Across the Pond, a Familiar Problem

Over the past few years, Canadian truckers no doubt feel as though they’ve searched the globe for ways to prevent lost wheels. But if they came to Britain looking for answers, they’d instead find an industry equally perplexed. Detached wheels kill an estimated three to five people a year, and lead to insurance claims totalling somewhere close to $130 million.

Truck and trailer makers blame bad maintenance practices while truck operators point to poor product design. Industry critics blame both, arguing that guilty operators are escaping punishment by hiding behind industry assertions wheel loss is a mystery no one understands. Meanwhile, the government keeps no official figures on wheel loss, and what police records do exist are sporadic and local in nature. Sound familiar?

Independent research commissioned by Disc-Lock Europe, the wheel nut manufacturer, and based on police findings, estimated there were between 2100 and 3200 wheel loss incidents of all types in 1991. “There is no doubt that in many cases wheel loss is caused by poor maintenance, or by the use of worn out or inferior fittings,” company representatives told an all-party meeting of Members of Parliament. While 71 MPs later signed a motion calling on Britain’s Dept. of Transportation to fund research into a new wheel design, nothing happened.

Interestingly, Britain looks to suffer more detached wheels than its European neighbors. Higher average speeds and tougher stop/start conditions may have an effect. It is equally unclear whether driving on the left side of the road is a factor (all mainland Europe countries drive on the right).

Most British cases involve the loss of a left-side wheel, usually from the rear, and almost always from a twin wheel assembly.

According to the London-based Institute of Road Transport Engineers (IRTE), most cases result from loose nuts as opposed to failed studs or wheels. Both IRTE and Sigma Engineering also claim stud tension is being lost without rotation of either stud or nut. How this happens is not fully understood, IRTE says, but the problem was the focus of a study by the British Standards Institute that resulted in quality assurance standards for British-made nuts and studs.

Unfortunately, the fact that 90% of the wheel fasteners on British trucks are made in Germany passed everyone by.

All the while, operators are angered that component manufacturers share no culpability when a charge is laid following an accident. In many cases, a truck owner charged with operating a dangerous vehicle receives an absolute discharge (guilty without punishment). Those found guilty as charged are fined up to $1200 (£500), though a minority of convictions have also been quashed on appeal.

SEARCHING FOR ANSWERS

If the trucking industry, equipment makers, and government are content to pin Britain’s wheel loss problem on a combination of poor maintenance practices and the use of inadequate wheel-end components, then stressing diligence on both counts seems to be the only concrete solution at hand:

Maintenance: A sure cause of wheel loss is the presence of dirt, dust, or paint between mating surfaces, according to IRTE. The collapse of this foreign material has been shown to loosen wheel fixings in tests by manufacturers. The resulting hammering of the studs can lead to their fracture. The wheel is lost, but not before tell-tale ovality of the wheel holes appears. An astute truck operator will not only look for such problems, he’ll understand the cause and address it.

All the official and unofficial advice available today calls for re-torquing after the wheel has been pulled, re-installed, and used for 100 kilometres or so to be standard practice. Still, this clearly doesn’t happen as it should. On the positive front, there is a growing feeling that studs and nuts should be periodically replaced. The application of a small amount of light lubricant is also generally considered desirable.

Equipment: European trucks and buses use one of three different types of wheel ends on heavy trucks and buses. All use 22-mm (7/8-inch) diameter studs, although this is not mandated by any country. Manufacturer-recommended torque values vary wildly between 332 pound feet and 553 pound feet (450Nm-750Nm) on hub-piloted wheels alone. Stud material tensile strength is typically 10 to 12 tons per square centimetre (65 to 75 tons per square inch). Ten holes are standard.

The most common wheel-end style is the ISO spigot-mount (hub-pilot) wheel. It was adopted throughout Western Europe during the early 1980s and is used on all models above about 12 tons (26,500-pound) gross weight. The wheel is located by a central spigot then clamped to the brake drum or hub by flat faced nuts, sometimes bearing captive washers.

The second type is the German DIN standard. Previously used only by mainland Europe manufacturers and now replaced by ISO, the wheels have spherically countersunk holes with nuts to match. The stud/nut assembly locates and clamps the wheel. Where twin wheels are used a free-floating spherically coned washer is positioned between the inner wheel and the drum or hub on each stud.

The third and similarly defunct fixing still in used on older vehicles is a British design that is similar to the DIN standard except that it uses conical holes and nuts. It, too, was replaced by the spigot-mount wheel on new vehicles and has now largely disappeared.

Often the trouble with equipment-related wheel loss centres on the studs. In 1994, the IRTE, following research into wheel stiffness by Exeter University, put forward a new 24-mm stud design featuring a waisted shank and rolled thread. The design puts fatigue performance beyond doubt, IRTE says, while permitting a weight-saving decrease in wheel nave thickness from 13 mm to 11 mm.

The design raises and then helps maintain clamping forces by inducing slight deformation in the wheel. Stud loads can safely be cut by 20% with the design. No on-highway vehicle maker has adopted the 24-mm stud, however, for fear of stepping away from the industry-standard 22-mm design.

The use of left-hand threads on left-side studs was dropped by European manufacturers in the 1970s. Today, only some imported Japanese vehicles have such fixings. IRTE research has shown that it is possible to deform left-hand threaded studs when attempting to remove a wheel believed to be secured with right-hand threads. This leads to a lack of tension in even correctly torqued nut/stud assemblies when the wheel is re-mounted. The seed of future stud failure is unwittingly sown by the mechanic.

Meanwhile there is a palpable unwillingness among new equipment makers to admit anything is wrong with existing designs, and among many truckers a resistance to greater scrutiny from inspectors.

Rightly or wrongly, this, combined with a continued lack of research funding, suggests wheel loss is here to stay.