Imagine driving a massive 16-wheeler truck barreling down a clear, steep road. Suddenly, you hear a crack and you glance out of your rearview mirror just in time to see one of your tires detaching from the truck.Heavy vehicle wheel separations are a deadly threat to everyone on the road. Did you know that wheel separations can be a result of sub-par maintenance standards? Follow this guide in understand what causes heavy wheel separations and how you can prevent them.
Wheel separation occurs when a wheel detaches from a vehicle while it's in motion. Some causes of this dangerous circumstances are:
- The wheel bearings can give out, compromising the wheel, brake drums and hub.
- Metal fatigue can cause the wheel unit to fail, causing detachment from the vehicle.
- The fastener system can also fail, which can result in the loss of both the wheel and the rim.
Some vehicles that have a dual wheel assembly can lose both wheels.Together, the brake drum and a set of dual wheels weigh approximately 350kg. This can equate to large freight traveling down a highway at high rates of speed.
The most common wheel design up until the 1980’s was the spoke system. This system mounted tires onto rims, which were supported in place by steel spoke wheels. They were then secured in place by bolts, studs, and lugs. Though the spoke system was popular, there was some risk associated with using them. Spoke systems would tend to wobble during mounting and when a driver traveled along rough roads or hit potholes. To overcome the inefficiencies of the spoke system, disc wheels were manufactured.
The earliest designs of the disc wheel were called stud-piloted wheels. This wheel system was far more sophisticated than its predecessor. It used a multifaceted two-piece threaded fastener that fortified both the inner and outer wheels of a dual wheel set. Over time, even this system began to show signs of imperfections. As the studs located and supported the wheel as it spun on a rotational axis, high levels of stress began to appear on both the studs and the wheel discs.
A new wheel system had to be developed to overcome the inefficiencies of past designs. This new system came in the form of the hub-piloted wheel, which has become the most common system to be used to this day. Hub-piloted wheels have a hole that has been precision cut at the center of the axle, which in turn activates on a curved extension or the seating pads ranging from the hub. As these parts interact with one another, they accurately lock onto the wheel around the center of the axle and restrain the wheel by activating the clamping power of the fasteners. This reduced stress upon the studs, a common weak point of the previous fastener system from an earlier design of stud-piloted wheels.
Detachment of Wheel Fasteners
One of the leading causes of wheel detachment is the detachment of the wheel fastener. The detachment of wheel fasteners occurs when the wheel nut unwinds itself from the stud and allows the wheel to separate from the vehicle. When this happens, the stud itself remains in place, but the wheel nuts are missing. This is the most common type of failure concerning wheel separation.
How it works: Once the wheel clamping force has been reduced to the point where relative motion occurs between the wheel and the hub, it becomes possible for the wheel nuts to unfasten themselves from the studs. For example, while a truck was traveling westward on a four-lane highway, one of the wheels of the left side of the last axle of the trailer separated itself. Reports state that the fastening system used was the standard 10-stud system used on most heavy vehicles. As the vehicle was in motion, the nuts unscrewed themselves from the studs causing the inevitable wheel separation.
According to the repair invoice of a brake maintenance job that was performed on the failed wheel assembly a mere three days before the incident, the left side of the axle had its brake linings replaced. After analyzing the driver’s logbook, it was confirmed that the vehicle had driven 571 km since the brake job. The report stated that the driver didn’t feel like there was anything wrong with the vehicle as he was driving. He had even checked the fasteners himself with a borrowed torque wrench from another transport driver.
Interestingly, Transport Canada has reported many incidents in which a wheel had been removed for maintenance purposes at least one week before a wheel detachment. A number of those wheels were removed for a brake replacement. Furthermore, each of the detached wheels were of the hub-piloted design. Since many of the reported cases involved the removal of the wheel for a brake replacement, it’s logical to assume that certain brake replacement procedures have an adverse effect on hub-piloted wheel fastener stability.
