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Overheard at PFE-Southwest

Written by Ed Sanow

The most mistaken topic in all of police fleet maintenance is probably the myth that brake pedals pulsate and steering wheels vibrate because of “warped” rotors. Of course, rotors don’t warp from heat to become wavy like potato chips. Rotors don’t warp – they wear unevenly.  

The problem of pedal vibration, incorrectly called rotor warp, occurs about 4,000 miles after the brake or tire change. Since it is caused by uneven and not heat, you can’t solve this wear problem by better control of the heat, i.e., by the use of specially processed rotors or by the use of drilled and slotted rotors. Instead, you solve the wear problem by fixing the shortcuts in rotor installation or the improper lug nut tightening.

“But the rotors look wavy.” Yes, they appear wavy due to either uneven wear around the swept area (if semi-metallic pads were used) or due to uneven build-up of friction material (if ceramic pads were used).

Do you want proof that it is a wear problem and not a heat problem? Measure the thickness of the rotor at the 12, 3, 6 and 9 o’clock position. If the rotor has warped by heat like a potato chip, then the thickness will be the same everywhere. If the rotor has been worn unevenly by intermittent contact with the brake pad, the thickness will be different – you will find one high spot and one low spot.

Further proof of the fact that it is uneven wear and not heat is one of the more common solutions. The shim put between the hub and the rotor to fix the problem has one high spot and one low spot! Match the high spot on the shim with the low spot on the rotor, and the problem is solved.

Now that you are looking at the problem as a wear issue, what causes it? The root cause of the uneven wear is one of two things. Either the rotor is out-of-true with the hub, which could be a poorly machined rotor or hub or both. Or the wheel was improperly torqued to the hub during the last tire change.

The last step in a professional brake job includes the use of a dial indicator to measure the total runout of the rotor mounted on the hub. It must be less than 0.002-inch, the OE spec for most vehicles. Two, for each tire and wheel change or rotation, be sure to torque the lugs in a star pattern and be sure to use either a torque wrench or torque sticks. Those two steps will virtually eliminate premature rotor wear, period.

 

Replacement Cycles

One critical point was omitted in the March-April “Police Lane” justifying the purchase of new vehicles – idle hours. Ford Fleet measures this at 33 miles of engine wear per hour of idling. Dodge Fleet has echoed that wear during idling, citing 29 miles per idle hour.

A 2.5-year-old, take-home vehicle has 125K miles on the odometer. It idles for just two hours a day. (Fill in your own actual figures.) The true miles, including idle time, for this most conservative example is over 166K miles. If this vehicle idled for six hours a day, the idle hour wear would double the odometer reading to 250K miles.

More than the engine gets idle wear. All of the moving parts on the engine and the engine-driven accessories see this wear-at-idle, like the power steering pump but especially the alternator. If the alternator is involved, so is the battery and all of the two dozen electronic modules.

If the engine is running, many parts of the transmission are involved. This certainly includes the torque converter, but also all of the seals and gaskets, since the trans fluid is under constant, full pressure. The fuel pump is in use, and is pumping about a half gallon of fuel per hour and is under constant, full pressure. Think about the electric motors running the cooling fans. Think engine coolant and the 100K-mile coolant life. Even a tight-fisted admin or council looks at a 250K-mile emergency vehicle differently than one with 125K miles.


Published in Police Fleet Manager, Jul/Aug 2012

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