When a tire succumbs to the inevitable destruction that accompanies low inflation pressure, the driver has a better than even chance of making it safely to the next exit. But when the wheels come off of a vehicle, the potential for disaster escalates at an alarming rate. The insurance industry refers to it as a “wheel-off” and it represents the majority of property damage claims year after year.
A fully inflated 100-pound truck tire and wheel assembly traveling loose at highway speed can crush a standard automobile like an aluminum can. It can also damage other vehicles as it slows to a stop and remains a serious hazard as long as it's in a traffic lane. And to make matters worse, the vehicle that lost the wheel(s) will usually be sitting just ahead of the accident(s) on the side of the road so everyone can see who's responsible.
Pundits will cry that torque is the solution to all of the evils related to a wheel-off. They'll swear that the proper amount of torque applied to a lug nut is the key to wheel retention. According to them, torque is the magic bean that grows into a spiraling stalk reaching into the clouds where tires never wear out and wheels stay shiny forever.
Back on earth, torque is simply a measure of twisting force; controlling that force allows a technician to approximate the correct amount of bolt tension between the lug nut and the back of the hub. Rather than proceed with a detailed explanation of the relationship between torque and bolt tension, or clamping force, let's use an analogy that everyone can understand.
Now stick with me on this, but installing a wheel is just like cooking a frozen pizza. My favorite brand requires an oven set to 400°F and about 18 minutes on a pizza stone. If I change the temperature and add or subtract time, I will not get the desired result. If I choose to grease the pizza stone or cook the pizza directly on the rack (even when the directions don't call for either), I'm probably calling for delivery. The people who manufacture my favorite brand of pizza have it down to a science and they printed it right on the label: 400°F for 18 minutes.
Wheel manufacturers collectively agree that standard disc wheels require 475 lb-ft. of torque. That number is just like the setting on the oven because it doesn't guarantee anything. A stud-piloted, or Budd, wheel requires a dry torque, while a hub-piloted, or unimount, wheel requires a lubricated torque. Anti-seize compounds are often improperly used to prevent inner and outer cap nuts on stud-piloted wheels from sticking together, which results in a higher amount of clamping force at 475 lb.-ft. On the other hand, failing to lubricate the studs and the flange nuts on hub-piloted wheels causes clamping force to fall short at 475 lb.-ft.
If debris or excessive paint is present between the mating surfaces of dual disc wheels, having the proper amount of torque will not prevent a wheel-off. As the wheels flex in service, the foreign material works its way out and creates a series of small gaps. Once again, magic torque dust cannot be sprinkled on the lug nuts to overcome the effects of joint settling.
Torque control and regimented installation procedures are the best methods for preventing a wheel-off. You cannot have one without the other, and both must stay within the manufacturer's specifications and guidelines. The correct amount of torque with the proper conditions will result in more than enough bolt tension and clamping force to keep the wheels on the vehicle. Change one thing, and you've got a burnt pizza no matter what temperature the oven reads.