The front and rear suspension have been redesigned to deliver low-to mid-speed agility and high-speed stability, nimble yet high quality ride comfort, with a light chassis and superior rigidity. A lightweight, highly rigid cross member and electric power steering also contribute to quick handling.
When the yaw gain (the force that turns a vehicle) is set at a high level to enhance the car’s nimbleness at the low-to-mid speeds, it tends to become excessive at high speeds, producing an oversensitive response in the car’s movements. To resolve this issue, we re-examined the geometry of the rear suspension. First of all, to ensure smooth movement at high speeds, we optimized the suspension links, and increased the grip of the rear wheels in response to impact (reducing the yaw gain).
Next, to ensure nimbleness of movement at low-to-mid speeds, we adopted a higher steering gear ratio (increasing yaw gain). By doing so, we simultaneously increased yaw gain at low-mid speeds and reduced yaw gain at high speeds, achieving improved nimbleness at the low-mid speed range and greater stability at the high-speed range.
The next challenge was to deliver agility at low-to-mid speeds at the same time as maintaining stability at high speeds, in terms of both vehicle movement and the steering force required. In other words, this refers to ensuring that the steering feel varies according to the speed of the vehicle. First of all, to ensure firm steering at high speeds, the front suspension caster angle and caster trail were increased to enhance self-aligning torque. Next, to ensure light and smooth steering at low-mid speeds, electric power steering controls were used to increase the power assistance at low-mid speeds, thus making the steering lighter. In this way we have created a steering feel that conveys security and a sense of oneness between car and driver, while also matching the car’s movements.
The next aim was ride comfort – the balance of agility with comfort. We revised the structure of the suspension to improve handling without making the springs and dampers stiff. First of all, to enhance the operational efficiency of the dampers, the mounts were set at a position that enables a greater lever ratio. By implementing this change, damping force and the rigidity of the top mount rubber could be reinforced, reducing the impact on ride comfort. The rear suspension trailing link attachment position was also shifted upwards. In this way, the direction of movement of the trailing links is adjusted to more easily absorb longitudinal impact shocks from the road, contributing to improved ride comfort. At the same time, as this also prevents the rear of the vehicle from rising, the vehicle has increased stability when stopping, which helps to reduce stopping distance.
To help to achieve the weight reduction target, we worked on the optimization of the cross member (suspension member) structure and engineering method. After first ensuring that the functionality requirements were met, CAE technology was then used to create a concept model to develop the optimum structure. When doing so, we verified at the same time that this was coordinated with the overall vehicle package. In the front, the center car section was extended and the longitudinal offset of the lower arm attachment position was reduced. In the rear, the longitudinal span of the cross member was extended and the longitudinal offset of the lateral link attachment position was reduced. Welding flanges were also removed from the front and the rear, to enhance the coupling rigidity of the welded sections. The structure thus adopted achieves both weight reduction and superior rigidity, contributing to the 14%* in weight reduction of the entire chassis, compared with current models.
* Figures are for a CD-segment car (Mazda6 class)