Under the microscope: the rear axle - A sophisticated solution

Nov 4, 2014
Rear-mounted engine, short wheelbase, compact dimensions: the outstanding manoeuvrability enjoyed by smart drivers presented the suspension engineers with a real challenge. The solution for stable handling performance coupled with a high level of comfort is an optimised De-Dion rear axle.
For high directional stability and to reduce the roll angle, the suspension designers strived for an instantaneous centre positioned as high as possible. This is the (virtual) point around which the vehicle tilts when cornering. The connecting line between the instantaneous centres on the front and rear axle is called the instantaneous axis or roll axis.
To reduce the lever with which lateral force affects the vehicle, the centre of gravity and the instantaneous centre (or the centre-of-gravity axis and the roll axis) should be as close to each other as possible. In practice, realising this means that the wheels must have constant toe and camber: when the body tilts (one-sided rebound), for example in a bend, the position of the wheels to the road should not change.
Complex multi-link rear wheel suspensions meet this requirement – a solution that cannot be implemented for smart primarily for reasons of space. A rigid axle would be a simple solution, but very unsatisfactory in terms of comfort due to the high unsprung masses with rear-wheel drive. The smart construction engineers therefore opted for a De-Dion axle. This is a rigid drive axle often used in classic sports cars where the differential is fitted separately from the axle. The differential is attached separately to the body to reduce the large unsprung masses of rigid drive axles. The wheels are connected with a rigid tube which is U-shaped in the case of the smart. This ensures that toe and camber do not change during compression.
To keep the rear axle mass as low as possible on the new smart, a rear-axle tube with variable wall thickness and rear-axle struts with a load step concept were developed.