How to design a light, big, and safe car

In an earlier post, we wrote about how reducing vehicle weight could both improve fuel economy and maintain highway safety. 

Many readers were curious to know more about how to make a vehicle lighter (which does not necessarily make it less safe) without making it smaller (which does make it less safe).  

This idea can feel counterintuitive, since historically the weight and size of a car have gone pretty much hand in hand. To unpack this thinking, here are some questions automakers are asking themselves, as they try to design out excess weight while maintaining vehicle size and safety. 

Question 1: "Can I use lightweight materials?"
Lightweight materials are the most direct way to decouple size from weight. Used correctly, they enable designers to escape the vicious circle of designing ever-heavier cars to achieve 5-star crash ratings. 

An aluminum door, for instance, weighs roughly 40% less and can absorb about 2.5 times more energy per pound than a comparably sized steel door. The all-aluminum Audi A8 has the safest crash ratings in its class at 35 mph. 

Stronger, higher grade steel is also an option. The Ford IMPACT program took 25% off the total weight of an F-150 by using high-strength steels, with no safety or performance compromises.   

Composites, like carbon fiber reinforced plastics, present the biggest opportunity to shave off extra pounds. That's because composite parts often weigh more than 60% less than comparable steel parts. These materials may be a decade away from mass production, but it's a question of when -- and not if -- we'll be seeing composite cars in our driveways. 

Question 2: "Can I integrate parts?"
Some of the cleverest vehicle designs in recent years have capitalized on the weight (and cost) savings of integrating parts. 

In addition to the lightweight seat concept mentioned in our earlier post, an integrated radiator support in the 2005 Ford F-150 replaces seven steel parts with one magnesium part, saving 25 pounds. This support was also integrated into the vehicle structure, where it improved torsional stiffness and crashworthiness. 

Question 3: "Can I get rid of unnecessary stuff?"
When the average vehicle model goes through a major makeover, it gains 150 to 200 pounds in added features, with no increase in size! 

We want to keep some of these features, like ABS, extra airbags, and side impact beams. Other features, such as complicated heating and cooling mechanisms in seats, could be eliminated by designing the cabin to manage temperature better, or even designing the seats to enable passive cooling. 

It's all about a design principle pioneered by Henry Ford: "Whenever anyone suggests to me that I might increase weight or add a part, I look into decreasing weight and eliminating a part!" 

Car designers who shed extra pounds from their original designs by answering the first two questions may find an unexpected unnecessary part in the third: that big engine. That's because lighter cars need smaller, lighter, less-expensive engines to meet the performance characteristics drivers demand. 

That same principle holds true for other parts. Designers who use lightweight materials and integrated parts may be able to go with lighter hinges for the aluminum doors or a simpler suspension to hold up a composite frame. We call this "weight decompounding." It occurs when weight loss in one area cascades into weight loss in others. 

Cars shed pounds in the real world
These questions are a simple way to shed light on the complex, aggressive, and iterative engineering process. But automakers can and are rising to the challenge -- and not just with futuristic concept cars. 

The new Mazda2 is 200 pounds lighter than its predecessor and passed its European crash test with the maximum 5 stars. 

Just last month, Nissan announced a new strategy for making its cars an average of 15% lighter in seven years, in order to improve fuel economy and driving experience. 

The new, $2,500 Tata Nano took getting rid of unnecessary parts to the extreme, saving over 10% of the mass (enabling it to use a smaller engine, among other things), but adding 20% more interior space than its closest rival by eliminating features such as a second windshield wiper.

The first generation Nano is not yet ready for the safety or emission standards of the European Union or the United States. The second generation, however, will be headed for Europe. 

The last question worth asking is, "Now that I've designed car that's up to 50% lighter and more fuel efficient, will it be outrageously expensive?" 

The answer is "not necessarily." In fact, in the near future, it may not cost more at all. 

Stay tuned for more on the business case for ultralight, safe vehicles.