Why Do Commercial Trucks Have Convex and Concave Wheels?

Side view of a tractor trailer on a highway

Photo: Scott Olson (Getty Images)

It’s amazing what your brain becomes blind to after years of repeated exposure. There was probably once a time, when I was very small, where I rode in the back back seat of my mom’s 1992 Dodge Caravan and stared out the pop-out window, wondering why tractor trailer and work truck wheels looked so paradoxically sunken and bulbous. And then I put that thought away for many years, as we do. But there are reasons trucks are the way they are, as we’ve discussed before. And today, we’re going to tackle the wheel question. Spoiler: it’s about packaging, cost saving and good old-fashioned common sense.

The front wheels of many commercial trucks are typically bowed out, where the area over the hub is bulged. That’s to fit the steering joints, bushings and bearings, as well as the suspension and brake systems, while keeping the axle as wide as possible for maximum stability. It also means that the center of the axis on which the wheels turn is as aligned with the center of the wheel and tire as it could be, and that too benefits dynamics and load-bearing capability.

Front quarter view of a Ford Super Duty

Here’s a Ford Super Duty, with the dual rear wheels opposing each other. The “face” of the outermost rear wheel you can see is just the other side of the front wheel.
Image: Ford

It’s when we look back that things get a little more interesting. If you haven’t seen both sides of your standard truck steelie before, you may not know that the outermost wheel on the back of a dually — a truck with two rear wheels a side — is identical to the wheels on the front axle, except flipped around so you’re looking at the reverse side. That wheel is actually bolted to the face of the innermost rear wheel, which is oriented the very same way the ones on the front axle are.

It’s an ingenious solution, because the benefits of the front axle geometry still apply to the rear axle, but you’re also getting the improved weight distribution and boosted payload capacity of four rear wheels. And should a tire give out and need replacing, all the wheels are the same. No need to fuss with specialized wheels for the front or back. How’s that for practicality?

That’s it. One of those simple answers to a simple question that floor you with its cleverness. Trucks: an endless well of satisfying engineering problem-solving that never stops giving.

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