Second generation od Saab 9-3 (2003–2012) includes a number of safety features, including innovations such as Saab Active Head Restraints (SAHR II) to reduce whiplash and ReAxs, a passive rear wheel steering design and passive toe-in to help reduce understeer under heavy braking. This effectively preloads the rear tires by applying some lateral force through toe-in as braking commences.
The adoption of a four-link rear suspension layout has allowed engineers to dial in a unique, passive rear-wheel-steer characteristic (Saab ReAxs) by carefully tuning both inboard and outboard suspension bushings.
When cornering, the kinetics at the rear axle induce a very slight deflection of both rear wheels in the opposite direction to the steering input, ie. toe-out for the outer wheel and toe-in for the inner wheel.
Depending on the radius of a bend and the consequent loadings at the rear axle, 1 degree of movement at the front wheels would typically produce a small but significant response of about one hundredth of a degree at the rear.
This is sufficient to prevent excessive understeer, where the driver is forced to apply progressively more steering input to turn the front end of the car, increasing the scrub angles of the front tires.
The ReAxs characteristic overcomes this “crabbing” effect, helping the tail of the car follow the direction of the front wheels, instead of its nose. For the driver, this gives the car a better balance, helping it turn in and respond more precisely to steering inputs.
