TORQUING WHEELS AND WHY IT IS SO IMPORTANT.
Brake pedal pulsation may be caused by improper machining of disc rotors, normal wear,
rust on the mounting surface or by improper torqueing of wheel nuts.
In reality, most brake pedal pulsation problems can be traced to improper torquing of wheel
nuts, which distorts the disc rotor, and causes runout. Runout of .07 mm or .003 inch may
cause DTV (‘Disc Thickness Variation’) after 5,000 to 6,000 kms on later model motor
vehicles.
DTV is caused by the disc pads scalloping out the high spots on a rotor with runout, as they
continue to rub along the swept surface area of the rotor, while the brakes are in the off
position. Once DVT develops brake pedal pulsation will become a problem.
To check for runout use a dial indicator. Also check that the rotor does not have a runout
problem because of scale build up between the ‘hat’ section of a hubless rotor and the hub
area.
Remember it’s the original torquing of the wheel nuts that caused the rotor runout that then
caused the DTV that finally caused the pedal pulsation problem!
To prevent this always, and we repeat ALWAYS, tighten the wheel nuts in a star pattern, to
the correct torque specification.
There are a variety of tools available to assure proper torquing of wheel nuts (always refer to
the vehicle manufacturer’s manual for torque specifications).
A quick and easy method of assuring that the wheel nuts are torqued correctly is by using a
torque stick that is rated for the correct amount of torque specified in the manufacturer’s
manual. However, it is possible to get an incorrect torque when using these torque sticks,
which is usually over torquing rather than under torquing of the wheel nuts.
This over torquing is possible if you hold your hand on the shaft of the torque stick while
applying pressure through an impact gun. By holding onto the shaft you may dampen the
vibrations, changing the built-in torque limiting properties of the torque stick.
Finally, after you have torqued the wheels correctly, be sure to explain to the vehicle owner
the importance of torquing wheel nuts correctly. In most cases when the vehicle comes back
to your workshop with brake pedal pulsation and you are satisfied that you have not distorted
the disc rotor, you’ll find the wheels have been removed since you worked on it, possibly to
have tyres fitted and they have incorrectly used an impact gun and created the problem.
In cases such as this, the customer returns the vehicle to your workshop, not understanding
how having the tyres fitted can create the brake pedal pulsation. It is also good customer
relations to let them know how much trouble you went to so as to do the job correctly,
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