Looking at Risks and Side Effects

New truck cab designs promise lower fuel consumption,– but also demand a rethinking of steering and wheelbase

EU directive 2015/719 permits longer truck cabs if they are able to enhance safety and aerodynamics.

However, this change also has implications for a truck’s steering and wheelbase. IAV is examining and optimizing new cab concepts on behalf of OEMs and has also presented a proprietary solution.
At present, the frontal area of a driver’s cab produces a high level of aerodynamic drag and excessive fuel consumption. Lengthening the cab and introducing larger curvatures would make it possible to improve the drag coefficient. But larger radii also demand a longer front end – precisely what the new EU directive is permitting. “As a result, the front end of a semi-trailer can be lengthened by as much as 80 centimeters”, explains Olaf Jacob, manager of the Truck Rolling Chassis project at IAV. “Its rear section can grow by up to 50 centimeters in length if aerodynamic tail end fairings are fitted to it.”

The measures to reduce fuel consumption, however, also come with side effects that need to be taken into consideration when modifying cab design. For instance, the legislator still demands that trucks must be able to drive through the turning circle (“BO-Kraftkreis”) defined by German Road Traffic Regulations. Even with a longer driver’s cab, they must remain within the prescribed radii when driving in a circle (12.5 meters on the outer circle, 5.3 meters on the inner circle). This can only be achieved if OEMs modify the steering and wheelbase of semitrailer trucks.

Higher load on the front axle

In addition, truck cabs are getting heavier and heavier. “Equipping them with comfort features, such as engine-independent air-conditioning, and the growing number of control units are constantly increasing their weight in spite of lightweight construction” Jacob says. “But this is leading to higher loads on the front axle.” Among other aspects, this has implications for the parking brake. Under the new conditions, it must reliably prevent the vehicle from moving even on a downhill gradient of up to 18 percent. Also still applicable is the requirement for at least 25 percent of the vehicle’s weight to be supported by the rear axle in order to transfer sufficient brake power to the road.

Possible countermeasures are: extending the wheelbase to the front; even greater use of lightweight design; layout changes, e. g. moving sub-frame components further towards the rear. “Our truck designers have a wealth of experience, such as in lightweight design”, Jacob says. “This enables us to examine and optimize our customers’ concepts.” IAV has also developed a design of its own for a truck tractor that improves aerodynamics and safety, and is also roadworthy. It involves lengthening the truck front end by 80 centimeters and moving the front axle by 40 centimeters.