JSAE Exposition 2015: Mazda2 features latest body structural technologies

Benefits include reduced weight, increased body rigidity, and improved crash safety

2015/06/09

Summary


Mazda Demio

Mr. Hidenori Matsuoka
Mr. Hidenori Matsuoka, Assistant Manager,
Body Development Dept., Vehicle Development Div.,
Mazda Motor Corporation

 The Automotive Engineering Exposition 2015 was held at Pacifico Yokohama on May 20 to 22 by the Society of Automotive Engineers of Japan, Inc. (JSAE). During the 2015 JSAE Annual Congress (Spring) that was held as part of the Exposition, Mr. Hidenori Matsuoka, assistant manager in the body development department of Mazda Motor Corporation's vehicle development division, lectured on the recent development of body structural technologies. He elaborated on the latest technologies that were incorporated in the all-new Mazda2 (sold as the Demio in Japan) for improved crash safety, body rigidity and other structural performances as well as further weight reduction. Special efforts were made in developing the Mazda2 by using conventional methods and steels without relying on high-cost engineering or material. A Mazda2 body was placed in front of the lecture hall for questions and answers session with the visitors. This report focuses on the body structural technologies used in the all-new Mazda2 that were introduced by the lecturer.



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Development themes for all-new Mazda2 and body development issues

 The following themes were chosen as body structural issues for development:

Development themes for all-new Mazda2 Issues of body development
* Attractive styling * Driving pleasure * Eco and clean * Peace of mind * Comfort * Affordable price * Emotional design * Body rigidity * Weight reduction * Crash safety performance * Easy to drive * Silence * Global production

 

 



35.6kg lighter body

 The vehicle weight of the all-new Mazda2 (a white body without doors, closures, fenders and paints) is 213.4kg, lighter by 10.2km than the previous model that weighs 223.6kg. This represents a weight reduction of 35.6kg (16%) in spite of the additional weight of 25.4kg (see Note) intended for increasing the vehicle size and improving performances to increase the car's overall market value.

Weight reduction in the all-new Demio Weight reduction in the all-new Mazda2

Source: Created by MarkLines based on the lecture

(Note) The vehicle size of the all-new Mazda2 is 160mm longer and 25mm higher than the previous model. It has an 80mm longer wheelbase, 10mm longer front overhang and 70mm longer rear overhang.

 Mazda's approach toward reducing the body weight of the all-new Mazda2 is called the multi-load path structure. This includes the optimization of the frame layout, parts shapes, and the frame cross-sectional shapes for optimizing performances, and expanded use of high-tensile steel sheet as introduced below.

 

 



Optimization of body frame layout

Body structure of the engine compartment
Body structure of the engine compartment

 The all-new Mazda2 represents a structural design approach called the multi-load path. It addresses a frame layout of high efficiency, optimization of parts shapes, etc., aimed at improving the body's structural strength and rigidity and crash safety performance.

 The red lines in the photo show how load is transmitted in a frontal crash. By optimizing the frame layout of the side members, hood ridges and suspension members, the crash load will flow smoothly from the front and be absorbed efficiently by the ruggedly structured cabin.

 

 



Increased ridge lines in frame cross-section improve efficiency of energy absorption

 As shown below, the cross-sectional shape at the front of the front side member has been changed from a rectangle to a cross. Increasing the number of ridge lines in the cross-section of the frame is a way to improve the efficiency of energy absorption in frontal crash.

Front side members Front side members
Front side members with increased ridge lines

 

 



Dispersing load in body panel

 The floor panel of the all-new Mazda2 is designed after the concept of dispersing the frame load running longitudinally from the front side members to the side sills over the floor panel. While the floor panel of the previous Mazda2 is made of 270MPa-class material (sheet thickness=0.6mm), the plate strength of the new Mazda2 has been increased to 440MPa (thickness=0.6mm). Moreover, the number of spot welds between the side member and the floor panel has been increased to improve the weld strength. This allows dispersing the frame load over the floor panel which results in increased structural strength of the entire body. This, in turn, enables discontinuing the use of the reinforcement frame found in the previous model. This alone contributes to 2.6kg weight reduction.

