Shopping carts veer because their wheels are usually swiveling castors that can rotate independently. When all four castors are free, the cart wanders, especially from a stationary start. The standard fix, used on most US grocery carts, is to lock the two rear castors so they can only roll forward and back, leaving only the front pair to steer. And once the cart is loaded, mass and inertia make any veering harder to correct, which is why a full cart feels much heavier to control than an empty one.
It’s common knowledge that for a product to sell, you need a good marketing strategy to advertise your product to the masses, a good sales team, an effective distribution system and many other things. But hey! Did you ever realize that there’s one inconspicuous, utterly common thing that might also play a role, even a tiny one, that helps products get picked from the supermarket shelves?
The Importance Of Shopping Carts
This may sound a bit odd at first, but shopping carts have played an important role in revolutionizing the transaction of commodities in stores and shopping marts. If you’re a person who’s spoiled by the comforts and luxuries of the 21st century, ask yourself this; if checking out different varieties of the same product meant that you had to traverse the entire length of a section in a grocery store, would you really bother?
Of course, some people would ‘go the distance’ to fetch the items of their favorite type or brand, but the average shopper, who doesn’t care if the toothbrush they’re using is made by Oral-B or Colgate, would just pick the one nearest to the billing counter.
The Design
Shopping carts (also known as shopping trolleys or shopping baskets in some parts of the world) are a great example of a simple machine at work. They consist of only two main parts: a metallic basket and a set of wheels. The basket has a handle attached to it (which helps in steering the cart), and it’s installed above a set of four small wheels that make pushing, pulling and steering the cart very convenient.
It’s quite clear that a shopping cart consists of very simple components, but it is of tremendous assistance to shoppers while they roam throughout the shopping mart looking for a particular flavor of cookie or a big bottle of anti-dandruff shampoo.
What’s The Problem With Shopping Trolleys?
In some countries, including India, the United Kingdom and Australia, there is a rather queer problem with trolleys; they seem to have a mind of their own! Suppose you try to turn a trolley towards, say, the left. It would definitely turn, but not towards the left; it would either go towards the right or move straight ahead. The same thing happens when you push them in the forward direction; it goes left or right unless you apply a surprisingly large amount of force to move it in the desired direction.
What’s The Reason Behind It?
The set of four wheels dictate the terms when it comes to steering a shopping cart. In such carts, all four castors are ‘floating’, which means that they swivel. This is a good thing when it comes to the amount of force the user needs to apply to get the trolley ‘moving’, but it’s bad news for steering and overall maneuverability. Since all four wheels can rotate on their respective axes, it becomes much more difficult to move the trolley in the desired direction, especially when you start moving it from a stationary position.
What’s The Solution?
Since the problem lies in all the castors moving in random directions of their own accord, you want to make sure that their random movement is somehow mitigated. Also, you don’t want to impact the maneuverability of the trolley (i.e., it should still turn easily).
The solution is fairly straightforward: limit the ‘swiveling ability’ of the pair of castors on the rear. This way, you can push the trolley with ease, while exercising better control over the direction of the trolley’s movement by steering the front castors. Many countries (e.g., the United States) already have trolleys designed in this way. Thankfully, the ‘fixed-wheel’ design is becoming increasingly common in other parts of the world too, thanks to its popularity in shopping carts for grocery stores.
In terms of applied force on shopping carts in grocery stores, it turns out that a simple design tweak can save a lot of ‘Newtons’ on a daily basis!
Why Do Shopping Trolleys Have Small Wheels?
Have you ever noticed that a shopping trolley rolls on surprisingly tiny wheels, while a bicycle or a wheelbarrow gets big ones? It isn’t an accident, and it isn’t just to save money. The wheels on a cart are swivel casters, a wheel hung on a little fork that can spin a full 360 degrees, and their size is a deliberate engineering compromise.
The biggest reason is stability. A loaded cart can carry a heavy, top-tall pile of groceries, and you really don’t want it tipping over when you swing it around a corner. Small wheels let the basket sit low to the floor, which keeps the cart’s center of mass low and stable. One caster patent puts it plainly: the need to keep the center of mass as low and as stable as possible requires the smallest diameter caster wheel which can still negotiate the obstacles the cart will meet. Tiny wheels also tuck the whole spinning fork into a compact housing under the basket, so the casters can swivel freely without sticking out and tripping you.
The trade-off is comfort over bumps. The same patent notes that the small diameter of conventional caster wheels makes negotiating even small obstacles difficult. A big wheel meets a bump or a floor gap low down, near the ground, and rolls over it easily. A small wheel hits that same lip much higher relative to its center, so a modest crack or a strip between floor tiles is enough to stop it dead, which is exactly why a cart judders and snags at the edge of a travelator or a car-park ramp. Since supermarkets have smooth, flat floors, designers happily accept that downside in exchange for a low, stable, easy-to-store cart. It is the same friction-and-geometry thinking behind how forces balance on a bicycle wheel.
Is It Easier To Push An Empty Or A Full Cart (And Should You Push Or Pull)?
Picture two carts at the supermarket: one empty, one heaped with a week’s shopping. The empty one is obviously easier to get rolling, and physics says exactly why. The friction that resists a sliding or rolling load follows a simple rule, f = μN, where N is the normal force pressing the wheels into the floor and μ is the coefficient of friction. Pile on groceries and you increase the cart’s weight, which increases N, which increases the friction you have to overcome. On top of that, a fuller, heavier cart has more inertia, so it resists both speeding up and changing direction. That is the real reason a loaded cart feels stubborn: it is harder to start, harder to stop, and far harder to steer back into line once a swiveling caster sends it veering.
What about push versus pull? For a heavy load on the ground, pulling can be the easier option, and again it comes down to the normal force. When you push something at a slight downward angle, the vertical part of your force adds to the load’s weight, pressing it harder into the floor, raising N, and so raising friction. When you pull at a slight upward angle instead, part of your force lifts the load a little, lowering N and lowering friction. With a shopping cart you mostly push because the handle is built for it and the casters need to trail behind your direction of travel, but the underlying physics is identical to how friction responds to the force pressing two surfaces together.
Why Are IKEA Carts So Hard To Push And Prone To Sliding Sideways?
If you have ever wrestled an IKEA cart down an aisle and felt it slide diagonally like a shopping trolley on ice, you have met the four-swivel design in its purest form. As we saw, most carts in the United States fix the two rear wheels and let only the front pair swivel, which keeps them tracking straight. In Europe, including on many IKEA flatbed trolleys, it is far more common to have all four wheels swivel.
Four swiveling casters are wonderful when you need to crab sideways into a tight gap or spin a long flat cart on the spot in a crowded warehouse store, which is precisely the situation IKEA designs for. The catch is that nothing is holding the cart to a straight line. A swivel caster only points the right way because of a small offset between its pivot axis and its wheel, which makes the wheel trail behind and line up with your direction of travel once it is rolling. From a standstill, or when you change direction, the four wheels can each be pointing somewhere different, so the cart lurches off at an angle until they all swing into line. Load it up with a heavy flat-pack and that extra inertia makes the wandering even harder to correct. So an IKEA cart isn’t broken, it is simply tuned for maneuverability over straight-line control, the exact opposite priority to a fixed-rear-wheel grocery cart.
