The benefits of making cars lose weight

With the ever-increasing levels of luxury and safety being built into the modern car, not to mention the fact that each new model tends to be larger than the one it replaced, controlling mass is becoming a bigger challenge. The problem with increased mass is that it not only hurts fuel economy, it also tends to make the car feel ponderous. Neither is good for those in search of a fuel-efficient ride that’s fun to drive.

Audi has been sweating the grams for years. The first car to undergo a serious diet was the full-sized A8 launched in 1994. It introduced the world to the Audi Space Frame (ASF). The benefit of using aluminum is simple - the current A8’s body-in-white tips the scales at just 218 kilograms. To put that into perspective, a steel body would weigh roughly two-and-a-half times that of the ASF.

The use of aluminum is reaching other areas of the car as well. For example, replacing the traditional steel front subframe, both lower control arms, front hubs and bumper bars with aluminum components drops the mass by 12.7 kg. An overlooked advantage is that an aluminum-intensive car is easier to recycle than its steel counterpart, which reduces its environmental baggage.

Another key diet area is the brake system. If the A8 W12 used cast-iron rotors, they would weigh 19.1 kg apiece. Using a cast-iron rotor with an aluminum hub drops that to 13.4 kg. The ultimate solution, at least from a weight perspective, is to use ceramic brakes — they weigh just 8.6 kg each, which represents an enormous reduction in mass. Then there’s the simple stuff. For example, replacing the usual wooden spare tire cover with one that uses a polyurethane honeycomb shaves another 2.5 kg, and so it goes.

There are other less obvious benefits to mass reduction. First, a smaller engine can be used without sacrificing the power-to-weight ratio and, therefore, performance. This not only cuts fuel consumption, it allows a smaller tank (which is lighter) to be installed without hurting the driving range. Dynamically, the dividends are just as big. Lopping 200 kg off the average car not only shortens its stopping distances, it cuts the distance needed to accelerate to 100 kilometres an hour by around 12 metres.

To drive the light-makes-might point home, there were a number of different exercises at Audi’s deep-dive Tech Day. The first involved pedaling a pair of bikes around a short circuit. One of the bikes was normal; the other had 7.5 kg added to each of its four wheels. The normal bike was a cinch to pedal. The heavy bike was a heart attack waiting to happen — it sure left me puffing.