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Proper Torque Prevents Rotor Warp
Brake rotors do not warp from heat. Instead, they wear unevenly. This uneven wear is caused by the brake pads themselves as they intermittently touch an out-of-true rotor. Either the rotor was installed out-of-true with the hub, or the tire was improperly torqued to the hub during the last tire change. You won’t solve a problem caused by wear by treating it as a problem caused by heat. Instead, look at tire/wheel lug nut torquing.
Star-pattern tightening with torque wrenches used to be the only way lug nuts were tightened. But that’s no longer the case. Today, it’s throw the tire and wheel over the lugs, hand-start the five lug nuts, then use an air wrench in a circle pattern one time around, and you are done. This is almost guaranteed to cause uneven rotor wear and the tell-tale pedal pulsations in just a few thousand miles.
The unevenly torqued rotor, or an out of sequence torqued rotor, will not be bent out of true when the rotor is cold. However, as the rotor heats up in normal use, it will expand unevenly. As the rotor heats up and expands, an excessive lateral runout will be caused, i.e., a high spot on one side and a spot on the other. The high spot will come into intermittent contact with the retracted pad during normal driving, i.e., without brake pedal pressure.
As this happens, the semi-metallic pad used with police cars will grind away the high spot. Over just a few thousand miles, the rotor will have a significant thickness variation, either from a worn away rotor or material transfer from the pad. This rotor thickness variation causes brake pedal pulsation and steering wheel vibration!
These causes of premature rotor wear can easily be fixed. First, use a star-pattern instead of a circle-pattern to tighten the lugs. The wrong tightening sequence, even with the right amount of torque, can cause the rotor runout problem. With a circle-pattern sequence, the most uneven area will be near the first and last tightened lugs. The last few lugs are pre-loading the rotor like a spring. What’s worse, even if perfectly tightened to 100 ft-lbs, the last few lugs are false readings. Some of the torque is absorbed in flexing the rotor, not tightening the lug.
Second, for those using a torque wrench, tighten the lugs with the socket wrench, then final tighten them with the torque wrench. Don’t overdo it with the socket wrench. The torque wrench must be allowed to tighten the lug at least a little. If it clicks out without tightening the nut at all, the nut could be way overtorqued. (That is what happens if you use a breaker bar for the initial tightening.)
Third, for those using an air wrench, use torque sticks, or set the air wrenches under the necessary torque and finish them off with a torque wrench. Uneven amounts of torque, even with the right tightening sequence, can cause the rotor runout problem.
The air wrench increases or decreases in torque as it is being used. The line pressure falls, the air compressor kicks in, the line pressure increases. Any torque variation over 10 percent is bad, i.e., one lug tightened to 95 ft-lbs and one tightened to 110 ft-lbs. The result is exactly the same “expands unevenly as it warms up” situation as in the circular tightening sequence situation.
It’s best if you remember the following: Rule One: star pattern tightening sequence; Rule Two: final tighten with torque sticks or a torque wrench. Follow these rules every time you replace a tire, and many of the premature rotor wear problems, and pedal pulsation, will go away.
Published in Police Fleet Manager, Jul/Aug 2010
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