Why Do Commercial Trucks Have Convex and Concave Wheels?

Author: May

Feb. 04, 2024

Automobiles & Motorcycles

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.

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It’s when we look to the 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.

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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 floors you with its cleverness. Trucks: an endless well of satisfying engineering problem-solving that never stops giving.

Semi-truck wheels aren't shaped the same on every axle? Why is that? The answer is simpler than you would expect.

Semi-trucks carry heavy loads across the country day after day, and we mostly ignore them. These behemoths of logistics have some interesting features and others that just make sense. Have you ever wondered why semi-truck wheels have different shapes? The front wheels are typically convex while the rest are concave; what gives?

Semi-truck wheels are shaped differently due to three practical factors

Semi-Truck Driving Down a Highway | Wiki Commons

It might sound extremely simple, but sometimes the right answer is simple. If you add one and one together, you always get two. There’s nothing complicated about this at all. The same goes for the semi-truck wheels.

Jalopnik tells us the front wheels and rear wheels of these big rigs are different because of:

  • Packaging
  • Cost Saving
  • Common Sense

Let’s explore.

The front wheels are convex; why?

Convex Front Wheel on a Semi-Truck | Wiki Commons

The front wheels do the steering for any vehicle. Although some cars and SUVs offer rear-wheel steering, this steering is limited to a small degree of change and is only meant to aid the front wheels. Because the front wheels handle the steering, there needs to be room to fit the steering joints, bushings, bearings, suspension, and brake systems required.

All of these items fit on the front axle, and the wheels are convex to keep the axle as wide as possible to offer maximum stability. Additionally, the center of the axis for wheel turn aligns with the center of the wheel and tire, or at least it’s as close as it can be. This is the right way to package the front wheels for the best performance in semi-trucks.

All rear semi-truck wheels help save money

Angled View of Rear Semi-Truck Wheels | Wiki Commons

It might be a little unbelievable, but the same reasons for the rear pairs of wheels on a dually pickup truck are the same as for semi-trucks and their trailers. Truckers get used to how these wheels are packaged, but most of us don’t know what’s done to make them work.

All rear axles on semi-trucks have pairs of wheels on each end. These wheels are identical but flipped around, creating the concave look of the outer wheels. Manufacturers attach the wheels this way allowing the two wheels to both attach to the same hub. This lower the cost associated with building these trucks.

For most commercial trucks, the rear wheels are identical to the front wheels, with the outer pair flipped around. This ensures fleet operators can buy several tires that will be used as required whenever one needs replacing. Semi-truck wheels could differ from one trailer builder to another, but they are typically the same.

Is the answer to the shape of semi-truck wheels really that simple?

Semi-Truck on the Road | Wiki Commons

Yes, it certainly is that simple. The packaging makes sense with the front wheels convex to make room for necessary components in a space where there is only one wheel at each end of the front axle. The use of single hubs for each pair of rear wheels cuts the overall cost of building and maintaining these big trucks and the trailers they pull. It’s an amazingly simple but sensible solution to what could have been a much more complicated conversation.

Next, learn why semi-trucks might be tailgating you, or enjoy this video below:

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