In my years of service as an Automotive Professional, I have seen too many reasons as to why you should torque those lug nuts. I’ve seen wheels come off, brakes damaged, broken/stripped lug nuts, bolts and studs, etc., and I have seen this repeatedly by Technicians as well as the Do-It-Your-Selfers for one simple reason - they don’t properly torque the Lug Nuts on their vehicle…
Lug Nut, and Lug Bolts, are designed with a specific grade of bolt/nut with a certain amount of “stretch.” Why do they Stretch? Through proper torque, which stretches the bolt, stretching is what allows the threads of the stud/bolt to tightly mate and secure to the counter part nut or threaded hole (the axle or wheel mounting hub) without working themselves loose. It’s almost like mechanically welding the nut to the bolt – except that you can remove and re-tighten it repeatedly. Yet, most bolt grades for this purpose, will retain it’s original size and properties (un-stretched) when torqued to spec. and then loosened - the properties of the bolt have a slight spring affect when loosened.
The below are examples of bolts in the various stages of torque being applied.
The first example is a bolt that is loose and no torque applied – un-stretched.
The second example is a bolt that is torqued to spec. There is a slight amount of stretch, but not enough to change metal/alloy properties of the bolt – thus returning back to it’s original shape for re-tightening.
The 3rd bolt example is a typical “Over-Torqued” bolt. Note that the bolt is not only stretched, but stretched to the point of changing the properties and “Yield Strength” of the bolt. The bolt now has a much lower Tensile/Yield strength and allows the bolt to flex, generate heat and fatigue – thus cracking, shearing, loosening, and breaking off are the result causing damage to the wheel and other related components.
Some other results of improper lug nut/bolt torque are as follows:
Warped Brake Rotors – brakes grabbing, pulsating or overheated.
Damage to the lug nut seating surface of alloy wheels.
Wheel hub damage – threaded wholes stripped out.
It’s very important to tighten lug nuts, incrementally, to the final torque spec. There are specific tightening sequences depending on what lug nut/bolt pattern you have – 4, 5, 6 or more… The below example shows the 3 typical bolt patterns and tightening sequences for most automotive applications.
Moreover, It is always a good idea to test drive the vehicle, then recheck your wheel lug nut/bolt torque – especially with alloy or painted wheels. I have seen too many times when you have a “false torque” due to paint, debris, corrosion, or a tight and binding centering hole over the wheel hub that allows the bolt or nut to come loose after driving. The wheels of a vehicle is probably the “MOST IMPORTANT” safety feature of the vehicle. Don’t take a chance of improper lug nut/bolt torque causing serious injuries to yourself, your family or other opposing motorists.
Below are a couple of great Internet sites to obtain free torque specifications for your vehicle. Additionally, most (probably all) Owners and Repair Manuals have these specs as well. So please, “Torque Those Lug Nuts!”
Lug Nut Torque Spec Sites: