The word on wheels

LAS VEGAS might appear to be a rather unusual place to go to talk about not gambling, but that's exactly what the National Association of Trailer Manufacturers did earlier this year.

When Ted Schorn made his presentation, “Wheels on Trailers: Elements of Assembly Process Control,” he said the attendees were there to avoid gambling … with their customers' safety and satisfaction. He said the mandate was to keep wheels on trailers in order to keep the trust of customers and placate the National Highway Traffic Safety Administration (NHTSA), whose intense interest has been piqued.

“The federal government thinks the loss of wheels is certainly a safety concern,” said Schorn, general manager of corporate quality for Enkei Corp, the largest aluminum wheel manufacturer in the world, and author of two of NATM's recommended practices.

“They will follow up if they hear about even one wheel off and they will ask you if you've had many other incidents, what kind of investigation you've done and whether you knew that wheel came off. It's kind of an uncomfortable situation. It's not just the potential loss of control of trailers, but the fact that this object could easily land in somebody's windshield in oncoming traffic.”

He said the problem was first identified in October 2003 as a result of a customer complaint, and is still occurring despite significant attention.

“The first complaint was that the manufacturer knew of 83 incidents that had come in by phone,” Schorn said. “I'm not saying that if that one dealership would've done the right things, this would've never happened. But I am saying that dealerships play an important role in communicating what's going on, both back to you and also toward the customer.”

Various investigations were conducted through July 2004, and recall campaigns were held in July and August.

“NHTSA discovered that these things were coming off all over the place and (concluded), ‘Boy, this must be a maverick industry,’” he said.

In September 2004, the Trailer Safety Industry Coalition was formed to deal with the issue, along with other common-interest issues. The TSIC features leaders of the trailer industry and its trade groups — the NATM, National Truck Equipment Association (NTEA), Recreation Vehicle Industry Association (RVIA), and National Marine Manufacturers Association (NMMA) — along with major wheel, axle, and trailer manufacturers.

TSIC first raised $178,000 from industry associations and key suppliers to put into a fund devoted to testing.

After conducting an investigation into wheel-attachment technology and wheel-separation issues affecting trailers under 26,000 lb GVWR, the TSIC's technical committee developed a series of recommended actions, “Wheel Mounting and Application for Trailer Use,” publishing them on December 20, 2004, and communicating them throughout the industry in an effort to eliminate or significantly reduce incidents of torque loss and wheel separation.

The technical committee continued its research through February 2006 and will publish “Understanding the Wheel Fastening System on Trailers” by this summer.

“We've been communicating with NHTSA every two to three months,” Schorn said. “All of the open investigations are now closed. No recall campaigns are going on and no further investigation is intended to be started. At this point, they're satisfied. The number of wheel incidents they know about has been dramatically reduced. They're giving us the opportunity to complete our work, to get this material to you, have it be understood and digested, and to finish our technical work.

“They're giving us an opportunity to get our act together and regulate our own industry so they don't have to beat us up. NHTSA frequently refers to itself as an elephant — very difficult to move, goes where it wants to, eats what it wants to, and if they sit on you, you give up. They'd prefer to have us — who are a bit more agile — deal with our own issues. All government agencies would prefer to use their resources on their pet projects.”

Schorn said that because dealerships put wheels on trailers, they need to follow the recommended practices.

“In some cases, we supply our trailers to dealerships without any wheels at all,” he said. “They're with the package but aren't assembled, for the purpose of economics and efficiency. So they're in a production setting sometimes. But certainly they're in the maintenance cycle and so they have an important place in this business. Customers and customer care are in the hands of the dealerships. Seldom do you sell direct to a customer right out of your factory. So you're relying on them — their customer communication on torque. And so it would be good for you to have a relationship with that dealership that gives you a warm and fuzzy feeling that they're doing what you'd do in handing off this trailer to your precious customer.

“But the other thing is also true: dealerships are to be sources of information to you. Some trailer manufacturers have organized systems of communication. Others don't. It is important for you to hear about troubles so you can take action inside your factory so the next set of trailers don't have that issue. The quicker that feedback cycle is, the better for you.”

The dynamics

Schorn said the wheel studs are affected by the force of the weight of the trailer (gravity) and friction in the road from turning. When the trailer is moving in a straight line, vertical loads combined with braking put forces on the wheel that tend to twist the wheel against the studs. The clamping friction between the hub and the wheel resist these twisting forces. The clamp load must be maintained or the studs will fatigue from cyclic bending and eventually break.

He said that during turning, friction between the road and the wheel works against the tendency of the trailer to follow its original line of travel. These forces act to pry the wheel loose from the hub.

