Cutting weight from a car body lets its battery shrink too

A chain reaction in the design

Cutting weight from a car body sets off a chain reaction. A lighter body needs less force to move and stop, so engineers can shrink the brakes, the motor and, in an electric car, the battery pack, while keeping the same range and performance. Engineers call the first cut the primary weight reduction, and everything that shrinks afterward the secondary weight saving. The United States Department of Energy states that cutting a car total weight by 10% typically buys a 6% to 8% gain in fuel economy, and that lighter materials can let an electric vehicle carry a smaller, cheaper battery for the same range.

A peer reviewed study of 77 real cars and crossovers built between 2007 and 2009 found that if every part of a car were open to redesign, cutting 1 kilogram from the body could theoretically free 0.95 kilograms of secondary saving elsewhere. The real world average across those same cars came to 0.47 kilograms. Once part of the design was already locked in before the redesign began, that fell to 0.12 kilograms. The researchers conclude that weight targets need to be set early, before the rest of the car is decided.

Bigger for electric cars

A study built to compare a battery electric car and a gasoline car under the same design finds the secondary saving runs bigger for the electric car. Researchers publishing in the Journal of Sustainable Development of Energy, Water and Environment Systems found a total to primary weight reduction ratio of 126% for the electric car at 200 kilometres of range, meaning every kilogram cut from the body becomes 1.26 kilograms once the smaller battery, motor and support structure are counted. The same simulation found 111% for a gasoline car of the same design. Subtracting the 100% baseline from each figure, MAOWCE calculates 26 extra kilograms saved elsewhere for every 100 kilograms removed from an electric car body, against 11 extra kilograms for a gasoline car body.

One car, measured in kilograms

A report from the European Aluminium Association, a group that represents aluminium producers, describes 1 real world example. An electric car of the Volkswagen Golf class can be made 187 kilograms lighter using aluminium instead of steel, the report states. The body dropped by 162 kilograms and the battery pack that followed shrank by 25 kilograms, holding range constant. That 187 kilogram cut equals 14.2% of the 1,327 kilogram steel version weight. The aluminium version also cost 635 euros less to build, and emitted 1.5 tonnes less greenhouse gas over its full life, according to the same report.

Ford spends the saving on battery

Ford, a carmaker, gives a real production example of the same trade. On its newest electric pickup platform, Ford replaced 146 separate structural parts with 2 aluminium castings, each 27% lighter than the best competing castings on the market, and cut the fasteners needed by 25%, according to WardsAuto. Ford also shortened and lightened the wiring on its newest electric vehicle by 4,000 feet and 22 pounds compared with its first generation electric SUV. Alan Clarke, Ford Executive Director of Advanced EV Development, explains that a heavier, cheaper part can be worth making lighter and more expensive if the weight it saves lets the car carry a smaller, cheaper battery instead, since the battery alone can account for about 40% of a battery electric vehicle total cost.