Differential thermal contraction can occur when wheels are mounted at shop temperatures in cold climates.  As the wheel components cool to ambient temperatures, clamping force is lost.  Wheel nuts which experience any rotation (i.e. back off) during this time will not regain their original torque / clamping force values.

Improper mating surfaces include both damaged and contaminated mating surfaces. Proper clamping force cannot be achieved with non-flat mating surfaces such as damaged or bent hubs and wheels, or worn or elongated bolt holes (raised metal).  Contaminants such as excess dirt, sand, rust, metal burrs, and paint on mating surfaces can wear away with use, causing a settling effect.  When present on the threads or between a nut and the wheel surface, these contaminants can also change the clamping force / torque relationship, resulting in "false torques" where much of the torque applied is used to overcome friction and is not converted into clamping force.

A loose wheel nut can originate from any of these sources individually, or more probably, from a combination of these sources - which makes the task of eliminating all loose wheel nuts very difficult.

 
Loose / Missing Wheel Nuts
 
Wheels are subjected to a variety of forces, including vertical forces from the vehicle and its cargo, road vibration and shock forces, cornering forces when the vehicle turns, and rotational forces from the turning of the wheel, especially during acceleration and braking.

When a wheel nut loosens these forces are redistributed among the remaining nuts and studs, but preferentially to the nuts and studs adjacent to the loose nut, causing these nuts to back off at reduced wheel force levels.  This loosening process accelerates with each successive nut that loosens, as the total clamping force drops and the stress concentration at the remaining nuts and studs increases.  At this point these studs can fracture due to fatigue or overstress, accelerating the process further - especially with heavy cargo loads.

Fines / Impound
Wheel / Component Damage
Downtime / Road Calls / Late Deliveries
 
Loose or missing wheel nuts, and the wheel and component damage caused by them, can result in both fines and impound.  Penalties vary from "points" and a few hundred dollars to several thousand dollars and 15 days impound for first offenses, with much stiffer penalties for subsequent infractions.

Wheel, hub, and mounting hardware damage caused by past loose wheel nuts or overtorquing greatly increase the risk of future loose wheel nuts and wheel detachments, and should be corrected.  Repair or replacement can easily reach well into the hundreds of dollars when considering both part cost and the labor involved.

In addition to the direct costs for inspection infractions and wheel/component damage, downtime is required to correct these problems.  If not corrected in the shop, the downtime for both vehicle and driver can extend to several hours and result in road call charges while the driver or inspector waits for the problem to be corrected.  These charges also can reach well into the hundreds of dollars at typical hourly downtime and road call rates.

Collectively, the total cost of a single event caused by loose or missing wheel nuts can be substantial.  This is especially so if the event results in a missed delivery time, the penalties for which can sometimes reach into the thousands of dollars per minute range.

Those separations which do not result in accidents can still be expensive.  The total costs for an extended road side service call, the driver/vehicle downtime, the replacement of a wheel and/or hub and mounting hardware, and a tire if the wheel is lost, can be substantial - not to mention the potential for late delivery penalties.  And fines for wheel separations can be stiff in some regions - for example, up to $50,000 per separation in Ontario.

The wheel separations which do result in accidents can create devastating consequences, sometimes maiming or killing those involved.  The majority of these accidents result from the detached wheel bouncing along the highway until it impacts oncoming traffic.  A small percentage result from the detached wheel hitting pedestrians, or causing the vehicle itself to overturn.

Those that result only in vehicular damage can be covered by liability or collision insurance, albeit with increased premiums in the future.  Those that result in personal injury or the unfortunate loss of life can generate legal suits reaching into the millions of dollars, affecting the fleets, their maintenance staff and management, and their customers and vendors.