The forces within the bolt that draw the wheel and hub together are increased during cornering, making it very important to not over-torque the bolt. Rapid or tight cornering increases these forces.

“The significant concern is that the forces that tend to pry the hub off will be greater than what that stud can hold — that you'll stretch the stud beyond its yield limit or proof load,” he said. “We want to make sure we have appropriate clamp so that we have some margin left in that stud so that it can give.”

How do wheels stay on? He said the bolt is stretched by the action of the lug nut when it is tightened. This stretch in the fastener is what generates clamp load in the bolt. The wheel is clamped to the hub by the force of the bolt trying to return to its original (shorter) length. When torque is applied to the lug nut, the resistance to turning the lug is measured. This should be related to the lengthening of the bolt and the subsequent result — clamping force.

He said proper assembly is critical. Generating the proper clamp load by measuring the applied torque is the simplest approach, but also very indirect.

“The last one-half or one-quarter turn is what applies the tension of the bolt,” he said. “This is what makes paint on threads bad. That's why it's so inestimably important that we don't increase or decrease the friction without knowing about it or accounting for it in the amount of torque we apply. Paint dramatically increases friction.”

He said the options are: length of bolt measurement, bolt tensile stress management, and angle measurement. The application of torque in a consistent manner, with consistent components, is very important.

Aspects of assembly

He said the critical aspects of assembly include:

  • Management responsibility.

    This involves establishment of a quality system of controls consistent with the criticality of the operations performed, planning, communication and follow-up, and training.

    It's management's job to plan and enable, check the work that is done, measure to see if the target has been hit, and take corrective action while sending a consistent message.

    “It's from a government point of view and a customer's point of view,” he said. “The trailer manufacturer's responsibility is to make sure these trailers are good, correctly built, and safe on the road. Nobody can shed that responsibility. I'd like to think of it this way: Management has a plan. They have to figure out what's going to happen in my plant, factory, company. It's my job as a boss to say, ‘I'm going to provide the things these people need to get their job done.”

  • Documentation and records.

    This involves revision management of instructions and assembly plans and record retention — torque confirmation for one year, calibration of your instruments for three years, audit for three years, and training for the length of employee service.

    “Nobody likes talking about paperwork,” he said. “But when something goes wrong, documentation is your only shot at figuring out what went wrong. It's for self-improvement or self-defense. If you don't have it, good luck. You'll have to try to figure out what you did two months ago. Also, somebody some day is going to say, ‘You did something wrong in your manufacturing. Prove to me you didn't. Prove to me you did due diligence, that you trained your workers to do this right, that you had work instruction available on the line.’ If you have no paper, it did not happen. It's your word against a person who got injured. Not having paperwork does not look good.”

  • Planning.

    Assembly plans must include a specific component list, acceptable torque range, tools to be used (for inspection/application), lubricant type and location, effective date, and revision level of the plan.

  • Assembly factors.

    They are a two- or three-step process, clean assembly, hand start (so there is no cross threading), star pattern with gradual increase in torque, proper calibrated equipment (controlled torque), and audit confirmation records and consistency.

    “Torque wrenches, depending on the type and style, can be off by 40%,” he said. “I have measured changes in repeatability of 30%. With three different operators, it goes to 40%.”

  • Measurement and calibration.

    Gauge control is required, including serialization (“put some unique identifier on and have records that track to that specific gauge that was used in that particular plant in this particular location for this particular operation”), at least an annual calibration by a qualified lab, records, and proper storage.

  • Nonconformance management.

    He said audits must record “as-received” and “as-corrected” readings.

    “When you have the numbers for the torque on the individual lug nuts on the wheels, you have to record the ‘as-received’ and ‘as-corrected’ condition,” he said. “I've looked at a lot of records, and a lot of people write, ‘OK … OK … OK. I checked them all, they're good.’ Or they'll put, ‘120 ft-lb, 120, 120, 121.’ They're writing down the way they left it, not the way it came in. An audit is supposed to tell you how good things were before. Now you can perform the re-work and fix them. But you have to know how bad it was before, so you can get to those guys.”

    Audit data must have trailer trackability, so you know which trailer had those readings. When sampling, if one trailer fails the check, all previous trailers (to the last good one) must be checked.

    If a wheel should come off, the internal response includes an investigation to determine the cause. What went wrong? The investigation is the manufacturer's responsibility, he said. Help should be sought from the component suppliers. Look at the assembly controls and records as well as the failed parts. Review the warranty database for similar incidents. Corrective actions must be implemented if there are any observed process-control, design, or material issues. He said the TREAD Act requires you to retain your records, even if no action is deemed necessary.

