Understanding Wheels and Torque

What do a pregnant woman from Maryland, a 48-year-old man from Ontario, and a driver in Seattle have in common? They are all victims of truck wheel-off incidents - a truck wheel flew off on the highway and struck their vehicles, in one case fatally.


"There are plenty of myths and outright falsehoods floating around repair shops - such as using anti-seize compounds as wheel lubricants - which only help to perpetuate the phenomenon of wheel-offs," says Kevin Rohlwing, senior vice president of training for the Tire Industry Association. "It's important that tire technicians receive the proper training to look at all aspects of clamping force - not just proper wheel torque - in order to ensure a wheel does not come off and create the potential for serious injury, or even death out on the roads."

Torque matters

The root cause of wheel-offs is in the way the wheels are installed in the first place, and that comes down to torque.

Rob MacMillan, an industrial process and systems guy working for Pinpoint Information Systems, has strong opinions on this. He's been working in the industrial fastening business for more than 20 years. He's sold wheel-installation machines to every major automobile and truck manufacturer in the business.

Obviously a truck-assembly facility is a lot different than a shop, but he also has a fair amount of in the- trenches experience, including working with a major fleet dealing with three wheel-off lawsuits.

"It is amazing to me," he says, "that all final assembly plants have very sophisticated machines that apply an elaborate fastening strategy that synchronizes the lug nuts, ensure an even distribution of clamp load, guard against deformation of the rotor and stretching of the studs, logs all data etc. - and yet when these vehicles hit their dealerships or fleet service facilities, they are removed and re-installed with a $50 impact wrench and a static click wrench."

Obviously it's not practical for service facilities to invest in the automated machines from the assembly plants, but MacMillan says shops could do much better than they currently do.

"The biggest issue with the installation of wheel lugs is the inconsistency of friction," he says. "The issues surrounding friction are compounded by the way all companies confirm the presence of installation torque. A torque specification is merely a way to predict how much clamping force (or squeeze force) there is in the system."

The problem, he explains, is that friction is variable, and friction dictates how much energy is consumed simply turning the nut and how much goes into clamping the joint together.

For example, if the desired installation torque was 100 pounds-feet and if a thread of the stud is damaged, it is not impossible for friction to consume 100% of the installation torque and leave zero clamping force in the system.

"To make matters worse," MacMillan says, "the effect of friction is minimized when the fastener is turning. Yet every truck company I have ever met confirms the installation torque once the fastener has stopped. The technician typically whacks the lug nut down with an unsophisticated pneumatic impact wrench and then confirms the torque with a click-style torque wrench.

"An impact wrench will apply torque to the nut as long as the operator happens to hold onto the trigger. This is subjective and varies from man to man."

Rolling time bombs

Two scenarios generally cause most of the problems in MacMillan's eyes. He calls the first one "Tighter is not better."

"The operator wants to make sure the lug nut is really tight. He holds the trigger long enough to be sure the lug is extra tight so it will never come loose. When he is done, he grabs his wrench. He clicks the nut. All is good."

The problem is, when you tighten a threaded joint, at some point the fastener begins to stretch, he says. This is a common fastening strategy in controlled industrial environments such as engine assembly, but all fasteners have an "elastic limit."

"If you exceed that limit, you have created a high probability that the fastener could break," says MacMillan.

"In an effort to be extra safe, the technician exceeds the elastic limit of the stud (which is impacted by age, use, corrosion, etc.), he clicks the lug nut and the wrench sees an adequate amount of friction so it indicates everything is OK.

"The reality is that you have a time bomb rolling down the street. You never know when the right series of conditions might stack together to make that stud break. It is impossible to predict."

Clicks and friction

MacMillan's second horror scenario is entitled, "The click wrench said it was good."

"The operator is very experienced and very diligent," as he describes the situation. "He knows the sound his tool makes when the nut is free-running and the noise it makes when the nut bottoms out on the wheel and starts to clamp the joint. He holds the tool on the nut, lets it bottom out, counts to five and lets go of the trigger. He grabs his torque wrench and clicks the nut. All is good."

To illustrate the point, MacMillan offers this cautionary tale: Imagine an operator grabs a handful of lug nuts and puts them in his apron. While in there, one comes in contact with a little bit of grease and another comes in contact with a little bit of dirt. When the technician installs those nuts, one will require a significantly different level of energy to make it turn.

If the tech typically waits for histool to impact the fastener for, say, five seconds at the bottom of the stud before he lets go of the trigger, the installation torque and the relative clamp load between those two fasteners will be dramatically different.

Now the fastener is obviously stopped. This is when the effect of friction is highest. (To manually push a car, you must overcome inertia and friction to get it rolling, but once it gets moving, friction is minimized and gets a lot easier). He takes his click wrench and successfully confirms the torque on both lug nuts. But the click wrench only measures resistance (friction). It is incapable of knowing what is friction and what is clamp force.

To get a real indication of how much torque is on the nut, you must get it moving, ignore the high breakaway torque (mostly friction), and then grab an accurate torque reading before you increase the installation torque by turning the nut too far.

"The hub nut with grease on it will lose less energy to friction than the nut with dirt on it, so there will be more energy left to clamp the wheel to the hub. The one with grease on it should make a better joint if it did not cause the stud to see too much torque and stretch beyond the elastic limit," MacMillan says.

"All I know is that I am afraid of both of them."

From the July/August 2012 issue of HDAJ