Why can the wheels of a supermarket shopping cart turn freely?
Time：Oct 22,2025
Why can the wheels of a supermarket shopping cart turn freely?
During the holiday, I wandered into the supermarket, casually pushing a little cart as I turned left and right—its wheels seemed to follow along as if they understood every word I said. Have you ever wondered: why is it so obedient?
The story has to start with the wheel.
Ancient humans discovered that placing logs beneath heavy objects made pushing them far easier than dragging them directly. Thus, the humble log evolved into the wheel, and with the addition of an axle, the transportation revolution began. Without the wheel, there would be no cities as we know them today—and certainly no supermarkets to explore.
Wheels are naturally "stubborn," stubbornly sticking to a single path no matter what. Want to make a turn? You’d have to lift the entire contraption off the ground. Later, someone added a "neck" to the wheel—a horizontally rotating bracket—and that’s how casters, commonly known as swivel wheels, were born. Thanks to this clever addition, wheels can now "shake their heads," allowing vehicles to finally "turn corners."
At the bottom of the shopping cart are four universal wheels: the frame and axle aren’t locked, allowing them to swivel freely by 360° around the vertical axis. When you push the cart forward, the wheels effortlessly "skim along" right behind the rotating axis. But as soon as you start rolling backward, friction instantly applies a torque to the wheels, causing them to "turn their heads" 180°—and obediently follow right behind again. Simply put, the wheels obey whichever direction the friction pushes them toward, which is exactly why the cart moves precisely where you intend it to.
Don't let its simple structure fool you—building it actually requires quite a bit of care and precision.
1. Steel plate shearing and punching to form brackets and brake pads in a single step;
2. Grease the bowl-shaped dish, arrange steel balls in two concentric circles—one above and one below—then press the bracket firmly into place to rivet it securely. This locks the steel balls permanently within the "track," effectively serving as bearings.
3. Rubber is melted and molded into wheels, the parting lines are polished, axle rings, screws, and nuts are installed, and finally, the product undergoes rigorous testing before being approved for operation.
Interestingly, the front wheel of a bicycle is actually an upgraded version of this same "universal joint" principle: the intersection of the front fork's extension line with the ground always lies ahead of the point where the wheel touches the surface. That’s why pushing forward keeps the bike remarkably stable, while pushing backward causes it to wobble like a snake. In fact, this tiny relative positioning plays an even more critical role in determining the overall stability of the bike than the wheel’s inertia itself.
Next time you're charging ahead with your shopping cart, don’t forget to give those four universal wheels beneath your feet a big thumbs-up—they’re the real unsung heroes behind your "shopping drift."
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