Maintenance and Idle
By: Ed Sanow
The most mistaken topic in all of police fleet maintenance
is 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 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
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.
One critical point was omitted in my August “On The Job” 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 ½ 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.