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Lose Some Weight, Will Ya?

July 15th, 2009

An article in the current issue of SAE‘s Automotive Engineering International (“Everything but the Engine”) discusses the effect that components other than the engine have on vehicle fuel efficiency.  Among the items discussed are transmissions and the benefits of DCTs (which I previously talked about here) and tire rolling resistance (which I previously talked about here).  (…Wow, I’m very timely, aren’t I?)  Other efficiency mechanisms discussed are reducing vehicle size, using advanced lightweight materials, improving aerodynamics, and reducing the weight of all the accessories that are packed into what have essentially become condominiums on wheels.

Buick Enclave Rear Suspension

Buick Enclave Rear Suspension

Let’s talk about weight.  Our cars need to go on a diet.  I remember last year reading a Road & Track article about the new BMW 1-series (finally, a small BMW to replace the formerly-small 3-series!), and my jaw hit the floor when I saw how much it weighs – 3373 lbs for a manual 135i?!  This is small car?  …At the other extreme, my wife’s Buick Enclave has a curb-weight just shy of 5000 lbs, despite the use of aluminum in some of the body panels and suspension components.  (The Enclave’s aluminum rear control arms are a thing of beauty, though.  GM did do a few things right.  …Am I the only one for whom control arm material is a criteria for purchasing a vehicle?)

More extensive use of light-weight materials such as aluminum, magnesium, high-strength steel, and carbon fiber can reduce weight, but it seems like automakers are more focused on making cars more bloated while using these materials to offset a portion of the weight-gain that would otherwise occur.  (They add 100 lbs of features, but only 75 lbs of weight!)  On the other hand, my race car weighs a scant 2500 lbs, and that’s with an old-school steel tub, a heavy steel cage, and 70 lbs of ballast in the floorboard.  And in its day it would carry 2 adults and 2 children (sans cage) plus their luggage.  Granted, it lacks all the amenities that consumers demand today.

Lightweighting is a good thing, with compounding benefits.  Reducing the weight of the vehicle means a reduction in the size of the engine needed to move it around, smaller brakes to stop it, and lighter suspension components with which to control it, all of which lead to further reduced weight!  At the same time, handling is improved and stopping distances are reduced.  So, why haven’t we seen this in our cars?  One answer is that many of the advanced materials are still too expensive to be cost-effective.  Secondly, consumers demand more and more comforts such as DVD players, Ipod adaptors, Bluetooth connectivity, Big Gulp holders, and hip room.  Finally, some argue that smaller and lighter-weight vehicles are less safe, backing it up with statistics showing increased fatalities for occupants of such vehicles.  And while it is true that today’s small vehicles won’t fare as well in a crash with a Ford Excursion as the Excursion will, this does not take into account the effect that smarter design and the use of advanced materials have on occupant safety.  Cars aren’t like billiard balls – there are many more dynamic forces at play, and heavier doesn’t necessarily mean safer.

Automakers need to build small, lightweight cars – not small versions of what they already build.

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