Backing a double-trailer combination can be done by truckers experienced with them, but can be very difficult for those who aren’t. Depending on the configuration, there can be two or three pivot points compared to the fifth wheel-kingpin connection on a single semi, and pushing the forward trailer so it backs just-so against the rear trailer is tricky.

That’s one reason a demonstration by ZF engineers a couple of weeks ago in Germany was impressive; another is that it wasn’t being done by a driver, but a guy with a tablet computer.

Engineers wrote a special application that sends steering and power signals to a tractor. On the tablet’s touch screen, the operator chose forward or reverse, then slid his finger over an image of the rear pup trailer, swinging it left or right.

The tractor’s electric power steering turned the front wheels to send the semitrailer and pup trailer in the direction desired by the operator, while the engine revved slightly and, working through an automated transmission, pushed or pulled the trailers as needed.

There was no one in the driver’s seat; it was all done through electronics. Moreover, the operator outside wasn’t directly steering the tractor; the steering responded to the tablet app’s signals, sent after the app calculated which way the tractor’s fifth wheel should push against semitrailer’s kingpin and thus against the drawbar of the pup trailer.

Why’d they do this? To encourage productivity -- to show that a vehicle longer and more complex than the single semis normally used in most of Europe could be employed to haul more cargo in one trip.

With little training, its driver could back it into position for loading or unloading, said engineers from ZF Friedrichshafen AG, which hosted a group of reporters on this demonstration.

Rigs like this are used in parts of Scandinavia, says our European correspondent, Sven-Erik Lindstrand, who sent a photo of one in Sweden.

Normal length is 25.25 meters (82.84 feet) and weight is 60 metric tons (135,000 pounds), soon to rise to 74 tons (166,500 pounds). More often it’s a long straight truck pulling a long dolly-mounted single semi.

It was raining the day of our visit to the demonstration facility near Aachen, in western Germany, so photography was difficult. But ZF had a video shot on a previous sunny day, so you can see how  the system works.  

It’s a little tedious because maneuvering is slow, as it would be if a driver were backing the rig. How’d you like to play with that app?