When four-wheel steering is a bad thing

Snaking away

Image by Unhindered by Talent via Flickr

I hope you’ll pardon my rant, but here’s something that really annoys me. British shopping trolleys. You’d think that when the inventor of the shopping trolley thought about how to make it easy to use, he’d have had a quick look around him at other four-wheel objects, and not try to reinvent the wheel. The vast majority of things on four wheels fall into two categories:

  • Their rear wheels are fixed, and the front wheels turn to steer the thing
  • Their front wheels are fixed, and the rear wheels turn to steer the thing

The second category largely consists of forklifts, construction vehicles and the like, where maneuverability under loaded conditions is important. There’s a hint in there somewhere for someone willing to keep their eyes open.

But in their distinctly finite wisdom, nearly all major purveyors of shopping trolleys in the UK have chosen to create a new category – where all four wheels twist freely, to choose their direction without limit.  Which is exactly what they do. I could go into the laws of physics to explain exactly why this is a terribly bad idea, but I’d no doubt get sarcastic comments about “maths people”. But all you need to keep in mind is the concept of inertia – an object moving in one direction will tend to keep moving in that direction. And the heavier it is, the more effort you need to put in to make it change direction.

Now, when you have two wheels that are fixed, all you need to do is twist the cart a little, and the fixed wheels do all the work for you, because they force the trolley to move at an angle (provided, of course, that your speed is not so high that you lose traction on the glossy supermarket floors. But in that case, you have more serious problems). When you don’t what happens is that the cart stays moving forwards, but at an angle. Sort of like a severe case of understeer and oversteer at the same time. In order to turn it, you actually have to get some weight in front of the trolley in order to apply a sideways force to the front end.

At least, I think so – I haven’t really managed to find a foolproof way of steering these things yet. Because you start off your shopping trip with a bit of confidence, trying a new technique, and because the empty trolley is light, and easy enough to control, you’re thinking “this new method I’m trying is really working!”. But as you fill it up, it gets heavier and heavier, and you have less and less control, and you start doing something akin to the time warp – a jump to the left, a step to the right, a pelvic thrust, and somehow you manage to make it round the corner without wreaking devastation on the muffin aisle. Or wiping out the three children desperately clinging to the side of the oncoming trolley whose flimsy female owner is slinging a month’s worth of pork pies and müller rice around the 2 for 1 cheese specials at the end of the aisle while doing her best to apply an unbalanced force to the metal mayhem, and generally managing only to appear unbalanced herself. And you leave the store with warbled knees and tired arms trying to think of something else to try to keep the beast under control next time.

The worst are the doddery old ladies, who are unable of applying the level of control necessary, and end up congregating in the corner of the store like playground bumper cars that’ve lost power and get shifted to one side to stay out of the way. I picture the store manager coming around every few hours with a broom and cleaning them out, perhaps giving them a bit of advice about using mobility-carts instead.

In the rest of the world, of course, trolleys have fixed rear wheels, and the only problem that causes is sideways maneuvering when trying to pass other shoppers in an aisle. Something that our four-wheel loosies are rather good at. But when it comes to steering and control, they’re streets ahead. A while back, I saw a documentary on skid training, where they replace the rear wheels of a car with swivel wheels in order to replicate conditions of zero control without having to use a skid pan. All they really need to do is put a steering wheel on a UK shopping trolley, stick the trainee in it, and give it a push. The Jackass approach…


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3 Responses to When four-wheel steering is a bad thing

  1. Jill says:

    I’m 100% with you here. We have them at some misguided stores here in the Boston area. At least we did, but come to think of it I have not experienced one lately, so maybe someone complained and they listened. Or maybe some children fell off while the mother was slinging pork pies and rice. Or a supermarket executive made a rare solo trip for groceries and discovered the problem first hand.
    Anyway, good luck. I feel your pain.

  2. Mark says:

    Here too in Oz trolleys are as you describe – but there is at least one advantage: you can rotate them in place which can be useful when turning them around in a narrow aisle.

    Take care, Mark

    • ntickner says:

      Yes – but how do you stop rotating? No – I do see the value in low speed manoeuvring, but doing anything with the vaguest sense of urgency is much more awkward…

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