Contrary to what some of us might suspect, the wobbly wheel of a shopping cart is not installed at the factory. If you need additional proof, wait until the grand opening of a new grocery store and test drive a brand new model yourself. Steering shopping carts easily is rarely a problem during the first few months of a grocery store's operation. It's only after the store has been open for a while that steering a cat becomes more and more of a challenge. There are actually a number of mechanical reasons why steering shopping carts is so difficult.
The wheels on modern shopping carts usually contain metal ball bearings which revolve around a metal axle. The wheels are more likely to be solid rubber, similar to office chair casters, than the pneumatic type found on cars. Over time, the ball bearings, axles and swivels may start to rust, which increases friction and makes the front wheels less maneuverable. Steering shopping carts with rusted swivels and axles is much more difficult than steering brand-new shopping carts.
Another consideration is the difference between the rear and front wheels of most shopping carts. The rear wheels are likely to be fixed and rigid, which is ideal for forward and backward motion, but not so good for side-to-side adjustments. The front wheels are free to move in any direction, but both wheels must be aligned for best results. Steering shopping carts with one front wheel out of alignment creates the frustrating sensation of wrestling for control. Replacing the rigid rear wheels with free-turning ones would only make the problem worse, since there could be four misaligned wheels in play.
Time and heavy usage are largely responsible for the frustrations shoppers face when maneuvering shopping carts down the aisles. Most grocery stores do not employ technicians who can make repairs on shopping carts. Over time, the most problematic carts are retired, or replaced with newer models. Before that day arrives, most shopping carts have been seriously abused by younger shoppers, left out in rainy parking lots, overloaded with groceries and used as makeshift scooters. When the wheels become dirty and deformed, and the casters have been forced out of alignment, steering shopping carts eventually becomes nearly impossible.
Shopping experts suggest test driving a few shopping carts before deciding on the best one to use. The wobbly wheel phenomenon may not become a factor until the shopping cart has reached a certain speed, so give all the candidates a good run whenever possible. Steering shopping carts through the aisles of a grocery store shouldn't have to be an endurance test.