How Does an Over-Torque Warp a Brake Rotor?by Jody L. Campbell
Every vehicle on the road has a specific wheel nut torque specification. Inexperienced do-it-yourself mechanics or even ones employed at repair facilities may have been guilty of over-torquing wheel nuts. Not only is there a specific torque specification, but there's also a proper way to tighten the lug nuts. By failing to do so correctly, wheel nuts can be over-tightened and warp the brake rotors and in some cases, damage the hub flange.
Identifying Rotor Warping
A warped rotor will give off a identifiable side effect when braking. While most can be felt at high-speed braking, some extreme cases of rotor warp can be felt even at low-speed braking. The brake rotor stands vertically to the tire. It has a recessed surface that hugs the hub of the vehicle. When the brakes are applied, the pads squeeze against the rotor to slow the vehicle down. If the rotor is warped, this causes the brake pads to squeeze against the warping. That sensation is then transferred to the brake pedal as a thumping pulsation and will cause the steering wheel to shake.
How Over-Torquing Warps Rotors
Wheel lugs flex under duress. Wheels nuts that are over-tightened or not tightened in the proper sequence can cause uneven tightening against the hub surface of the rotor. This is most often caused by mechanics who tighten lug nuts improperly using a pneumatic gun without the use of a torque stick or a do-it-yourself mechanic tightening them out of sequence with a lug wrench. Because one or more of the wheel nuts is not as tight as the other(s), the particular tightening balance off-centers the rotor. Once a rotor warps, it is difficult to undo.
The pattern to employ when torquing wheel nuts is to snug one wheel nut up with a torque wrench or a pneumatic gun with a torque stick. Do not fully tighten the wheel nut, but simply snug it to the hub flange. Choose the next wheel nut in the opposite side of the first one you tightened and so on. An X pattern should be used for four wheel nuts, while a star-pattern should be used for five or more wheel nuts. Once you have snugged the wheel nuts, go over them again, employing the same pattern, until the torque specification has been obtained.
Hand torque wrenches are almost always preferred more so than pneumatic guns with torque sticks. However, torque sticks come in a variety of thickness which in theory, only allow you to tighten the wheel nuts so tight. Hand torque wrenches can feature a specific torque specification or may feature adjustable settings to obtain the correct specification. A clicker ratchet-style hand torque wrench is favorable because it will pronounce an audible click once the specification has been obtained on the wheel nut.
Never tighten the wheel nuts in a circular pattern. This can tighten one side of the wheel before the opposite side and cause the wheel to be off-centered. Always use a torque wrench set to the correct torque specification for your vehicle (which can be found in the owner's manual) and always use a X or star pattern when torquing. Never use a pneumatic gun without a torque stick. The gun may feature as much as 300 to 600 pounds of torque in the forward position. Rarely are torque specs higher then 150-foot pounds on most light-duty trucks and are much lower for passenger vehicles. Again, if using a gun with a torque stick, be sure to employ the "X" or star pattern when torquing wheel nuts.
- photo_camera Image by Flickr.com, courtesy of Saquan Stimpson