Two Piece Flange Nuts
Two piece flange nuts can also play a role in the wheel detachments of large vehicles. It should be noted that even the smallest pieces of mechanical machinery can lead to catastrophic failure. In North America, Hub-piloted wheel systems rely upon two-piece flange nuts. Requirements specify that there must be a minimum and maximum torque to tension ratio when using two-piece flange nuts. To meet the minimum requirements, there must be at least 30,000 lbs (133 kN) of tension in the stud when there are 370 lbs of torque on the flange nut. To meet the maximum requirement, there must be less than 62,100 lbs (276 kN) of tension in the stud when there are 500 lbs (678 N-m) of torque on the flange nut.
Tests have revealed that when flange nuts are reused, or when they age, their torque and clamping force can be significantly reduced (by as much as 50%). If a flange nut were to receive such a decrease, there’s a very real possibility that it would not be able to meet industry standards. Furthermore, because hub-piloted wheels rely entirely upon clamping power to prevent wheel detachment, even a slight decrease in torque and clamping force can represent a huge risk to a wheel detaching from a vehicle.
Temperature and wheel separation
It turns out that temperature can play a role in wheel separation. The Ottawa Transit Authority (OC Transpo) conducted an internal investigation on a newer bus that was using a dynamometer after the vehicle in question experienced a wheel separation due to the loss of flange nuts. The initial theory speculated that the event might have been triggered due to a change in temperature associated with the brake system.
Throughout a given work day, an urban bus can make hundreds of scheduled stops as it picks up and drops off passengers. The more a brake is used, the hotter the wheel assembly can get. Tests were conducted to simulate an urban bus on a typical business day. It was discovered that as the temperature in the wheel assembly increased, the flange nut needed more rotational displacement to reach the proper levels of torque. After the temperature had cooled down, the effects of thermal contraction resulted in the fastener becoming over-torqued. Ultimately, it was concluded that as the wheel assembly heated up, clamping power decreased considerably.
This experiment prompted the bus company to reevaluate their braking systems. Urban buses regularly experience hot wheel assembly temperatures due to their need to stop multiple times throughout the day. As a result, the bus company increased the torque levels of their buses from the manufacturing standard of 475 ft-lb (644 N-m) to 600-650 ft-lb (813-881 N-m). This was done as a way to counteract the effects of temperature related wheel separations. As an added precaution, The Ottawa Transit Authority specified that flange nuts can’t be torqued while the wheel assembly is still hot. This would prevent extra clamp force and prevent stud stress.
When a wheel separates from a truck, the Ontario Highway Traffic Act deals with it in a very straightforward manner. If a wheel separation incident occurs, the offender may be fined up to $50,000. With this act in place, a wheel separation becomes an absolute liability offense. This means that the incident is completely the fault of the offender and there is no defense. The accused cannot defend themselves by trying to prove that the wheel separation wasn’t their fault (called a defense of due diligence).
The problem, however, is that from the perspective of a forensic engineer, the inability to defend one’s self in a court of law discourages an investigation into the actual causes of the wheel separation. If the problem cannot be investigated, the actual cause of wheel separation may never be truly discovered. The engineering community is deprived of the opportunity to discover if there is an inherent mechanical flaw that can be contributed to wheel separation. As you could very well imagine, there are those who stand in opposition to this law.
The idea of absolute liability for occurrences in which wheel separation takes place implies that all truck wheel systems are mechanically sound. It also implies that all wheel separations are the result of poor maintenance practices. Indeed, there have been many instances in which poor maintenance have played a role in wheel separation, but there have been other cases where it was mechanical failure that led to wheel separation and not human error.
Whether a wheel separation occurs due to human error or a mechanical issue, it’s best to keep industry best practices in mind to avoid possible catastrophe. There are a number of best practices that should be observed when servicing truck tires and wheels to avoid wheel separation. These include:
- Proper bearing installation and adjustment
- Proper wheel and hub preparation
- Proper fastener torque
- Thoroughly examining reused parts
- Daily inspections
As time progresses, the trucking/engineering community will continue to investigate why tire separation happens. The Ontario Highway Traffic Act and its absolute liability offense Act may slow investigations down due to its approach of implying that wheel separations are always the fault of the operator no matter what. Only time will tell if tire separation is exclusively human error or a mechanical design flaw.