Fueling Mass Reduction is a Weighty Small Car Issue

July 17, 2010/Steve Tackett

MOTOR MATTERS GREEN WHEELING BY DAN CARNEY

A car built heavier for maximum crash safety protection requires a bigger engine, transmission, frame, brakes and suspension, all of which bring additional weight.
Referred to as “road-hugging weight” is satisfactory for crash safety and cushy comfort. But it is anathematic to fuel efficiency and sprightly performance.
“As we went for product excellence, reducing mass was our lowest priority,” observed just-retired General Motors vice president Bob Lutz, speaking specifically of GM, but his remarks reflect the state of the industry. “On our next generation, we are going to maintain excellence, but get a lot of the mass out,” he said.
Mazda has begun mass reduction effort with the introduction of its new 2011 Mazda2 subcompact car. Naturally, as a subcompact, the Mazda2 is a light car, but Mazda engineers went to even extra effort to whittle away mass from every component, in the expectation that all the small savings would add up to a large total reduction in mass, boosting gas mileage in the process.
The result is a car with a 2,306-pound curb weight, which the company says is the lightest in its class. This is critical, because with rising federal requirements for manufacturers’ Corporate Average Fuel Economy, carmakers are pursuing all avenues to trim fuel consumption of their vehicles.
Sprightly performance is important for a company like Mazda, which trades on its “Zoom-Zoom” image to differentiate itself from competitors like the Toyota juggernaut. Cutting unnecessary weight serves both purposes, so it has become a key ingredient of Mazda’s product development philosophy, one that we can expect to see spread to other manufacturers.
When developing the new 100-horsepower Mazda2, engineers made the body shell stronger through the substitution of high-strength steel for normal “mild” steel. The high-strength steel provides the same strength and safety as a larger — and heavier — amount of conventional steel. The result is a body that weighs 50 pounds less than it would have if Mazda had used its previous approach. They also increased the number of welds and structural adhesive (that’s glue to you and me) to make the shell a stronger unit without adding any more steel. That was good for another 13 pounds saved.
Cutting the mass of the frame meant that the suspension parts didn’t need to be as strong to do the same job, so engineers slashed another 28 pounds from the car there. They substituted a single-piece stamped lower suspension arm for the previous fully boxed welded-together part, which is more technically appealing to gearheads, but unnecessarily costly and heavy.
“It has equivalent stiffness because with the lighter loads, the deflection (from cornering and braking) will be the same,” explained product development engineer Dave Coleman.
Smaller hinges and latches for the doors and hood saved 5.5 pounds. Even optimizing the wiring harness by locating components nearer to one another to cut down on the heavy copper wiring that connects them saved 6.4 pounds.
These items all saved cost too, so the engineers felt emboldened to blow some cash on pricey neodymium speaker drivers — the same stuff used in headphones — in place of heavy iron magnets, cutting the weight of the stereo speakers in half.
The Mazda2’s optional four-speed automatic transmission is a couple gears short of the fashionable six-speeds, but it also contributes to reduced weight because of its fewer components and smaller size.
The four-speed does extract a penalty in fuel economy, unlike many of today’s six-speed automatics, which can match or exceed the efficiency of a manual transmission. The subcompact Mazda2 scores 29 miles per gallon in the city and 35-mpg highway when equipped with the standard five-speed manual transmission, and loses 2 mpg on each test with the automatic. The 2011 Mazda2 starts at $13,980.
Watch for more new models to employ these techniques to whittle away mass in every vehicle segment to boost fuel economy without resorting to costly alternative powertrain technologies. The benefit for Mazda and driving enthusiasts is the extra Zoom-Zoom that comes from having less junk in the trunk. — Dan Carney, Motor Matters

Copyright, Motor Matters, 2010