Method of dispersing load from the frame to the floor panel Method of dispersing load from the frame to the floor panel

 

Dashboard panel area
Dashboard panel area (t represents "thickness.")

 The same approach applies to the dashboard panel as well. Dispersing load from the front side member over the dashboard panel has enabled the removal of the reinforcement frames. The dashboard panel of the previous model was made of 270MPa-class, 0.8mm thick material. The dashboard of the new Mazda2 is made of a 440MPa-class, 1.0mm thick high-tensile steel. As a result, the reinforcement frame shown by dotted lines in the photo was removed in the new model.

 

 



Performance Maximizing Section

 In designing the all-new Mazda2, the engineers pursued the best cross-sectional shape that would prevent the deformation and buckling of the panels constituting the body frame. This has led to a "performance maximizing Section" that maximized the flexural strength of the hollow frame. The highly efficient cross-sectional shape is determined by three points:

 (1) Constrictions at proper positions in sides (2) The angle between the planes connecting the upper and side planes is made closer to a right angle. (3) The width of the upper plane is minimized.

 Compared with ordinary cross-sections, the strength is nearly quadrupled in the new cross-section when used in the thin sheet range of high-tensile steel. This cross-section is used in various places of the body to achieve 5.2kg weight reduction overall in the Mazda2.

 

oncept of performance maximizing cross-section Second cross member
Concept of performance maximizing cross-section Source: Created by MarkLines based on the lecture Second cross member

 

Rear side rail area Bumper beam area
Rear side rail area Bumper beam area

 

 



Expanded use of high-tensile steel sheet

 The use ratio (by weight) of 780MPa-class or stronger high-tensile steel sheet is 10.6% in the previous model and 30% in the all-new Mazda2. A 1180MPa-class high-tensile steel is used in the bumper beam. The material change, including the expanded use of high-tensile steel sheet, has achieved 6.7kg weight reduction.

Use ratio of high-tensile steel sheet Use ratio of high-tensile steel sheet (by weight)

Source: Created by MarkLines based on the lecture

 

Use of high-tensile steel Use of high-tensile steel

Source: Mazda2 PR material

 

 



Breakdown of 35.6kg weight reduction

 The graph shows the breakdown of 35.6kg weight reduction. The main contributors are the performance maximizing cross-section (11.2kg), the optimized frame layout based on the multi-load path approach (7.7kg) and shape optimization (5.6kg). The effect of the material substitution (6.7kg) results mainly from the expanded use of high-tensile steel sheet. A special contributor is the high strength structural foam material (1.0kg) used in the B-pillar.

Contributors to 35.6kg weight reduction Contributors to 35.6kg weight reduction

Source: Created by MarkLines based on the lecture

 

 



Increased body performances

 The all-new Mazda2 has achieved 35.6kg weight reduction while, at the same time, achieving improved crash safety, body rigidity and other structural performances that were key points in its development.

 The all-new Mazda2's crash safety has proved to be among the highest in various crash tests.

 As for the body rigidity, the torsional rigidity which is an index in a static test has improved 22% and the torsional resonant frequency which is an index in a dynamic test has improved 14% respectively from the previous model. The improvement in the body rigidity leads, other than the improved rigid feel, to the improvement in steering response, high-speed stability, ride comfort in various scenes, suppressed vibrations and quiet operation. For instance, the road noise has been reduced by 5dB.

 The all-new Mazda2 was named the 2014-15 Car of the Year Japan. The innovative body structural approach contributed significantly to the improvement of the car's total performance through weight reduction and improvement of various performances.

Increased body rigidity Increased body rigidity

Source: Created by MarkLines based on the lecture

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