Quote:
Originally Posted by nicknaz
If there is so much Al.. why is the car still 3500lbs
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Because it's a modern roadster, built for today's market and focusing on tomorrow's sensibilities.
Open-topped cars were traditionally wobbly affairs, so compromised for handling and ride, not to mention safety. That won't fly now, so the structure beneath those alloy panels is built up to a strength rating, instead of down to a weight. Make a rigid car first, then add lightness. Modern safety and comfort equipment, multiple airbags, automatic climate control, iDrive; all add weight, but just try selling a car without them...
Instead, the weight saving is strategic; bonnet/hood, trunk/bootlid, front wings, these overlap the ends of the wheelbase and lowering their weight concentrates more weight inside the wheelbase, improving handling. The plastic bumpers/fenders further this philosophy, whilst being cheaper to replace after a bump. The light alloy roof lowers the centre of gravity when raised and demands smaller, lighter motors to open and close it; ditto the boot/trunklid which opens to stow the roof. The doors are all-steel aren't they? This greatly improves rigidity in a crash as steel is less flexible than aluminium/aluminum; with the doors open it's just the floorpan that has to take all the punishment. Steel is also better for side-impact protection, not to mention feeling more weighty and sounding more solid when you close the doors; perceived quality is very important in the showroom. Does the suspension use any alloy components? Do the brakes have separate alloy hats/bells? These measures would help reduce unsprung weight which is very useful if kept to a minimum.
At the end of the day, the car can't be all-that light as an all alloy construction, or composites like carbon fibre, would simply make it too expensive for its market segment. Being open-topped requires the roll-over hoops (high strength steel?) and reinforced windscreen frame and glass (triple laminated?). These add significant weight. The Run Flats are due to packaging; the small rear end has to contain BMW's sophisticated rear suspension, the (small) fuel tank, the entire gubbins to stow the roof, usable boot/trunk space, the driveshaft and diff,, the enclosures for the subwoofers behind the seats if fitted, enough room to recline the seats themselves for comfort... there's nowhere for a spare wheel, even a space saver. The car I'm looking at has normal tyres, thankfully; runflats would be murder on our roads.
The lack of LSD is quite vexing. The obvious reason is BMW don't want the car competing with their more elite M Power machinery, but I like to think it's to make the car more of a GT; LSDs are quite twitchy I've heard.