Fascinating inner workings of trucks

We see them every day next on the road - huge truck tractors, "horses," towing massive trailers.

We see them every day next on the road - huge truck tractors, "horses," towing massive trailers.

They bear legendary names like Freightliner, International, Iveco, Mack, MAN, Mercedes, Scania, Volvo. They haul their cargoes by day and by night across the length and breadth of our country and continent.

And yet the man in the street knows little about the inner workings of these giants of the highway. Let's take a peep into the fascinating world of the big rigs.

First, the engines. They are all turbocharged diesels, mostly straight sixes, though Mercedes has a V8 and a V6. The capacities range from about 11-litres to 16- litres. Without turbocharging they would have been considerably bigger. The diameter of each piston is typically about 130mm and the stroke about 150mm - think of two one-litre paint tins on top of each other. Compare that to a typical 1,6-litre passenger car engine where each cylinder's swept volume is slightly smaller than a half-litre oil can. The maximum power output of the turbodiesels used in the trucks range from 280 kW to 370 kW, compared to about 80kW on the 1,6-litre petrol engine.

Even more astounding is the maximum torque developed by these big diesels. It ranges from 1800Nm to 2400Nm, whereas it would be about 150Nm on the 1,6-litre passenger car engine. Given proper maintenance, the truck engines can routinely go 800000km without a major overhaul, and the magical 1 million km is not unheard-of in trucking circles.

One really has to stand next to one of these engines to appreciate how big they are. The 15-litre straight 6 Cummins engine used on Freightliner and International horses, for instance, has the same mass as a VW CitiGolf with four occupants. This doesn't include the clutch on the Cummins engine, but it does include the 45 litres of oil and 24 litres of coolant in the engine.

The gearboxes on these super trucks, like many other features, are in a state of rapid development. The days when Oshkosh drivers were famous for having left legs twice as strong as their right legs, courtesy of the Oshkosh's fierce clutch, are long gone.

Nowadays gear changes, if not automatic, are by fingertip control. And on trucks that still retain a clutch pedal, the action is featherlight. Many trucks have 12-speed gearboxes, some 13- or 14-speed, often including one or two "crawler" gears for low-speed manoeuvring.

Two reverse gears are usually provided, one a crawler. When the gearbox is in automatic mode, the changes are mostly controlled by an integrated electronic management system with uncanny precision.

The horses all have single wheels on their steering front axles - it would be impractical to steer double wheels.

They drive through both of their tandem rear axles, which each carry double wheels at either end. The horse alone rides on 10 tyres.

Speaking of the number of tyres, the most common trailer designs have either 3 or 4 pairs of double wheels, making a total of 12 or 16 tyres just on the trailer.

This means that the combined horse-and-trailer frequently has either 22 or 26 of the huge 315 x 22,5 radial ply tyres, each costing just under R3000 new, and lasting perhaps 400000km a casing, by which time they will have been retreaded once or twice. The average is 1,5 recaps a casing.