The Importance of Correct Wheel Nut Torque

 

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 wrench 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 wrenches, 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 wrench 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 wrench.

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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