Automotive Q&A: The importance of proper torque on lug nuts

Tag : Nut Socket

Q. A little while back I had a flat tire, which was changed alongthe road by AAA. I then had the flat tire fixed by a gas stationand put back on the car. My neighbor accompanied me to the gasstation and when we were on the way home he said I should have thewheel nuts "torqued" by someone, as the young man at thegas station didn't do this. I was hoping you might explain to mewhat this is. I didn't want to sound ignorant by asking him what hemeant by this.
A. Your neighbor is correct about the importance of tighteningwheel lug nuts. Excessively tightened lug nuts can strip thethreads, distort brake rotors, damage the wheel, and possibly shearoff the lug stud. Insufficiently tightened lug nuts can come loose,with catastrophic consequences.
It sounds like the fellow at the service station used either a lugwrench or impact gun to install the wheel lug nuts, rather than atorque wrench. A simple lug wrench comes in either an L or crossshape and with the right amount of grunt tightens lug nuts prettywell. An impact gun uses compressed air and a hammering effect toquickly and noisily zip the lug nuts tight. The problem with impactguns is they're so powerful, one can easily over-tighten the nuts,leading to the above difficulties. A torque wrench measures thetightening force applied, allowing the user to accurately followthe manufacturer's recommendations.
Torque is the twisting force one applies to the wrench. It'smeasured in the US in pound feet (or foot pounds if you prefer thatphrasing). If you have a 1-foot-long wrench and applied 80 poundsof force to the end of it, you'd have 80 lb-ft of torque, thecorrect rate for some, but not all, lug nuts. A 2-foot wrench with40 pounds applied would apply the same torque. It's also importantto tighten the lug nuts in a crisscross pattern to avoid wheel andbrake rotor distortion. Believe it or not, there's a manufacturer'storque specification for just about every nut and bolt on a car,particularly on the engine and drivetrain.
Brake rotor distortion is a fairly common fault, causing apulsation or chattering effect when one applies the brakes.Improper lug nut tightening and/or excessive use of the brakes(overheating) are the two most likely causes. In their quest toreduce vehicle weight, many manufacturers have thinned down thebrake rotors, which unfortunately makes them more susceptible todamage.
If your lug nuts were tightened by hand I wouldn't be too concernedabout the possibility of over-tightening. If an impact gun was used- without a torque stick (a clever long-shaft socket designed tolimit torque) it's probably worth the trouble to have the lug nutsre-checked.