator with about 0.050-inch preload, and then zero the gauge. Slowly rotate the rotor/hub a full 360 degrees and note runout. If more than about 0.0025- inch runout is found, correction is needed. Before assuming that you need to replace a warped rotor, place matchmarks at one of the wheel fastener locations (mark the stud and that fastener location on the rotor hat). Remove the rotor and re-install at about a 45-degree clock position and re-check runout. You may be able to minimize/ correct the runout by changing the clock position of the rotor relative to the hub. Wheel fastener torque can affect rotor runout. Excessive and/or uneven applied fastener torque, especially with regard to a thin-hat rotor design, can easily distort the rotor. While shop time is valuable, the use of an impact wrench is not recommended for wheel installation. Especially in the case of alloy wheels and thin-hat brake rotors, use only a calibrated torque wrench, and always follow the proper tightening sequence in order to evenly spread the clamping load across the hub face. Another possible cause of a brake "pulse" may be the result of front wheel hub service on a vehicle equipped with ABS, when only one hub has been replaced. If an ABS warning light illuminates and stays on, and your scanner indicates an issue at a specific location (right front wheel speed sensor, for example), remove the sensor and inspect for con- taminants before replacing the sensor. ABS wheel speed sensors pick up wheel speed signals from a tone ring that is built into the hub or CV outer joint. A specific air gap exists between the tone ring and sensor. If the tone ring is part of the hub and only one hub is replaced during service, the new hub will feature a tight per-spec air gap, while the op- posite hub may be worn resulting in excessive air gap. If one front hub needs to be replaced, it's best to replace both. A differential in the signals can cause a false ABS light activation. Rust/debris buildup at the air gap location also can cause issues. If the vehicle is equipped with ABS, the OE system bleeding steps must be fol- lowed to ensure that the ABS module functions properly. If in doubt, always refer to the vehicle's service manual. Original equipment or aftermarket brake rotors may feature slots that are intended to help clean the pads and evacuate pressure gases. Especially for emergency vehicle applications such as police vehicles, when rotors are replaced, rotors and pads must provide the same type of performance in order to maintain superior braking.

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