Abstract
Vehicle is becoming more and more heavier recently to meet high safety
requirements in the market and also to improve comfort customers demand. Accordingly,
weight of body structure, which supports a whole vehicle, is also increasing. This trend also
applies to open sports car category, including NEW MX-5. Body in white of the preceding
MX-5 weighed 230Kg, whereas the competitors´ trend is now 285kg.
Because the preceding MX-5 was a world-famous lightweight sports car, the light weight was
an important task in the development of the 3rd generation model to assure its kinematic
performance. However, delivering just lightweight was not enough. To exhibit the kinematic
performance as a sports car also required strength and stiffness to support suspension, small
yaw moment of inertia and low center of gravity. The car also had to meet high safety
requirements. For the body structure, which is the frame of vehicle, target was set such that
all requirements are satisfied and yet the preceding MX-5 weight level should be achieved.
Lightweight body structure could be delivered by increasing strength of the material even if
thinner sheet metal is used. However, lightweight structure had to be devised that can still
secure stiffness as a sports car even with thinner sheet metal. Ultra high-strength steel had to
be developed that efficiently absorbs crash energy even with thinner sheet metal. Scenario
was first drawn to achieve the target and work got underway.
Firstly, to devise lightweight structure, large-section members were laid out continuously
between wheelbases that directly link to the body stiffness so that thinner sheet metal would
not cause the stiffness deterioration. To be more specific, backbone frame was set at the top
of the tunnel to devise better connecting method between front and rear frame members. The
parts were also made straight as much as possible to allow us to use the high-strength steel.
Secondly, for the development of the super high-strength material, material selection and
verifications of workability, crash load by shape and weld strength were repeatedly performed.
As a result, the mass productionability was achieved. New process of hot stamping process
was also developed. This successfully improved strength of the bumper reinforcement,
leading to weight reduction.
Adoption of such a lightweight structure and ultra high-strength steel allowed the body static
stiffness to improve compared to the preceding models: bending stiffness by 22% and
torsional stiffness by 47%. Body deformation value (dynamic stiffness) during driving also
improved. The structure satisfied the latest safety standard. While there was impact of the
larger vehicle size, weight of the body in white was reduced by 13 kg from the preceding
MX-5, achieving 217 kg. Weight of the body structure including major components, such as
bumper reinforcement bolted to body for serviceability and production efficiency, was also
reduced by 1.6 kg, delivering 247.5 kg.
Keywords - Weight Saving, Ultra High-Strength Steel, Body Stiffness, Light Weight Sports Car, High Mount Backbone Frame