Yet they don't.
There are few credible statistics on wheel-off incidents, but those that do exist suggest the trucking industry is getting better at prevention. Still, news reports of death by flying wheel aren't entirely rare. What to do?
In December, we looked at bearings, a key source of wheel failures. For this article, we'll focus on another major factor: fasteners.
Keep 'em cleancopy here is bold
There are many ways for a truck or trailer wheel to fail and come adrift. Yet even with a hub-piloted disc, the wheel least sensitive to poor maintenance, the problem can often be traced to simple cleanliness.
The obvious cause of failure may be a loose fastener, usually due to a torquing problem during installation. What kept it from having enough torque to hold things together in the first place, though?
Chances are, the wheel components were just plain dirty or rusty. It doesn't take much dirt or much rust to change things after a few thousand miles. Even a small amount of dirt or rust, or something as small as excess paint drips between mating surfaces, will cause a loss of clamping force over time.
Note that I didn't say "can cause," but "will cause."
It's simple fact: If there's any rust, dirt, excess paint or even oil in the wrong place, your torque spec is out the window.
A common failure in disc wheels, for example, is a crack running between bolt holes around the wheel. Its likely causes are a loose wheel nut or worn mating surfaces. Dirt and corrosion will just make it worse.
Similarly, the crack that forms from the bolt hole to the center hole of a disc wheel is usually the result of loose inner cap nuts caused by foreign material between the wheel and the hub or drum. That prevents a flush contact, and it means the fastener never did manage to make things perfectly tight. Worse, it also means the fastener has been encouraged to loosen up.
Before reusing any undamaged wheel, at the very least use a wire brush or file to thoroughly remove grease, oil, old rubber, dirt, road film and corrosion. This is especially critical in three key areas: where the tire seats, where components fit together, and where the wheels mount on the vehicle.
Be certain to check around the studs on the hub and drum as well as in and around bolt holes and chamfers on the wheels to ensure that they're clean and flat. Sanding, light sandblasting or a solvent bath may be needed.
A lot of people routinely repaint steel wheels at every tire change to help prevent rust and corrosion. If you do it yourself, first make certain that all surfaces are clean and flat and then start with a fast-drying metal primer. It's critical to avoid paint build-up on the wheel mounting surfaces and in the bolt holes. Paint should be dry and hard before the wheel is installed.
Another method of wheel refinishing is powder coating, commonly used on things such as lawn furniture, though it's not something you can do yourself. It's claimed to offer a superior, chip-resistant finish that lasts 40% longer than paint, and it creates uniform coverage with no runs or excessive thickness, which can also affect clamping force. The best part may be that it's quick. From start to finish, rims can be ready to have tires remounted in just an hour.
It starts with a wash and then bead-blasting that takes the wheel down to bare metal, at which point it's easy to inspect for cracks and measure bolt holes accurately. Then the powder coating is applied, and finally the wheel is baked in an oven for 20 minutes or so. As with paint, there's a wide of array of standard custom colors available. Cost is comparable to paint.
Nuts and bolts
If you look at RP 656 in the Recommended Practices Manual published by the American Trucking Associations' Technology & Maintenance Council, you'll find these telling words in the third paragraph:
"Wheel-end fasteners have a finite service life..."
"There are no industry standards or guidelines for wheel bolt service life since service life will vary by application, duty cycle, geographic location, and maintenance practices."
"Most fastener problems can be avoided by using a few simple instruments... such as a wire brush, an oil dropper, and a calibrated torque wrench."
We know what the wire brush is for, so let's move to the oil dropper. There we'll find some major confusion. The differences between hub-piloted and stud-piloted fastening systems are too fundamental to go into here, and the former has pretty much taken over the market anyway.
You MUST apply two drops of 30-weight oil to the leading threads when re-assembling a hub-piloted wheel. It helps prevent corrosion and promotes even torquing. But you MUST NOT apply any oil anywhere in a stud-piloted system. It's assembled and torqued dry.
In both cases, you MUST NOT use any kind of anti-seize compound on the threads. As RP 656 puts it, "Many of these compounds are constructed of inconsistent material that could significantly alter the designed torque-tension relationship between the mating fasteners."
Now for the torque wrench.
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 and says 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 fleet maintenance shop, but his comments are nonetheless worth hearing. He actually has a fair amount of in-the-trenches experience, including working with a major fleet dealing with three wheel-off lawsuits, trying to help them create a process-driven wheel-maintenance regime.
"It is amazing to me," MacMillan 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 contends fleets could do much better than they currently do.
"The biggest issue with the installation of wheel lugs is the inconsistency of friction," he goes on. "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 by 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 unsophis