BMW flips structure around for EV safety

MUNICH—Two stripped-down versions of BMW crossovers occupy corners of a small white-painted conference room.

Both have metallic silver chassis and door-less bodies in shiny black. On one, a corner of the front end is crumpled; the other has a shallow indent midway along the frame.

The damage was the result of crash tests: The first represented an offset head-on collision at 56 kilometres per hour; the second, the vehicle taking a sideways hit into a pole at just over half that speed.

The dummies survived both collisions.

BMW says the vehicles represent the future of car construction, to arrive first in its four-seat MegaCity Vehicle, or MCV, due in 2013.

The less that battery-powered cars weigh, the longer and faster they go between charges. The batteries are so much heavier than internal-combustion components that it’s especially important to slim down the remaining parts.

The Mini E, which I wrote about earlier this month, weighs 260 kilograms more than its gasoline-powered equivalent. The next step along BMW’s progress toward the MCV, a battery-powered “1” series coupe known as the ActiveE — scheduled for test distribution next year but, like the Mini, not destined for mass production — will tip the scales equally hard.

Which brings us back to those black and silver models. They’re a return to the olden days of body-on-chassis construction, but with a big difference.

In the past, the chassis was thick steel, and strong; the body, the same material, only much thinner and weaker. In most accidents, collision forces bounced from the chassis into the passenger compartment, causing gruesome damage.

BMW says it’s flipping things around: The MCV chassis is aluminum — relatively soft. The body screwed (and perhaps glued) to it is made of carbon fibre — strong and rigid. When the car smacks into something, the chassis crumples and absorbs the impact while the passenger compartment remains intact. There’s so little deformation, the company says, that the doors open normally even if the chassis is badly damaged.

(The battery pack — 96 cells in a shallow rectangular box inside the chassis frame — withstood the crash tests.)

This set-up weighs far less than all-metal construction: Carbon fibre is 30 per cent lighter than aluminum and half the weight of steel. To demonstrate, BMW put two bumper supports on display. Lifting the steel version was a strain. The carbon-fibre variety could be easily hoisted in one hand.

Carbon fibre originates with petroleum — evidence that there are far better uses for oil than burning it.

It has allowed Formula One drivers to stroll away from devastating crashes. But since the racers are hand-built, they have little to teach in terms of mass production.

Aircraft builders also use carbon fibre, but in a form too expensive for automotive use. BMW has a cheaper version, just as strong but too thick for aircraft. And the company is developing mass-production techniques that make body components in two steps, compared with the four or five required to stamp steel panels. The process takes minutes, rather than the seconds for each stamping. but it’s a work in progress.

The fibres are spun into a yarn that’s woven into sheets resembling cloth. The strands in each sheet run one way. The sheets can be layered with the strands oriented in various directions to produce whatever qualities the part requires.

The layered sheets are pre-formed, then, placed in molds where, under intense heat and pressure, they’re shaped and impregnated with resin to produce parts that are stronger than steel, and, as a bonus, won’t corrode.

Carbon fibre also permits swooping, complex shapes that aren’t possible in steel or aluminum.

Four years from now, we’ll see how well it works.

Travel was provided freelance auto writer Peter Gorrie by the automaker.

peter.gorrie@sympatico.ca

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