+ Click to Enlarge ROBOT IN CONTROL: Volvo uses a computer-controlled robotic steering system for accurate repeatability of steering moves and to avoid drivers' subjectivity and differing driving techniques.

February 18, 2006

Volvos have gained an excellent reputation as versatile tow cars and Volvo customers around the world regularly use their cars to tow trailers, horse boxes, boats or caravans, so expect high standards of towing performance.

As a result, towing tests are an important part of Volvo's rigorous durability testing when it is developing new cars and it is probably the only car manufacturer to have its own bespoke range of trailers and caravans, plus a special robot to test the towing performance and stability of its cars.

Volvo has helped create a new ISO standard for towing tests.

Volvo's testing confirms that the ideal characteristics for a good tow car are good performance with high torque from low revs while its gearbox, steering, brakes and suspension all combine to play an important role in determining how the car will function when towing.

Its suspension must not be too soft, and preferably level, when towing as the angle of the car's wheels and therefore their traction on the road can change under heavy loads.

It is important that the steering still reacts predictably and gives the driver a feeling of the road without transferring any movements of the trailer back through the steering wheel as, if the driver instinctively tries to correct these, it usually makes any instability worse.

Stefan Svensson, responsible for vehicle dynamics at Volvo Car Corporation, explained: "When towing, you should always try to maintain a steady course and avoid unnecessary movements of the steering wheel."

Which is where the robot comes in... to avoid inconsistency or subjectivity about how a car functions when towing - it could vary from driver to driver - Volvo has developed a robot that steers the car and is programmed to make exactly the same movements of the steering wheel from test to test to destabilise the trailer.

"The robot always makes the same movements of the steering wheel so we can avoid any human error and the robot test immediately exposes a car if it does not have good towing characteristics," Svensson said.

Volvo has contributed to the development of towing test methods which have now become an ISO standard. The tests involve Volvo's engineers measuring the lateral acceleration of the trailer, the angle between the car and the trailer, the angle of the steering wheel, the yaw angle of the car and the actual speed of the car.

All measured values must be within certain criteria for the car to be approved by Volvo's test engineers.

Volvo performs its towing tests in extreme conditions to create major loads on the towing hitch as well as major stresses on the transmission, suspension and the rest of the car.

Fully loaded trailers

Test drivers tow fully loaded trailers through endless driving cycles in all weathers on Volvo's test tracks near Gothenburg in Sweden and in Arizona in the US - extremes of heat and cold.

If that's not extreme enough, Volvo also tests its cars towing fully loaded trailers on a 23.3km climb with an average gradient of seven percent out of Death Valley, California where temperatures regularly exceed 40 degrees.

All Volvo cars will have to pass this extreme test.

The cars are also subjected to excess stresses such as repeatedly towing a fully loaded trailer up a 20 percent gradient for which the engineers have a fleet of 35 trailers/caravans of various sizes and weights - including twin-axle caravans weighing 1600kg and horse transporters weighing more than two tonnes.

As you would imagine, Volvo's tow hitches are also tested to the same extremes as its cars and specifically developed for each model.

The car's bodywork is designed with all the anchorages needed for the hitches, which are available in swan-neck and detachable varieties.