How Do Anti-Theft Wheel Locks on Shopping Carts Know to Trip at the Edge of the Parking Lot?

Today’s perimeter is just a buried wire carrying electric pulses.

I know anti-theft wheel locks on shopping carts are designed to trip at the edge of the lot, but how? And how come half the time the wheels are locked when the cart is quite close to (or even inside) the store, nowhere near the edge of the lot? —The Squeaky Wheel

We’ve come a long way since Elmer Isaacks patented the first shopping cart wheel lock back in 1968. Until then, anyone could make off with a shopping cart at will and abandon it in the nearest park, pond or private yard whenever they got bored. Thank God such things are unheard of in today’s wheel lock-enabled utopia.

Isaacks’ wheel lock enforced the cart perimeter with a row of magnets under the pavement. When a cart rolled over the line, a magnetic mechanism would push a rod through a hole in the wheel, like a stick through the spokes of a bicycle.

Modern systems are more sophisticated, if less elegant: Today’s perimeter is just a buried wire carrying electric pulses. An induction coil in the wheel picks up these pulses and passes them to an onboard computer, which, sensing its moment, responds by activating an internal, motor-driven braking system.

These different approaches to the same problem have one thing in common: Both can be defeated by simply keeping the locking wheel a few feet above the perimeter line, usually by doing a quick shopping-cart wheelie as you cross. (Some stores now use two locking wheels on opposite corners, making this exploit impossible without hoisting the entire cart over your head, Hulk style.)

So, if locking wheels are so easy to beat, why bother? Well, they’re still better than nothing; every cart saved is $150 you don’t spend. But it’s also because these systems don’t just fight cart theft, many also fight “pushout theft,” also known as “filling your cart and walking out without paying.”

When a cart enters the store, the lock is armed, and will engage if it goes out the door without first going through a checkout line. It’s a cute system, but you can see how it might get confused and lock up an innocent cart occasionally. Since the average pushout costs $560, it’s worth the trouble to the retailer—especially when that trouble falls mostly on folks like you and me, Squeaky, with our square-wheeled carts.

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