Maintaining stability on the road

TEXT: ANDERS NORDNER

Just as he turns on to the motorway off-ramp, a driver’s mobile phone rings. For a second or two, his attention wanders. As a result, his speed on the off-ramp curve is somewhat higher than planned. This could have disastrous consequences − unless he has a vehicle equipped with ESP, the electronic stability programme.

ESP is an automatic system that acts to stabilise a vehicle in extreme situations when there is a high risk of skidding, overturning or jack-knifing.

Put simply, the ESP system compares the route of the vehicle with its desired direction. In case of excessive divergences the system intervenes, without involving the driver, and applies the wheel brakes as appropriate, reducing engine torque as much as necessary to get the vehicle back on track again.

“The difficulty with ESP systems is to strike the right balance between safety and driveability,” says Jolle IJkema, a vehicle dynamics engineer at Scania. “Providing absolute safety is no problem, but then the vehicle can’t be driven any more.”

Scania’s philosophy is to lean in the direction of safety and refine the response of the ESP system by means of continuous improvements.

Yaw control and roll-over protection

The ESP system consists of two sub-systems with different functions: yaw control and roll-over protection.
The purpose of yaw control is to help the vehicle continue in the desired direction of travel. Yaw control intervenes primarily in case of slippery, low-friction surfaces and activates the brakes on individual wheels in order to decrease the tendency for the vehicle to turn out of control.

The purpose of roll-over protection is to prevent the vehicle from toppling over in case of high lateral acceleration. Roll-over protection intervenes primarily on dry surfaces with high friction. It applies the brakes on all wheels in order to lower vehicle speed − the most important factor in reducing the risk of roll-over.

Every time the ESP system is activated, this information is stored in the system memory. If the system is activated often, this is an indication that the driver may need further training.

At present, Scania is offering ESP on its tractor units, but the European Union has decided to require ESP on all new two- and three-axle tractors beginning in July 2011 and on all two- and three-axle trucks beginning in July 2012.

ESP can save lives

The Swedish Road Administration has calculated the effect of different measures for improving road safety.

The greatest effect is achieved by installing median barriers on roads. But passive crash safety or accident prevention systems such as ESP can also be very beneficial.

For each 100 road fatalities, the Road Administration estimates that the following number of lives can be saved by using various measures:

Measure    Lives saved
Median separation    51
Centre- and road-edge rumble strips   19
Lane departure systems    8
Belted truck drivers    5
Anti-slide systems on trucks (ESP)   3

Source: Swedish Road Administration, publication 2008:136