Why do commercial vehicles have convex and concave wheels

Why do commercial vehicles 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. When I was very young, I probably once rode in the back seat of my mom’s 1992 Dodge Caravan and stared out the hinged window and wondered why tractor trailers and work truck wheels looked so paradoxically sunken and bulbous. And then I pushed that thought aside for many years, as we do. But there are reasons why trucks are the way they are, as we’ve already discussed. And today we will deal with the wheel question. Spoilers: It’s all about packaging, cost savings, and good old-fashioned common sense.

The front wheels of many commercial vehicles are typically outwardly bowed, with the area above the hub being bulged. This means that the steering joints, bushings and bearings as well as the suspension and braking system fit, while the axle remains as wide as possible for maximum stability. It also means that the center of the axis around which the wheels rotate is optimally aligned with the center of the wheel and tire, which also benefits dynamics and load capacity.

Front view of a Ford Super Duty

Here is a Ford Super Duty with the two rear wheels facing each other. The “face” of the outermost rear wheel that you can see is just the other side of the front wheel. Image: Ford

If we look backwards, things get a little more interesting. If you’ve never seen both sides of your standard truck Steelie, you might not know that the outermost wheel on the back of a Dual – a truck with two rear wheels per side – is identical to the wheels on the front axle except reversed, so you’re looking at the back. This wheel is actually bolted to the face of the innermost rear wheel, which is oriented the same as that on the front axle.

An ingenious solution, as the advantages of the front axle geometry still apply to the rear axle, but you also get the improved weight distribution and increased payload of four rear wheels. And should a tire fail and need replacing, all wheels are the same. No need to bother with front or rear specific wheels. How’s that for practicality?

That’s it. One of those simple answers to a simple question that will blow your mind with its cleverness. Trucks: an endless source of satisfying technical problem solving that never stops giving.