The bit that lets a lorry bend
A big articulated LORRY is really two machines. At the front is a tractor unit with the ENGINE and the CABIN; behind it is a TRAILER carrying the LOAD. They join at a flat steel disc called the fifth wheel, with a strong pin that drops into the middle of it. That one pivot is how something sixteen metres long can still get round a roundabout: the front half turns and the back half swings along behind.
It has another advantage. A driver can unhook a full trailer, hook up an empty one and leave again, so the expensive part — the RIG with the engine in it — spends its time driving rather than waiting to be loaded.
Why lorries have so many wheels
All those extra wheels are not about grip. They are about spreading weight. Road surfaces wear out roughly in proportion to the fourth power of the weight pressing down through each AXLE, which means doubling the load on one axle does something like sixteen times the damage. Splitting the same weight between more axles is gentler on the road by an enormous margin.
That bit of maths writes the rules. In Britain a lorry can weigh up to 44 tonnes, but only with six axles underneath it. It is also why an ordinary car barely counts: cars are so light in comparison that engineers working out how long a road surface will last mostly ignore them and count the lorries.
Every truck is shaped by its cargo
A TANKER is a tube because a cylinder holds pressure evenly and has no corners for liquid to sit in. Inside there are usually baffles — walls with holes in them — to stop the liquid surging forward and shoving the truck along when the BRAKES come on. A DUMPTRUCK carries a hydraulic ram under its body to tip the back up. A FLATBED has nothing above the deck at all, so a crane can drop a girder or a boat onto it from any side.
The real giants never touch a public road. Mining dump trucks carry more than 400 tonnes in a single LOAD, stand as tall as a house, and roll on tyres taller than a grown adult. They are usually delivered in pieces and bolted together where they will work.
The truck that jumps over cars
Long-distance drivers once kept each other company on CB radio, complete with nicknames and their own slang, and a CONVOY rolling together is still a familiar sight. The modern version is quieter: satellite navigation, and a tachograph recording every hour at the wheel, because tiredness is the biggest danger in HAULAGE.
Then there is the MONSTER truck, which does no useful work whatsoever. The first one, Bigfoot, was a PICKUP that a mechanic kept making taller through the late 1970s, until in 1981 he drove it over two scrap cars in a field to see what would happen. Someone filmed it, crowds turned up wanting to watch it again, and an entire sport grew out of one man steadily ruining his own truck.