How It Works: Wheel separations

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Few things are more terrifying than the thought of a flying truck wheel crashing through your windshield. Despite safety regulations and industry policies, they’re still coming off. And while you might think it’s just big trucks, an alarming number of consumer vehicles and trailers regularly lose wheels as well — and that can be just as deadly. It’s all about the right maintenance done at regular intervals.

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Separations in both commercial and consumer vehicles generally occur when the wheel comes off its mounting studs, or when component failure causes the hub and wheel to detach. Although it’s far less common, a tire that’s improperly inflated can also come off the rim.

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How It Works: Wheel separations Back to video

The wheels go on over threaded studs and held on with nuts that are tightened up against them, and that’s where things can start to go wrong. If the nuts aren’t tight enough, there’s a possibility they can gradually unscrew and fall off. But even if they stay on, the wheel can wobble on the stud, which can widen the mounting hole in the rim as well as weaken the stud.

While it seems that “tighter is better” might fix the issue, it doesn’t. Too-tight nuts are as bad as those that are too loose. Over-tightening the nut can stretch the stud, which can actually reduce the clamping force that holds the wheel on the hub. It can also crack or cross-thread the wheel nut, or even warp the brake rotors

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Manufacturers usually specify the clamp force and how much torque is required when installing the wheel nuts, depending on the application. However, not all tire installers may bother to check the specifications and will just tighten the nuts until they’re snug instead of using a torque wrench to verify it. It’s also recommended that the nuts be re-torqued after the vehicle’s been driven for a specified distance — usually 50 to 100 kilometres — but many drivers don’t bother going back to the shop.

It’s rather telling that in one study, many wheel separations on light-duty vehicles happened not long after the vehicle had been serviced — either for tire replacement, brakes, or other repairs that required removing the wheels — and the driver had not come back in the recommended time for the nuts to be re-torqued. Other studies show that lost wheels often spike in fall and spring, concurrent with large numbers of drivers taking their vehicles for their seasonal tire changeover.

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Having the nuts re-torqued after a tire change or wheel service is considered vital for big trucks, but not all fleet operators want to take their vehicles off the road long enough for a quick service. Experts studying big-truck wheel separations also noted that with many failures, the hub and wheel hadn’t been properly cleaned before installation. This means dirt, debris, or corrosion can act like spacers between the hub and wheel, leaving it loose when they’re eventually dislodged during driving.

In some cases, though, the wheel stays attached, and it’s the hub, bearing, or other suspension component that fails and breaks away. It’s more likely on heavy trucks, but it can happen on consumer vehicles as well. In a few cases it’s an error during servicing, such as a technician installing a part incorrectly or forgetting to put a cotter pin on the end of the hub, or the truck being overloaded. But more likely, it’s due to lack of maintenance, including insufficient lubrication, corrosion, or leaking seals.

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On a newer consumer vehicle, it’s likely that steering and suspension components that used to require regular scheduled maintenance are now permanently lubricated and sealed. However, some older vehicles may still require their wheel bearings to be greased — as well as trailers, especially boat trailers that are regularly dunked in water. Seized bearings are the main reason you see trailers broken down on the side of the highway.

Drivers should also take their vehicles in for service if they hear a crunching or clunking sound in the front end, or if the front tires are wearing unevenly. This could indicate a ball joint that’s about to fail. These components attach the hub and wheel to the steering assembly; if you see a vehicle with one front wheel tucked in under it, that’s a broken ball joint.

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To help prevent your own wheel issues, have your vehicle serviced according to its recommended schedule. When you have your wheels removed, take them back for a checkup within the suggested interval.

You can’t guarantee that others are taking the same safeguards, but there are some precautions you can take to give yourself a fighting chance. When you’re scanning traffic when driving, take a quick glance at other vehicles’ tires. If you see a bit of a wobble, keep your distance. It might just be a hubcap that’s not on straight, but if it falls off, drivers swerving to avoid it can cause a crash. Be especially careful around trucks, cars, and trailers that look like they could use some maintenance. If something does let go, you don’t want to be in the line of fire.