Vehicle light-weighting of late has gained a lot of importance across the automotive industry. With the developed nations like the U.S. setting stringent fuel economy targets of 54.5 mpg by 2025, the car industry's R&D is taking light weighting to a whole new level, besides improving engine efficiency. The commercial vehicles on the other hand are also gradually catching up when it comes to using alternate material for weight reduction. This paper will discuss light-weighting in the context of buses though.
For a typical bus, the contribution of shell structure weight in the bus body weight is more than 40%. This qualifies as the area with a huge potential for weight saving. On the other hand the shell structure forms the base skeleton of the bus body providing it with adequate strength and stiffness for meeting both functional (bending & torsional stiffness) and passive safety requirements (rollover compliance).
There are certainly many ways to achieve light weight within the strength and safety requirements. Common ones are to completely replace the existing structure material with higher yield strength material with a possible reduction in section thickness. The other one is to selectively replace conventional steel at specific areas. While the first approach with homogenous material has certain advantages in procurement and manufacturing, it is not optimal in the design perspective and results in higher costs.
This paper discusses the second approach, multi-material concept, by applying it to an existing 8M City bus and studying the design implications and long term benefits.