    “You have to go back and find the root cause of that problem,” he said. “Somebody substituted material you didn't authorize? Somebody forgot to tighten something? Something happened that means some point of control failed. You need to find and fix that leak.”

    The external response includes instructing the dealership regarding how to handle the situation. The manufacturer's liability is at risk in a poor response. Parts should not be reused, especially the studs, lug nuts, and wheel (even if they appear undamaged). Service should include the use of fresh matching materials, assembled together to the proper torque via a torque wrench. Parts involved should be sent back to the trailer manufacturer for review/analysis. Based on the warranty database review and the evaluation of the cause of the incident, NHTSA notification should be considered.

  • Transporter responsibilities.

    Follow specific instructions from the trailer manufacturer, including the torque range to be held, the method of measurement, the frequency of checks, and the records to be provided.

    “Eighty percent of wheel-off incidents occur within the first 100 miles, which mostly means that's a time when the transporter has responsibility,” he said. “So at the very least, we need to give them some instruction about what torque range they need to hold.”

Recommended practices

Here are the recommended practices:

Component Guidelines

  • Surfaces of contact on an aluminum wheel (the nut seat and the mounting surface) must be free of paint, contamination, and damage. Smooth, clean surfaces provide the most uniform clamping pressure and best retain torque.

  • Surfaces of contact on a steel wheel (the nut seat and the mounting surface) must be free of excessive paint, contamination, and damage. Smooth, clean surfaces provide the most uniform clamping pressure and best torque retention.

  • Surfaces of contact on the axle (the flat hub surface and the threaded studs) must be free of excessive paint, oils, grease, contamination, and physical damage.

  • Lug nut geometry must match that of the wheel nut seat. The threads and nut seat must be free of paint, oils, grease, and other contamination.

  • Stud length must be sufficient that, after mounting the wheel to the hub, the lug nut is engaged to a depth at least equivalent to the diameter of the stud. For example, a lug nut threaded on a ½" diameter stud should thread on for a depth of at least ½".

Assembly Guidelines

Assembly of the wheel onto the hub is a critical, safety-related process. The proper method of assembly and the consistency of the torque applied to the wheel fasteners are important factors in ensuring reliability of the fastening system and retention of the wheel to the trailer.

The trailer manufacturer, distributor/dealer, and end user must consistently follow proper torquing technique in order to ensure the hub and wheel are properly seated and use caution to prevent anything from interfering with the flat, full designed mating contact of wheel mounting surface and hub. Excess paint, oil, and grease must be removed from the fastener contact surfaces (the mounting surfaces, studs, and lugs) or not applied at all. Adherence to the recommended “do's” and “don'ts” set out below will minimize the likelihood of faster torque loss and wheel separation.


  • Obtain confirmation from each component manufacturer that its component is appropriate for application, meets the appropriate component guidelines, and is compatible with the other components in the wheel system.

  • Develop and distribute a list or manual of proper assembly and torquing procedures consistent with these guidelines and specific technical information provided by component manufacturers.

  • Train appropriate personnel (factory and field) in proper assembly and torquing procedures.

  • Insist on consistent, strict adherence to these assembling and torquing procedures.

  • Conduct and document regular audits or checks to verify compliance with assembly and torquing procedures.

  • Investigate and correct any obstruction at the center bore of a wheel, resulting from a poor fit between the ornamental cap and the wheel.

  • Remove oil and grease from threaded fasteners (studs and lugs).

  • Mask or shield (cover) all fastener contact surfaces (mounting surfaces and studs) before painting axles, whether for improved cosmetics or for corrosion protection.

  • Only use an impact wrench with torque stick as a tool initially to lightly secure the wheel, applying a crisscross or star pattern.

  • Use a calibrated torque wrench to complete the torque-fastening process, applying the same crisscross or star pattern.

  • Re-torque periodically during the trailer's initial towing and thereafter in accordance with the component suppliers' recommendations.

  • Maintain records of the maintenance and torque checks performed by transporters, noting any loss of torque or any corrective measures taken.

  • Investigate any customer claim involving wheel loss.


  • Don't deviate from the component manufacturers' recommendations regarding compatible components without a competent engineering review.

  • Don't substitute any component for the components the suppliers have specified without a competent engineering review.

  • Don't deviate from the component suppliers' fastener torque specifications without a competent engineering review.

  • Don't use adhesive products to maintain fastener tension.

  • Don't use lubricants or oils on threaded fasteners (studs or lugs) to make applying the torque easier, unless assembly specifications require it.

  • Don't apply any additional paint on fastener contact surfaces (mounting surfaces/hub faces or studs).

These recommended practices are available at,,, or

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